Agricultural food processing consists of a variety of value chains and generates different types of agricultural waste through the value chain from farm to fork [13,14]. Large quantities of valuable wastes are produced during the harvesting and processing stages, and these wastes should be studied and analyzed to extract their valuable parts sustainably. For example, paddy straws are produced in large quantities during paddy harvesting. These straws are commonly found in the field and are often used as fodder for animals and as bedding for livestock [15]. The remaining part is burned by the farmers when ready for subsequent cultivation. Additionally, rice husk frequently becomes a material for burning and is discarded in landfills [16]. The destruction of this valuable biomass without proper use will cause irreversible damage to the environment and all living beings on the planet. Bran is another by-product of the rice processing value chain. After harvesting and milling, the by-products of the rice industry can be subjected to industrial symbiosis to exploit their full value [2,13].

Rice farming is integrated with geography, soil type, water availability, harvesting and processing techniques, and market behavior [17,18]. Farmers grow different types of rice in different regions because each grows best in specific soil types and climates. Additionally, access to water and proper irrigation systems boost agricultural production [19,20]. Many small and medium rice farmers still depend on labor [14]. Traditional techniques are slow and dependent on workability and experience. Modern technology and equipment are helpful in rice-producing areas but expensive [13,14,21,22]. Consequently, different production conditions affect the rice industry’s waste [21,22].

After completion of the harvesting process, the paddy is transferred to the rice mills to be processed into white or brown rice. The paddy is subjected to a series of operational procedures during the rice milling process to remove straw particles, half-filled seeds, husks, bran, and germ. Several milling processes exist, such as one-stage milling and multi-stage milling [2,30]. Compared to multi-stage mills, the one-stage milling technique produces fewer by-products. The large-scale industrial milling process has several steps, such as cleaning (removing chaff, dead seeds, seeds that are only half full, and stones), parboiling, de-husking, peeling, polishing, and grading [2,30,31]. In addition, specific varieties of rice will be washed in hot water for a certain amount of time to remove the husk, enhance its size, and obtain a better shape of the grains [31].

There are several ways to remove rice husks from rice seeds. The germ particles and outer bran are removed after the husking in a series of huller reels and pearling cones, where the waxy cuticle is sheared off by friction between the high-speed abrasive cone and its casing [30,31,32]. As a result, rice bran is generated as a by-product [33]. The milling gap between the cone and the cover can be changed. Therefore, the grinding ratio can be changed by raising or lowering the cone [30,31]. Typically, in most rice mills, the rice passes through several cones, each with a higher milling rate than the previous one [30]. Since the milling space between the cone and the casing is adjustable [31], the milling rate can be varied by raising or lowering the cone [29]. The bran from the different stages is usually quantified as one product [33]. Next, rice from the pearlier is passed through polishers to get a finer appearance to the rice grains [2,26]. In this process, some parts of the starchy kernel are removed. This by-product is called fine bran if it is included within the inner bran layer. Finally, the mixture of whole and broken rice from the polishers are subjected to the sieving process and graded according to the standard at which the rice is sold [27].

The FAO Agricultural Outlook predicts that paddy production will rise to 52603 metric tons by 2027 compared to 2018 [40]. Due to factors including the increase in agriculturally usable land, technological advancements, and faster population growth in recent decades, global agricultural output has expanded dramatically [40].

As agricultural waste generates economic benefits, agricultural waste recycling is not meant to degrade value like other industrial waste recycling does [42]. Due to the nature of systemically implemented operations, recycling must be compared to materials that remain the same or lose performance when recycled. Due to their inherent propensity for rapid spoiling, agri-food supply chain management may need to be more sustainable and efficient [43]. Having a systemic vision and viewpoint that prioritizes the concepts of complexity and networks is essential for solving this challenge [13,42,43,44]. According to this method of thinking, a system is a collection of interconnected individuals whose behaviors are determined by their connections. When all of these elements are considered, they form a holistic system with more worth than just the sum of its individual parts. From this point of view, designing the agri-food scenario using a systemic approach is a viable method to begin a paradigm shift that entails switching from linear to circular structures.

Rice straw is the vegetative part of the rice plant (Oryza sativa L.). Rice straw consists of the plant’s stem, leaves, and pods and is generated after being cut off during harvest. Rice straw comprises cellulose, lignin, waxes, silicates, and minerals. In general, animals are often fed with rice straw, and rice straw can be utilized for creating compost, paper, cow bedding, and crafts; it also offers energy to specific industries and covers agricultural areas [45,46]. The rice straw of the current year is usually burned before the subsequent plowing to prepare the field.

Variety, cultivated area, seasons, nitrogen fertilizer, plant maturity, plant health, and several other environmental and human variables significantly impact the chemical composition of any biomass [15,47]. Changes in chemical and physical parameters affect the yield and quality of the final product. Heterogeneity is thus seen as detrimental to the manufacturing process. Additionally, this impacts how by-products are used at the end of their life cycle. Therefore, compositional analysis and structural characterization should be considered to enhance the effectiveness of the valorization options. Rice straw has a greater silica concentration but less lignin than the straws of other cereals [48]. In order to maximize silica amount in the stem ratio, it is advised that the rice straw be shortened as much as possible [2]. Cell walls may contain silica, or silica may be soluble in water. They are eliminated with urine, where they sometimes crystallize. Since rice straw has a high oxalate content (1–2% of dry matter) and is known to lower Ca concentrations, adding supplemental Ca is often recommended [49]. Variety, time duration between harvest and storage, amount of nitrogen fertilizer used, plant maturity (lignin content increases with maturity), plant health, and environmental conditions affect the quality of rice straw [15]. Although rice straw is a rich energy source, it contains only 2–7% protein and is indigestible due to its high silica content. Therefore, it is considered a coarse and low-quality food source [50]. Minerals such as sulfur may be a limiting factor when considering it as fodder [51]. Other conditions usually involve:

• Excessive amounts of neutral detergent fiber (NDF) lead to decreased feed consumption and fat-corrected milk output [52].
• There is not enough P, Cu, Zn, Ca, and NaCl to meet the needs of animals [53].
• In comparison to corn silage, it contains less energy, has an unpleasant taste, and uses nitrogen less effectively [54].

A significant waste product in the value chain of rice processing is rice bran. It is mainly used as animal feed and is regarded as a healthy source of fiber for pets because of its high nutritional content. Additionally, farmers may get it at a significant discount because of its availability. Due to the high fat and fiber content of rice bran, up to 40% of it is added to the diets of cattle, dogs, pigs, and chickens [60,61,62]. Additionally, rice bran is a valuable feed for many animals since it contains 14–18% oil. Therefore, dehulled rice bran may be utilized in more value-added processes than ordinary rice bran [2].

The composition of rice bran has a significant role in defining its possible valorization options. Rice bran’s physical and chemical properties are influenced by several aspects concerning the grain and the milling procedure [63]. Rice variety, environmental circumstances, grain size and form, distribution, chemical components, strength of the outermost layer, and breaking resistance are the primary elements affecting rice grain [64]. Additionally, the type of grinding machine is the main factor related to the processing conditions, and the grinding process of different layers of rice grain at different depths shows different chemical compositions [63,64].

Rice bran contains various nutrients, including carbohydrates, proteins, minerals, and lipids. It has a high carbohydrate (cellulose and hemicellulose) content and is simple to employ to create microbial products with added value [65]. As a result, before the valuation procedure, it is required to assess the composition. Because rice bran is employed in value-added goods as a microbial product or as a food additive, it is generated during several phases of the rice milling process, which are eventually combined and discharged as rice bran. As a consequence, the chemical composition varies significantly [2]. In addition, the chemical composition of raw rice bran and de-oiled rice bran varies in fiber concentration [55]. 

Rice bran stands out compared to other cereal grains due to the tocotrienol, tocopherol, γ-oryzanol, and β-sitosterol contents [66]. This is significant since there is mounting evidence that these substances may help to lower levels of total plasma cholesterol, triglycerides, and low-density lipoprotein while raising levels of high-density lipoprotein [66]. In addition, ferulic acid and soluble fiber (including β-glucan, pectin, and gums) are found in the indigestible cell walls of rice bran. While the United States Department of Agriculture (USDA) nutritional database values for crude rice bran are often utilized in animal diet formulation [67], caution must be taken since they may not account for changes across rice cultivars [68].

The variety of rice bran utilized determines the chemical content and quality of the end product. According to Hong and his co-workers [69], the fatty-acid content of rice bran oil varies based on the type of rice bran utilized. According to the same paper, rice bran oil, which includes a high concentration of free fatty acids, has several drawbacks when used as fuel in diesel engines in the winter season.

Rice husk is the outer covering of the rice grain and is produced as a by-product of the rice milling process. It is also called hull and chaff [39,76]. In agricultural nations, this is the most prevalent agricultural by-product. In particular, rice husk is utilized as the primary source of energy in rice mills, poultry farming, and silica-rich cement [56,77,78]. Additionally, small quantities are used as construction materials and fertilizers [79]. However, most rice husks eventually wind up in landfills or are burned in the open air, significantly polluting the environment. The calorific value of rice husk is considerably high, roughly 16,720 kJ/kg [80]. As previously stated, many millers directly burn or gasify rice husk as their primary energy source [16]. Rice husk ash is another type of waste produced during this burning procedure. This additional waste, which makes up around 25% of the original volume of rice husks, has a significant adverse effect on the environment [38].

Due to photosynthesis and biochemical interactions, silica and a barrier layer are formed on the rice plant’s stem and husk surfaces [81]. These layers have developed to shield the rice plant and its grains from environmental changes such as temperature variations, excessive water evaporation, and microbial assault [81]. Approximately 20–30% of the rice husk is made up of mineral components, including silica and metallic residues containing magnesium (Mg), iron (Fe), and sodium (Na). Calcium (Ca), manganese (Mn), and potassium (K) are further examples of trace elements [82]. Rice husk mainly comprises organic compounds, including cellulose, lignin, and hemicellulose, making up around 70–80% of the total weight [37,83]. Rice husk is maturing into a raw material prospective in the manufacturing sector. However, when rice husk accumulates to the point that it poses a severe threat to the local ecosystem, it is classified as agro-waste. As a result, these adverse effects on the environment must be softened via a process of valorization or value addition. Therefore, it is crucial to conduct a physicochemical investigation and determine the composition of the material.

Rice straw could also be valorized for four different purposes: energy production, animal feed, fertilizer, and other uses. By pyrolyzing rice straw, bio-oil, biochar, and syngas may be generated. Numerous chemical substances are found in rice straw bio-oil, including alcohols, acids, furans, aromatics, ketones, phenols, and pyranoglucose [47]. Alcohol and pyranoglucose are created as a consequence of the pyrolysis of cellulose, while hemicelluloses are used to create ketones [47]. The metabolic process through which carbohydrates are changed into alcohols or acids is known as fermentation, as shown in Equations (1) and (2). Second-generation biofuels are made from cellulose feedstock (Equation (1)). Physical, chemical, or biological pretreatment and fermentation are all viable routes to their production. While its lack of competition from other feedstock substrates is an advantage, its need for highly efficient lignohemicellulose enzymatic breakdown is a drawback. Although the commercialization of second-generation ethanol facilities shows promise, the longevity of these plants will primarily rely on the market availability of the feedstocks at affordable costs [90]. Bacteria convert carbohydrates into lactic acid (Equation (2)). Numerous chemical or physical pretreatments are required, followed by enzymatic hydrolysis to convert fermentable sugars from lignocellulosic materials into ethanol or lactic acid. In addition to its many uses in the food and beverage industries, lactic acid and its derivatives also have a wide range of applications in the pharmaceutical, cosmetic, and manufacturing industries [91,92]. Numerous studies have demonstrated that rice straw can be utilized to make second-generation biofuels [47,93,94,95,96]. Typically, bacteria and yeast turn carbohydrates into lactic acid and sugars into alcohol. Trichoderma reesei, which was derived from decaying rice straw waste, produces cellulases that break down cellulose in the rice straw to glucose, which is then fermented with yeasts such as S. cerevisiae to make ethanol [93,97,98].

The anaerobic digestion process may convert rice straw into biogas [75,99,100]. Anaerobic digestion is a sustainable process that converts organic waste into usable energy. Generating green energy from rice straw is an effective way to lessen the effects of global warming [75]. Around the globe, rice straw is utilized directly as an energy source for heating rooms by direct burning, firing clay pots, and cooking [101]. Additionally, small grids in certain nations such as Nigeria have a higher potential for using rice husks and straw as a source of rural power [102,103]. Umar and co-workers [102,103] claimed that rice straws have the potential to generate 1.3 million MWhy-1 energy in a country such as Nigeria. A 36 MW power plant in Sutton, Ely, Cambridgeshire, was constructed in 2000, producing more than 270 GWh annually while using 200,000 tons of rice straw [104]. Another work shows that Sri Lanka has a total energy capacity of 2129.24 ktoe/year of primary energy from rice straw and rice husk and a capacity of 977 Mwe, allowing it to produce 5.65 TWh of electricity per year [16].

According to literature, rice straw may be used efficiently for composite preparation [105,106,107,108,109,110,111]. Furthermore, rice straw microfibrils at 5% increase the characteristics of rice straw polypropylene composites [107]. Another study highlights many uses of rice-straw cement bricks for load-bearing walls [106]. Rice straw can also be used to lower the price of cement bricks with sufficient thermal insulation, appropriate mechanical qualities, and fire resistance [112,113,114,115]. Furthermore, rice straw-based composites with adhesives generated from starch can be used as ceiling panels and bulletin boards [109]. Finally, following proper pretreatment, rice straw could also be utilized to produce fiberboard [116].

Rice straw has a low value as a feeding material, despite its use as bedding for cattle [15]. In contrast to ruminants, which depend on symbiotic bacteria to break down cellulose in the gastrointestinal system, all vertebrates lack the enzymes necessary to dissolve β-acetyl bonds [52]. Additionally, dried rice straw contains low nutrient value owing to its low amount of protein and high amounts of lignin and silica. However, this may be addressed by pretreating it with ammonia or urea [48]. To increase the nutrient availability of rice straw, it can be converted into silage. Therefore, some researchers have focused on improving rice straw harvesting technologies for silage production [120]. Other studies have explored several practical examples of silage processing, including using different additives to enhance fermentation quality and adding yeasts such as Candida tropicalis [121,122,123]. Feed intake, digestibility, rumen fermentation, and microbial N synthesis efficiency are improved after urea treatment of rice straw [124].

Rice straw has been proposed as a low-cost adsorbent for purifying contaminated water [125]. However, straw surface composition and metal speciation significantly impact the adsorption capacity, which changes with metal ions and water pH [126,127]. On the adsorbent surface of rice straw, methyl/methylene, hydroxyl, quaternary ammonium, ether, and carbonyl groups predominate; adding additional quaternary ammonium or incorporating carboxyl groups enhances its adsorption capability [128]. However, competing cations and chelators in the solution are likely to result in decreased sorption capacities [129]. Furthermore, most heavy metal ions exhibit maximal adsorption capacities around pH 5. In contrast, very acidic circumstances promote Cr adsorption [130], which could be the result of the reduction of Cr(VI) to Cr(III). Moreover, cellulose phosphate derived from rice straw that has been treated with NaOH and then reacted with phosphoric acid in the presence of urea has a more remarkable ability to absorb heavy metals. This ability is increased when microwave heating is used to produce it [131]. The addition of epoxy and amino compounds to rice straw by reacting with epichlorohydrin and trimethylamine results in a high sulfate adsorption efficiency, demonstrating the material’s anion exchangeability [132]. Like rice husk, straw can be used as an adsorbent for different water contaminants, such as alkali and phenolic chemicals, that can usually be recovered using anionic species [133]. Various adsorbents from rice straws have also been developed to remove dyes from wastewater. An example of cationic dye application is rice straw treated with citric acid, which increases the specific surface area and pore size. These treated straws have been used to absorb crystal violet or methylene blue from an aqueous phase [134]. It has been observed that the addition of activated rice straw causes a significant reduction in microalgae in water, which has been attributed to the synergistic effects of humic chemicals and H₂O₂ created by the straw breakdown [135]. According to a different investigation, water and methanol extracts from rice straw controlled the cyanobacterium Anabaena sp. but promoted Chlorella sp. To prevent the development of Anabaena sp., rice straw extraction is an economical and ecologically beneficial option, but it may not work as well on other cyanobacteria and microalgae [136].

Rice straw is utilized as organic fertilizer for various crops in many places throughout the globe. It can also be used as a soil conditioner to replace the organic matter in the soil [112]. In addition, rice straw is also a growth medium for mushrooms [137]. Adding biochar derived from rice straw to the soil makes it possible to enhance the characteristics of the soil by lowering its pH, cation exchange capacity (CEC), nutrient availability, and nitrate leaching [138,139,140]. 

Considering the concept of circular economy and green product technology, the biorefinery plan would be the best option for managing and utilizing rice bran [145]. In addition to providing more nutrients than other cereal grains, rice bran has more lipids, protein, and calories [146,147,148]. Rice bran is vulnerable to oxidative rancidity; thus, heat stabilization is necessary to avoid spoilage and rancidity [149]. Rice bran oil is widely recommended around the world due to the presence of several beneficial natural and healthy bioactive ingredients. Companies have been encouraged to manufacture stabilized rice bran and rice bran products to improve the health of organisms because of the unique mix of lipids, minerals, and nutrients found in rice bran, including calcium, phosphorus, and magnesium [65]. In addition, several researchers have found that the manufacturing of de-oiled rice bran and rice bran oil is in great demand worldwide [62,150,151].

The production of biodiesel from rice bran is actively marketed all over the globe. However, rice bran oil must be removed from the rice bran to produce biodiesel via a transesterification process [152]. Several methods have purportedly been utilized to produce biodiesel from rice bran oil, including acid-catalyzed and base-catalyzed transesterification and lipase-catalyzed transesterification. However, each technique has different environmental effects as well as technological and economic benefits and drawbacks [153,154,155,156].

Some researchers have examined bioethanol synthesis from rice by-products such as rice bran, defatted rice bran, and rice washing drainage [157,158,159,160]. After pretreating stripped rice bran with diluted acid and detoxifying it, the Pichia stipitis NCIM 3499 strain generated an ethanol concentration of 12.47 g/L [161]. Additionally, another study found that biological pretreatment with the fungus Aspergillus niger increased ethanol output [162]. Numerous scientists have attempted to manufacture lactic acid from dehulled rice bran using various microbes [163,164,165,166]. Another study discovered that many Bacillus coagulans isolates could grow in denatured rice bran enzymatic hydrolysates without adequate nutrients, with the majority producing concentrations of lactic acid more significant than 65 g/L and yields greater than 0.85 g/g [163]. They stressed in the same paper that manufacturing lactic acid from dehulled rice bran might be economically viable.

Due to its numerous similarities to gasoline, biobutanol is the most ecologically benign substitute for traditional fossil fuels. Additionally, when HCl and enzyme treatments are used together, they can remove 41.18 g/L of sugar from dehulled rice bran and 36.2 g/L of sugar from rice bran [167]. Another study reported that both defatted rice bran hydrolysates and rice bran hydrolysates could be fermented in bioreactors with nutrients to make butanol at a rate of 12.24 g/L and 11.4 g/L, respectively [163].

Rice bran can be used as an adsorbent for polluting substances because it has a granular shape, is chemically stable, does not dissolve in water, and is easy to get. Its surface has several active sites that can remove pollutants [39]. How well these sites work depends on the chemical nature of the solution and whether or not there are other ions in the solution besides the ones to be trapped. Additionally, various functional groups on the surface of rice bran, such as hydroxyl and carbonyl groups, are responsible for its high adsorption effectiveness [39]. The existence of these groups is supported by the ATR–FTIR spectrum, which exhibits rice-straw-like peaks. Some researchers have tried to figure out the best way to remove arsenic from water using a fixed-bed column system made of rice bran.

Another excellent substitute for conventional fossil fuels is hydrogen, which, when oxidized, merely produces water vapor (H₂O). Additionally, hydrogen has a higher energy content for mass units than traditional fuels, ranging from 112 to 142 kJ/g [168,169]. Photo and dark fermentation and their combination are all capable of producing bio-hydrogen [170]. Some studies have investigated hydrogen generation from rice bran and defatted rice bran using isolated bacteria from the same substrates. They identified E.ludwigli IF2SW-B4 as the most promising strain. When rice bran was utilized as a substrate, 545 mL/L of bio-hydrogen was produced [171]. The whole biotechnology process will be more economical once enzymes are produced utilizing low-cost ingredients. For the environmentally friendly and more effective release of fermentable sugars from different affordable and sustainable biomasses such as rice bran, enzymatic hydrolysis is used [171]. Researchers have conducted several investigations to synthesize enzymes from defatted rice bran and rice bran [172,173,174]. 

South Asian countries such as India, Pakistan, Bangladesh, and Sri Lanka were among the best in the world in utilizing rice husk from 1970 to 1985 [175]. In addition, governments and other organizations engaged in rice farming and the post-harvest process have provided essential direction and strong support for rice husk management. Rice husk differs from other agricultural wastes in several important physicochemical aspects, including high silica concentration, low density, high porosity, and a significant outer surface area [176]. Because of these qualities, rice husk is more valuable than other waste materials. As a result, it covers a range of industrial applications.

In water treatment, using activated carbon for the adsorption process to remove heavy metals from industrial effluents is appealing. Numerous functional groups, including hydroxyl, methyl/methylene, ether, and carbonyl are present on the rice husk’s adsorbent surface, contributing to the material’s enhanced adsorbent efficiency [39]. The existence of these groups is supported by the ATR–FTIR spectrum, which mainly exhibits bands at 3307 cm⁻¹, 2921 cm⁻¹, 2000–2500 cm⁻¹, 1654 cm⁻¹, 1034 cm⁻¹, and 788 cm⁻¹ representing hydroxyl groups, C–H groups, C≡C or C≡N bonds, C=O groups, C–O and C–H bonds, and Si–O bonds, respectively.

The presence of several types of polar groups on the surface of rice husks results in a significant cation exchange capacity, indicating a potential efficacy in physisorption mode [177]. Rice husk treated with H₃PO₄ showed enhanced copper absorption capacity [178]. Some studies found that chemically treated rice husk can absorb cationic dyes such as methylene blue [179,180] and malachite green [181]. To study the absorption of fluoride from aqueous solutions, some researchers produced rice husk by chemically impregnating it with nitric acid, followed by physical activation [182]. According to their findings, the highest absorption of fluoride was 75% at a pH of 2, and the ability to absorb fluoride decreased as the pH rose from 2 to 10. Ahmaruzzaman and Gupta [183] confirmed this conclusion. When modified rice husk is cross-linked with poly(methyl methacrylate-co-maleic anhydride), nanoparticles are formed that can be used to absorb heavy metal ions (such as Pb(II)) and dyes (such as crystal violet) [184]. Researchers have discovered that novel green ceramic hollow fiber membranes made from rice husk ash can act as an adsorber and separator to remove heavy metals from water effectively [185]. Treating rice husk with H₂SO₄ and NaOH prior to heating enhances the product’s capacity to absorb phenol [186].

Biomass derived from agricultural waste has been identified as a rich source of feedstock for biochar production; however, at present, farmers, and other stakeholders such as millers, practice open field burning or open dumping to dispose of these by-products. Compared to low-cost traditional treatment procedures (boiling, chlorination, sand filtration, and solar disinfection), biochar adsorbent offers various advantages. It is also suitable for low-income countries because of its availability, cheap cost, and accessible technology. Low-cost conventional approaches mainly destroy pathogens, while biochar can remove a wide range of pollutants from drinking water. Existing processes, such as chlorination, emit carcinogenic by-products, and boiling concentrates chemical contaminants. Pyrolysis temperature, vapor residence time, and other chemical and physical alteration variables influence the properties of biochar. Compared to conventionally activated biochar, rice husk biochar activated by a single phase of KOH-catalyzed pyrolysis under CO₂ has a larger surface area and greater capacity for phenol adsorption [187]. The gold-thiourea complex can be effectively adsorbed using biochar derived from rice husks that have been heated to 300 °C and have particular silanol groups and oxygen functional groups [188]. Rice husk-activated carbons are effective in removing phenol [189], chlorophenols [190], basic dyes [191,192], and acidic dyes [193,194] from water, as well as heavy metal ions such as Cr(VI) at low pH [195,196], Cu(II), and Pb(II) [197].

Rice husk pellets provide an alternative to diesel oil and coal for energy generation in small-scale power plants. Through pyrolysis and gasification processes, they may also be used to produce biodiesel [202]. Rice husk, subjected to a thermochemical conversion process, may provide an inexhaustible supply of gaseous and liquid fuel. Thermochemical processes such as combustion, gasification, and pyrolysis are often regarded as the primary means of producing secondary energy substances. Fermentation and transesterification are also critical biochemical steps in ethanol and biodiesel production [175,203,204]. The briquettes made from rice husks with starch or gum arabic as binders burn stronger and more efficiently than timbers [205]. Another study describes a reactor that uses rice husk combined with sawdust or charcoal to generate high-grade fuel [206]. In order to obtain charcoal, which has a comparatively high calorific content, rice husk is subjected to carbonation using starch as a binder and either ferrous sulfate or sodium hypophosphite, which promote ignition [135]. Economically viable primary pyrolysis oil, suitable as boiler fuel oil and for the manufacture of catalytically treated, upgraded liquid products, can be obtained by fluidized-bed rapid pyrolysis with the catalytic treatment of rice husk [207].

Materials derived from rice husks have been used in the world’s most advanced technical equipment and industries. For example, the Indian space agency has figured out how to extract high-quality silica from rice husk ash. This high-purity silica might also increase its use in the information technology sector [175]. In addition, the same publication has stated that other scientists have discovered how to extract and purify silica from rice husk ash to produce semiconductors. In addition, several researchers have pointed out the prospects of using silicon-based compounds extracted from rice husk and ash in various industries [208,209].

Due to its microscopic particle size, high solution pH, and low supportive electrolyte content, rice husk ash is an effective adsorbent for heavy metals, including lead and mercury [212,213]. In addition, the fluoride-absorption capacity of rice husk ash treated with aluminum hydroxide is enormous [214]. Both the effective removal of phenol from aqueous solutions and the adsorption of various dyes, including indigo carmine, Congo red, and methylene blue, has been accomplished using rice husk ash [180,215,216,217,218]. Due to its high silica concentration and the existence of mesopores and macropores, rice husk ash is a promising adsorbent for removing contaminants from biodiesel [219]. Zou and Yang [220] examined different approaches for generating silica and silica aerogel from rice husk ash. Epoxy paints can use rice husk ash as a filler, and the inclusion of rice husk ash can improve a variety of qualities, including wear resistance, elongation, and scratch resistance [221]. In addition, a paper’s printing quality might be enhanced by using rice husk ash. Because rice husk ash contains more silica, it can improve the paper’s surface quality, and the coating layer it generates reduces the quantity of ink penetrating the paper [222]. Additionally, some researchers have studied pigments made from rice husk and ash [175].