rice hull research paper

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Rice Husk Research: From Environmental Pollutant to a Promising Source of Organo-Mineral Raw Materials

Baimakhan satbaev.

1 RSE Astana Branch, National Center on Complex Processing of Mineral Raw Materials of the Republic of Kazakhstan, Nur-Sultan 010000, Kazakhstan; ur.liam@anatsa-cnf (B.S.); ur.kb@s_ilagrun (N.S.); ur.liam@veyabtas_nesra (A.S.)

Svetlana Yefremova

2 National Center on Complex Processing of Mineral Raw Materials of the Republic of Kazakhstan RSE, Almaty 050036, Kazakhstan; ur.liam@65nemraj (A.Z.); ur.liam@sa_vokebnalbak (A.K.); ur.liam@ter-vse (S.Y.)

Abdurassul Zharmenov

Askhat kablanbekov, sergey yermishin, nurgali shalabaev, arsen satbaev, vitaliy khen.

3 Scientific and Technical Society (STS), KAHAK, Almaty 050010, Kazakhstan; ur.tsil@nehkiylativ

Associated Data

Data sharing is not applicable to this article.

Rice husk is a large-tonnage waste left from rice production. It is not subject to humification and therefore becomes a serious environmental pollutant. Due to the presence of two essential elements—carbon and silicon—in its composition, rice husk is a promising organo-mineral raw material. The known methods for processing of rice husk are associated with the formation of even more aggressive waste. The creation of a waste-free technology for processing this plant material requires a detailed study. Rice husk of Kyzylorda oblast was studied using IR, SEM, TA, TPD-MS, EPR, and TEM methods. It was determined that under a temperature up to 500 °C, the ligno-carbohydrate component of rice husk decomposes almost completely. Three main peaks are recorded during the decomposition: hemicellulose at 200 °C, cellulose at 265 °C, and lignin at 350–360 °C. This process is endothermic. However, above of 300 °C the exothermic reactions associated with the formation of new substances and condensation processes in the solid residue begin to prevail. This explains the increase in the concentration of paramagnetic centers (PMCs) in products of rice husk carbonization in the range of up to 450 °C. Further increase in temperature leads to a decrease in the number of PMCs as a result of carbon graphite-like structures formation. The silicon–carbon product of rice husk carbonization (nanocomposite) is formed by interconnected nanoscale particles of carbon and silicon dioxide, the modification of which depends on the temperature of carbonization. The obtained data allow management of the rice husk utilization process while manufacturing products in demand based on ecofriendly technologies.

1. Introduction

Within the last few years, works on plant waste processing have multiplied all over the world, creating a wide range of products to be used in various areas of economic activity. Kazakhstan too is focusing on environmental protection and ecological development [ 1 , 2 ]. The “ Birge—taza Qazaqstan ” campaign is being carried out, aiming to foster environmental values in society and cultivate a caring attitude toward nature. The country’s leadership is keen to develop agribusiness, inevitably contributing to the growth of vegetable agricultural waste. This strongly calls for the implementation of the country’s Green Economy Concept [ 3 ]. Since Kazakhstan is a rice-growing country, it is now experiencing the problem of efficient processing of rice production waste—rice husk, which is globally one of the major environmental pollutants [ 4 ]. It is known that the performance characteristics of finished materials and the scope of their application largely depend on the technological parameters of processing of raw materials. Hence, as it was revealed by an earlier review of the literature [ 5 ], the studies of rice husk are mainly devoted to the research of conditions and operating parameters of processes of its refining. There are positive results of approbation of rice husk and products of its processing in different areas of practical use [ 5 , 6 ]. The two most common of them are the production of activated carbon [ 4 , 7 , 8 , 9 , 10 ] and the extraction of silicon dioxide as a source of silicon for the production of pure metal and its compounds, concrete, cement, and refractory ceramics [ 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 ].

However, the large number of methods of processing rice husk, proposed by various researchers, does not eliminate the problem of its utilization. In Kazakhstan, industrial processing of this waste is not implemented, and rice husk continues to accumulate. Due to a number of environmental and economic reasons, other countries are holding back the organization of such industrial enterprises [ 5 ]. Firstly, the majority of the proposed methods for rice husk processing lead to the formation of new waste. These are either the acidic effluents generated during the cleaning of rice husk, toxic gases released during its incineration, or dumps of finely dispersed ash [ 21 ]. Secondly, they do not ensure the profitability of production [ 5 ]. It is obvious that a technology is needed that would ensure the maximum utilization of this agricultural waste. However, the creation of an effective technology requires a comprehensive study of not only the recycling process, but also of the raw materials themselves. As a rule, the research in the field of rice husk processing is traditionally limited to determining its composition.

Consequently, the present work is devoted to the study of Kyzylorda oblast rice husk structure by the methods of IR spectroscopy (IR) and scanning electron microscopy (SEM), determining the qualitative and quantitative composition of its organic and mineral components, the study of the decomposition of rice husk by the method of thermal analysis (TA) while determining the role of the constituent components in this process by the method of temperature-programmed desorption mass spectrometry (TPD-MS), the study of the paramagnetic centers formation in the course of rice husk carbonization and the carbonization temperature influence on it, and the study of the supramolecular structure of carbonized rice husk. The obtained data make it possible to provide insight into rice husk as an organo-mineral raw material containing two important elements—carbon and silicon. The optimum decomposition temperature for rice husk has been identified. It is shown that the destruction of rice husk occurs as a result of the disintegration of energetically weak links and removal of easily moving groups with an increase in concentration of paramagnetic centers. The temperature ranges for endo- and exothermic processes leading to the formation of graphite-like structures on the basis of carbon-containing components have been identified. The dynamics of the transformation of silicon dioxide from one form to another with an increase in the temperature of heat treatment of rice husk have been traced. Such data make it possible to effectively manage the processes of using rice husk to produce a silicon–carbon nanocomposite with a unique structure or as an independent ingredient for the production of in-demand products.

2. Materials and Methods

2.1. materials.

Rice husk from different rice-growing farms of Kyzylorda oblast was used as the object of study. The combined batch of rice husk was washed with cold distilled water at a ratio of 1:5 in order to remove the residual flour and dust. Washed rice husk was separated from the liquid by filtering through a mesh with cell size of 5 mm and left on the mesh until an air-dry condition was reached. Rice husk in an air-dry condition was put into the drying oven, where it was kept at a temperature of 105 °C until it reached a constant weight.

2.2. Methods of Analysis

2.2.1. infrared spectroscopy.

IR spectra were recorded on a Specord M80 spectrophotometer (Carl Zeiss, Jena, Germany) in the form of press tablets with KBr in the range of 4000–400 cm −1 . The IR spectrum presented is the average of three measurements. Mineralogical analysis was performed by comparing the obtained IR spectra with correlation diagrams of group frequencies, as well as with reference IR spectra of monominerals [ 22 , 23 ].

2.2.2. Scanning Electron Microscopy

Scanning electron microscopy and electron probe microanalysis were performed on a Superprobe 783 microanalyzer (JEOL, Tokyo, Japan). Analyses and photography of secondary and backscattered (composition) electrons were performed by using an INCA Energy Dispersive Spectrometer (Oxford Instruments, London, England). To avoid the formation of a charge on the analyzed materials, which are capable of deflecting the electron beam, the samples were precoated with a thin structureless gold film in a fine coat ion sputtering apparatus (JEOL, Tokyo, Japan). To clarify the distribution of individual elements, footage of characteristic X-ray radiation of corresponding elements was taken.

2.2.3. Determining Rice Husk Composition

Rice husk composition determination was performed as per the methods described in [ 24 ]: the cellulose content was identified by the method of Kurschner and Hoffer, using a nitric acid alcoholic solution. For comparison, the cellulose amount was identified by determining hardly hydrolyzable polysaccharides using 80 wt.% sulfuric acid. The hemicellulose content was identified by determining easy hydrolyzable polysaccharides using 2 wt.% hydrochloric acid. The quantitative identification of lignin was performed using 72 wt.% sulfuric acid as per Komarov’s modification. The total quantity of extractive substances was identified by treatment with an alcohol–benzol mixture in a Soxhlet apparatus, as well as with hot water. The content of mineral components in rice husk was identified based on silicate chemical analysis.

2.2.4. Thermal Analysis

Thermal analysis was performed on a Hungarian Paulik F.–Paulik J.–Erdey L. system derivatograph Q-1500D (MOM, Budapest, Hungary). The smooth heating of the sample was performed to a temperature of 1000 °C with a temperature increase rate of 12 °C min −1 in the atmosphere of the exhaust gas.

2.2.5. Temperature-Programmed Desorption Mass Spectrometry

TPD-MS was performed on a MX-7304A monopole mass spectrometer (Electron, Sumy, Ukraine) with electron ionization, which was re-equipped for thermal desorption measurements. A sample weighing 1–20 mg was placed on the bottom of a quartz-molybdenum ampoule and before the start of the experiment was pumped out at room temperature to a pressure of ~5 × 10 −5 Pa. The programmed linear heating of the sample was performed at a speed rate of 0.15 °C s −1 to a temperature of ~750 °C. The volatile products of thermolysis directly entered the ionization chamber of the mass spectrometer through a high-vacuum valve 5.4 mm in diameter, ionized and fragmented under the action of electrons. After the separation by masses in a mass analyzer, the intensity of the ion current of the products of desorption and thermolysis was recorded by a VEU-6 secondary electron multiplier (“Gran” Federal State Unitary Enterprise, Vladikavkaz, Russia). The registration and analysis of mass spectra and curves of dependence of the pressure of volatile destruction products on the temperature of the sample (P-T) were performed by the automated computer-based data recording and processing system. The registration of mass spectra was performed in the range of 1–210 amu. During the TPD-MS experiment, about 240 mass spectra were recorded. During the thermal desorption experiment, the samples were heated rather slowly and the rate of evacuation of volatile thermolysis products was high, allowing us to neglect the diffusion effects; therefore, the intensity of the ion current was proportional to the rate of desorption.

2.2.6. EPR Spectroscopy

EPR spectroscopy of the initial and carbonized rice husk in the range starting from 200 °C to 800 °C with a step of 50 °C was performed on an upgraded EPR IRES-1001-2M homodyne spectrometer (JEOL, Tokyo, Japan), which operates in the 3 cm wavelength range, at room temperature and optimal conditions for registration of spectra: a magnetic field of 120 Oe with a microwave power of 16 mW and modulation amplitude of 1 Oe. These conditions of spectra recording were chosen as optimal because the microwave power value eliminated the effects of EPR saturation and the magnetic modulation amplitude eliminated broadening of the EPR line. EPR spectra were recorded between 3 and 4 components of the reference sample, which were ions Mn 2+ in MgO. G-factor and an EPR line width of tested samples were determined using known reference sample parameters. The concentration of free radical states of tested samples was calculated by comparison of areas of their spectra and the calibrated reference sample (third line of EPR spectrum ions Mn 2+ in MgO).

2.2.7. Transmission Electron Microscopy (TEM)

Transmission electron microscopy studies were performed on different instrumental equipment:

• - On the EM-125K device (Sumy electronic devices plant, Sumy, Ukraine) by the method of direct observation of translucency by using the microdiffraction. The samples were prepared by the method of dry preparation, i.e., by the method of dry application of the agent to a collodion backing film and by the method of one-stage carbon replicas with extraction. During the microdiffraction studies, the photographing of diffraction patterns was performed.
• - On the Transmission Electron Microscope Philips EM 301 (Philips, Amsterdam, Netherlands) at an accelerating voltage of 80 kV in the range of electron microscopic magnifications of 13–80 thousand times. The images were recorded with an Olympus C-3040 digital camera, which was operated via computer using the Image Scope M software (Systems for microscopy and analysis (SMA), Moscow, Russia). The objects were prepared as follows. A small quantity of the sample was ground in an agate mortar. The resulting powder was applied to an object copper grid previously coated with an amorphous carbon backing film. The object grid with the applied sample was fixed into the microscope object holder and inserted into the microscope column [ 25 ].

2.2.8. Rice Husk Carbonization and Extraction of Silicon Dioxide

Rice husk carbonization for further research was performed in a shaft furnace SSHOL-8/11 (Tula-Term, Tula, Russia) in an atmosphere of exhaust steam gases. For this purpose, the reactor was filled with 200 g of rice husk, hermetically sealed with a cap that has a tube for exhaust gas removal and placed into the working area of the furnace. Heating to a specified temperature (in the range of 200 to 1000 °C with a step of 50 °C) was performed at a speed rate of 15 °C min −1 , keeping the sample at this temperature for 30 min.

In order to extract silicon dioxide, rice husk carbonized at 650 °C was heat-treated at 800 °C in the open air in order to burn off the carbon. The process was performed in a rice husk carbonization unit. During the process, the reactor was closed with cap, which, in addition to the gas outlet tube, had the air inlet tube that supplied the air inside of the reactor through the whole layer of carbonized rice husk. The process was conducted for 1 h. The yield of the resulting product of yellow-white color in terms of rice husk was equal to 14.8 wt.%.

The separation of carbon and silicon dioxide in carbonized rice husk was performed with the help of a sodium hydroxide solution with a concentration of 70 g dm −3 at a solid:liquid ratio (S/L) of 1:15. The content of silicon dioxide in the carbon residue was equal to 2–3 wt.%.

3.1. Infrared Spectroscopy Study of Rice Husk

As can be seen from the IR spectrum ( Figure 1 ), rice husk of Kyzylorda region has a complex functional composition. In general, the spectrum is characterized by band widening. This may be due to various reasons: irregular intra- and intermolecular interactions, overlapping absorptions of different types of vibrations, and absorption at frequencies slightly different from each other. As a result, the bands merge, and many of them do not have independent peaks but are recorded as a shoulder on more intense lines. All of this creates certain difficulties in the spectrum interpretation.

IR spectrum of rice husk.

In the IR spectrum of rice husk, a series of bands of different intensity are observed with maxima at 3410, 2975, 1640, 1460, 1425, 1375, 1320, 1160, 1065, 1035, and 895 cm −1 . They can be caused by vibrations of functional groups of cellulose, which is the main component of plant tissues [ 26 ]. Cellulose is a polysaccharide whose molecules (C 6 H 10 O 5 ) n are long chains with a spatially regular structure. These chains consist of β-D-glucose (β-D-glucopyranose) links connected by glucoside bonds 1–4 [ 24 ].

Bands at certain wavenumbers can be caused by vibrations of the functional groups of different compounds that make up rice husk. For example, the band at 1160 cm −1 can also belong to the vibrations of the C–O bonds of oxygen-containing groups of hemicelluloses. Hemicelluloses are found in the cell walls of plants and are composed of polysaccharides containing elementary units of five to six carbon atoms. Most of the hemicelluloses are not homogeneous polysaccharides, but mixed. Mixed polysaccharides are composed of various monosaccharide residues linked by glucoside bonds at various positions. Therefore, a broad band with a maximum at 3410 cm −1 can correspond to stretching vibrations of hydroxyl groups included in hydrogen bonds of hemicelluloses as well [ 27 ].

The bands with maxima at wavenumbers 1595 and 1512 cm −1 are typical for skeletal vibrations of aromatic rings. Their presence indicates the presence of lignin [ 28 ]. Lignin is a natural polymer built from the structural elements of C 6 C 3 oxygen derivatives of phenylpropane produced from carbohydrates [ 29 ].

According to paper [ 28 ], the band at 1640 cm −1 can also be caused by vibrations of carbonyl groups conjugated with condensed nuclei. The peak with a maximum at 1725 cm −1 is due to vibrations of conjugated aldehyde and ketone groups and non-conjugated carbonyl and carboxyl groups [ 30 ]. The opinions of researchers differ on the interpretation of the absorption band at 1275 cm −1 . Some believe [ 31 ] that this band is due to asymmetric stretching vibrations of C-O-Si bonds in the methoxyl groups of lignin. Others attribute it mainly to bending vibrations of methylene CH 2 groups [ 32 ].

The bands at 1085, 795, and 465 cm −1 are characteristic bands of the silica component [ 22 ]. No clear absorption bands corresponding to the presence of C-O-Si or Si-CH 3 bonds in rice husk were recorded. However, the intense band with a maximum at 1085 cm −1 has shoulders at 1070 cm −1 and 1040 cm −1 , as well as diffuse absorption bands at 1275 cm −1 and 1225 cm −1 , typical for the valence vibrations of C-O-Si or Si-CH 3 , respectively [ 33 ].

3.2. SEM Study of Rice Husk

Figure 2 a shows a longitudinal section of rice husk, characterizing its shape. Rice husk has a base formed by heterogeneous fibers. A wave-like “shell”, the inner filling of which is represented by a loose mass of plant tissue, is held at the base. Numerous cracks give visible friability to the specimen. Dispersed particles, possibly introduced as a result of mechanical destruction of rice husk, are observed in the deep cracks. Figure 2 shows the distribution of carbon ( Figure 2 b), silicon ( Figure 2 c), and oxygen ( Figure 2 d) in a longitudinal section ( Figure 2 a) of rice husk. The image was obtained in backscattered electrons (in the characteristic emission of C, Si, and O, respectively). The distribution of elements in the figure is characterized by the cluster of light dots. As shown in Figure 2 c, silicon is predominantly located in the outer surface layer and is also localized in some places of the plant tissue. Local accumulations on the inner layer are also explained by the presence of destroyed surface layer particles. The distribution of oxygen ( Figure 2 d) follows the shape of silicon and carbon. However, the image contrast in the case of carbon and oxygen is lower in comparison to silicon. This may be explained as follows. It is known that the higher the atomic number of the element and the greater the difference between the atomic numbers of the elements being compared, the higher the probability of backscattering and the higher the image contrast. The presence of silicon dioxide in rice husk composition distinguishes it from other plant materials.

SEM micrographs of the rice husk sample: ( a ) longitudinal section of rice husk; ( b ) distribution of carbon; ( c ) distribution of silicon; ( d ) distribution of oxygen.

After rice husk thermal treatment, a redistribution of chemical elements was registered by SEM ( Figure 3 ). As a result of the destruction of the rice husk organic component, fusion of the material occurs ( Figure 3 a). Deep channels are formed in the places of burnout of longitudinal fibers and along the contour of the “shell”. A peculiar carbon matrix is formed ( Figure 3 b). It is evenly filled with a silicon-containing phase ( Figure 3 c). As can be seen from the accumulation of white dots in Figure 3 d, the oxygen distribution corresponds to the silicon distribution form. Thus, the thermal destruction of rice husk leads to the formation of a silicon-carbon composite.

SEM micrographs of the rice husk carbonized at 650 °C sample: ( a ) longitudinal section of rice husk carbonized at 650 °C; ( b ) distribution of carbon; ( c ) distribution of silicon; ( d ) distribution of oxygen.

3.3. Rice Husk Composition

Due to the intricate structure of plant tissue, all of its constituent components are very closely related to each other. This makes it difficult to separate them. Thus, cellulose in plant cell walls is closely related to hemicelluloses, lignin, and extractive substances, while lignin is partially penetrating the cellulose microfibrils. Some hemicelluloses (cellulosans) form associates with cellulose and cannot be removed from plant tissue without noticeable damage to the cellulose itself. Therefore, the techniques described in Section 2.2.3 were used to perform the most accurate quantification of rice husk composition.

When treating rice waste with a mixture of concentrated nitric acid and ethyl alcohol, the lignin is nitrated and partially oxidized, and transferred to the alcohol solution. Hemicelluloses are hydrolyzed for the most part. Alcoholic medium moderates the oxidizing and hydrolyzing effect of nitric acid on cellulose. The cellulose content in the studied sample of rice husk, determined by this method, was 33 wt.%. This figure agrees well enough with the amount of cellulose (30 wt.%) established by the method of determination of hard-to-hydrolyze polysaccharides using 80 wt.% sulfuric acid. Hemicellulose content was determined according to the method of determination of easily hydrolyzable polysaccharides using 2 wt.% hydrochloric acid. Their quantity in the composition of rice husk was 18 wt.%. Since the composition of extractive substances is extremely diverse and quantitative isolation of individual components is rather complicated, the total amount of extractive substances soluble in the alcohol–benzene mixture (i.e., resins) and hot water was determined in the composition of rice husk, which was 2.0 wt.% and 6.1 wt.%, respectively. Unlike polysaccharides (cellulose and hemicelluloses), which are hydrolyzed to simple sugars, lignin is resistant to the action of mineral acids. Therefore, its content was determined using 72 wt.% sulfuric acid at room temperature on rice husk previously deresinated with an alcohol and benzene mixture. The reaction mixture was then diluted with water and boiled. The amount of lignin determined by this way was 26 wt.%.

By the silicate chemical analysis scheme, it was found that rice husk contained mineral components such as silicon dioxide (~14.0 wt.%), calcium oxides (~0.2 wt.%), magnesium (~0.1 wt.%), iron (~0.02 wt.%), aluminum (<0.1 wt.%), potassium (~0.4 wt.%), sodium (~0.03 wt.%), and other elements at impurity levels.

3.4. Investigation of Rice Husk Thermal Degradation Process by Thermal Analysis

Thermal analysis of rice husk in the region of 50–150 °C captures on the DTA curve an endo effect related to the loss of free water ( Figure 4 ). The decomposition of rice husk in the exhaust gas atmosphere ends at 770 °C, as can be seen on the TG curve. The DTG curve shows that the bulk of rice husk decomposed with the highest rate at 238 and 265 °C. This process runs with heat absorption, since two blurred endo effects are fixed on the DTA curve at these temperatures. The exothermic effect recorded on the DTA curve at 300 °C is caused by the formation of new substances and condensation processes in the solid residue. It is alternately replaced by endo (450, 550 °C) and exo (500, 600 °C) effects caused by structuring and burnout of carbonaceous residue. Rice husk mass loss reached 79.5%. The excess mass of the residue (20.5wt.%) over the mineral component in the composition of rice husk (~15wt.%) is explained by the presence of carbon formed during condensation–destruction processes and subsequent graphitization (since TA was performed in the atmosphere of the exhaust gas) and is probably firmly bound to silicon dioxide.

Rice husk derivatogram.

3.5. Study of Rice Husk Pyrolysis by TPD-MS

Analysis of correlation curves between the pressure of volatile degradation products of plant materials and heating temperature (P/T) showed ( Figure 5 ) that rice husk decomposes in the temperature range of 150–500 °C, with the maximum decomposition being fixed at T max ~265 °C. The data obtained are in good agreement with the TA results. Comparison of Figure 5 and Figure 6 indicates that the release of the bulk of rice husk pyrolysis products at T max ~265 °C is caused by cellulose degradation due to the desorption of pyran and furan derivatives ( m / z 98). The formation of pyran derivatives is caused by the dehydration of elementary links in the pyranose form; the formation of carboxylic acids followed by decarboxylation promotes the formation of furan derivatives; at the same time, the appearance of aromatic structures at this temperature indicates the sequential course of intermolecular and intramolecular aldol condensation reactions [ 34 ]. The pyrolysis stage with T max ~350 °C occurs due to the degradation of aromatic compounds of lignin, which proceeds in a wider temperature range with the formation of phenol ( m / z = 94, T max = 350 °C), pyrocatechol ( m / z = 110, T max = 290–330 °C), cresols ( m / z = 107, T max = 365 °C), tropylium ion, C 7 H 7 + ( m / z = 91, T max = 340 °C), 4-vinyl-methylguaiacol ( m / z = 164, T max = 290 °C), benzene ( m / z = 78, T max = 360, 545 °C), naphthalene ( m / z = 128, T max = 520 °C), and 4-vinylphenol ( m / z = 120, T max = 220, 290 °C). The formation of these compounds is caused by thermal transformations of corresponding aromatic links and functional groups of lignin [ 35 ]. The stage with T max ~200 °C corresponds to the destruction of hemicelluloses [ 36 ]. However, due to their close relationship with other plant matter constituents, the above compounds predominate in the pyrolysis products at 150–250 °C ( Figure 6 b). At the same time, the ion with m / z = 60 (HOCHCHOH + ) observed in Figure 6 a is known to be the most intense marker ion in the mass spectra of carbohydrate pyrolysis products and is usually detected already at 150 °C [ 37 , 38 ].

Temperature–pressure (P-T) curves of rice husk.

TPD-MS results: ( a ) mass spectrum of pyrolysis products of rice husk at 347 °C obtained after electron impact ionization; ( b ) TPD-curves of the ions with m / z 164, 128, 120, 110, 107, 98, 94, 91, and 78 under pyrolysis of rice husk.

3.6. Investigation of Rice Husk Structural Changes during Carbonization by EPR Spectroscopy

When studying the structural changes of rice husk during heat treatment up to 800 °C using EPR spectroscopy, it was found that the material already exhibits an EPR signal at room temperature. This signal can be caused by the formation of free radicals under mechanical impact during rice husk grinding. The parameters of the EPR spectra are presented in Table 1 . For illustrative purposes, the dependence of paramagnetic centers (PMCs) content on the processing temperature of rice husk is shown in Figure 7 . The concentration of paramagnetic centers reaches a maximum value (1000 × 10 16 spin g −1 ) at 450 °C. There is a general tendency of reducing the width of the EPR line while increasing temperature of heat treatment, as it follows from Table 1 . However, despite general narrowing of the line, it broadens at certain temperatures (400 and 450 °C). Obviously, at these temperatures two kinds of paramagnetic centers are formed (free radicals and clusters as a result of closure of the former) with similar values of g-factor, the superposition of their signals leads to the broadening.

Dependence of the concentration of paramagnetic centers on the carbonization temperature of rice husk.

Parameters of EPR spectra of original and carbonized rice husk.

Comparing the pattern of PMCs concentration changes as the plant raw material processing temperature increases with the results of TA, IR spectroscopy, and TDS-MS, it can be concluded that the growth of PMCs amount when heating rice husk to 450 °C is caused by an increase in the concentration of free-radical states (FRS) as a result of splitting of energy-weak bonds and removal of easily mobile groups. Carbonization of plant material is inevitably accompanied by the formation of condensed carbon rings that form a graphite-like structure. This leads to a decrease in the value of the PMCs index. Consequently, on the ascending part of the paramagnetic centers concentration dependence on the treatment temperature ( Figure 7 ), the ΔH decrease ( Table 1 ) is explained by intensification of exchange interactions in the FRS spin system as their concentration increases, while on the descending part—by a decrease in the dipole–dipole interaction and appearance of delocalized π electrons in graphite-like structures. However, treatment of rice husk carbonized at 650 °C with a sodium hydroxide solution causes PMCs concentration to increase to 1.1 × 10 19 spin g −1 . This is obviously associated with the removal of silicon dioxide from carbonizate due to the breaking of C-SiO 2 bonds and formation of a large amount of FRS. In the studied temperature range of rice husk heat treatment, gradual decrease of g-factor values also occurs, which approach the free electron g-factor value (g = 2.0023) in graphite structures. Thus, we can conclude that rice husk structural transformations during heat treatment undergo a free radical formation stage followed by the formation of hexagonal meshes of cyclically polymerized carbon.

3.7. Characteristics of Carbonized Rice Husk Supramolecular Structure

The presence of particles of different morphology and sizes was recorded in rice husk carbonized at different temperatures (600, 650, and 1000 °C) using transmission electron microscopy ( Figure 8 ). Mainly, there are lamellar translucent and dense particles ( Figure 8 a); aggregates formed by translucent particles of round or oval shape of 15–20 nm and larger ( Figure 8 b). However, more attention should be given to hybrid structures, which are a combination of two phases ( Figure 8 c,d): a lamellar formation of one phase permeated by another denser dispersed (10–20 nm) phase. Moreover, the hybrid particles can be porous (pore size 15–20 nm, Figure 8 c). Particles whose microdiffraction pattern ( Figure 8 e,f) is represented by rings (0.337; 0.210; 0.122 nm), symmetric reflexes (0.46; 0.406; 0.337; 0.251; 0.245; 0.236 nm) and broad rings with a set of interplanar distances of 0.253; 0.212; 0.152; 0.121 nm were determined in the rice husk carbonizate by microdiffraction studies along with amorphous particles. According to paper [ 39 ], this indicates the presence of the following carbon and silica-containing phases: C (26–1080), H 2 Si 14 O 29 ⋅5.4H 2 O (31–584), SiC/Unnamed mineral, syn. (29–1129).

TEM micrographs and microdiffraction patterns of carbonized rise husk: ( a ) rice husk particles carbonized at 650 °C; ( b ) rice husk particles aggregates carbonized at 1000 °C; ( c ) porous hybrid formation obtained by rice husk carbonization at 600 °C; ( d ) cluster of differently shaped particles of rice husk carbonized at 650 °C; ( e ) microdiffraction patterns from the particles shown in Figure 8 a; ( f ) microdiffraction patterns from the particles shown in Figure 8 d.

To study the structure of silica, carbonized rice husk was heat-treated at 800 °C in open air to burn out the carbon component. Figure 9 shows clusters of dense particles of 20 nm and larger ( Figure 9 a) and translucent rounded particles of 20 to 100 nm in diameter ( Figure 9 c). The microdiffraction pattern from dense particles is represented by reflexes with a set of interplanar distances of 0.432; 0.375; 0.286; 0.261; 0.231; 0.207; 0.154; 0.15 nm ( Figure 9 b), corresponding to a mixture of SiO 2 (12–708), SiO 2 (18–1169), and H 2 Si 2 O 5 (27–606) [ 39 ]. The appearance of a 0.428 nm diffuse ring in the microdiffraction pattern from the translucent particles ( Figure 9 d) indicates the nucleation of the SiO 2 /Tridymite-20H, syn. (14–260).

TEM micrographs and microdiffraction patterns of rice husk silica: ( a ) cluster of dense particles; ( b ) microdiffraction pattern from dense particles; ( c ) cluster of translucent particles; ( d ) microdiffraction pattern from translucent particles.

4. Discussion

Analysis of rice husk and products of its heat treatment showed the following. Rice husk is an organo-mineral material. As a large-tonnage waste of rice production, it is a large-scale environmental pollutant, since due to the presence of silicon dioxide it is not subject to the humification process. At the same time, the combined presence of carbon and silicon dioxide in its composition opens up broad prospects for its use as a valuable raw material. The value of rice husk lies in ensuring the formation of a unique silica–carbon structure of the materials produced from it.

The state of carbon and silicon dioxide in the resulting products depends on the processing conditions of plant raw materials. It is possible to get carbon with an amorphous or graphite-like structure. The situation is similar to silicon dioxide. Starting as amorphous, various crystalline forms of silicon-containing products can be obtained: cristobalite, tridymite, and even silicon carbide as a result of the interaction of silicon dioxide with the resulting carbon. Carbonized rice husk is made up of different-shaped particles. The hybrid structures formed by particles of carbon and silicon dioxide have the greatest interest among them.

Due to the fact that silicon dioxide is connected strongly with organic components in rice husk after the thermal destruction of plant tissue, it does not form an isolated structure but rather has a strong interaction with carbon (up to chemical bond), and as a result it builds a nanocomposite ensemble. Obviously, being formed inside the plant cell in the form of soluble silicic acid (which the presence of was registered by a TEM microdiffraction technique even in carbonized rice husk), silica diffuses through the membranes of plant tissue to its outer surface, causing silicification of the cellulose scaffold, which was confirmed by the results of SEM study of the elements distribution in the rice husk structure. Some of it remains in the inner layer. In both cases, the formation of bonds between silica tetrahedra, carbohydrates, and lignin is possible. According to TA and EPR spectroscopy, the temperature of 650 °C appears to be the necessary and sufficient temperature for the carbonization process of rice husk, although according to TPD-MS data, rice husk decomposes in the range up to 500 °C in three stages corresponding to the decomposition of the main components (hemicellulose, cellulose, lignin). At 650 °C, the destruction of the initial raw materials is completed, as shown by TEM study of the supramolecular structure with the formation of a silicon–carbon nanocomposite in which some of the C-SiO 2 bonds remain intact. This was confirmed by the results of different techniques used. Firstly, as it was found during TA, the residue mass after rice husk decomposition in the exhaust steam gas atmosphere (under analogous conditions carbonization of the feedstock was also performed for the study by other methods) exceeded the mass of the mineral component in rice husk due to the formation of cyclically polymerized carbon and probably SiC, as evidenced by the EPR and TEM results. A subsequent attempt to obtain carbon freed from silicon dioxide by the chemical way was unsuccessful. The residual content of SiO 2 in carbonized rice husk treated with an alkaline solution was 2–3 wt.%. Moreover, after this treatment of the obtained product, according to EPR spectroscopy data the number of free radicals increased significantly as a result of the destruction of the C-SiO 2 bonds present in the silicon–carbon composite. Thus, the results of studies carried out by different methods within the framework of the present work are in good agreement and complement each other.

Carbonized rice husk with a unique structure has a wide range of applications: as an active filler for elastomers [ 40 ], a reducing agent in electrothermal metallurgical processes [ 6 ], a sorbent [ 41 ], and a feed additive [ 6 ]. However, in a number of processes, it seems economically more profitable to use rice husk as raw material with the formation of the necessary compounds directly during the process of obtaining the final product. Examples of such processes are the production of new generation refractory materials by the method of self-propagating high-temperature synthesis (SHS) [ 42 , 43 , 44 , 45 ] and the production of plate material in the process of vapor-explosive hydrolysis without the use of any types of synthetic plastics [ 46 ].

The modern concept for the development of the refractory industry consists of the transition to the production of resource-saving refractories of a new generation, distinguished by increased environmental safety and wear resistance, as well as ensuring an increase in the quality of the final product. The feasibility of creating a new generation of refractories is due to the increasing requirements of consumers, as well as the need to improve the operating conditions of refractories and reduce energy costs in their manufacturing. For the development of refractory production, the Republic of Kazakhstan has sufficient raw materials. There are significant reserves of refractory clays, quartzite, chrome ores, small deposits of magnesite, zircon, talc-magnesite, bauxite, man-made raw materials represented by waste from the mining and metallurgical industry [ 47 , 48 ], as well as mineral raw materials for the production of composite materials and ceramics based on corundum, zircon, andalusite, and barite.

Carborundum (silicon carbide) is often used to increase the strength of refractory materials. As part of the present work, we recorded silicon carbide formation during the carbonization process of rice husk. This process was studied and described in detail in papers [ 21 , 49 ]. Therefore, it seems reasonable to introduce rice husk into the charge to produce refractories. When a specially prepared charge is heated to 950 °C, the process of self-propagating high-temperature synthesis will take place. Under the conditions of this SHS process, the organic component of rice husk will be carbonized to form nanoscale carbon. As a result of carbon interaction with silicon dioxide present in rice husk in its active form, silicon carbide will be formed. On the one hand, this will reduce or completely eliminate the consumption of silicon-containing ingredients to be introduced into the charge. On the other hand, enrichment of reaction products with highly fire-resistant (low porosity and high density) compounds will contribute to the formation of durable and dense refractory, increasing thermal durability of refractory lining to be used in chemically aggressive environments of ferrous and nonferrous metallurgy, energy, chemical industry, and construction materials production.

At present, the issues of using plant fibers from agricultural waste in the manufacturing of building boards are quite pressing. There are a number of problems that need to be solved at the same time [ 50 , 51 , 52 , 53 , 54 , 55 , 56 , 57 , 58 ]:

• - Release of volatile organic substances, aldehydes, and terpenes, a small amount of which causes adverse effects on health.
• - Unsatisfactory physical and mechanical properties of the resulting materials, providing mainly as use for decoration and furniture production, but limiting their use in the construction industry.
• - Use of a large number of highly toxic and inflammable artificial organic polymers (formaldehyde, epoxy, and other resins such as binders, various types of hardeners, plasticizers, and adhesives) since almost all technologies are based on pressing plant biomass. Strengthening the mechanical properties usually requires increasing the amount of resin binder. Increasing fire resistance and improving other properties such as sound absorption, impact resistance, and thermal conductivity is associated with a more complex composition of the blend, i.e., increasing the number of ingredients and different methods of processing can increase in the cost of the finished material.

In this regard, there is independent interest to try and use rice husk for the production of building boards by the method of steam explosive hydrolysis, which determines the effective decomposition of organic raw materials [ 46 ]. The method is based on implementing hydrothermal degradation of polysaccharides by alternating the stages of pressing with charge heating and decompression to form a silicon–lignin solid residue. Plasticizing properties of lignin will allow eliminating the use of synthetic binders. The presence of silicon dioxide in the board composition will contribute to increasing its strength. This direction of research is attractive because it opens the prospect of developing the production of efficient construction boards on the basis of ecofriendly technology using renewable raw materials.

5. Conclusions

Rice husk of Kyzylorda region has a complex functional composition. The main components are polysaccharides (48–52 wt.%) and lignin (26 wt.%). A distinctive feature of rice husk is its high ash content due to the presence of silicon dioxide (14 wt.%). Silicon dioxide is predominantly evenly located in the outer surface layer of rice husk and in the form of local accumulations in its internal surface layer.

Thermal destruction of rice husk occurs at up to 500 °C in three stages. Hemicelluloses decompose at 200 °C. The maximum decomposition at 265 °C is caused by the destruction of cellulose. In the range of 350–360 °C, the destruction of lignin takes place. The decomposition process of rice husk is endothermic. Above 300 °C, exothermic reactions predominate due to the formation of new substances and condensation processes in the solid residue.

In the process of rice husk carbonization at 450 °C, the concentration of paramagnetic centers increases due to the splitting of energetically weak bonds and the removal of easily mobile groups. A further increase in temperature to 800 °C is accompanied by a decrease in the number of PMCs as a result of the formation of graphite-like structures since the g-factor value approaches free electron g-factor value (g = 2.0023) in graphite structures. The preferred carbonization temperature is 650 °C.

Carbonized rice husk is a silicon–carbon nanocomposite formed by nanosized particles of carbon and silicon dioxide with the presence of silicon carbide. Thanks to its unique structure, the silicon–carbon nanocomposite has a wide range of applications. The studies performed and the results obtained make it possible in the future to test rice husk as an independent charge ingredient in the preparation of refractories by the SHS method and building plates by the vapor-explosive hydrolysis method.

Acknowledgments

The authors would like to thank N.T. Cartel, Director of the Chuiko Institute of Surface Chemistry, National Academy of Sciences of Ukraine, academician of the National Academy of Sciences of Ukraine; T. Kulyk, Head of the Laboratory of the Kinetics and Mechanisms of Chemical Transformations on Solid Surfaces of the Chuiko Institute of Surface Chemistry, National Academy of Sciences of Ukraine, PhD in Chemistry, and Senior Researcher for performing of the TPD-MS; V. Levin, Head of the Mineralogy Laboratory of the K.I. Satpayev Institute of Geological Sciences and Candidate of Geological and Mineralogical Sciences for performing of the scanning electron microscopy; L. Komashko, microscopist of the D.V. Sokolskiy Institute of Fuel, Catalisys and Electrochemistry JSC, V. Matveev and F. Pisarev, microscopists of the Frumkin Institute of Physical chemistry and Electrochemistry Russian academy of sciences for performing of the transmission electron microscopy; Y.A. Ryabikin, Leading Researcher of the Institute of Physics and Technology and Candidate of Physical and Mathematical Sciences for performing of the EPR spectroscopy.

Author Contributions

Conceptualization, S.Y. (Svetlana Yefremova) and B.S.; methodology, S.Y. (Svetlana Yefremova); investigation, S.Y. (Svetlana Yefremova), B.S., A.K., S.Y. (Sergey Yermishin), N.S., A.S., and V.K.; data curation, S.Y. (Svetlana Yefremova); writing—original draft preparation, S.Y. (Svetlana Yefremova); writing—review and editing, A.Z.; supervision, A.Z. and B.S.; project administration, S.Y. (Svetlana Yefremova) and B.S.; funding acquisition, B.S. All authors have read and agreed to the published version of the manuscript.

This research was funded by the Ministry of Education and Science of the Republic of Kazakhstan over the years, including grant numbers 1123/GF3 and AP05132122.

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Data availability statement, conflicts of interest.

The authors declare no conflict of interest.

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The use of waste materials in the papermaking process is at the forefront of the packaging world. It is no secret that traditional packaging harms the environment and damages sustainability. Deforestation and the subsequent loss of habitat are both severe factors that plague traditional cardboard packaging. In hopes that we can minimize these issues, we should explore the possibility of implementing organic waste materials into our packaging to increase sustainability./p>

Introduction

In 1978, the UN defined sustainability as, “meeting the needs of the present without compromising the ability of future generations to meet their own needs.” If we want to ensure the future of our planet, the importance of sustainability cannot be overstated. /p>

The demand for paper-based materials contributes significantly to deforestation, a phenomenon that has drastic effects on sustainability. “About 3.5 billion cubic meters of wood is harvested worldwide each year (Food and Agriculture Organization, 1995), of which 500 million cubic meters (or 14 percent) is used for pulp and paper.” Deforestation leads to significant environmental problems such as climate change, the loss of habitats, and a subsequent decrease in biodiversity in affected regions. /p>

The utilization of organic waste materials instead of wood can lessen the severity of deforestation and increase sustainability. Rice hulls are one such organic waste material, and its exploration can allow for the possible benefits explored above and minimize the risk of climate change./p>

Benefits of Rice Hulls

Rice hulls can bring many potential benefits due to the certain qualities that they hold. The implementation of rice hulls in the papermaking process can dramatically increase sustainability due to their abundance, protective qualities, and their general efficiency./p>

Most rice husks find themselves in landfills. Nearly twenty percent of a rice harvest is composed of hulls by mass, and roughly 7 million hectares of forest are lost annually according to the Rainforest Action Network (RAN). Utilizing even a fraction of these materials in papermaking could drastically reduce wood consumption, and thus lessen the impact of deforestation. 

This abundance of rice hulls can already be utilized in the papermaking process. Even if five percent of cardboard has rice hulls implemented into the material, we could see a significant reduction in deforestation. 

Despite their weight, rice hulls are a strong and durable material. The husks are water-resistant and are known to be good insulators. Having a package that resists such environmental conditions would be economical and preferable for consumers, especially when combined with the rice hulls' green nature./p>

Perhaps the greatest benefit brought by the implementation of rice hulls is the general efficiency they bring. Their lightweight nature eases shipping as weight is often a limiting factor when it comes to mass-cargo shipping and managing large supply chains. Their nature as a byproduct of rice is also helpful, as food for the general populace along with a possible packaging material is being produced in one.

Building upon the weight reduction, packages with rice hulls would be able to reduce costs and be transported with greater fuel efficiency. In an article by Kristy Flowers, she argues the following, “The benefits of reduced packaging help reduce production costs and the cost savings are passed onto the consumer.” Not only is their implementation environmentally friendly and logistical, but it’s also economical because of the cheapness of rice husks. She goes on to argue, “Reduced packaging… more packages… can be transported at the same time and reduce the number of trips required. Less fossil fuel is needed and so less pollution is generated with the use of PBHs.” The use of rice hulls can therefore reduce the number of trips trucks, ships, and planes take to transport packages over time. 

The benefits of implementing rice husks are numerous. However, some drawbacks also exist. The effects of lignin on paper strength and the increased biodegradability time for rice hulls should both be addressed and explored so that, if we implement rice hulls into papermaking, we do so in a responsible and proper manner.

Possible Drawbacks

While rice hulls have massive potential benefits, their implementation in papermaking suffers due to lessening paper quality. Rice hulls contain a yellowish fiber called lignin, which tends to weaken the paper’s strength. This decreases its ability to be recycled. Each time a piece of paper is recycled, the cellulose fibers, which comprise it, grow shorter and weaker. Composing a paper with a majority of rice hulls in its composition can thus shorten its lifespan.

Biodegradability is a primary concern when it comes to packaging. Over time, the environment naturally decomposes waste materials. Using the table below, we can see that paper products such as cardboard generally degrade in a few months. Rice hulls, however, tend to take upwards of a year. Thus, despite being a biological product, they take longer to decompose naturally, a possible hindrance to sustainability.

Yet, the rate of degradation is nothing compared to other products. Glass and plastic bags, two mainstream packaging products, are far less efficient in terms of degradability. Just because rice hulls degrade slower compared to paper products does not mean that they are inherently less sustainable due to how both products are produced.

Separating the production and degradation processes creates a clearer picture. To produce cardboard, deforestation and loss of habitat occur. Producing rice hulls, on the other hand, is a byproduct of producing food. Therefore, it is arguable that despite the fast rate of degradation undergone by cardboard, rice hulls are still more environmentally friendly. 

While these drawbacks are something to be considered, they are not complete hindrances to implementing rice hulls in papermaking. To counteract the effect of lignin, a smaller percentage of rice husks can be implemented into the paper mixture. This way the paper will retain some of its strength while decreasing wood consumption and thus, deforestation.

To conclude, rice hulls are a good example of an organic waste material that has the potential to revolutionize the packaging industry. Exploring involving the implementation of organic waste materials into daily objects such as cardboard can drastically reduce climate change and have a positive impact on the environment. 

References:

https://www.un.org/en/academic-impact/sustainability

https://www.ran.org/the-understory/how_many_trees_are_cut_down_every_year/

http://sustainability.asn.au/blog/wp-content/uploads/2019/05/chart-01.png

https://www.norwexmovement.com/rice-husks/

https://www.kristykorganics.com/post/benefits-of-using-rice-hulls-pbh

https://www.riceland.com/rice-hull-product

https://www.researchgate.net/publication/283082426_Rice_Hulls_as_a_Cushioning_Material

https://www.wcibags.com/2017/01/24/biodegradable-bags-compostable-bags-and-recyclable-bags-what-does-it-all-mean/

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• Published: 29 November 2019

Efficiency of rice husk ash and fly ash as reactivity materials in sustainable concrete

• Mohamed Amin 1 &
• Bassam Abdelsalam Abdelsalam 1  

Sustainable Environment Research volume  29 , Article number:  30 ( 2019 ) Cite this article

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Many environmental problems occur due to rice husk burning and emissions from coal-fired power stations. This paper presents the recycling of rice husk ash (RHA) and fly ash (FA) from power plants as reactivity materials for producing sustainable (green) concrete. This research aims to investigate the efficiency of RHA and FA replacement ratios on fresh and hardened properties of concrete mixtures. The experimental program consisted of 21 concrete mixtures, which were divided into three groups. The cementitious material contents were 350, 450 and 550 kg m −3 for groups one, two and three, respectively. The replacement ratios from the cement content were 10, 20 and 30% respectively, for each recycle material (RHA and FA). The slump and air contents of fresh concrete were measured. The compressive strength, splitting tensile strength, flexural strength, modulus of elasticity and bond strength of hardened concrete as mechanical properties were also analyzed. The compressive strength was monitored at different ages: 3, 7, 28, 60 and 90 d. The water permeability test of hardened concrete as physical properties was conducted. Test results showed that the RHA and FA enhanced the mechanical and physical properties compared with the control mixture. The cementitious content of 450 kg m −3 exhibited better results than other utilized contents. In particular, the replacement ratios of 10 and 30% of RHA presented higher mechanical properties than those of FA for each group. The water permeability decreased as the cementitious content increased due to the decrease in air content for all mixtures. The water permeability loss ratios increased as the cementitious content decreased.

Introduction

Rice husk ash (RHA) is an agricultural waste byproduct, and its disposal presents a major challenge by waste managers. RHA from parboiling plants exerts critical environmental threat; thus, approaches for its reduction are urgently needed. RHA material is considered a real super pozzolan due to its richness in silica, the content of which is approximately 85–90% [ 1 ]. The incorporation of high-volume fly ash (FA) (60% by binder weight) in cement paste/mortar physically and chemically influences the microstructure of the cementitious system. The replacement of cement with FA increases the water-to-cement ratio and causes low early age strength. Xu and Sarkar [ 2 ] indicated that such replacement is responsible for producing 1–3 μm spaces between particles in the paste at the early ages. The usage of FA as pozzolan in the Thai concrete industry has significantly increased during the past decade because of its enhancement of concrete features. The mechanism responsible for the enhancement has been well documented [ 2 , 3 , 4 ]. The degree of hydration of the cement paste is low at low water/cement (w/c) ratios. At the age of 7 d, approximately 50% of the cement is hydrated; at 7–90 d, the degree of hydration is increased by a few percent [ 4 ]. Chopra et al. [ 5 ] concluded that the increases in strength of approximately 25% at 7 d, 33% at 28 d and 36% at 56 d are attributed to the increases in RHA content from the control mixture to the 15% cement alteration. The increase in RHA content of up to 15% increases the compressive strength of the concrete, but above this value, the strength is reduced due to the decreased hydration reaction and cement content [ 5 ]. Habeeb and Mahmud [ 6 ] conducted the X-ray diffraction graph and showed that the ash from burning husk at a temperature less than 690 °C is in amorphous form because of the broad peak on the 2θ angle of 22° [ 6 ]. Provis [ 7 ] stated that alkali-activated materials are inorganic binders resulting from the reaction of an alkali metal source (solid or dissolved) with a solid silicate powder-like FA and slag. FA has been increasingly regarded as an appropriate raw material for alkali-activated concrete because of its availability and adequate composition of silica and alumina [ 8 , 9 , 10 ]. Alkali-activated FA mortars, regardless of the type of activator used, are generally more durable than ordinary Portland cement mortars under experimental conditions [ 11 , 12 ]. Also, alkali-activated FA concrete when cured at an elevated temperature has excellent mechanical and durability features [ 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 ]. Cement and concrete production has resulted in environmental burdens. For example, the cement-producing trade accounts for 5–7% of the total carbon dioxide phylogenesis emissions [ 20 ]. One of the foremost pressing challenges that the housing industry faces is the deterioration of concrete structures. Thus, the scientific community should promote industrial ecology (utilization of commercial by-products) and establish the principles of property management in concrete production. These goals will attain a ‘green’ combined style and brand-new rigorous approach towards the construction of sturdy structures (for a given service life) with minimum environmental burden [ 21 ]. The pozzolanic activity of RHA depends on the silicon oxide content, silicon oxide crystallization and size and area of ash particles. In general, ash should contain a restricted quantity of unburnt carbon. RHAs with amorphous silicon oxide content and huge specific surface area are often made through the combustion of rice husk at controlled temperature, and these factors are principally liable for its high reactivity [ 22 , 23 , 24 ]. As the advanced evolution of associated reactions with cement is not entirely represented, accelerated pozzolanic tests can be used to approximate the RHA reactivity. This case can be provided that inherent characteristics, such as reactive silicon oxide, cannot be rated as an absolute index of RHA reactivity in amalgamated cement [ 25 ].

This study aims to solve various environmental problems, such as RHA and FA, and preserve the natural resources simultaneously in the cement industry. The properties of sustainable concrete containing high replacement ratios of RHA and FA (up to 30% of cement) are analysed. Furthermore, this work monitors the effect of these replacement ratios with various contents of cement (reaching 550 kg m −3 ).

Materials and methods

Portland cement CEM I–52.5 N was used in all mixtures. The cement was tested in accordance with the ES 4756–1/2013 [ 26 ]. Table  1 presents the physical and chemical features of cementitious materials.

The fine aggregate (clean and rounded) utilized in this experiment was natural siliceous sand with a particular specific gravity of 2.67, bulk unit weight of 1680 kg m −3 and fineness modulus of 2.85. The coarse aggregate was local crushed limestone (dolomite) with a specific gravity of 2.70, bulk unit weight of 1700 kg m −3 and maximum nominal size of 13 mm, according to ES 1109/2008 [ 27 ]. The ratio of fine to coarse aggregate is approximately 1:2.

RHA was obtained by burning husk under an uncontrolled temperature. The gathered ash was sifted through a British Standard (BS) sieve with a size of 75 μm to remove large particles. The produced RHA exhibited a grey color. Energy-dispersive X-ray (EDX) composition analysis and transmission electron microscopy (TEM) analyses were applied on the produced RHA. EDX test showed that produced ash contains 96.2% silicon dioxide (SiO 2 ) and 0.47% calcium oxide (CaO), as presented in Table 1 . The result indicates that RHA is a more reactive material than cement and FA. The chemical compositions of RHA in this study are comparable with those of Akeke et al. [ 1 ]. The TEM test indicated that particle size varies between 15 and 52 μm.

FA is an industrial by-product of coal-fired power stations; the FA utilized in the current study is categorized as class F in accordance with the requirements of ASTM C618–19 [ 28 ]. Table 1 presents the chemical composition of FA as determined via X-ray fluorescence.

Superplasticizer (high rang water-reducing admixtures)

A high-performance superplasticizer admixture of the aqueous solution of modified polycarboxylate basis (Viscocrete-5930) was used to increase the workability of concrete mixtures. Viscocrete-5930 complies with ASTM C494/C494M-17 [ 29 ], with a specific gravity of 1.11. The dosage was approximately 3% to compensate the reduced water and enhance the workability of mixtures with cementitious contents of 450 and 550 kg m −3 .

As shown in Table  2 , the water-to-cementitious material ratio (w/c) was set to 0.55 for mixtures with 350 kg m −3 cementitious content. The w/c was reduced to 0.25 to improve the compressive strength of concrete mixtures with cementitious contents of 450 and 550 kg m −3 .

Mixture proportions

Twenty-one concrete mixtures were prepared in this study and divided into three groups. Each group consisted of seven mixtures. This experimental work used three cement contents: 350, 450 and 550 kg m −3 for groups one, two and three, respectively. RHA and FA were used for all series as replacements of cement with various ratios. The mixtures in every group were classified as follows: control mixture; three mixtures containing 10, 20 and 30% of RHA; and three mixtures using 10, 20 and 30% of FA. The ratio of fine aggregates to coarse aggregates was maintained at 1:2. The mixtures were designed to use 0.55 w/c with 350 kg m −3 of cementitious materials. The w/c was reduced to 0.25 for groups two and three to improve the compressive strength of the concrete. Superplasticizer was added to the concrete with 3% of cementitious contents for groups two and three to compensate the reduced water. Table 2 shows the mixture proportions.

The experimental mixing steps are explained as follows: The fine and coarse aggregates were initially mixed for 1 min. Then, cementitious materials were added, and the quantities were remixed for 3 min. Water was added to the mixture through the mixing process with respect to the superplasticizer addition. Subsequently, the mixing process was continued for 3 min.

Test procedure

The consistency of fresh concretes was measured in terms of slump values (ASTM C143/C143M-15a) [ 30 ] and air content values (ASTM C231/C231M-17a) [ 31 ]. The compressive strength of concrete was evaluated on cube-shaped specimens (150 mm) at 3, 7, 28, 60 and 90 d (BS 1881–116) [ 32 ]. The splitting tensile test was conducted at 28 d on cylinder samples (150 × 300 mm) (ASTM C496/C496M-17) [ 33 ]. The flexural strength test was performed at 28 d (ASTM C78/C78M-18) [ 34 ]. The prism specimens (100 × 100 × 500 mm) were utilized for the flexural strength test. The average values of the three specimens for each testing age and all strengths were recorded. Cylinder forms (150 × 300 mm) were prepared to determine the modulus of elasticity at 28 d (ASTM C469/C469M-14) [ 35 ]. The bond strength was tested by pulling steel bar from cylinder samples. Permeability was measured at 28 d on specimens with a diameter of 150 mm and length of 150 mm to determine the depth of water penetration in concrete.

Results and discussion

Table  3 presents the fresh and hardened properties of concrete mixtures. The fresh properties include slump and air content results. The hardened properties consist of mechanical and physical properties. The mechanical features include the compressive, splitting tensile, flexural and bond strengths and modulus of elasticity. The water penetration (permeability) is the only physical feature tested in this investigation.

Slump results

The slump test was conducted to study the effect of RHA and FA replacement ratios on the slump of concrete mixtures. In group one, the slump was decreased by 12, 18 and 24% for mixtures M2, M3 and M4 with RHA replacement ratios, respectively, compared with the control mixture. By contrast, the slump of mixtures with FA replacement ratios was decreased by 6, 12 and 18% for mixtures M5, M6 and M7, respectively, compared with control mixture M1. In group two, the slump was increased by 11, 22 and 28% for mixtures M9, M10 and M11 with RHA replacement ratios, respectively, compared with control mixture M8. Contrarily, the slump for mixtures with FA replacement ratios was increased by 22, 28 and 33% for mixtures M12, M13 and M14, respectively, compared with control mixture M8. In group three, the slump was increased by approximately 21, 26 and 37% for mixtures M16, M17 and M18 with RHA replacement ratios, respectively, compared with control mixture M15. However, the slump of mixtures with FA replacement ratios was increased by 26, 32 and 42% for M19, M20 and M21, respectively, compared with control mixture M15. Generally, the slump value is increased by using a superplasticiser with cementitious materials of 450 and 550 kg m −3 . The greatest slump occurred in mixture M21 due to the usage of 550 kg m −3 of cementitious materials containing 30% FA. Figure  1 presents the slump test results.

Effect of using cementitious materials (CM) on the slump test of concrete mixes

• Air content

The experimental results showed that the RHA and FA replacement ratios cooperated in reducing the air content. In addition, the air content decreased with the increase in cementitious materials. The RHA replacement ratio of 30% from a cement content of 550 kg m −3 significantly reduced the air content by 1.1% compared with all mixtures.

Compressive strength

The results from three cubes were averaged to determine the compressive strength of each concrete mixture. The compressive strength test was conducted at five different ages: 3, 7, 28, 60 and 90 d. The results showed a good early compressive strength wherein all mixtures containing 450 and 550 kg m −3 of cementitious materials achieved structural concrete at 3 d. By contrast, the mixtures using 350 kg m −3 of cementitious material achieved structural concrete at 7 d. Figure  2 shows the effects of RHA and FA replacement ratios from cement content on the compressive strength at various ages. The maximum RHA replacement ratio was 10% of cement content. The increment percentages at 28 d were 21, 22 and 17% for mixtures M2, M9 and M16, respectively, compared with the control mixture of each group. On the contrary, the maximum FA replacement ratio was 20% of cement content for all groups. Subsequently, the compressive strength at 28 d increased by 16, 20 and 15% for mixtures M6, M13 and M20, respectively, compared with the control mixture of each group. However, the 10% replacement ratio of RHA from cement contents improved the compressive strength compared with that of FA. Figure  3 presents that the compressive strength at 60 d increased by around 6–9% compared with that at 28 d. The increasing ratio of the compressive strength at 90 d was between 11 and 14% compared with that at 28 d. The compressive results at different ages agreed with those of Chopra et al. [ 5 ]. The increasing percentage of compressive strength was referred to the interaction of SiO 2 in the RHA with free calcium hydroxide, thereby reducing the internal air voids in concrete structure. The RHA contains a higher ratio of SiO 2 (approximately five times) than cement.

Compressive strength of concrete mixtures ( a ) 350 kg m −3 CM, ( b ) 450 kg m −3 CM, ( c ) 550 kg m −3 CM

Development of compressive strength ( a ) group I, ( b ) group II, ( c ) group III

Tensile strength

Cylindrical specimens were tested to determine the tensile strength of concrete mixtures using RHA and FA as replacements of cement content. Figure  4 a presents the effect of RHA and FA replacement ratios on the tensile strength of mixtures with 350 kg m −3 of cementitious materials. The concrete mixtures containing RHA recorded a higher tensile strength than that incorporated with FA. The increment percentages were 19, 13 and 3% for mixtures M2, M3 and M4 using RHA ratios, respectively, compared with control mixture M1. In group two, the tensile strength of mixture M13 which contained 20% of FA was closed up to the tensile strength of mixture M9 which used 10% of RHA as replacement for cement content. The highest tensile strength of group two was 8.1 MPa for mixture M9, which increased by 21% from control mixture M8, as shown in Fig.  4 b. In group three, the tensile strength was close to the tensile results in group two, as displayed in Fig. 4 c. Finally, the tensile results showed that the best cement content was 450 kg m −3 , which was used in group two for comparison with other cementitious contents. In addition, the replacement ratio of 20% of FA agreed with the tensile results of 10% of RHA in all groups. The tensile strength ratio from the compressive strength showed that the increment ratio of tensile strength decreased as the cementitious materials increased. The highest tensile strength ratio was approximately 10% from compressive strength for mixture with 30% of RHA and a cementitious content of 350 kg m −3 .

Tensile strength of concrete mixes ( a ) 350 kg m −3 CM, ( b ) 450 kg m −3 CM, ( c ) 550 kg m −3 CM

Flexural strength

The 10% replacement ratio of RHA with all cementitious contents improved the flexural performance of concrete mixture. Figure  5 a presents similar flexural strength for M1 and M7, which indicates the insignificant effect of using 30% of FA with cement content of 350 kg m −3 . On the contrary, the 30% replacement ratio of FA improved the flexural strength of concrete with cement contents of 450 and 550 kg m −3 , as exhibited in Fig.  5 b and c. Thus, a significant enhancement in flexural strength was achieved by using RHA and FA with 550 kg m −3 cement content. The percentage of flexural strength to compressive strength ranged from 14.3 to 14.9%, indicating that fine materials contributed to the enhancement of the flexural strength and compressive strength.

Flexural strength of concrete mixes ( a ) 350 kg m −3 CM, ( b ) 450 kg m −3 CM, ( c ) 550 kg m −3 CM

Bond strength

For reinforced concrete, a suitable bond between steel bars and the surrounding concrete must be developed. The RHA and FA ratios contributed to the improvement of the bond strength between the steel and concrete. The bond strength was increased by approximately 20, 14 and 4% for mixtures containing 10, 20 and 30% of RHA, respectively, with 350 kg m −3 cement content compared with control mixture M1. However, the bond strength using FA replacement ratios was 5.6, 5.8 and 5.1 MPa for mixtures M5, M6 and M7, respectively, whereas that of control mixture M1 was 5 MPa, as presented in Fig.  6 a. The bond strength between steel and concrete was developed by using cement contents of 450 and 550 kg m −3 , as shown in Fig. 6 b and c. The 10% replacement ratio of RHA improved the bond strength for each group, especially with a cement content of 550 kg m −3 . On the contrary, the 20% replacement ratio of FA enhanced the bond strength of concrete, which agreed with mixtures using 10% of RHA. The bond/compressive strength percentage was calculated to evaluate the efficiency of pozzolanic materials (RHA and FA). The bond/compressive strength ratios were approximately 16.1, 16.6 and 17.0% for cementitious contents of 350, 450 and 550 kg m −3 , respectively. The correlation showed that the bond strength of concrete mixtures improved with the increase in cementitious contents.

Bond strength of concrete mixes ( a ) 350 kg m −3 CM, ( b ) 450 kg m −3 CM, ( c ) 550 kg m −3 CM

Modulus of elasticity

The cylinder specimens were tested to determine the modulus of elasticity of concrete mixtures using RHA and FA replacement ratios. The elasticity test was conducted by recording the strain values corresponding compressive loads. Figure  7 shows the effect of RHA and FA replacement ratios on the modulus of elasticity of concrete mixtures. The modulus of elasticity of mixtures containing 450 kg m −3 of materials was slightly similar to that of mixtures containing 550 kg m −3 of materials. The replacement with RHA and FA from cement content improved the modulus of elasticity of concrete mixtures by various percentages. The best replacement ratios were 10% of RHA and 20% of FA for each cement content. The highest modulus of elasticity was 46.5 GPa for mixture M16, which contained 450 kg m −3 of cementitious materials, with 10% of RHA replacement ratio.

Modulus of elasticity for mixes using 350, 450 and 550 kg m −3 CM

• Water permeability

Permeability is a key factor influencing the durability of concrete. This parameter is particularly important in reinforced concrete, because the concrete must prevent water from reaching the steel reinforcement. Therefore, the durability, corrosion resistance and resistance to chemical attack of concrete are directly related to its permeability. The permeability of mixtures using 350 kg m −3 of cementitious materials indicated that using 30% of RHA reduced the permeability of concrete by 44% from control mixture M1. This reduction was due to the decrease of the air content in this mixture compared with all mixtures with the same cement content, as shown in Fig.  8 a. The best replacement ratio is 30% RHA with cement contents of 450 and 550 kg m −3 , which decreases the water permeability of concrete mixtures due to the lowest air content, as presented in Fig. 8 b and c. The RHA and FA replacement ratios from cement contents were beneficial for decreasing the air content and water permeability of concrete mixtures.

Permeability of concrete mixes ( a ) 350 kg m −3 CM, ( b ) 450 kg m −3 CM, ( c ) 550 kg m −3 CM

Conclusions

The percentage of SiO 2 in RHA is 96.2%, which is higher than that in FA.

The slump results of concrete mixtures increased with the increase in cementitious contents, especially by using RHA and FA.

The best replacement ratios of RHA and FA were 10% and approximately 20% with cement contents of 350, 450 and 550 kg m −3 . The mixtures with RHA exhibited a higher compressive strength than those containing FA. The increment percentages of compressive strength at 28 d were 21, 22 and 17% for mixtures M2, M9 and M16, respectively, compared with the control mixture of each group (RHA mixtures). Furthermore, the improvements of compressive strength were 16, 20 and 15% for mixtures M6, M13 and M20, respectively (FA mixtures).

The concrete mixtures containing RHA presented higher tensile strength than those with FA. In addition, the tensile results showed that the best cement content was 450 kg m −3 , which was used in group two, compared with other cementitious contents. The tensile strength ratio from compressive strength revealed that the increment rate of tensile strength decreased as the cementitious materials increased.

The flexural strength was significantly enhanced by using RHA and FA replacement ratios with 550 kg m −3 cement content. The percentage of flexural strength to compressive strength ranged from 14.3 to 14.9%, indicating that fine materials contributed to the enhancement of the flexural strength and compressive strength.

The 10% replacement ratio of RHA achieved a high bond strength for each group, especially with a cement content of 550 kg m −3 . On the contrary, the 20% replacement ratio of FA enhanced the bond strength of concrete, which agreed with the mixtures using 10% of RHA. The bond/compressive strength ratios were approximately 16.1, 16.6 and 17.0% for cementitious contents of 350, 450 and 550 kg m −3 , respectively. The correlation showed that the bond strength of concrete mixtures improved with the increase in cementitious contents.

The replacement with RHA and FA from cement content improved the modulus of elasticity of concrete mixtures by various percentages. The best replacement ratios were 10% of RHA and 20% of FA for each cement content. The highest modulus of elasticity was 46.5 GPa for mixture M16, which contained 550 kg m −3 of cementitious materials with 10% of RHA replacement ratio.

The permeability of mixtures using 350 kg m −3 of cementitious materials indicated that the usage of 30% of RHA reduced the permeability of concrete by 44% from control mixture M1. This reduction was due to the decrease of the air content in this mixture compared with all mixtures with the same cement content. In addition, the best replacement ratio is 30% of RHA with cement contents of 450 and 550 kg m −3 , which decreased the water permeability of concrete mixtures due to the lowest air content.

The RHA and FA replacement ratios from cement contents were beneficial for decreasing the air content and water permeability of concrete mixtures. This result achieved the main goal by using high replacement ratios of RHA and FA for improving the durability of concrete.

Availability of data and materials

All data generated or analyzed during this study are available in Egypt.

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Acknowledgements

The authors wish to thank the assistance of M. Ashraf and A. Mohammed final year students, in carrying out some of the experimental work.

This work was supported by the laboratory of civil constructions department, Faculty of Industrial Education, Suez University for funding support.

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Amin, M., Abdelsalam, B.A. Efficiency of rice husk ash and fly ash as reactivity materials in sustainable concrete. Sustain Environ Res 29 , 30 (2019). https://doi.org/10.1186/s42834-019-0035-2

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According to the statistics supplied by the IRRI (1988), the world’s paddy rice ( Oryza sativa ) production in 1987 was 470 million MT. Most of this tonnage is produced in Southeast Asia. A major derivative of the rice crop is the hull, a fibrous, nondigestible commodity representing some 20% of the dried paddy on-stalk (Yoshida 1981). Dried paddy on-stalk yields 52 wt% of white rice, 20% hull, 15% stalk, and 10% bran. The remaining 3% is lost in the conversion process. If all the paddy rice available were commercially milled, 98 million MT of hulls would have been produced in 1987. Because of their abrasive character, poor nutritive value, low bulk density, and high ash content, only a small percentage of the hulls can be disposed of for certain low-value applications such as chicken litter, juice-pressing aids, and animal roughage. If not properly utilized, rice hulls will create a growing problem of space and pollution in the environment. In some countries, rice hulls and straw are used as fuel in parboiling paddy rice. It is likely that hull utilization will increase in light of the high cost of fuel and the energy crisis confronting the world population.

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Two decades of rice research in Indonesia and the Philippines: A systematic review and research agenda for the social sciences

• Ginbert P. Cuaton   ORCID: orcid.org/0000-0002-5902-3173 1 &
• Laurence L. Delina   ORCID: orcid.org/0000-0001-8637-4609 1  

Humanities and Social Sciences Communications volume  9 , Article number:  372 ( 2022 ) Cite this article

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• Development studies
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While rice studies are abundant, they usually focus on macro-level rice production and yield data, genetic diversity, cultivar varieties, and agrotechnological innovations. Moreover, many of these studies are either region-wide or concentrated on countries in the Global North. Collecting, synthesizing, and analyzing the different themes and topic areas in rice research since the beginning of the 21st century, especially in the Global South, remain unaddressed areas. This study contributes to filling these research lacunae by systematically reviewing 2243 rice-related articles cumulatively written by more than 6000 authors and published in over 900 scientific journals. Using the PRISMA 2020 guidelines, this study screened and retrieved articles published from 2001 to 2021 on the various topics and questions surrounding rice research in Indonesia and the Philippines—two rice-producing and -consuming, as well as emerging economies in Southeast Asia. Using a combination of bibliometrics and quantitative content analysis, this paper discusses the productive, relevant, and influential rice scholars; key institutions, including affiliations, countries, and funders; important articles and journals; and knowledge hotspots in these two countries. It also discusses the contributions of the social sciences, highlights key gaps, and provides a research agenda across six interdisciplinary areas for future studies. This paper mainly argues that an interdisciplinary and comparative inquiry of potentially novel topic areas and research questions could deepen and widen scholarly interests beyond conventional natural science-informed rice research in Indonesia and the Philippines. Finally, this paper serves other researchers in their review of other crops in broader global agriculture.

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Introduction

Rice feeds the majority of the world’s population and employs millions, especially in developing countries in the Global South (Muthayya et al., 2014 ). Rice consumption has increased globally over the last decade. Statista data show that, in the cropping year 2020/2021, the world population consumed about 504.3 million metric tons of rice, increasing from 437.18 million metric tons in 2008/2009 (Shabandeh, 2021 ). These data highlight the crop’s global contribution and importance, especially in realizing the Sustainable Development Goals (SDGs), the blueprint for global prosperity (Gil et al., 2019 ). The SDGs call for systems transformation, including in agriculture, guided by the principles of sustainability and equity, driven by the leave-no-one-behind aphorism, to address the root causes of perennial poverty and chronic hunger.

Pathologist M. B. Waite ( 1915 ) pointed out that the apparent indicator of progress in modern agriculture is the application of scientific research and the subsequent modification and improvement of farming systems based on those research. For example, the Green Revolution resulted in increased agricultural production in developing countries due to the transfer of agrotechnological innovations from countries in the Global North to countries in the Global South. Although, we acknowledge that this project came with a cost (Glaeser, 2010 ; Pielke and Linnér, 2019 ; Pingali, 2012 ).

Regional rice studies have proliferated in Europe (Ferrero and Nguyen, 2004 ; Kraehmer et al., 2017 ), the Americas (Singh et al., 2017 ), Africa (Zenna et al., 2017 ), the Asia Pacific (Papademetriou et al., 2000 ), and South Asia (John and Fielding, 2014 ). Country studies on rice production have also emerged in Australia (Bajwa and Chauhan, 2017 ), China (Peng et al., 2009 ), and India (Mahajan et al., 2017 ). Scholars have also systematically reviewed rice’s phytochemical and therapeutic potentials (Sen et al., 2020 ), quality improvements (Prom-u-thai and Rerkasem, 2020 ), and its role in alleviating the effects of chronic diseases and malnutrition (Dipti et al., 2012 ).

These extant studies, however, are limited on at least three fronts. First, their foci were on rice production, yield, and operational practices and challenges at the macro level. Second, there have been zero attempts at synthesizing this corpus since the 21st century. Third, there are also no attempts at examining the various rice research areas that scholars, institutions, and countries need to focus on, especially in developing country contexts, and their nexuses with the social sciences. This paper addresses these gaps by unpacking and synthesizing multiple rice studies conducted in the emerging Southeast Asian economies of Indonesia and the Philippines from 2001 to 2021. A focus on these developing countries matters since they are home to over 35 million rice farmers (IRRI, 2013 ).

We conducted our review from the Scopus database, using a combination of bibliometric and quantitative content analyses. Section “Results and discussions” reports our results, where we discuss (1) the most relevant and influential rice scholars and their collaboration networks; (2) the most rice research productive institutions, including author affiliations, their countries, and their research funders; and (3) the most significant articles and journals in rice research. This section also identifies 11 topic areas belonging to four major themes of importance for rice research in the two countries. Section “Contributions from and research agenda for the social sciences” provides a research agenda, where we identify and discuss the contributions of our review in terms of future work. Despite the preponderance of rice research in the last two decades and more in Indonesia and the Philippines, contributions from the social sciences remain marginal. Thus, in the section “Conclusion”, we conclude that emphasis is needed on expanding and maximizing the contributions of social scientists given the many opportunities available, especially for conducting interdisciplinary and comparative rice research in these Southeast Asian countries.

Review methods and analytical approach

We used bibliometric and quantitative content analyses to systematically categorize and analyze more than two decades of academic literature on rice in Indonesia and the Philippines. Bibliometric methods, also known as bibliometrics, have grown to be influential in evaluating various research fields and topic areas. Bibliometrics mushroomed because of the increasing availability of online databases and new or improved analysis software (Dominko and Verbič, 2019 ). Bibliometrics quantitatively and statistically analyze research articles using their bibliographic data, such as authors, affiliations, funders, abstracts, titles, and keywords. These data are analyzed to identify and assess the development, maturity, research hotspots, knowledge gaps, and research trends (Aria and Cuccurullo, 2017 ). For example, bibliometrics have been used in reviewing hydrological modeling methods (Addor and Melsen, 2019 ), business and public administration (Cuccurullo et al., 2016 ), and animals’ cognition and behavior (Aria et al., 2021 ).

This review article used bibliometrix , a machine-assisted program that offers multiple options and flexibility to map the literature comprehensively (Aria and Cuccurullo, 2017 ). We run this program using R Studio version 4.1.2 (2021-11-01; “Bird Hippie”) for its source code readability, understandability, and easy-to-do computer programming (Cuaton et al., 2021 ). We used bibliometrix in three critical analytical phases: (a) importing and converting data to R format, (b) identifying our dataset’s collaboration networks and intellectual and conceptual structures, and (c) processing, presenting, and analyzing our dataset. Bibliometrix, however, is unable to produce specific data that we want to highlight in this paper; examples of these are our coding criteria on interdisciplinarity and author gender, where such information was not captured in the articles’ bibliographic data in Scopus. We addressed these issues by conducting a quantitative content analysis (QCA) of our dataset. QCA is a method to record, categorize, and analyze textual, visual, or aural materials (Coe and Scacco, 2017 ). QCA has been applied in other reviews, such as in energy research development in the social sciences (Sovacool, 2014 ), the concepts of energy justice (Jenkins et al., 2021 ), and in examining agricultural issues in Botswana (Oladele and Boago, 2011 ) and Bangladesh (Khatun et al., 2021 ).

Search strategies

We constructed our dataset from the Scopus database, which we accessed via our institution’s online library on 14 November 2021. Scopus is a scientific database established in 2004 and owned by Elsevier Ltd. (Elsevier, 2021 ). We excluded other databases, such as Google Scholar, ScienceDirect, Web of Science, and EBSCO, suggesting one potential bias in our review (Waltman, 2016 ; Zupic and Čater, 2015 ). Our decision to exclusively use Scopus arises from two main reasons. First, the database has broader coverage than others, including the abovementioned (Falagas et al., 2008 ). Scopus includes new and emerging journals published in developing countries like Indonesia and the Philippines, our focus countries. Second, Scopus has a user-friendly interface and its search options allow researchers to flexibly explore its universe of indexed articles based on authors, institutions, titles, abstracts, keywords, and references (Donthu et al., 2021 ).

We followed the PRISMA 2020 Guideline (Preferred Reporting Items for Systematic reviews and Meta-Analyses) (Page et al., 2021 ) in our search for potential rice-related studies in Indonesia and the Philippines (see Fig. 1 ). We used the initial search string: “rice” AND “Indonesia*” OR “Philippine*” (asterisk or “*” was used as a wildcard search strategy) and limited the year coverage from 2001 to 2021. Our first round of searches resulted in 3846 documents (results as of 14 November 2021). We filtered these documents by including only peer-reviewed, full-text English articles on rice. We did not include any documents from the grey literature (e.g., news items, press releases, government or corporate reports), and other document types indexed in Scopus such as reviews, books, conference papers, errata, comments, editorials, and short reports.

Our initial result of 3846 documents (results as of 14 November 2021) was filtered by including only peer-reviewed, full-text English articles on rice, resulting in 2243 eligible documents.

We also excluded articles with irrelevant keywords by using the following combined queries:

This resulted in 2243 eligible documents. We downloaded these documents as raw files in BibTex format and imported them to Biblioshiny , a web interface in Bibliometrix, where they were further filtered. Our verified final dataset comprises 2243 full-text English articles cumulatively written by 6893 authors and published across 909 journals (see Table 1 ).

Structure and analytical approach

We examined the authors’ profiles based on their gender, relevance in the study, and global impact. For gender, we coded them into ‘man,’ ‘woman,’ and ‘undetermined’ because some did not put enough information that helps in gender identification. We identified their gender by counter-checking their Scopus profiles to their verified accounts in Google Scholar, ResearchGate, Publons/Web of Science, or institutional profiles. We measured the authors’ relevance and impact against their (a) productivity, (b) citations, and (c) H-indices. We acknowledge, however, that some Filipino and Indonesian scholars, whose papers may not be indexed in Scopus, could also be prolific based on different parameters, but we excluded them. We proceeded to map the collaboration networks of these authors to identify “who works with whom on what.” A collaboration network illustrates nodes (circle shape) as authors and links (connecting lines) as co-authorships (Glänzel and Schubert, 2005 ).

Institutions, countries, funders

Following Sovacool ( 2014 ), we categorized the authors’ institutions into four: (1) University and research included authors who are researchers, instructors/lecturers/professors, other academic faculty from various non-university research think tanks, institutes, and national and local research centers; (2) Government consisted country or state departments, bureaus, ministries, and other government regulatory bodies; (3) Interest groups and NGOs included intergovernmental bodies, such as the United Nations Food and Agriculture Office (FAO) and international organizations like the International Rice Research Institute (IRRI) and Oxfam; and (4) Banking and finance encompassed players from the finance sector, including multilateral development banks such as the Asian Development Bank (ADB), World Bank, and the International Fund for Agricultural Development (IFAD). After coding and categorizing, we analyzed the authors’ institutional collaboration networks.

We identified the country’s productivity and coded them by global region based on their geographical location: (a) Asia, (b) Australia, New Zealand, and South Pacific, (c) Europe, (d) North America, (e) South America, and (f) Africa. We did this to show how various countries have been researching rice in Indonesia and the Philippines since the 21st century.

We then constructed a country collaboration map as a visual macro-representation of countries working together on rice research using these data. Bibliometrix, however, measured the country’s productivity based on the corresponding authors’ affiliations. We, therefore, noted two critical points here. First, many corresponding authors may have multiple institutional affiliations. For example, one corresponding author may belong to more than two affiliations (e.g., a corresponding Filipino author may have concurrent institutional affiliations in Japan, Australia, and New Zealand). Second, the corresponding authors may not necessarily be nationals of that country. Note that the unit of analysis is based on the corresponding authors’ institutional affiliations at the time of publication and not on their country/ies of citizenship or nationality. Despite these, our findings still provide insight into the macro-level productivity of countries conducting rice research in Indonesia and the Philippines.

We analyzed the funders using Scopus’ in-house Analytics Tool and determined their relevance based on the number of articles mentioning them in the Funding source or Acknowledgment section in the paper. We categorized the funders into six: (1) government (e.g., ministries, departments, or regulatory agencies), (2) research (e.g., research councils, research centers, and national academies), (3) foundations and non-government organizations (NGOs), (4) universities, (5) private companies and corporations, and (6) intergovernmental organizations/IGOs, including multilateral development banks.

Articles and journals

In terms of interdisciplinarity, we coded the articles as (a) interdisciplinary, (b) disciplinary, or (c) unidentified by using the authors’ department or division affiliation/s as a proxy to determine their disciplinary training. We coded an article as interdisciplinary if it belonged to any of the three criteria: (1) it had an author that had training or belonged to a department/division in at least two conventional disciplines (e.g., agriculture, anthropology, sociology, biology); (2) it had an author that had a self-identified interdisciplinary department (e.g., interdisciplinary division, sustainability, agriculture economics, etc.); or (3) it had at least two authors with different disciplinary training or expertise (e.g., business and economics; crop science and political science, etc.). We coded an article as disciplinary if its author/s had only belonged to one conventional department/division affiliation (e.g., Division of Agriculture, Department of Economics, Division of Environmental Science, etc.). On the other hand, we coded an article as undetermined when the authors had only indicated the name of their institutions or did not indicate their departmental or division affiliations (e.g., only the University of the Philippines, IRRI, Universitas Gadja Mada, etc.).

We examined the articles based on their local relevance and global influence. Bibliometrix measured the articles’ relevance based on their “local citations” or citations received from the 2243 articles of our sample dataset. We did this to determine which papers are considered relevant by authors studying various areas of rice research in Indonesia and the Philippines. Global influence is measured based on the articles’ citations from the global research community or other scientific works beyond our sample dataset. We also conducted a co-citation analysis of the cited references. Co-citation is the frequency by which articles cite together two or more articles relevant to the topic areas of inquiry (Aria and Cuccurullo, 2017 ). Bibliometrix had identified some co-cited articles published before our timeline of interest (i.e., pre-2001) which provide scholars with a more profound understanding of rice research in the two countries.

On the other hand, Bibliometrix identified the most relevant journals based on the number of papers the journals had published and the local citations of the articles. These data guide readers and researchers on which journals to look for on rice studies in Indonesia and the Philippines.

Knowledge hotspots

Bibliometrix creates a thematic map that allows researchers to identify which study areas have been adequately explored and which areas need further investigation or re-investigation to identify knowledge hotspots and research gaps (Aria and Cuccurullo, 2017 ). Della Corte et al. ( 2019 , pp. 5–6) discussed the major themes in Bibliometrix in the following:

“Themes in the lower-right quadrant are the Basic Themes , characterized by high centrality and low density. These themes are considered essential for a research field and concerned with general topics across different research areas.

Themes in the upper-right quadrant are the Motor Themes , characterized by high centrality and density. Motor themes are considered developed and essential for the research field.

Themes in the upper-left quadrant are the highly developed and isolated themes or Niche Themes . They have well-developed internal links (high density) but unimportant external links, which could be interpreted as having limited importance for the field (low centrality).

Themes in the lower-left quadrant are known as Emerging or Declining Themes . They have low centrality and density, making them weakly developed and marginal.”

Contributions from and research agenda for the social sciences

As interdisciplinary environmental and social scientists, we also focused our review on the social studies of rice in the two countries. This section highlighted the gaps between the natural and the social sciences in rice research and advanced a research agenda for interdisciplinary and comparative social scientists.

Limitations

As in any systematic review, we acknowledge certain limitations to our work. We discuss four of these.

First, to keep a certain level of reliability, we focused only on peer-reviewed full-length research articles written in the English language and indexed in the Scopus database. Therefore, we may have excluded some relevant articles, including those written in Filipino, Indonesian, and other local or indigenous languages and published in local or international journals but are not indexed in Scopus. Our review also excluded conference papers, commentaries, book reviews, book chapters, conference reviews, data papers, errata, letters, notes, and non-academic publications like policy briefings, reports, and white papers.

Second, in our quantitative content analysis, we acknowledge the highly cis-heteronormative approach we used in coding the author’s gender as “man” or “woman.” We identified these genders from the names and pictures of the authors in their verified Scopus, Publons/ Web of Science, and institutional profiles. It is not our deliberate intention to neglect the varying genders of researchers and scientists beyond the traditional binary of man or woman.

Third, we recognize that our analysis cannot directly identify how much each funder provided as the unit of analysis in bibliometrix may depend on how prolific researchers were in publishing articles despite smaller funds. For instance, one research project supported by Funder A with US$1 million may have published only one article based on their project design or the funder's requirement. Since the authors published only one paper from this project, the data could show that Funder A only funded one research. Another research project, supported by Funder B, with only US$300,000 in funding, may have published more than five papers; therefore, more articles counted as funded by Funder B. This issue is not within the scope of our review.

Lastly, it should be noted that the future research works we discussed were highly influenced by our research interests and the general overview of the literature, and thus neither intend to cover nor aim to discuss the entire research topics that other scholars could study.

Despite these limitations, we strongly argue that our review provided relevant insights and proposed potentially novel topic areas and research questions for other scholars to explore, especially social scientists, in deepening and widening rice research in Indonesia and the Philippines. To end, we hope that researchers heed our call to conduct more interdisciplinary and comparative rice-related studies in these two emerging Southeast Asian countries.

Results and discussions

Our dataset comprises 2243 peer-reviewed journal articles cumulatively written by 6893 authors who cited around 80,000 cumulative references. The average annual publications from 2001 to 2013 were only 57 papers but elevated to hundreds beginning in 2014 (Fig. 2 ).

The average number of annual publications on rice research in Indonesia and the Philippines from 2001 to 2013 was only 57 papers but elevated to hundreds beginning in 2014.

Of the 159 authors, one had a duplicate profile; thus, we identified 158 authors publishing on rice studies; the majority (66%) are men. The top 50 most prolific scholars produced a little over 25% (567 articles) of the total articles. Australian ecologist Finbarr Horgan topped this list ( n  = 21), followed by Bas Bouman and Grant Singleton—each with 20 articles. The top 10 authors with the highest number of publications have affiliations with the IRRI, the University of the Philippines, the University of Gadjah Mada, and the Philippine Rice Research Institute (PhilRice). For the full list of prolific scholars with at least 10 articles published, see Supplementary Table 1 .

In terms of the authors with the most local citations, although Finbarr Horgan has the most documents, Johan Iskandar ( n  = 36 citations) from the Universitas Padjadjaran, who studies rice genetic diversity, is the most cited. Local citations refer to the citations received by authors from our sample dataset of 2243 articles. Muhidin Muhidin from the Universitas Halu Oleo and Ruhyat Partasasmita from the Universitas Padjadjaran, followed him with 30 and 28 local citations, respectively. Common to these three authors are their Biology background/expertise and interest in rice genetic diversity. To check the top 20 most locally cited scholars, refer to Supplementary Table 2 .

The H-index is the author-level measure of publications’ productivity and citation impacts (Hirsch, 2005 ). Bas Bouman (H-index = 18) leads the top 10 scholars among rice-related researchers in Indonesia and the Philippines. Yoshimichi Fukuta (H index = 13) and Shaobing Peng (H index = 13) followed him. These three authors are affiliated with or have collaborated with the IRRI. To check the top 10 scholars with the highest H-indices, refer to Supplementary Table 3 .

Figure 3 reveals the top 80 authors who collaborate across eight major clusters of rice research. The Red cluster shows Finbarr Horgan as the most prominent author with at least four significant collaborators in pest management, specifically on rice stemborers (Horgan et al., 2021 ), anthropods’ biodiversity in tropical rice ecosystems (Horgan et al., 2019 ), and virulence adaptations of rice leafhoppers (Horgan et al., 2018 ). In the Purple Cluster, Yoshimichi Fukuta has multiple publications with at least six collaborators in the study of rice blast (Ebitani et al., 2011 ; Kadeawi et al., 2021 ; Mizobuchi et al., 2014 ). In the Brown cluster, Bernard Canapi from the IRRI has collaborated with at least five scholars in the study of rice insect pest management (Cabasan et al., 2019 ; Halwart et al., 2014 ; Litsinger et al., 2011 ), farmers’ preference for rice traits (Laborte et al., 2015 ), and the drivers and consequences of genetic erosion in traditional rice agroecosystems in the Philippines (Zapico et al., 2020 ). The Gray cluster shows that Siti Herlinda has collaborated with at least four scholars to study anthropods in freshwater swamp rice fields (Hanif et al., 2020 ; Herlinda et al., 2020 ) and the benefits of biochar on rice growth and yield (Lakitan et al., 2018 ).

The authors’ collaboration networks show eight major clusters of rice research in Indonesia and the Philippines.

Institutions

Author affiliations.

In terms of institutional types, Fig. 4 shows that most rice researchers in Indonesia and the Philippines have affiliations with “University and research.” Figure 5 shows the top 20 institutions in terms of research productivity led by the IRRI, the University of the Philippines System, the PhilRice, the Institute Pertanian Bogor/IPB University, and the University of Gadja Mada. These 20 institutions produced 66% of the articles in our dataset.

The majority of rice researchers in Indonesia and the Philippines have affiliations with “University and research”.

The top 5 most productive institutions in terms of rice research in Indonesia and the Philippines are the IRRI, the University of the Philippines System, the PhilRice, the Institute Pertanian Bogor/IPB University, and the University of Gadja Mada.

Scholars affiliated with the IRRI have written the most papers (at least 19% or 358 articles) in our dataset. The range of topics covers both regional and country studies. Some regional examples include the drivers of consumer demand for packaged rice and rice fragrance in South and Southeast Asia (Bairagi et al., 2020 ; Bairagi, Gustafson et al., 2021 ). Country studies, for example, include an investigation of rice farming in Central Java, Indonesia (Connor et al., 2021 ), the cultural significance of heirloom rice in Ifugao in the Philippines (Bairagi, Custodio et al., 2021 ), and the distributional impacts of the 2019 Philippine rice tariffication policy (Balié and Valera, 2020 ).

The University of the Philippines System, with rice scholars affiliated with their campuses in Los Baños, Diliman, Mindanao, and Manila, produced the next largest number of papers (more than 200 or 10%) on topics about rice pests and parasites (Horgan et al., 2019 , 2021 ; Vu et al., 2018 ), weed control (Awan et al., 2014 , 2015 ; Fabro and Varca, 2012 ), and climate change impacts on rice farming (Alejo and Ella, 2019 ; Ducusin et al., 2019 ; Gata et al., 2020 ). Social studies of rice conducted by the University of the Philippines researchers include indigenous knowledge on climate risk management (Ruzol et al., 2020 , 2021 ), management options in extreme weather events (Lopez and Mendoza, 2004 ), agroecosystem change (Aguilar et al., 2021 ; Neyra-Cabatac et al., 2012 ), and the development and change over time of rice production landscapes (Santiago and Buot, 2018 ; Tekken et al., 2017 ).

PhilRice, a government-owned corporation under the Department of Agriculture (Official Gazette of the Philippines, 2021 ), is the third most prolific rice research-producing institution (122 papers) on topics ranging from nematodes or rice worms (Gergon et al., 2001 , 2002 ) and arthropods (invertebrates found in rice paddies) (Dominik et al., 2018 ), hybrid rice (Perez et al., 2008 ; Xu et al., 2002 ), alternate wetting-and-drying technology (Lampayan et al., 2015 ; Palis et al., 2017 ), and community development strategies on rice productions (Romanillos et al., 2016 ).

The IPB University, a public agrarian university in Bogor, Indonesia, investigates rice productivity and sustainability (Arif et al., 2012 ; Mucharam et al., 2020 ; Setiawan et al., 2013 ), irrigation (Nugroho et al., 2018 ; Panuju et al., 2013 ), extreme weather events such as drought (Dulbari et al., 2021 ), floods (Wakabayashi et al., 2021 ), and emerging social issues such as food security (Putra et al., 2020 ), land-use change (Chrisendo et al., 2020 ; Munajati et al., 2021 ), and sustainability (Mizuno et al., 2013 ). This university has 23 research centers, including those which focus on environmental research; agricultural and village development; engineering applications in tropical agriculture; Southeast Asian food and agriculture; and agrarian studies.

Universitas Gadja Maja in Yogyakarta, Indonesia, hosts 21 research centers, including its Agrotechnology Innovation Centre. It carries out research incubation and development activities, product commercialization, and integration of agriculture, animal husbandry, energy, and natural resources into a sustainable Science Techno Park. Some of their published studies focused on drought-tolerant rice cultivars (Salsinha et al., 2020 , 2021 ; Trijatmiko et al., 2014 ), farmers’ technical efficiency (Mulyani et al., 2020 ; Widyantari et al., 2018 , 2019 ), systems of rice intensification (Arif et al., 2015 ; Syahrawati et al., 2018 ), and climate change adaptation (Ansari et al., 2021 ).

In terms of institutional collaboration, the IRRI tops the list with at least eleven collaborators (Fig. 6 ), including the Japan International Center for Agricultural Sciences, the PhilRice, the University of the Philippines System, and the Indonesian Center for Rice Research.

The IRRI, as an international organization focused on many aspects of rice, is not surprising to have the greatest number of institutional collaborators ( n  = 11 institutions).

Rice studies’ authors are from at least 79 countries; the majority of them are working in Asia (79%), followed by Europe (13%) and North America (9%). At least 90% of rice scholars are in Indonesia, and more than 51% have affiliations in the Philippines, followed by Japan, the USA, and China. For the list of the top 20 most productive countries researching rice in Indonesia and the Philippines, see Supplementary Table 4 . Figure 7 shows a macro-level picture of how countries have collaborated on rice-related projects in Indonesia and the Philippines since 2001, suggesting that rice research in both countries has benefited from international partnerships.

A macro-level picture of how countries have collaborated on rice-related projects in Indonesia and the Philippines since 2001. It suggests that rice research in both countries has benefited from international partnerships.

Only around 47% (1050 studies) of our dataset acknowledged their funding sources, where most received financial support either from governments (45%), research (27%), or university funders (16%) (Fig. 8 ). To see the top 15 funders that supported at least 10 rice-related research projects in Indonesia and the Philippines from 2001 to 2021, refer to Supplementary Table 5 . Of over 150 rice research funders, Indonesia’s Ministry of Education, Culture, and Research (formerly the Ministry of Research and Technology) funded ~6% (62 out of 1050 studies). The Japan Society for the Promotion of Science and Japan’s Ministry of Education, Culture, Sports, Science and Technology came in as the second and third largest funders, respectively.

The majority of rice research projects in Indonesia and the Philippines were funded by governments (45%), research (27%), and university institutions (16%).

Half of all articles in the dataset were borne out of interdisciplinary collaboration. More than a quarter of the articles, however, were unidentified, showing an apparent undercount of the total number of disciplinary collaborations. Most of these collaborative pieces of work (~61%) belong to the natural science subject areas of agricultural and biological sciences; biochemistry, genetics, and molecular biology; and environmental science (see Table 2 ). Note that the cumulative number of articles in Table 2 is more than the total number of the sample dataset since an article may belong to multiple subject areas as indicated by its authors in Scopus. Less than 9% (354) of all papers were written by social scientists, highlighting their marginal contribution to rice research. The social studies of rice can increase our understanding of the many facets of rice production, including their socio-political, economic, and cultural aspects.

Our review shows that there are 10 major networks of rice research co-citations (Fig. 9 ). The papers by Bouman et al. ( 2005 ), Bouman et al. ( 2007 ), Bouman and Tuong ( 2001 ), and Tuong and Bouman ( 2003 ) were co-cited by scholars studying the relationship between water scarcity management vis-à-vis rice growth and yield (the purple cluster in Fig. 9 ). Papers by Yoshida et al. ( 2009 ), De Datta ( 1981 ), and Peng et al. ( 1999 ) were co-cited by scholars researching the genetic diversity, yield, and principles and practices of rice production in Indonesia (the red cluster in Fig. 9 ). Papers by Ou ( 1985 ), Mackill and Bonman ( 1992 ), Sambrook et al. ( 1989 ), Kauffman et al. ( 1973 ), Iyer and McCouch ( 2004 ), and Mew ( 1987 ) were considered essential references in studying rice diseases (blue cluster in Fig. 9 ). The top-cited article on rice research in Indonesia and the Philippines, based on their overall global citations, is a study on water-efficient and water-saving irrigation (Belder et al., 2004 ). This study detailed alternative options for typical water management in lowland rice cultivation, where fields are continuously submerged, hence requiring a continuous large amount of water supply (Belder et al., 2004 ). Global citations refer to the citations received by the articles within and beyond our sample dataset of 2243 articles. To see the top 10 most globally cited articles on rice research in Indonesia and the Philippines, refer to Supplementary Table 6 .

There are 10 major networks of rice research co-citations in Indonesia and the Philippines.

The journal Biodiversitas: Journal of Biological Diversity published the most number of papers on rice research in the two countries. Biodiversitas publishes papers “dealing with all biodiversity aspects of plants, animals, and microbes at the level of gene, species, ecosystem, and ethnobiology” (Biodiversitas, 2021 ). Following its indexing in Scopus in 2014, Biodiversitas has increasingly published rice studies, most of which were authored by Indonesian researchers. To see the top 10 most relevant journals for rice research in Indonesia and the Philippines based on the number of documents published since 2001, refer to Supplementary Table 7 .

Based on their local citations, the journals Field Crops Research , Theoretical & Applied Genetics , and Science are the most relevant. Field Crops Research focuses on crop ecology, crop physiology, and agronomy of field crops for food, fiber, feed, medicine, and biofuel. Theoretical and Applied Genetics publishes original research and review articles in all critical areas of modern plant genetics, plant genomics, and plant biotechnology. Science is the peer-reviewed academic journal of the American Association for the Advancement of Science and one of the world’s top academic journals. To see the top 30 most relevant journals for rice research in Indonesia and the Philippines based on the number of local citations, refer to Supplementary Table 8 .

The most used keywords found in 2243 rice research papers published between 2001 and 2021 in Indonesia and the Philippines are food security, climate change, drought, agriculture, irrigation, genetic diversity, sustainability, technical efficiency, and production (Fig. 10 ). We found 11 clusters across four significant themes of rice research in these countries (Fig. 11 ).

The most used keywords found in 2243 rice research papers published between 2001 and 2021 in Indonesia and the Philippines are food security, climate change, drought, agriculture, irrigation, genetic diversity, sustainability, technical efficiency, and production.

There are four major themes composed of 11 clusters of rice research in Indonesia and the Philippines since 2001.

Basic themes

We identified four major clusters under ‘basic themes’ (refer to Fig. 11 ):

The Red Cluster on studies in the Philippines related to rice yield and productivity, drought, nitrogen, the Green Revolution, and the use and potential of biomass;

The Blue Cluster on studies in Indonesia related to food security, climate change, agriculture, upland rice, irrigation, technical efficiency, and sustainability vis-à-vis rice production;

The Green Cluster on rice genetic diversity, bacterial blight diseases, resistant rice genes, aerobic rice, and brown planthoppers; and

The Gray Cluster on the nutritional aspects of rice, including studies on biofortified rice cultivars.

Agriculture suffers from climate change impacts and weather extremes. Rice researchers in Indonesia and the Philippines are identifying drought-tolerant rice cultivars that can produce high yields in abiotic stress-prone environments (Afa et al., 2018 ; Niones et al., 2021 ). These hybrid cultivars are vital for increasing rice productivity, meeting production demand, and feeding the growing Filipino and Indonesian populations (Kumar et al., 2021 ; Lapuz et al., 2019 ). Researchers have also looked at alternative nutrient and water management strategies that farmers can use, especially those in rainfed lowland areas during drought (Banayo, Bueno et al., 2018 ; Banayo, Haefele et al., 2018 ). There were also studies on the socio-cultural dynamics under which farmers adapt to droughts, such as how past experiences of hazards influence farmers’ perceptions of and actions toward drought (Manalo et al., 2020 ).

Motor themes

We identified three significant clusters of ‘motor themes’ (refer to Fig. 11 ):

The Pink Cluster on yield loss and integrated pest management of rice fields;

The Blue-Green Cluster on biodiversity, ecosystem services, remote sensing, and water productivity; and

The Orange Cluster on the antioxidant properties of rice bran and black rice.

In both countries, pests, including weeds (Awan et al., 2014 , 2015 ), insects (Horgan et al., 2018 , 2021 ), and rodents (Singleton, 2011 ; Singleton et al., 2005 , 2010 ), have significant impacts on yield loss in rice production and human health. To address these, many farmers have embraced chemical-heavy pest management practices to prevent yield loss and increase economic benefits. Pesticides began their use in Indonesia and the Philippines and rapidly expanded from the 1970s to the 1980s (Resosudarmo, 2012 ; Templeton and Jamora, 2010 ). However, indiscriminate use of pesticides caused an ecological imbalance that exacerbated pest problems (Templeton and Jamora, 2010 ) and contributed to farmers’ acute and chronic health risks (Antle and Pingali, 1994 ; Pingali and Roger, 1995 ).

Integrated pest management was introduced, applied, and studied in both countries to address these issues. This approach combines multiple compatible pest control strategies to protect crops, reduce pesticide use, and decrease farming costs (Gott and Coyle, 2019 ). For example, Indonesia’s 1989 National Integrated Pest Management Program trained hundreds of thousands of farmers and agricultural officials about its principles, techniques, and strategies (Resosudarmo, 2012 ). In the Philippines, the government of then-President Fidel V. Ramos (1992–1996) prohibited using hazardous pesticides and instituted a “multi-pronged approach to the judicious use of pesticides” (Templeton and Jamora, 2010 , p. 1). President Ramos’ suite of policies included deploying Integrated Pest Management “as a national program to encourage a more ecologically sound approach to pest control” (Templeton and Jamora, 2010 , p. 1). This pesticide policy package benefited the Philippine government in terms of private health costs avoided (Templeton and Jamora, 2010 ).

To address weed problems, farmers traditionally use manual weeding, a labor-intensive practice. However, as labor costs for manual weeding increased, herbicide use became economically attractive to farmers (Beltran et al., 2012 ). Herbicide experiments were made to address common rice weeds including barnyard grass ( Echinochloa crus-galli ) (Juliano et al., 2010 ), crowfoot grass ( Dactyloctenium aegyptium ) (Chauhan, 2011 ), three-lobe morning glory ( Ipomoea triloba ) (Chauhan and Abugho, 2012 ), and jungle rice ( Echinochloa colona ) (Chauhan and Johnson, 2009 ). Knowledge gained from these experiments contributed to the development of integrated weed management strategies.

Yet, many factors come into play when farmers decide to use herbicides. Beltran et al. ( 2013 ) reported that farmers’ age, household size, and irrigation use are significant determinants of adopting herbicides as an alternative to manual weeding. Beltran et al. ( 2013 ) further showed that economic variables, like the price of the herbicide, household income, and access to credit, determined farmers’ level of herbicide use (Beltran et al., 2013 ). Their study highlights the complex decision-making process and competing factors affecting weed management in the Philippines.

Apart from weeds, insects, like brown planthoppers ( Nilaparvata lugens ) and green leafhoppers ( Cicadella viridis ) and their accompanying diseases, affect rice production. In Java, Indonesia, Triwidodo ( 2020 ) reported a significant influence between the insecticide use scheme and the brown planthopper ( Nilaparvata lugens ) attack rates in rice fields. Brown planthopper attacks increased depending on the frequency of pesticide application, their varieties, and volume (Triwidodo, 2020 ). In the Philippines, Kim and colleagues ( 2019 ) developed a rice tungro epidemiological model for a seasonal disaster risk management approach to insect infestation.

Some social studies of integrated pest management included those that looked at the cultural practices that mitigate insect pest losses (Litsinger et al., 2011 ) and farmers’ knowledge, attitudes, and methods to manage rodent populations (Stuart et al., 2011 ). Other social scientists evaluated the value of amphibians as pest controls, bio-monitors for pest-related health outcomes, and local food and income sources (Propper et al., 2020 ).

Niche themes

We identified two ‘niche themes’ consisting of studies related to (a) temperature change and (b) organic rice production (refer to Fig. 11 ). Temperature change significantly affects rice farming. In the Philippines, Stuecker et al. ( 2018 ) found that El Niño-induced soil moisture variations negatively affected rice production from 1987–2016. According to one experiment, high night temperature stress also affect rice yield and metabolic profiles (Schaarschmidt et al., 2020 ). In Indonesia, a study suggests that introducing additional elements, such as Azolla, fish, and ducks, into the rice farming system may enhance rice farmers’ capacity to adapt to climate change (Khumairoh et al., 2018 ). Another study produced a rainfall model for Malang Regency using Spatial Vector Autoregression. This model is essential as rainfall pattern largely determines the cropping pattern of rice and other crops in Indonesia (Sumarminingsih, 2021 ).

Studies on organic rice farming in the Philippines include resource-poor farmers’ transition from technological to ecological rice farming (Carpenter, 2003 ) and the benefits of organic agriculture in rice agroecosystems (Mendoza, 2004 ). Other studies on organic rice focused on its impacts on agricultural development (Broad and Cavanagh, 2012 ) and climate resilience (Heckelman et al., 2018 ). In Indonesia, Martawijaya and Montgomery ( 2004 ) found that the local demand for organic rice produced in East Java was insufficient to generate revenue enough to cover its production costs. In West Java, Komatsuzaki and Syuaib ( 2010 ) found that organic rice farming fields have higher soil carbon storage capacity than fields where rice is grown conventionally. In Bali, farmers found it challenging to adopt organic rice farming vis-à-vis the complex and often contradictory and contested administration of the Subaks (MacRae and Arthawiguna, 2011 ) and the challenges they have to confront in marketing their produce (Macrae, 2011 ).

Emerging or declining themes

We identified two clusters of ‘emerging/declining themes’ or areas of rice research that are weakly developed and marginal (refer to Fig. 11 ). The Purple Cluster (emerging) studies rice straw, rice husk, methane, and rice cultivation, while the Light Blue Cluster (declining) pertains to local rice research.

In this section, we present and discuss the contributions of the social sciences, highlight key gaps, and provide a research agenda across six interdisciplinary areas for future studies. In Table 3 , we summarized the various topic areas that other scholars could focus on in their future studies of rice in Indonesia and the Philippines.

Economic, political, and policy studies

Political scientist Ernest A. Engelbert ( 1953 ) was one of the earliest scholars to summarize the importance of studying agricultural economics, politics, and policies. Engelbert ( 1953 ) identified three primary reasons scholars and laypeople alike need to understand the nature of political processes in agriculture. First, the rapid change and highly contested political environment where agriculture operates often places agriculture last on national policy agenda. Second, the formulation of agricultural policies intersects with contemporary national and economic contexts by which these policies revolve. Third, understanding the political processes around agriculture can help avoid political pressures and machinations aimed at undermining agricultural development.

Politics play a crucial role in better understanding rice- and agriculture-related policies, their evolution, dynamics, challenges, developments, and futures. Grant ( 2012 , p. 271) aptly asks, “Who benefits [from government policies, regulations, and programs]?” . Knowing, understanding, and answering this question is crucial since policymaking is a highly contested process influenced and negotiated not only by farmers and decision-makers but also by other interest groups, such as people’s organizations and non-government organizations. On the other hand, understanding macro- and micro-economic government arrangements come hand-in-hand in analyzing how policies impact farmers and consumers. Using tariffs as an example, Laiprakobsup ( 2014 , p. 381) noted the effects of government interventions in the agrarian market:

“… when the government implements consumer subsidy programs by requiring the farmers to sell their commodities at a cheaper price, it transfers the farmers’ incomes that they were supposed to earn to the consumers. Moreover, the government transfers tax burdens to the farmers via export taxes in that the agricultural industry is likely to purchase the farmers’ commodities as cheaply as possible in order to make up for its cost.”

The two countries have compelling economic, political, and policy-oriented rice studies. Some examples of this type of research in the Philippines are the following. Intal and Garcia ( 2005 ) argued that the price of rice had been a significant determinant in election results since the 1950s. Fang ( 2016 ) analyzed how the Philippines’ colonial history bolstered an oligarchy system, where landed elite politicians and patronage politics perpetuated corruption to the detriment of rice farmers. Balié and Valera ( 2020 ) examined rice trade policy reforms’ domestic and international impacts. San Juan ( 2021 ) contends that the 2019 Rice Tariffication Law of the Philippines only encouraged the country to rely on imports and failed to make the local rice industry more competitive.

In Indonesia, some political studies on rice production are the following. Putra et al. ( 2020 ) analyzed how urbanization affected food consumption, food composition, and farming performance. Noviar et al. ( 2020 ) provided evidence that households in the rice sub-sector have achieved an insufficient level of commercialization in their rice production. Rustiadi et al. ( 2021 ) investigated the impacts of land incursions over traditionally rice farming regions due to Jakarta’s continuous expansion. Satriawan and Shrestha ( 2018 ) evaluated how Indonesian households participated in the Raskin program, a nationwide rice price subsidy scheme for the poor. Misdawita et al. ( 2019 ) formulated a social accounting matrix and used a microsimulation approach to assess the impacts of food prices on the Indonesian economy.

Future work

Social science researchers could further explore and compare the local, regional, and national similarities and differences of the abovementioned issues or conduct novel research related to land-use change, land management, urbanization, food and agricultural policies, trade policies, irrigation governance, and price dynamics. Comparative social studies of rice could also lead to meaningful results. As social policy scholar Linda Hantrais noted:

“Comparisons can lead to fresh, exciting insights and a deeper understanding of issues that are of central concern in different countries. They can lead to the identification of gaps in knowledge and may point to possible directions that could be followed and about which the researcher may not previously have been aware. They may also help to sharpen the focus of analysis of the subject under study by suggesting new perspectives.” (Hantrais, 1995 , p. n/a).

Sociological, anthropological, and cultural studies

Biologists dominated agricultural research until the mid-1960s (Doorman, 1991 ). Agriculture, in other words, was no social scientist’s business. However, this situation gradually changed when governments and scholars realized the long-term impacts of the Green Revolution from the 1950s to the 1980s, which underscores that the development, transfer, and adoption of new agrotechnology, especially in developing countries, is driven not only by techno-biological factors but also by the socio-economic, political, and cultural realities under which the farmers operate. Since then, sociologists, anthropologists, and cultural scholars have become indispensable in answering the “how”, “what”, and “why” agrarian communities follow, adopt, utilize, or, in some cases, prefer local/traditional production technologies over the technological and scientific innovations developed by engineers, biologists, geneticists, and agriculturists. Nyle C. Brady, a soil scientist and the former Director-General of the IRRI pointed out:

“… we increasingly recognize that factors relating directly to the farmer, his family, and his community must be considered if the full effects of agricultural research are to be realized. This recognition has come partly from the participation of anthropologists and other social scientists in interdisciplinary teams … during the past few years.” (IRRI, 1982 ).

Since the late 19th century, many rice studies have tried to answer the roles of social scientists in agricultural research. Social sciences have contributed to agricultural research in many ways, especially regarding technology adoption by farmers (DeWalt, 1985 ; Doorman, 1990 ). Doorman ( 1991 , p. 4) synthesized these studies and offered seven roles for sociologists and anthropologists in agricultural research as follows:

“Accommodator of new technology, ex-post and ex-ante evaluator of the impact of new technology, an indicator of the needs for new technology, translator of farmer’s perceptions, broker-sensitizer, adviser in on-farm research, and trainer of team members from other disciplines.”

Social studies of rice are especially critical in Indonesia and the Philippines—home to hundreds of Indigenous cultural communities and Indigenous peoples (Asian Development Bank, 2002 ; UNDP Philippines, 2010 ). Regardless of the highly contested debates surrounding “indigeneity” or “being indigenous,” especially in Indonesia (Hadiprayitno, 2017 ), we argue that Indigenous cultural communities and Indigenous peoples have similarities (i.e., they are often farming or agrarian societies) but also recognize their differences and diversity in terms of their farming practices, beliefs, traditions, and rituals. These socio-cultural factors and human and non-human interactions influence rice production; thus, these differences and diversity bring front-and-center the importance of needs-based, community-driven, and context-sensitive interventions or projects for rice farming communities. These are research areas best explored by sociologists, anthropologists, and cultural scholars.

Today, agriculture’s sociological, anthropological, and cultural research have gone beyond the classic technology adoption arena. In Indonesia, studies have explored farmers’ technical efficiency in rice production (e.g., Muhardi and Effendy, 2021 ), the similarities and differences of labor regimes among them (e.g., White and Wijaya, 2021 ), the role of social capital (e.g., Salman et al., 2021 ), and the reciprocal human–environmental interactions in the rice ecological system (e.g., Sanjatmiko, 2021 ). Disyacitta Nariswari and Lauder ( 2021 ) conducted a dialectological study to examine the various Sundanese, Javanese, and Betawi Malay words used in rice production. Rochman et al. ( 2021 ) looked into the ngahuma (planting rice in the fields) as one of the inviolable customary laws of the Baduy Indigenous cultural community in Banten, Indonesia.

In the Philippines, Balogbog and Gomez ( 2020 ) identified upland rice farmers’ productivity and technical efficiency in Sarangani. Aguilar et al. ( 2021 ) examined the drivers of change, resilience, and potential trajectories of traditional rice-based agroecosystems in Kiangan, Ifugao. Pasiona et al. ( 2021 ) found that using the “modified listening group method” enables farmers’ peer-to-peer learning of technical concepts. Sociologist Shunnan Chiang ( 2020 ) examined the driving forces behind the transformation of the status of brown rice in the country.

Social scientists could further look into the social, cultural, technological, and human–ecological interactions in the temporal and spatial studies of different rice farming regions in Indonesia and the Philippines. Other topics could include the cultural practices and the techno-social relationships of rice farmers (e.g., Shepherd and McWilliam, 2011 ) and other players in the rice value chain, local and indigenous knowledge and practices on agrobiodiversity conservation, historical and invasive pests and diseases, agricultural health and safety, farmer education, and aging agricultural infrastructures. Lastly, future researchers can explore the impacts of adopting rice farming technologies in the different stages or processes of the rice value chain. They can look into its short- and longer-term effects on farmers’ livelihoods and conduct comparative analyses on how it improves, or not, their livelihoods, and whether farmers regard them better compared to the traditional and indigenous practices and beliefs that their communities apply and observe in rice farming.

Social and environmental psychology

Our review yielded no article published on the social and environmental psychology aspects of rice farming in Indonesia and the Philippines, suggesting a new research frontier. The increasing demand for and competition over agricultural and natural resources due to climate change and population expansion (Foley et al., 2011 ) opens up new and emerging sociopsychological dilemmas for society to understand, answer, and, hopefully, solve. Social and environmental psychologists can help shed light on these questions, such as those related to understanding farmers’ pro-environmental agricultural practices (Price and Leviston, 2014 ), sustainable sharing and management of agricultural and natural resources (Anderies et al., 2013 ; Biel and Gärling, 1995 ), and understanding the psychosocial consequences of resource scarcity (Griskevicius et al., 2013 ). Broadly, social psychology examines human feelings, thoughts, and behaviors and how they are influenced by the actual, imagined, and implied presence, such as the effects of internalized social norms (Allport, 1985 ). Social psychologists look at the many facets of personality and social interactions and explore the impacts of interpersonal and group relationships on human behavior (American Psychological Association, 2014b ). On the other hand, environmental psychology examines psychological processes in human encounters with their natural and built environments (Stern, 2000 ). Environmental psychologists are interested in studying and understanding people’s responses to natural and technological hazards, conservation, and perceptions of the environment (American Psychological Association, 2014a ).

Using the Asian Journal of Social Psychology and the Journal of Environmental Psychology as benchmarks, we recommend that scholars explore the following uncharted or least studied areas of rice research in Indonesia and the Philippines: sociopsychological processes such as attitude and behavior, social cognition, self and identity, individual differences, emotions, human–environmental health and well-being, social influence, communication, interpersonal behavior, intergroup relations, group processes, and cultural processes. Researchers could also investigate the psycho-behavioral areas of nature–people interactions, theories of place, place attachment, and place identity, especially in rice farming. Other topics may include farmers’ perceptions, behaviors, and management of environmental risks and hazards; theories of pro-environmental behaviors; psychology of sustainable agriculture; and the psychological aspects of resource/land management and land-use change.

Climate change, weather extremes, and disaster risk reduction

Indonesia’s and Philippines’ equatorial and archipelagic location in the Pacific Ring of Fire (Bankoff, 2016 ; Parwanto and Oyama, 2014 ), coupled with their political, social, and economic complexities (Bankoff, 2003 , 2007 ; UNDRR and CRED, 2020 ), expose and render these countries highly vulnerable to hazards, such as typhoons, strong winds, tsunamis, storm surges, floods, droughts, and earthquakes. The accelerating global climate change increases the frequency and intensity of some of these hazards, such as prolonged droughts, torrential rainfalls causing floods, and super typhoons (IPCC, 2014 ). For example, torrential flooding, induced by heavy rains caused by low pressures and southwest monsoons, has been damaging lives and livelihoods, including rice production (Statista, 2021 ). The 2020 droughts caused over 12 trillion pesos (~US$239.40 billion) of economic losses in the Philippines (Statista, 2021 ) and affected millions of Indonesians (UNDRR, 2020 ). Prolonged drought in Indonesia has also exacerbated fire hazards, which caused transboundary haze pollution in neighboring countries, like Singapore and the Philippines, inflecting environmental health damages (Aiken, 2004 ; Sheldon and Sankaran, 2017 ; Tan-Soo and Pattanayak, 2019 ). Increasing sea-level rise due to anthropogenic climate change puts cities like Jakarta and Manila at risk of sinking in the next 30–50 years (Kulp and Strauss, 2019 ). The high vulnerability, frequent exposure, and low capacities of marginalized and poor Indonesians and Filipinos turn these hazards into disasters (Gaillard, 2010 ; Kelman, 2020 ; Kelman et al., 2015 ), negatively affecting rice agriculture.

Given these contexts, climate change, weather extremes, and disaster risks, vis-à-vis its impacts on the rice sector, are issues of profound interest to scholars and the Indonesian and Philippine governments. In the Philippines, climate adaptation studies include re-engineering rice drying systems for climate change (Orge et al., 2020 ) and evaluating climate-smart farming practices and the effectiveness of Climate-Resiliency Field Schools in Mindanao (Chandra et al., 2017 ). In Indonesia, where some rice farming communities are vulnerable to sea-level rise, scholars are experimenting to identify rice cultivars with high yields under different salinity levels (Sembiring et al., 2020 ). Hohl et al. ( 2021 ) used a regional climate model to develop index-based drought insurance products to help the Central Java government make drought-related insurance payments to rice farmers. Aprizal et al. ( 2021 ) utilized land-use conditions and rain variability data to develop a flood inundation area model for the Way Sekampung sub-watershed in Lampung, Sumatra. Others also looked at the science behind liquefaction hazards caused by irrigation systems for wet rice cultivation in mountainous farming communities like the 2018 earthquake-triggered landslides in Palu Valley, Sulawesi (Bradley et al., 2019 ).

Examples of climate mitigation-related studies in the Philippines include investigating the social innovation strategies in engaging rice farmers in bioenergy development (Minas et al., 2020 ) and evaluating the environmental performance and energy efficiency of rice straw-generated electricity sources (Reaño et al., 2021 ). Doliente and Samsatli ( 2021 ) argue that it is possible to combine energy and food production to increase farm productivity and reduce GHG emissions with minimal land expansion. Other studies have looked into the potential of alternate wetting and drying irrigation practices to mitigate emissions from rice fields (Sander et al., 2020 ).

Future work could explore the following topic areas: demand-driven research and capacity building on climate information and environmental monitoring; nature-based solutions for climate mitigation and adaptation; water–energy–food nexus in rice farming; the nexus of climate change and conflict in rice farming communities; the potentials and pitfalls of social capital in farmer’s everyday adaptation; just energy transitions in rice farming; vulnerabilities from and traditional/local/indigenous ways of adapting to climate change, including the various learning strategies communities use for its preservation; and examples, potentials, and barriers in adopting climate-smart agriculture technologies and practices.

Demographic transitions and aging farmers

Farmers are in various stages and speeds of aging globally (Rigg et al., 2020 ). Evidence of aging farmers in the Global North has been reported in Australia (O’Callaghan and Warburton, 2017 ; Rogers et al., 2013 ), the Czech Republic (Zagata et al., 2015 ), England (Hamilton et al., 2015 ), Japan (Poungchompu et al., 2012 ; Usman et al., 2021 ), and the United States of America (Mitchell et al., 2008 ; Reed, 2008 ; Yudelman and Kealy, 2000 ). Similarly, in the Global South, HelpAge International ( 2014 , p. 21) reported that “there has been a universal trend of an increase in the proportion of older people… attached to agricultural holdings… across [Low and Middle-income Countries in] Asia, sub-Saharan Africa, Latin America, and the Caribbean.” Moreover, farming populations are aging rapidly in East and Southeast Asia (Rigg et al., 2020 ) and southern Africa (HelpAge, 2014 ). Despite this, the literature on aging farmers in Southeast Asian countries remains scant, except for case studies conducted in some villages and provinces in Thailand (Poungchompu et al., 2012 ; Rigg et al., 2018 , 2020 ) and the Philippines (Moya et al., 2015 ; Palis, 2020 ).

Rice farmers’ quiet but critical demographic transformation in Indonesia and the Philippines has not received much attention from scientists, policymakers, and development practitioners. The impacts of aging farmers on the micro-, meso-, and macro-level agricultural processes and outcomes are important issues that require urgent attention. Studies done in other countries could guide future work to explore these questions in Indonesia and the Philippines. These include aging’s potential negative implications in terms of agricultural efficiency and productivity (e.g., Tram and McPherson ( 2016 ) in Vietnam, and Szabo et al. ( 2021 ) in Thailand), food security (e.g., Bhandari and Mishra ( 2018 ) in Asia), farming continuity and sustainability (e.g., O’Callaghan and Warburton ( 2017 ) in Australia, Palis ( 2020 ) in the Philippines, and Rigg et al. ( 2018 , 2020 ) in Thailand), aging and feminization of farm labor (e.g., Liu et al. ( 2019 ) in China), cleaner production behaviors (e.g., Liu et al. ( 2021 ) in Northern China), youth barriers to farm entry (e.g., Zagata and Sutherland ( 2015 ) in Europe), and health and well-being of aging farmers (Jacka, 2018 ; Rogers et al., 2013 ; Ye et al., 2017 ).

Other critical new topics include the (dis)engagement and re-engagement of young people in rice farming; gender dynamics—including structures and systems of inclusion and/or exclusion—in rice production; the impacts of migration and return migration to farming households; community-based and policy-oriented case studies that provide examples of successfully engaging and retaining youth workers in farming; and social protection measures for aging farmers, to name a few.

Contemporary and emerging challenges

One of the biggest and most visible contemporary global challenges is the Covid-19 pandemic. Most pronounced is the pandemic’s impacts on the healthcare system and the economic toll it caused on the lives and livelihoods of people, including rice farmers. Only 0.18% (4 articles) of our dataset have investigated the impacts of Covid-19 on rice systems in Indonesia and the Philippines. Ling et al. ( 2021 ) assessed the effects of the pandemic on the domestic rice supply vis-à-vis food security among ASEAN member-states. They found that Singapore and Malaysia were highly vulnerable to a pandemic-induced rice crisis, while Brunei, Indonesia, and the Philippines are moderately vulnerable. They argued that Southeast Asian rice importers should consider alternative import strategies to reduce their high-risk reliance on rice supply from Thailand and Vietnam and look for other suppliers in other continents.

Rice prices did not change in the early months of the pandemic in Indonesia (Nasir et al., 2021 ); however, as the health emergency progressed, distributors and wholesalers incurred additional costs due to pandemic-induced mobility restrictions (Erlina and Elbaar, 2021 ). In the Philippines, San Juan ( 2021 ) argues that the global rice supply disruption due to the pandemic proves that the country cannot heavily rely on rice imports; instead, it should work on strengthening its domestic rice supply. To realize this, he recommended drastic investments in agriculture and research, rural solar electrification, and the promotion of research on increasing rice yields, boosting productivity, and planting sustainably as feasible steps on the road to rice self-sufficiency.

The ways and extent to which the pandemic negatively affected or exacerbated the vulnerabilities of rice farmers and other value chain actors remain an understudied area in the social studies of rice. Scholars could study the pandemic’s impacts in conjunction with other contemporary and emerging challenges like climate change, weather extremes, aging, conflict, and poverty. Scholars could also explore the medium- and longer-term impacts of the pandemic on rice production, unemployment risks, rice supply and nutrition security of farming households, and the potential and extent to which economic stimulus can benefit rice farmers, to name a few. Most importantly, the pandemic allows researchers and governments to assess the business-as-usual approach that resulted in the disastrous impacts of the pandemic on different sectors, including rice farmers, and hopefully devise strategies to learn from these experiences.

From our review of 2243 articles, cumulatively written by 6893 authors using almost 80,000 references, we conclude that a voluminous amount of rice research has been conducted in Indonesia and the Philippines since 2001. As in other reviews, (e.g., on energy research by Sovacool, 2014 ), our results show that women scholars remain underrepresented in rice research in Indonesia and the Philippines. While interdisciplinary collaboration is abundant, most of these studies belong to the natural sciences with minimal contributions from the social sciences, arts, and humanities. University and research institutions contributed the most to rice research in Indonesia and the Philippines: from hybrid rice cultivars, water management, and technology adoption to socio-cultural, political, economic, and policy issues. Influential scholars in the field were affiliated with the IRRI, which can be expected given the institute’s focus on rice, and key agriculture-focused universities and government bureaus such as the University of the Philippines and the PhilRice in the Philippines, and the Institut Pertanian Bogor University and the Universitas Gadja Maja in Indonesia. We also discussed some examples of economic, political, and policy studies; social, anthropological, and cultural research; social and environmental psychology; climate change, weather extremes, and disaster risk reduction; demographic transitions; and contemporary and emerging issues and studies on rice in the two Southeast Asian countries. Ultimately, we hope that this systematic review can help illuminate key topic areas of rice research in Indonesia and the Philippines and magnify the crucial contributions from and possible research areas and questions that interdisciplinary and comparative social scientists can further explore.

Data availability

The dataset analyzed in this study is available in the Figshare online repository via https://doi.org/10.6084/m9.figshare.17284814.v2 . All codes about Bibliometrix are available at https://bibliometrix.org/ .

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Acknowledgements

The work described in this paper was substantially supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. HKUST 26600521). Partial funding was also made available by the HKUST Institute for Emerging Market Studies with support from EY.

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GPC made substantial contributions through this paper’s original conceptualization, data curation and processing, formal analysis, methodology, writing the original draft, and reviewing, revising, and finalizing the paper. LLD made substantial contributions to its conceptualization, funding acquisition, supervision, and reviewing and editing of the review paper.

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Cuaton, G.P., Delina, L.L. Two decades of rice research in Indonesia and the Philippines: A systematic review and research agenda for the social sciences. Humanit Soc Sci Commun 9 , 372 (2022). https://doi.org/10.1057/s41599-022-01394-z

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DOI : https://doi.org/10.1057/s41599-022-01394-z

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Apr. 2, 2024

Rice student making a difference in community through lung cancer research.

Priyanka Senthil is maximizing her opportunities at Rice University through her research and advocacy work related to lung cancer screening.

Senthil, a third-year student at Rice majoring in health sciences and minoring in medical humanities, has been working on clinical research related to improving lung cancer screening guidelines and treatment for the past three years.

“I’ve always been intrigued by lung cancer after I learned that it’s the deadliest cancer in the U.S. and the world,” Senthil said. “And that’s because a lot of people who are at high risk for lung cancer are not getting screened, which is the best way to catch lung cancer early. That’s what really drew me into both the research and advocacy work that I do now.”

Senthil is the executive director of the  American Lung Cancer Screening Initiative  (ALCSI), which is a national nonprofit organization of over 300 students and doctors across the country dedicated to raising awareness of and access to lung cancer screening.

Senthil is the president of ALCSI’s Rice chapter, which recently held a White Ribbon Build event where participants painted and signed wooden white ribbons for lung cancer patients.

“We have held over 490 community events and taught over 25,000 individuals about lung cancer screening,” she said. “We’ve also worked with mayors, governors and national leaders to issue proclamations and public service announcements encouraging constituents to get screened in addition to passing legislation around lung cancer screening.”

Senthil has published 10 papers related to lung cancer screening, including a first-author review  paper  that provides an update on the current lung cancer screening guidelines.

In March, she and her team published a  study  in the “Journal of Clinical Oncology” showing that using smoking duration, instead of pack-years, to determine lung cancer screening eligibility is more equitable and greatly increases opportunities for early lung cancer detection.

She also recently gave an oral presentation at the 2024 Academic Surgical Congress on a study regarding disparities in the surgical treatment of lung cancer between males and females in the United States. The study has found that females are more likely than males to receive inferior treatment for early-stage non-small-cell lung cancer.

Senthil said her research has made her passionate about finding ways to address underserved communities when it comes to accessibility of cancer treatment.

“I think it’s really important for groups to be doing research that highlights limitations and disparities in order to push for health policy change,” she said. “It goes back to trying to be a voice for people in communities that are oftentimes silenced or just not heard.”

Senthil’s time as a student at Rice has provided her with ample opportunities to pursue these passions boundlessly, she said.

“I have absolutely loved my time at Rice,” she said. “What I really liked about Rice early on was how collaborative the atmosphere is. Everyone is here to help and push everyone else up, and I really love that about Rice.

“Secondly, it’s the opportunities. We’re right next to the Texas Medical Center, which is the largest medical center in the world, so if you’re interested in research or anything related to medicine, there isn’t a better place to be.”