Rice straw and rice husk-based absorbents for oil spill response

2.1 Effects of spilled oil on the environment, humans, and animals

The location, proximity to environmentally sensitive regions, and environmental conditions are known to influence oil dispersion from accidents at sea (Zhang et al., 2019). Since different species exhibit varying degrees of susceptibility to toxicity as well as physical pollution, the kind and volume of oil spill, as well as the resistance of the affected animals, largely determine the extent of the damage (Li et al., 2016). In fact, the physical features, chemical makeup, and natural weathering processes of an oil spill all affect the ecological system (Lee et al., 2015).

Mesozooplankton, copepods, and micro-zooplanktonic larvae can be severely impacted by polycyclic aromatic hydrocarbons, dispersants, and crude oil, which can cause acute, chronic, or sublethal toxicity because they cannot escape contaminated waters. In addition, growth inadequacies, physiological abnormalities, and a higher death rate result from oil spill pollution (Almeda et al., 2014, 2013). Particularly, smaller invertebrates are susceptible to the narcotic impacts of crude oil in this regard, resulting in altered genes, slowed growth, increased mortality rates, metabolic disorders, and infertility (Hook et al., 2014). Due to direct toxicity and cleanup attempts, semi-terrestrial crustacean populations have decreased by over 66.7% as a result of oil-contaminated sandy shorelines (de la Huz et al., 2005; Singh et al., 2020). More seriously, oil spills could also affect seabirds, in which the spilled oil adheres to their feathers, lowering their insulation and waterproofing, causing hypothermia, malnourishment, or drowning, especially in the Arctic and subarctic areas (Fritt-Rasmussen et al., 2016). When oil seeps into coastal areas, the hazards to shorebirds rise. For example, spilled oil causes birds’ feathers to deteriorate. Moreover, birds can ingest spilled oil during preening, and birds can consume oil-contaminated prey. Additionally, the cleanup operations produce disturbances for marine animals (Ma et al., 2022). Ingestion, inhalation, and skin contact are how marine animals are exposed to oil spills, causing long-term severe repercussions, such as habitat degradation, food scarcity, behavioural changes, and reproductive issues (Bi et al., 2025; Helm et al., 2014).

Fish populations near spawning grounds or larval drift zones are at greater danger under the oil spill disaster (Hjermann et al., 2007; Rooker et al., 2013). Infants are particularly susceptible since their membranes are delicate and their bodies are still developing their ability to eliminate poisons. According to studies, even a small amount of oil and some chemicals may cause major damage or even death to them. They may become malformed, have difficulties in growing, become easier prey for predators, or starve as a result of these oil spill occurrences (Hicken et al., 2011; Langangen et al., 2017; Sørhus et al., 2015). In addition, it might be disastrous if oil spills get into locations such as salt marshes, coral reefs, tidal flats, rocky shorelines, and mangroves, seaweeds, algae, and kelp forests, because these areas are interconnected (Duke 2016). In the case of oil-covered corals, they are found to develop poorly, lose vitality, become malformed, turn white, or lose their color. Additionally, oil contamination can lead to unusual root growth, leaf loss, fewer seedlings, impaired photosynthesis, and even genetic disruption in maritime plants (Naidoo et al., 2010). When oil covers the mangroves’ breathing passages, they suffer severe damage because the spilled oil prevents oxygen from reaching the roots (Lewis et al., 2011; Singh et al., 2020). More significantly, large oil spills along the coast present a major threat to the health of humans and the ecosystem (Abdellatif et al., 2020; Ferreira et al., 2022). Oil spills are a major concern for fishermen and other coastal workers, in addition to harming the ecosystem, in which both locals and beach visitors suffer as a result.

Oil spills are harmful to people’s health, particularly for those who are cleaning up the oil spill mess. Headaches, respiratory issues, and irritation of the skin and eyes might result from exposure to the spilled oil (Peres et al., 2016). Alternatively, fishing communities and native people are found to be dramatically affected by oil spills, even heavy stress (Picou and Gill 1997), causing long-term health consequences such as hormone disorders, weakened immune systems, and genetic damage (Laffon et al., 2016, 2013). In addition to human health impacts, oil spills have also caused remarkable effects on society. For example, the community of Cordova, Alaska, had financial difficulties as a result of the 1989 Exxon Valdez tragedy. In addition, oil spills increased criminality, domestic issues, and fighting (Gill et al., 2012). Many indigenous groups were forced to rely on store-bought food after losing access to their hunting and fishing grounds (Miraglia 2002). More dangerously, pollution from oil spills disrupts people’s livelihoods and damages their culture (Bi et al., 2025). Furthermore, oil spills also affect public safety and pose a risk to individuals. Oil spills harm ecosystems, harm marine life, and result in long-term financial issues. Businesses along the shore, tourism, and fishing all suffer significant financial losses. Additionally, food becomes unhealthy when oil enters the food chain (Zhang et al., 2019). In general, the environment, culture, economy, ecosystem, society, and human are all negatively impacted by oil spill incidents and disasters (D’Andrea and Reddy 2014; Kapsalis et al., 2021).

2.2 Properties and weathering of spilled oil

Depending on where they come from and how much processing they undergo before being released into the environment, oil spill properties differ significantly (Fakhru’l-Razi et al., 2009). Light crude, heavy crude, bitumen, and intermediate versions are among the several kinds of extracted petroleum that are accessible (Merv Fingas 2016; Martínez-Palou et al., 2011). Each source of petroleum has its own chemical signature, and thus, before refinement, the chemical composition, density, and concentrations of sulphur, heavy metals, and other volatile compounds in crude oils vary depending on the oil sources (Chaerun et al., 2004; Rauckyte et al., 2006). Polycyclic aromatic hydrocarbons, n-alkanes, isoprenoids, steranes, terpenoids, and volatile compounds make up the majority of oil (Merv Fingas 2016; Schädle et al., 2014). However, it was found that different chemical elements arise after an oil spill because of oil deterioration over time (Radović et al., 2014). Therefore, during cleanup, determining the chemical characteristics of oil spills becomes crucial, including density, viscosity, and water interactions, which influence the physical behaviour of spilled oil. As a result, these traits together determine their effects on the environment and ecosystem (Lee et al., 2015, 2011).

Several chemical, ecological, and physical changes occur whenever oil spills into the sea. These modifications are referred to as weathering (Reed et al., 1999). Weathering has an impact on the consequences of an oil spill. Nonetheless, each phase’s importance changes based on environmental circumstances, such as the spill’s location, duration, kind of oil, and surrounding conditions (Merv Fingas 2016), which alter the oil’s composition over time. The primary weathering processes include dispersion, dissolution, sinking, sedimentation, evaporation, emulsification, spreading, and biodegradation (Mishra and Kumar 2015), in which the oil disperses in weathering processes depend on its density and viscosity. Furthermore, the thickness of the oil layer, the extent of the impacted region, and oil dispersion are all influenced by external variables, such as temperature, wind direction, and ocean currents. Dispersed oil can damage marine life by combining with water at varying depths (Singh et al., 2020).

The physical and chemical properties of spilled oils are found to affect the solubility of the oil spill in the seawater. Also, sea temperature, wind speed, turbulence, and physical dispersion are the other crucial parameters affecting the solubility of oil spills. To gauge how harmful substances like benzene, toluene, ethylbenzene, and xylenes are to marine life, their degraded forms should be assessed. Indeed, it was found that one of the quickest weathering mechanisms is evaporation. The elimination of volatile chemicals, such as gasoline, lighter polycyclic aromatic hydrocarbons, and lower molecular weight alkanes, could accelerate the evaporation process. Besides, other factors affecting evaporation rates include temperature, wind speed, and wave movement. Both the viscosity and density of oil spills may increase throughout the evaporation process; as a result, viscous oil fractions may sink to the ocean floor. Light crude oils may lose up to 75% of their total volume to evaporation, whereas heavier crude oils retain over 90% of their content (Li et al., 2016).

Emulsification occurs when water and oil mix, and water-in-oil or oil-in-water emulsions are two types of emulsifications that appear in oil spills. Turbulence and wave energy have an impact on these emulsions’ stability; it may increase oil viscosity by 1000 times and oil volume by 2–5 times, making oil cleaning efforts difficult (Fingas 2004). Sunlight also causes another problem called photo-oxidation. Heterocyclic molecules, petroleum hydrocarbons, and polycyclic aromatic hydrocarbons can all be changed by sunlight. The creation of oxygenated molecules with improved water solubility is the outcome of this process. However, even with a minimal oxidation process, photo-oxidation, which makes up a very minor portion of weathering overall, can nevertheless yield toxic chemicals. Alternatively, sedimentation occurs when components of the oil settle on the ocean floor, and oil characteristics are thus changed by sedimentation, particularly in muddy locations. One of the main weathering processes is biodegradation, which is caused by marine fungi and bacteria that break down hydrocarbons through enzymatic processes. This process occurs near sediments, inside water columns, or at the sea surface. Temperature, salinity, oil content, oxygen concentration, the state of the ecosystem, and nutrition availability all affect the rate of biodegradation (Singh et al., 2020; Azevedo et al., 2014). In general, the impacts of oil spills are depicted Fig. 2

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Fig. 2. Impact of oil spill interactions on the environment (Zhang et al., 2019).

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4.1 Factors affecting absorption capacity

The efficiency of cellulose-based absorbents like rice straw and rice husk in oil spill remediation is contingent upon many aspects, including (i) the attributes of the sorbent material, (ii) the features of the spilled oil, and (iii) marine environmental circumstances. The quality of the sorbent material is important in influencing oil absorption capability and retention. In addition, the features of spilled oil impact the efficacy and simplicity of recovery, whereas environmental factors, including wind, waves, and temperature, influence absorption effectiveness (Chau et al., 2025). Table 2 presents a summary of these parameters.

4.2 Absorption mechanism

The fact shows that the oil absorption capacity (OAC) of cellulose is contingent upon the kind of oil, the structure of cellulose, and the surrounding environmental conditions. It was indicated that capillary action is considered as the principal mechanism facilitating oil absorption into the porous structure of cellulose (Doshi et al., 2018; Hoang and Pham 2021b; Lv et al., 2018). Moreover, the contact angle between oil and the absorbent surface is also found to affect the absorption efficiency, in which reduced contact angles could improve oil uptake. This effect arises from the inverse relationship between capillary force and contact angle, indicating that a diminished angle leads to enhanced capillary activity (Payne et al., 2012b). In general, various celluloses exhibit different OAC values, which are because of variations in cellulose structure and pore dimensions. Smaller pores are more efficient for low-viscosity oils since they provide enhanced capillary forces that assist in retaining the absorbed oil. In contrast, high-viscosity oils are more appropriate for celluloses with bigger pores. Fig. 3 illustrates the method of oil absorption into cellulose (Chau et al., 2025).

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Fig. 3. Absorption mechanism of absorbents with porous structure.

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The ability of oil to spread across the surface of an absorbing material to enable sorption relies on molecular-level interactions, including van der Waals dispersion forces, Debye and Keesom forces, π–π interactions, hydrogen bonding, the hydrophobic effect, and double-layer forces (Hubbe et al., 2013) Van der Waals interactions between two large bodies can be roughly estimated by summing the interactions of all molecular pairs across the two surfaces. Since most oils are nonpolar and do not engage in hydrogen bonding, van der Waals forces are expected to play a major role in governing how oil behaves at interfaces, particularly with water (Abdullah et al., 2010; Hammer et al., 2010; Zhu et al., 2010). The strength of these dispersion forces is influenced by how easily molecules can be polarized and the distance between them. The other two components of van der Waals forces - Debye and Keesom forces -become more relevant when impure oils come into contact with polar substances such as water or cellulose. Oil may spread across cellulosic materials with the aid of Debye forces, which are interactions between a permanent dipole and an induced dipole. This is particularly true if the oil includes highly polarized aromatic or sulfur-containing chemicals. When polar molecules in oil interact with water, Keesom forces—which happen between molecules with permanent dipoles—also come into play (Hiemenz and Rajagopalan 2016; Pasichnyk et al., 2008). The interaction between oil and ionic-charged materials is thought to be significantly influenced by π-bonding. For instance, cations on the surface of cellulosic sorbents or in aqueous solutions can interact with π-electrons to produce attractive forces (Lu et al., 2013; Mahadevi and Sastry 2013). Prior research has investigated the role of aromatic π-electrons in oil–water interactions (Furutaka et al., 2001). Generally, oil containing sulphur (frequently present in crude oil as thiophenes) and other highly polarizable atoms may show greater dispersion forces (Samokhvalov 2011). Furthermore, many oils include some polar components, which improve the oil’s capacity to spread over various surfaces in addition to strengthening its cohesiveness inside it (Hubbe et al., 2013; Simanzhenkov and Idem 2003).

Hydrogen bonding is essential for oil dispersion on water-wet surfaces, especially cellulosic sorbents. The strong capacity of cellulose to form hydrogen bonds across the water-cellulose interface is the cause of this. However, the nonpolar molecules of oil cannot form hydrogen bonds with water. Water-wetted cellulose fibers tend to absorb less oil compared to their dry counterparts, which can be explained by this difference (Payne et al., 2012a). Overall, hydrogen bonding is a significant factor in aqueous systems involving oil and surface interactions (Claesson et al., 2006). Although each pyranose unit in cellulose has three hydroxyl groups, only some are available for hydrogen bonding with water under typical conditions (Kadla and Gilbert 2000). As a result, cellulose surfaces are not completely, and certain crystalline faces can even exhibit hydrophobic characteristics (Biermann et al., 2001). Besides van der Waals forces that help oil spread on surfaces, oil also tends to reduce the total oil–water interfacial area. This reduction is thermodynamically favorable because it allows more water molecules to exist in their natural bulk state, where they have freedom of movement. Moreover, the presence of micro- or nanoscale air bubbles has been found to extend the range of the hydrophobic effect (Faghihnejad and Zeng 2012; Meyer et al., 2006). Just outside the surface, ions are influenced not only by electrostatic forces but also by other types of interactions with the surface. In case of two close surfaces while their diffuse electric layers significantly overlap, an electrostatic double-layer force is generated. Wang et al. (Wang et al., 2010) demonstrated that this double-layer effect offers a key role in estimating the forces between mineral surfaces in water, particularly when a surfactant resembling the asphaltene component of crude oil is present. Since asphaltenes tend to accumulate at the interfaces of oil phases (Natarajan et al., 2011), this scenario is particularly relevant for crude oil adsorption onto sorbent materials. As reported, the interactions between the asphaltene compound surfaces coated were strongly influenced by variations in pH, the concentration of monovalent salts, and especially the presence of Ca²⁺ (Wang et al., 2012).

Generally speaking, interactions at the phase boundaries determine an oil phase’s capacity to disperse across a sorbent surface and ultimately enter the solid’s pores. Fundamentally, these interactions are frequently represented as a confluence of almost separate forces, including van der Waals, hydrogen bonding, and electrostatic forces. Rosenholm (2010) reported that these oversimplified models could not adequately represent intricate real-world systems, especially as they frequently ignore the impact of contaminants and the interaction between various molecular contact mechanisms. Future studies should evaluate if adding the hydrophobic effect to these models significantly improves their usefulness.

5.1 Natural rice straw and rice husk

Remediation of oil spills using adsorption is a cost-effective and efficient method. Sustainability is promoted by using agro-industrial or agricultural waste as absorbents. The effectiveness of rice straw in the remediation of crude oil has been studied. With an adsorption capacity of 6.67 grams of oil per gram of rice straw, Tayeb et al. (2020) evaluated its absorption capacity and found removal efficacy of 94.7% for low-density oils. In another study of Hoang et al. (2018a), rice straw was utilized to absorb oil spills, including kerosene, fuel oil, diesel oil, and crude oil. They discovered that the type of oil affects rice straw’s ability to absorb it. According to their findings, rice straw has the best capability for absorbing crude oil, suggesting that it may be able to absorb more viscous oils. The factors influencing rice straw’s ability to sorb oil were examined by (Taufik et al., 2021). In comparison to untreated samples, they observed that pre-treatment at higher temperatures increased surface area, leading to better absorption. Using a central composite design, the optimal parameters achieved as 0.148 g/cm³ (power density), a 25% (v/v) concentration of oil, and a 10-minute heat treatment. The oil sorption effectiveness of raw rice husk and raw rice straw for absorbing diesel oil was assessed by Catalan et al. (2023), who used a methodical packing arrangement to control bulk density. The findings showed that a commercial sorbent pad had mean sorption rates of 7.25%, raw rice husk 2.97%, and raw rice straw 4.71%. Raw rice straw’s improved performance was attributed to its functional groups, porosity, and surface roughness. In a study of Lunsamrong et al. (2024), they studied the OAC from rice straw, and found that the optimal adsorption capacity was 175.67%. The ideal conditions were a contact area of 6.25 cm², an adsorption time of 30 minutes, and a sorbent mass of 30.10 g. The results suggest that fibers from rice straw might be helpful in cleaning up wastewater and oil spills. The effectiveness of a straw and chalk composite absorbent in wastewater treatment was examined by Pirestani et al. (2018). At 15 minutes of equilibrium, the highest OAC of 28.85% was observed, while at 3 minutes, the minimum OAC of 17.82% was noted. Performance was significantly impacted by the sorbent’s content; at 2 g/L, 61.05% of OAC was obtained. However, at lower concentrations (0.25–0.5 g/L), the OAC tended to reduce. Therefore, it can be said that extending the contact duration improves the straw absorbent’s ability to remove oil from wastewater, which qualifies it for use in industrial wastewater treatment. According to (Ianov et al., 2022), rice straw has an OAC of up to 10 g/g.

In a study of Hassanein et al. (Hassanein et al., 2014), they found that the size and weight of the straw affected the OAC. When compared to their shorter or longer counterparts, medium-sized straws performed better OAC, with the highest OAC being 10.0 g/g. In another study, Souare et al. (2024) employed both rice straw and loofah to create a biodegradable membrane, which had an 11.19 MPa of tensile strength, a water flow of 2057.37 L/m²/h, and a separation efficiency of 99.06%. In comparison with conventional membranes, the loofah and rice straw membranes demonstrated enhanced mechanical qualities, showing their appropriateness for extensive, economical environmental applications. The OAC of rice straw in both seawater and freshwater was assessed based on its particle size and fractional composition. The sequence of oil absorption in seawater was as follows, according to the results: Particle size distribution: medium-sized (26.8 g/g) > tiny (15.5 g/g) > extremely small (9.5 g/g) > long (7.36 g/g). In freshwater, the corresponding values were 25.0, 14.2, 8.0, and 5.4 g/g, respectively (Ahmed and Abd El Aziz 2014; Shaikhiev et al., 2024).

Similar to rice straw, rice husk is also a potential absorbent for spilled oil. The oil removal ability of the absorbents is due to their waxy fiber surfaces and large lumens, which facilitate internal capillary movement of the oil (Lim and Huang 2007). In another study of Bazargan et al. (2014), rice husk has undergone low-temperature alkali treatment to create a lignocellulosic sorbent material and eliminate silica content. The resulting optimized rice husk-based cellulosic sorbents exhibit high RMG380 marine diesel absorption, attributed to their low bulk density and fluffy structure, with uptake reaching up to 20g/g. According to Razavi et al. (Razavi et al., 2014), reducing the particle size of raw rice husk led to a decrease in oil adsorption efficiency - from 50% to 30% for crude oil, 65% to 20% for spent oil, and 70% to just 0.01% for engine oil. This decline was likely due to the destruction of microcavities. The adsorption behavior of crude and spent oils followed the Freundlich isotherm model, whereas engine oil adsorption conformed to the Langmuir model. Moreover, Aminuddin et al. (2023) indicated that rice husk could be efficiently employed as an absorbent for engine oil. They found that the use of rice husk could have an oil absorption capacity of 2.4 g/g. In general, the oil absorption capacity of rice husk is quite low, showing that rice husk could be suitable for small-scale oil spills under calm water. In a study of Ali et al. (2012), they compared the OAC for kapok fiber, sugarcane bagasse, and rice husks. They found that both kapok fiber and sugarcane bagasse demonstrate outstanding oil sorption performance across all oil types, with sorption capacities exceeding 10 g/g. In contrast, rice husks show the lowest OAC among the tested sorbents. Consequently, rice husk may not be suitable as a sorbent material for either water or general oil removal. However, it can still serve as a reasonably effective sorbent for engine oils, as their sorption capacity for these oils exceeds 5 g/g. This result is because the pore volume of rice straw is very low, while its lumen is small.

Rice straw-based sorbents can serve several valuable functions during nearshore and onshore clean-up activities. Nevertheless, wherever possible, the use of large amounts of sorbent material should be limited to reduce secondary issues related to waste handling and disposal. For this reason, extensive shoreline application of sorbents should be reserved for circumstances in which alternative clean-up methods are unlikely to be practical or effective, as shown in Fig. 4(a). When positioned close to the shoreline, sorbent booms can efficiently intercept oily run-off generated during shore-washing operations, such as high-pressure cleaning of contaminated rocks, or be deployed in intertidal areas to capture oil that has been refloated or remobilized. Additionally, under appropriate environmental conditions - particularly where water flow through the boom remains low - snare booms can be successfully installed across industrial water intakes to reduce the entry of floating, high-viscosity oil, as depicted in Fig. 4(b) (ITOPF Ltd 2012). A recent report by ITOPF Ltd described the utilization of various natural organic materials, including paper pulp, peat, chicken feathers, sawdust, straw, and bagasse for manufacturing oil recovery booms, as depicted in Fig. 4(c) (ITOPF Ltd 2012). They noted that unmodified natural organic sorbents exhibit poor reusability, which may lead to increased operational costs. In addition, these materials possess low mechanical strength and are prone to damage or deformation during deployment. In another study, Pagnucco and Phillips (2018) explored the use of human hair as a sorbent material for boom fabrication to contain and recover crude oil spills. Hair-based booms were compared with natural sorbents, such as recycled cellulose and cotton by-products, as well as synthetic polypropylene booms, to evaluate their oil and seawater adsorption capacity and buoyancy retention. Oceanic mesocosm experiments simulating the real oil spill demonstrated that human hair exhibited a substantially higher oil absorption capacity in comparison to other materials tested, with the approximate OAC of 0.84 g/g. This enhanced performance may be attributed to the heterogeneous structure of mixed human hair, which provides a wider range of adsorption sites (Fig. 4d) (Hoang et al., 2021a; Pagnucco and Phillips 2018).

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Fig. 4. (a) Using sorbent for recovering oil spill under large scale; (b) Sorbent-based snare strung across for catching floating oil spill; (c) Sorbent booms constructed by straw (ITOPF Ltd 2012); (d) Crude oil absorption rate for natural organic sorbent types (Hoang et al., 2021a; Pagnucco and Phillips 2018).

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Rice husk and rice straw are examples of natural sorbents that include inherent oils or waxes that increase their preference for oil over water (Al-Majed et al., 2012). Without these ingredients, cellulose-based absorbents frequently hold onto more water and may sink if water absorption reaches a particular point. Cellulose-based absorbents decompose naturally, unlike synthetic polymer fibers, making them an environmentally friendly choice for cleaning up oil spills. Although biodegradability is advantageous, other properties, like as chemical composition, flammability, cohesiveness, mechanical strength, and reusability, must also be considered when choosing a sorbent. These characteristics have an impact on how well natural cellulose-based absorbents work to clean up oil spills, particularly when containment booms are used. Apart from their biodegradability, rice husk and straw have the potential to be efficient absorbents for the first reaction to minor accidents or as backup solutions for handling bigger ones. However, the properties of the sorbent material alone should not be used to assess the oil absorption capability. For oil spill cleanup to be as effective as possible, the right application environment must be taken into account.

5.2 Modified rice straw and rice husk

Natural cellulose-source materials like rice straw and rice husk are generally more affordable, widely available, and environmentally friendly, making them appealing for efficient waste management (Mahato et al., 2020; Hoang et al., 2021a). However, their main limitation lies in their relatively poor oleophilic and hydrophobic properties because hydroxyl groups present on the surface of cellulose fibrils and along its molecular backbone form intermolecular hydrogen bonds, while also interacting with water molecules (Fürtauer et al., 2021; Zamparas et al., 2020). One approach to improving the oil absorption performance of these natural sorbents is through thermal, chemical, and biological treatment or their combinations for surface and structure modification, aiming to increase the pore size and enlarge the absorption surface area.

Thermal treatment is one of the most widely used techniques, as it can modify the physical properties of materials while maintaining the chemical composition of the fibers (Andrade et al., 2020). When lignocellulosic fiber bundles are heated to temperatures between 100 and 200°C for different durations, the drying process causes the bundles to separate into individual filaments. Alternatively, non-cellulosic compounds or chemical components with lower transition temperatures may be depolymerized (Ahmad et al., 2019). Low-temperature procedures, such as drying, can aid in improving these qualities by eliminating surface contaminants, which speeds up the absorption of oil (Lam et al., 2018). On the other hand, the material may be carbonized by high-temperature processes like pyrolysis, which greatly increases the material’s ability to absorb oil and its ability to distinguish between water and oil (El Gheriany et al., 2020). Indeed, the ability of rice straw to absorb oil was assessed in a research of Taufik et al. (2021), using both untreated and heat-treated varieties. Compared to the treated rice straw, which had more pore gaps visible, the untreated rice straw shown in Fig. 5(a) had more noticeable silica grooves. According to reports, hemicellulose, cellulose, lignin, silica, and potassium are the primary constituents of rice straw. Consequently, lignin and silica in rice straw degrade more quickly in the presence of heat (Bhattacharyya et al., 2020). The surface of the rice straw became coarser with wrinkles, grains, and grooves as a result of heat treatment. In fact, Fig. 5(b)‘s SEM pictures of the treated rice straw revealed a loosened surface structure, which facilitates the absorption of diesel (W. C. Li et al., 2017). Earlier studies have indicated that such roughening enhances the surface area available for absorption (Hadidi et al., 2020; Onwuka et al., 2018). Fig. 5(c) illustrates that the heat-treated rice straw had a substantially higher sorption capacity (2.3 g/g) and oil absorption efficiency (51.67%) than the untreated rice straw, which had a lower absorption capacity of 1.9 g/g. Furthermore, the treated rice straw’s water absorption was much lower (4.17%) than that of the untreated sample (12.08%). These outcomes are the consequence of the rice straw’s surface being changed by heat treatment, which produced a rougher texture by adding wrinkles, grains, and grooves. Furthermore, heat treatment modifies the rice straw fibers’ morphological and structural qualities, such as their diameters, surface tension, chemical composition, and mechanical traits (George et al., 1998). Previous research has indicated that such surface roughness enhances accessible surface area (Hadidi et al., 2020)(Ibrahim et al., 2018), it is beneficial for absorption (Onwuka et al., 2018). Reduction of water uptake also supports the preference for using treated rice straw.

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Fig. 5. Rice straw’s SEM and oil absorption capacity; (a) SEM of untreated rice straw before wetting with engine oil, (b) SEM of treated rice straw before wetting with engine oil, (c) Oil absorption capacity of untreated and treated rice straw (Taufik et al., 2021).

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In addition to heat-treated rice straw, thermal modification of rice husk has also been shown to significantly enhance the oil uptake performance. Kenes et al. (2012) showed that the adsorption capacity of rice husk-based sorbents is increased with the increase in density of petroleum liquids. As a result, rice husk with high temperature treatment at 700 °C showed significantly superior sorption behaviour as compared to untreated material. This enhancement is mainly related to the alteration of the pore structure that minimizes the loss of retained hydrocarbons. Among the tested fuels, gasoline with the lowest density of 0.734 g/cm³ demonstrated the lowest adsorption capacity, while heavy crude oil with a density of 0.937 g/cm³ had the highest adsorption capacity. Notably, the thermally processed rice husk had a maximum adsorption capacity of around 15 g/g for heavy crude oil. Comparable trends of liquid density versus sorption efficiency have been reported for previous studies (Husseien et al., 2009). Moreover, Genieva et al. (2012) analyzed the effect of pyrolysis conditions on rice husk-derived ashes to produce black and white rice husk ash under controlled environments. Combustion of raw rice husk in air in a fluidized bed reactor produced white rice husk ash with high reactivity and medium porosity. In contrast, pyrolysis under oxygen-limited or inert conditions produced black rice husk ash that was silica interspersed with amorphous carbon and gave a highly porous structure (Maiti et al., 2006). Subsequent adsorption experiments showed that diesel fuel adsorption at equilibrium was almost 5g/g for black rice husk ash and only about 2.8g/g for white rice husk ash. This difference stems from the difference in surface chemistry, with black ash having a largely nonpolar surface and white ash having a polar surface. Since diesel fuel is composed mostly of nonpolar hydrocarbons, the adsorption profile of diesel fuel on black rice husk ash was always more (Sharma et al., 2010).

Razavi et al. (2015) carried out a comparative study on the sorption of oil with raw rice husk, black rice husk ash, and white rice husk ash. Fourier transform Infrared spectroscopy (FTIR) analysis (Fig. 6a) showed that functional groups related to lignin, cellulose, hemicellulose, and inorganic compounds have a large effect on oil adsorption behaviour. A broad absorption peak located around 3420 cm^-1 is associated with the O-H stretching vibrations of surface hydroxyl groups and adsorbed water molecules found in black and white rice husk ash (Ahmaruzzaman and Gupta 2011; Chen et al., 2011). Several peaks at 1600-1700 cm^-1 are ascribed to C-OH groups, also to deformation of water molecules, and C=C and C=O bonds in hemicellulose structures (Foo and Hameed 2009). Meanwhile, the prominent band at 2920 cm^-1 reflects C-H stretching vibrations of lignin, cellulose, and hemicellulose, which confirms that both Si-OH surface groups and organic polymer chains are responsible for the enhanced oil affinity. Their adsorption data indicated that adsorption of oil on black rice husk ash followed the Langmuir isotherm, while the adsorption on raw rice husk and white rice husk ash (except engine oil on raw husk) was reasonably presented by the Freundlich isotherm. Overall oil removal efficiency was in the order of black rice husk ash > raw rice husk>white rice husk ash, as shown in Fig. 6(b). Maximum adsorption capacities of black rice husk ash were reported to be 2000 mg/g for engine oil, 1250 mg/g for spent oil, and 1000 mg/g for crude oil. Importantly, a decrease in particle size of raw rice husk from 2 mm to 0.035 mm at alkaline pH values (8-10) resulted in drastic decrease of engine oil removal efficiency - from 70% to almost zero (Fig. 6c). This trend is in line with the previous results that excessive grinding damages the pore networks in natural sorbents and thus affects the oil retention (Lee et al., 1999; Husseien et al., 2009). In contrast, particle size variation had little effect on oil adsorption by black and white rice husk ash. Kumagai et al. (2007) also found that porosity has only a minor impact on oil uptake from pyrolyzed rice husk. The decreased sensitivity to surface area and pore volume might be attributed to high thermal degradation in pyrolysis, which results in a more uniform material structure (Mansaray and Ghaly 1998). However, the adsorption of spent oil by thermally modified rice husk was greatly influenced by the particle size, presumably because of the moderate viscosity of the spent oil. In such a case, the larger pores related to the coarser particles favor strong interaction with oleophilic components, enhancing the sorption efficiency.

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Fig. 6. (a) FT-IR spectra of raw rice husk, black rice husk ash, and white rice husk, (b) Effect of the contact time on the engine oil adsorption of by raw rice husk, black rice husk ash, and white rice husk (pH = 8–10, temperature = 23°C), (c) Effect of the particle size on the engine oil adsorption (pH = 8–10, temperature = 23°C) (Razavi et al., 2015); (d, e) SEM of untreated and treated rice husk before wetting with engine oil, (f) Oil absorption capacity of untreated and treated rice husk (Aminuddin et al., 2023); SEM images of TRMC before (g) and after crude oil sorption (h, i) Effect of initial crude oil concentration on oil absorption capacity, (j) Percentage of oil removal after 4 sorption–desorption cycles (Kovo G. Akpomie et al., 2018).

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In another study of Farias et al. (2023), rice husk was utilized as an absorbent to remove lubricating oil from water through four different methods of treated rice husk, such as acid treatment, alkaline treatment, thermal treatment, and no treatment. They discovered that the acid-treated sample had the largest surface area (3.71 m²/g), whereas the untreated rice husk had the smallest (0.79 m²/g). The thermally-treated rice husk was found to have the best OAC, while all of the absorbents showed excellent OACs, ranging from 1650 to 2000 mg/g. Additionally, Aminuddin et al. (2023) investigated the main variables affecting engine oil sorption effectiveness using rice husk; they evaluated the OAC of raw and heat-treated rice straw. Indeed, the rough and intricate surface roughness in the case of rice husk is seen in SEM micrographs. Heat treatment dramatically changed the rice husk structure, leading to a material with much higher porosity, as shown in the SEM photos. There was a noticeable difference between the surfaces of the heat-treated and untreated rice husk samples. In contrast to the untreated sample (Fig. 6d), the heat-treated rice husk’s exterior wall showed a greater number of button-like formations with fine holes (Fig. 6e). Significant structural changes were brought about by the high-temperature treatment, such as a rise in the number and size of pores, the creation of new pore types, the fusion of smaller holes into larger ones, and changes in the surface and volume properties of the pores. The samples of heated rice husk showed the highest oil absorption efficiency (48.95%) and the highest OAC (2.7 g/g). However, as shown in Fig. 6(f), their water absorption was lower (4.28%) than that of the untreated (8.68%) and biocharcoal (9.71%) samples. As an alternative, the application of thermally-treated rice husk–montmorillonite composite (TRMC) was examined by Kovo G. Akpomie et al. (2018). As seen in Figs. 6 (g and h), there were noticeable changes in the surface morphology of TRMC following crude oil adsorption in comparison to its pre-process appearance, along with a decrease in porosity. This alteration shows that crude oil has been successfully absorbed inside the sorbent’s porous structure and that its active sites are being used effectively. In addition, Fig. 6(i) shows how the preliminary crude oil concentration could affect the sorption capacity and removal efficiency by TRMC. Because of variations in the interfacial tension between the sorbent surface and the oil–water combination at higher concentrations, the percentage removal decreased as the oil concentration increased (Gulistan et al., 2016). Furthermore, the decrease can be the consequence of active sites being saturated, which would leave fewer accessible for further adsorption. The maximal OAC of TRMC, as examined by the Langmuir isotherm, was observed as 9.7 g/g, exceeding that of several sorbents that have been reported in the literature. Notably, after three regeneration cycles, TRMC maintained its functionality, indicating strong reusability (Fig. 6j). After repeated usage, the initial crude oil sorption effectiveness dropped from 82.2% to 69.5%; however, this decline is probably the result of aggregation of oil or irreversible binding inside the sorbent structure, which causes the loss of active sites.

Depending on its source, cellulose, a polymer composed of β-glucose units joined by β-1,4 glycosidic bonds, has a different molecular weight. Since effective oil-water separation requires both hydrophobicity and oleophilicity, cellulose’s inherently very hydrophilic surface is not ideal. To address this issue, cellulose might be chemically altered to become rougher and stiffer. Improved and more efficient adsorption results from these changes, which expand the surface contact area (M. A. Hassan et al., 2014; Zamparas et al., 2020).

Cellulose undergoes a chemical process called “mercerization,” which improves its affinity for oil and makes it suitable for cleaning up oil spills (Zamparas et al., 2020). In fact, in order to extract cellulose, Lunsamrong et al. (2024) explored using 5 wt.% NaOH of rice straw at 90 °C. They discovered that the treated cellulose’s contact area grew to 6.25 cm², which led to a maximum OAC of 175.67%. Furthermore, the impact of the alkali concentrations used to treat rice husk on the OAC was assessed by (Bazargan et al., 2014). As seen in Fig. 7(a), it was found that the alkali content significantly impacted yield. Additionally, they said that the structural characteristics of rice husk were altered by alkali treatment, producing a material with a noteworthy capacity for oil sorption. However, surface functional groups and microporosity were not main variables impacting sorption performance, according to tests conducted using Brunauer–Emmett–Teller and Fourier Transform Infrared methods. Rather, as Fig. 7(b) illustrates, the temperature was shown to have an impact on the OAC. It is evident that increasing the treatment temperature enhances the decomposition efficiency of the husk. A greater degree of structural breakdown consequently leads to a reduction in bulk density and an improvement in liquid absorption capacity. Therefore, the absorption capacity for marine oil was found to be strongly inversely correlated with bulk density. Indeed, the alkali-modified rice husk exhibited a lower bulk density, facilitating greater internal diffusion of oil within the sorbent structure (Bazargan et al., 2014). Indeed, the BET surface areas of rice husk after being treated by weak alkali and strong alkali were 2.6 and 3.5 m²/g, respectively, compared to 2.3 m²/g for non-treated rice husk, leading to the highest OAC of strong alkali-treated rice husk of 20.1 g/g. Moreover, Liu et al. (2022) developed an oil-absorbing felt composed of discarded rice straw fibers, as shown in Fig. 7(c). Molds were used to create the fibers, and they underwent chemical treatment to acquire superhydrophobic and super-oleophilic properties. First, the fibers from the leftover rice straw were chopped and crushed into tiny pieces. In order to create microstructures, the fibers were then submerged and agitated in an aqueous NaOH solution (2.5 mol/L) at the ambient temperature. Following treatment, a 0.02 mol/L solution of citric acid was used to neutralize the alkaline solution, and deionized water was used to completely rinse the fibers. As illustrated in Fig. 7(c), superhydrophobic rice straw felts with various geometries - including cake-like, square, cylindrical, and rectangular forms - were readily fabricated, demonstrating potential applicability in specialized areas. As seen in Figs. 7 (d and e), the resultant superhydrophobic rice straw felts had exceptional superhydrophobic and superoleophilic qualities. The microstructural data shown in Fig. 7(f) showed surface characteristics that were unevenly dispersed and resembled microscale strips. In comparison to both untreated straw fibers and conventional wool-based oil-absorption felts, the superhydrophobic rice straw felts demonstrated noticeably greater oil absorption capabilities (Fig. 7g). Additionally, as Fig. 7(h) illustrates, all evaluated mixes had oil/water separation efficiencies greater than 95%. They also performed exceptionally well in unfavorable conditions, including high turbulence, acidic environments, and UV light. To enhance oil adsorption, Pachathu et al. (2016) chemically treated rice straw with NaOH and surfactants. The maximal oil removal efficacy of the 313 K microwave-assisted therapy was 98.72%. The adsorption behaviour was analyzed by isotherm and kinetic models, with the Langmuir model showing a peak adsorption capacity. Moreover, it was reported that 18% NaOH solution pretreated rice straw, followed by exposure to a 2% cetyltrimethylammonium chloride solution for 60 minutes at 20°C, resulted in a substantial enhancement of its OAC, from 0.83 g/g to 8.49 g/g (Shi et al., 2022).

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Fig. 7. (a) - Effects of NaOH concentration in the rice husk treatment process on oil absorption capacity, (b) - Effects of temperature on oil absorption capacity (Bazargan et al., 2014); (c) - Superhydrophobic rice straw felts with various geometries, (d and e) - Water and diesel oil droplets on Superhydrophobic rice straw felts, (f) – SEM of Superhydrophobic rice straw felts, (g) - Weight gain of various absorbents, (h) Oil absorption efficiency of various oil/water mixtures (Liu et al., 2022).

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Acid treatment involves the use of stronger mineral acids like HCl, H₂SO₄, or H₃PO₄ to partially hydrolyze the amorphous regions of biomass, primarily targeting hemicellulose and lignin. This process aids in eliminating silica, waxes, and other impurities while exposing free hydroxyl (–OH) groups and thus enhancing the surface area. As a result, the treated biomass becomes more hydrophilic, which enhances its interaction with polar contaminants (W. U. Hassan et al., 2014). Indeed, Li et al. (2023) improved the oil absorption capacity of rice straw by subjecting it to acid treatment and altering it with sodium dodecyl sulfate through electrostatic interactions. Figs. 8 (a and b) presents the tubular structures of unmodified rice straw (Fig. 8a1) and modified rice straw (Fig. 8b). It could be seen in Figs. 8(b2 and b3) that a porous, loosely organized fiber surface of the modified rice straw. Moreover, the surface area of treated rice straw (Fig. 8b3) increases in comparison with untreated rice straw (Fig. 8a), hence improving hydrophobic characteristics (Li et al., 2023). As shown in Fig. 8(c), the unmodified rice straw exhibits strong hydrophilic behavior. In contrast, Fig. 8(d) demonstrates that, after modification, the rice straw becomes significantly more hydrophobic, as evidenced by the increase in contact angle from 5.14° to 71.60°. This substantial rise confirms the enhanced hydrophobic nature of the modified rice straw (Wang et al., 2018; Tran et al., 2020). Under optimum circumstances (10% H₂SO₄, 90-minute reaction at 90 °C, followed by 2% sodium dodecyl sulfate treatment at 20 °C for 120 minutes), the oil sorption capacity of rice straw was augmented by 4.16 g/g. In another study of Feng et al. (2021), they used rice straw to fabricate a cost-effective and environmentally friendly separation layer featuring a rough surface structure and abundant anionic functional groups through a straightforward acid–base treatment. In the first phase, the raw rice straw was mechanically ground into fine granular particles, which were then subjected to acid treatment by dispersing the powder in a 1.25% H₂SO₄ solution under mild boiling conditions. The acid-treated material was subsequently rinsed with distilled water till the time neutral pH could be achieved. Subsequently, it was immersed in the 1.25% NaOH solution under the same reaction conditions as the acid treatment. Finally, distilled water was used to wash the alkali–acid-treated rice straw. The samples for experiments are the acid-alkali rice straw (A1A2-RS), alkali-acid rice straw (A2A1-RS), acid-treated rice straw (A1-RS), and alkali-treated rice straw (A2-RS). As shown in Fig. 8(e) (1-2), unlike the RS sample through which a water droplet could not permeate within 60 s, the A1-RS exhibited superhydrophilicity, achieving a water contact angle of 0° within 120 ms of contact. Furthermore, the A2-RS, A1A2-RS, and A2A1-RS samples demonstrated even shorter infiltration times for water droplets (Fig. 8e (3-5)). This enhanced wettability was the result of superior water absorption as well as retention capacities associated with the increased surface roughness of the alkali-treated samples (Tao et al., 2014). A similar phenomenon was observed when oil was used as the probing liquid in air, which is ascribed to the highly porous structure and elevated surface energy of A1A2-RS (Fig. 8e6). As illustrated in Fig. 8 (f1), the oil-repellency of the materials was markedly enhanced, with the contact angle increasing from 114° for RS to 146° for A1A2-RS. The progressive increase in surface roughness observed for RS, A1-RS, A2-RS, A2A1-RS, and A1A2-RS indicates that the hierarchical surface structure plays a crucial role in governing the oleophobicity of the material (F. Li et al., 2017). In general, super-hydrophilic materials exhibit super-oleophobic or oleophobic behavior in aqueous environments due to the formation of a stable hydration layer that provides a strong oil-repellent interface (Kota et al., 2012). Consequently, the pre-wetted A1A2-RS demonstrated excellent underwater oleophobicity and extremely low oil adhesion toward various types of oil, including both heavy and light oils. The contact angles of underwater oil A1A2-RS for different oils ranged between 142° and 147°, confirming its comprehensive applicability (Fig. 8f2). As shown in Fig. 8(g), all tested oil-in-water emulsions achieved separation efficiencies exceeding 96%, with the total organic carbon content of the filtrate remaining below 30 mg/L. Permeation flux, another key parameter in separation performance (Fig. 8h), is primarily influenced by the material’s porosity and wettability (Gao et al., 2015). Notably, although emulsions containing heavier or more viscous oils exhibited slightly lower flux values than those of lighter oils, the differences were minimal. This consistency indicates that the A1A2-RS separation layer possesses broad applicability and stable performance across a wide range of oil–water emulsions. To compare the OAC for various treatments, Farias et al. (2023) conducted a comparison of the OAC of raw rice husk with thermal-treated rice husk, alkali-treated rice husk, and hydrochloric acid-treated rice husk. They found that hydrochloric acid-treated rice husk has the highest pore volume (0.0068 cm³/g), as well as the largest surface area (3.71 m²/g), while thermal-treated rice husk has the largest pore size (20.6 nm). The study revealed that all tested absorbents exhibited rapid absorption behavior for lubricating oil, reaching equilibrium within approximately 15 minutes. During the first 5 minutes, the absorbents achieved 97%, 82%, 84%, and 88% of their maximum OACs for thermal-treated rice husk, raw rice husk, alkali-treated rice husk, and hydrochloric acid-treated rice husk, respectively. Among them, thermal-treated rice husk demonstrated the highest efficiency in removing lubricant oil from water, attaining an adsorption capacity of 1977 mg/g, implying that pore size is more important than pore volume and surface area in sorbing high viscosity oil.

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Fig. 8. (a and b) SEM of unmodified rice straw (a1–a3), modified rice straw (b1–b3) with sodium dodecyl sulfate; (c, d) Contact angle of unmodified rice straw (c) and modified rice straw (d) (Li et al., 2023); Images of the water droplet permeation on rice straw (e1), acid-treated rice straw (e2), alkali-acid rice straw (e3), alkali-treated rice straw (e4), acid-alkali rice straw (e5), oil droplet permeation on acid-alkali rice straw (e6), Perchlormethane contact angles underwater for samples (f1); Underwater oil contact angle of multiple oils for acid-alkali rice straw (f1), g) Total organic carbon contents of filtrates, (h) fluxes for multiple emulsions (Feng et al., 2021).

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In contrast, acetylation is an esterification process where hydroxyl groups in cellulose and hemicellulose are replaced with acetyl (–COCH₃) groups through reactions with acetic acid or acetic anhydride (Oushabi 2019). The efficiency of this reaction can be improved by using catalysts such as N-bromosuccinimide, pyridine, or various organic and inorganic acids (Asadpour et al., 2016; Elias et al., 2015). By adding acetyl groups, the fiber surface becomes more hydrophobic, which lowers its attraction for water and improves its ability to absorb nonpolar materials, such as oils and organic solvents. Furthermore, acetylation improves biomass’s chemical and thermal stability, increasing its resilience to adverse environments and expanding its usage in industrial and environmental applications (Cardoso et al., 2025; Onwuka et al., 2016; Zamparas et al., 2020). According to Mahmoud et al. (2020), acetylated fibers could absorb 24.54 g of oil per gram of fiber, compared to 13.75 g/g for untreated fibers. They highlighted the capacity of acetylated fibers in maintaining their biodegradability and improving oil absorption efficiency in comparison with the synthetic equivalents, making them environmentally feasible for oil spill cleanup. The identical results, showing increased OAC in acetylated fibers, were also reported by Quek et al. (2020), demonstrating enhanced OAC in acetylated fibers. Additionally, using acetic anhydride at temperatures between 100 and 120 °C for 1 to 4 hours, Sun et al. (2002) investigated the acetylation of rice straw without the use of solvents, either with or without four tertiary amine catalysts. In 0.5 hours at 120 °C, the treated straw’s weight increased by 15.4% when a 7% catalyst was used, compared to 11.2% when no catalyst was used. Depending on the degree of acetylation, the OAC of acetylated straw ranged from 16.8 to 24.0 g/g. Significant hydrophobicity in the modified straw prevented water absorption, making it a low-cost, efficient natural sorbent for oil cleanup. Since its hydrophobic nature inhibits water absorption, acetylated rice straw generally has several benefits, such as lower cost, high OAC, quick oil uptake, and simple desorption. Additionally, the sorbent may be reused for oil spill remediation several times since the absorbed oil can be retrieved by simple squeezing. For oil spill remediation applications, acetylation of rice straw and other biodegradable lignocellulosic materials, such as cotton wool, sugarcane, paper, and wood, represents an economical, technically feasible, and ecologically benign method (Adebajo et al., 2003). In a study of Thi et al. (2016), they used acetic anhydride at a 1:10 straw-to-acetic anhydride ratio, a reaction temperature of 90°C, and a reaction period of 120 minutes with tertiary amines acting as catalysts to acetylate rice straw in order to improve its hydrophobic qualities. According to the findings, acetylation considerably raised the modified rice straw’s maximum OAC from 7.92 g/g to 17.9 g/g.

For the combination treatment for rice straw and rice husk, Wang et al. (2015) prepared cellulosic sorbents derived from rice husk through both chemical and biological pretreatment methods. The chemical pretreatment using NaOH and H₂O₂ yielded sorbents with a cellulose content as high as 87.75%. After that, acetylated sorbents produced from NaOH/H₂O₂-treated and Aspergillus flavus-treated rice husks were evaluated and compared with raw and non-acetylated rice husk sorbents for their oil sorption capacities. As reported, NaOH/H₂O₂ pretreatment followed by acetylation resulted in a noticeable reduction in the lignin-associated aromatic ring, confirming effective delignification (Zuluaga et al., 2009). It could be concluded that after chemical pretreatment, characteristic bonds such as O–H, C=O, C–O–C, O–CH₃, and Si–O–Si were weakened compared with those in the raw rice husk spectrum, indicating enhanced OAC. For the OAC experiments illustrated in Fig. 9(a), the acetylated chemically treated rice husk sorbent exhibited the highest OAC, achieving 19.66 g/g for RMG 386 oil and 16.39 g/g for corn oil. These were followed by the acetylated biologically modified sorbent, with capacities of 12.75 g/g for RMG 386 and 10.25 g/g for corn oil. All things considered, both varieties of acetylated rice husk sorbents showed noticeably higher oil absorption effectiveness (OAE) in aqueous conditions. Additionally, in the RMG 386/water and corn oil/water systems, the acetylated chemically treated sorbent exhibited a low water absorption of 1.46 g/g and 1.74 g/g, respectively, in contrast to the non-acetylated chemically treated sorbent’s 12.66 g/g and 11.58 g/g. Oil affinity is a gauge of oleophilicity, whereas water absorption capability directly indicates a material’s hydrophobicity. Sorbents lacking hydrophobicity have a tendency to absorb water and oil at the same time, leading to lower OAE. Acetylated sorbents, on the other hand, have greater intermolecular interactions between acetyl groups and oil molecules, which promote selective OAC and prevent absorbed oil from gravitationally draining (Wang et al., 2015). Therefore, in real-world applications, acetylated, modified cellulosic sorbents that combine oleophilicity and hydrophobicity are perfect for attaining high water resistance and OAC. In a similar manner, Wang et al. (2020) extracted cellulose from raw rice husk using a biological delignification method, then acetylated it to add hydrophobic properties. Both bacterial and fungal strains were used in the delignification procedure, and their respective performances were compared. In particular, three fungal strains (Trametes versicolor, Phanerochaete chrysosporium, and Aspergillus flavus) and one bacterial strain (Bacillus licheniformis) were used to treat rice husks. The FTIR spectra of the treated rice husks, shown in Fig. 9(b), show many distinctive absorption peaks that correspond to the aromatic rings of lignin during 45 days of biodegradation (da Silva et al., 2012; Liu et al., 2009). The carbonyl groups in hemicellulose are responsible for the observed peak linked to C=O stretching, whereas the C–H deformation vibrations are connected to both cellulose and hemicellulose constituents (Balat 2008; De Rosa et al., 2010; Ibrahim et al., 2010). Furthermore, cellulose and hemicellulose both exhibit absorption bands corresponding to C–O–C vibrations, but cellulose is uniquely characterized by the absorption of C–H deformation bonds (Cao et al., 2013). Significant changes in the structural makeup of the rice husk after treatment are shown by the biomodified spectrum fluctuations. Overall, rice husks infused with Aspergillus flavus and Phanerochaete chrysosporium revealed more crystalline structures in contrast to those treated with Bacillus licheniformis and Trametes versicolor. The most effective of them was the white-rot fungus strain Aspergillus flavus, which produced a modified material with up to 55% (w/w) cellulose. The OAE was further improved by further acetylation, as shown in Fig. 9(c). The combination of delignification and acetylation treatment achieved a maximum OAC of 20 g/g. Alternatively, rice straw was also hydrothermally treated and esterified to create a biomaterial that enables oil spill adsorption. The esterifying agent in this procedure was vinyl acetate, the solvent was N, N-dimethylformamide, and the catalyst was K₂CO₃. Temperature of 90°C, catalyst concentration of 1.5 g/cm³, esterifying agent content of 20%, with a reaction time of 6 hours, were found to be the ideal reaction conditions. With maximal OAC and water uptake capabilities of around 9.7 g/g and 0.5 g/g, respectively, the modified material showed a significant improvement in hydrophobicity (Wang and Liu 2017).

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Fig. 9. (a) Comparison of oil (in RMG 386 oil and corn oil) absorption capacity with water absorption capacity on various rice husk-based absorbents (Wang et al., 2015); (b) - FTIR spectra of rice husks treated with various bacterial strain after 45 days, (c) – Oil absorption capacity of rice straw treated by A. flavus and acetylated rice husk compared to raw rice husk after 30 min (Wang et al., 2020).

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In addition to thermal and chemical treatment, alternative treatments like cellulose aerogels (Thai et al., 2020), and surface changes (Dan et al., 2020; Xie et al., 2020) have been utilized to enhance the OAC of cellulose-derived absorbents like rice straw and rice husk. Investigations into functional upgrades in the development of porous cellulose materials have been conducted to augment hydrophobicity and promote OAC (Li et al., 2019; Nie et al., 2021; Oberlintner et al., 2021). Aerogels are extensively porous three-dimensional structures created by substituting the liquid phase of a gel with gas by regulated drying (Long et al., 2018). Ultra-high porosity (up to 99.8%), low density (0.004–0.500 g/cm³), and substantial surface area (100–1600 m²/g), make them suitable as petroleum-derived sorbents (Dilamian and Noroozi 2021; Bidgoli et al., 2019). These positive properties make them promising candidates for a wider range of applications (Dilamian and Noroozi 2021; Bidgoli et al., 2019). Nanocellulose-based aerogels have attracted increasing attention for oil–water separation, their superior biodegradability compared to synthetic polymer, as well as carbon-based aerogels, is a beneficial trait (Sun et al., 2017; Štefelová et al., 2017). Alternatively, Chhajed et al. (2022) isolated nanocellulose from rice straw using a chemo-mechanical method. They organized various nanocellulose fractions into a porous configuration. An optimum density, porosity, and morphology were used. As shown in Fig. 10(a), the untreated rice straw exhibits a smooth and flat surface, their dimensions are in the micrometer range (approximately 50–200 μm in width and several μm in length). Chemical treatment of the primary and secondary cell walls was disrupted, resulting in the dissolution of non-cellulosic components like lignin, hemicellulose, and pectin, which helped to isolate the cellulose fibers. The extracted cellulose fiber morphology is depicted in Fig. 10(b). It reveals a symmetrically dense structure caused by hydrogen bonding; this dense structure has fiber diameters in the range of 10 to 25 μm. Hydrogen bonding helps in the stabilization of micro–nano cellulose fibers, forming an ultrafine, uncharged, and entangled network as illustrated in Figs. 10 (c and d). High-magnification images reveal that the diameter is in the nanometer range (approx. 30 ± 10 nm) and the length extends to several micrometers. Following cellulose extraction, the crystallinity of the cellulose fibers increased relative to that of the raw rice straw, which can be attributed to the removal of amorphous components such as lignin and hemicellulose (Gupta et al., 2021; Verma et al., 2021). The effects of nanofibrillated cellulose concentration on the density and porosity of the prepared aerogels are presented in Fig. 10(e), in which the aerogels exhibited extremely low densities ranging from 6 to 23 kg/m³ as the micro–nano cellulose fiber concentration increased from 0.5 wt% to 2 wt%. The porosity of the samples correspondingly decreased with increasing fiber content. The sorption capacity of the polylactic acid-coated aerogels ranged from 28 to 70 g/g (Fig. 10f), and the materials demonstrated excellent reusability, maintaining efficient performance for at least ten consecutive sorption–desorption cycles (Fig. 10g). Furthermore, Dilamian and Noroozi (2021) presented a simple technique for synthesizing ultra-lightweight (2.2–24 mg/cm³), extremely porous (98.4%–99.8%) natural cellulose aerogels from rice straw via freeze-drying. The resultant aerogels exhibited an interconnected porous architecture having a specific surface area of 178.8 m²/g with a mesopore volume of 0.8 cm³/g, as seen in Fig. 10(h). The remarkable adsorption selectivity of the hydrophobic aerogels was further validated, as illustrated in Figs. 10(I and j). When the crosslinked cellulose aerogel was brought into contact with chloroform droplets at the bottom of the beaker, it absorbed the entire volume within 7s, demonstrating the ultrafast adsorption capability of the material. Similarly, the crosslinked cellulose aerogel completely absorbed the toluene layer on the water surface within 120 s. After the sorption of organic solvents, the aerogels remained afloat due to their low density and hydrophobic nature. The adsorption capacities of crosslinked cellulose aerogel for various nonpolar liquids and oils were quantitatively evaluated, as presented in Fig. 10(k).

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Fig. 10. FE-SEM images: (a) Raw rice straw, (b) Cellulose fibers, (c) micro–nano cellulose fiber at low magnification, (d) micro–nano cellulose fiber at high magnification, (e) Density and porosity in relationship with micro-nano cellulose fiber, (f) Absorption capacity of sample for various oil types, (g) Reusability of sample (Chhajed et al., 2022); (h) Preparation process of cellulose fiber aerogels from raw rice straw, (i) Chloroform removal, (j) Toluene layer on the water surface, (k) A mass-based adsorption capacity of cellulose fiber aerogels, (l) Reusability of cellulose fiber aerogels (Dilamian and Noroozi 2021).

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Superhydrophobic cellulose aerogels exhibit excellent uptake capability for nonpolar organic liquids. These materials display very high adsorption capacities for solvents like hexane, acetone, toluene, dimethylformamide, and chloroform, and range from about 98-170 g/g. The amount of liquid absorbed is controlled by internal properties of the absorbed material, such as surface tension, polarity, and density. In addition, chemically crosslinked cellulose aerogels exhibit excellent affinity toward oils with a range of viscosities - including crude oil, pump oil, and motor oil with sorption capacities ranging from 112 to 130 g/g. The better adsorption behavior is mainly attributed to Van der Waals attractions and intermolecular forces that allow the adhesion of the oil on the surface of the aerogel to be high. The hydrophobic nature of the cellulose framework leads to a decrease in the surface energy, which favors the selective absorption of nonpolar liquids. Moreover, the interlinked porous structure and capillary action help enhance liquid retention even further. As shown by the dashed reference lines in Fig. 10(k), between about 78-92% of the theoretical pore volume is taken up when oil and solvent are adsorbed, showing that the pore dimensions are good for efficient liquid entrapment. Mechanical resiliency tests (Fig. 10l) show that the crosslinked aerogels maintain more than half their original adsorption capacity after five cycles of compression and recovery, indicating their durability and reusability. Collectively, these results prove the beneficial properties of the aerogels, which feature high porosity, hydrophobic surface, quick absorption kinetics, high sorption capacity, and good recyclability. In another study, Tran et al. (2020) prepared aerogels reinforced with rice straw fiber using the freeze-drying method, and then made them hydrophobic by methyltrimethoxysilane chemical vapor deposition. The resulting materials had excellent water repellency, with contact angles greater than 150°. These aerogels had low thermal conductivity (0.034-0.036 W/mK), ultra-low density (0.05-0.06 g/cm³), and high porosity (about 97%), which makes them suitable to be used in insulation applications. Further, they exhibited good mechanical properties, with oil absorption capacities up to 13 g/g and a Young’s modulus value as high as 47 kPa. Furthermore, Vo et al. (2022) reported the development of a scale-up process for separating cellulose from rice straw for the fabrication of aerogels. Alkali pretreatment was effective in the removal of most of the lignin and silica, and gave an intermediate product with 73% cellulose. Subsequent bleaching with hydrogen peroxide further removed any remaining impurities, and the purity of the cellulose was raised to about 90%. Aerogels derived from this refined cellulose showed low density, great specific surface area, and highly porous networks, which gave them the great oil-water separation performance and maximum OAC of 21.7 g/g.

Polymer grafting has also been widely used to improve the sorption behavior of fibrous materials, synthetic and bio-based (Zamparas et al., 2020). In graft polymerization, the side chains are covalently attached to a polymer backbone, and this affects surface chemistry and enhances the performance of the materials. Among hydrophobic vinyl monomers, butyl acrylate has been found to be especially useful in terms of the generation of natural-based absorbents because of its ability to improve swelling and oil absorption. Therefore, Tung et al. (2022) successfully achieved the synthesis of oil-absorbing material by grafting the butyl acrylate monomer on rice straw using 2,2’-azobisisobutyronitrile as an initiator and divinyl benzene as a crosslinker. The maximum sorption capacity of the resulting copolymer was found to be 20.56 g/g with a divinyl benzene content of 1.5%. Reusability tests for seven cycles of adsorption-desorption revealed that about 70% of the initial efficiency was retained. Porous polyurethane foams are another class of oil sorbents that are widely used because of their inherent hydrophobicity and open-cell structure (Santos et al., 2017). Although these foams are able to swell and recover repeatedly, the sorption capacity often decreases with each cycle (Keshawy et al., 2020). Incorporation of fillers into polyurethane matrices has been found to improve performance by creating composite foams that have better adsorption properties (Maia et al., 2022). Ianov et al. (2022) obtained a composite sorbent by incorporating rice straw into elastic polyurethane foam. Optimal formulations, using polyurethane part A:part B ratios from 1:0.6:0.16 to 1:0.6:0.72, gave the OAC of about 13 g/g. The composite selectively absorbed oils, including diesel, industrial oil, and vacuum oil, and minimal water absorption with a selectivity factor of 0.4 after 120 min. Tayeb et al. (2020) proposed a hybrid adsorbent composed of rice straw and a synthetic hydrophobic agent (RP18) and came out with successful removal of the crude oil present in contaminated water. Their results showed a removal efficiency of 94.7% at low oil concentrations with an absorption capacity of 6.67g/g and 12g/g for rice straw and the RP18 sorbent, respectively. Further investigation by Hoang and Pham (2021b), they reported that the OAC is significantly impacted by the size and proportion of rice straw that is introduced into the porous polyurethane matrix. In fact, rice straw was mixed with pristine polyurethane with mass ratios of 5% and 25% to create the rice straw–polyurethane-based absorbent. Fig. 11(a) shows the four sample types for assessment, which were 5% straw with 0.5 mm length (S1), 25% straw with 0.5 mm length (S2), 5% straw with 3 mm length (S3), and 25% straw with 3 mm length (S4). Two straw lengths, 0.5 mm and 3 mm, were employed. As shown in Fig. 11(b), the sample with 25% rice straw and a length of 0.5 mm (S2) had the greatest oil absorption capability. Similar results could also be reported in another study of this group (Hoang et al., 2018b).

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Fig. 11. (a) As-prepared rice straw/polyurethane-based absorbents depending on rice straw mass and size, (b) Relationship between oil absorption capacity and rice straw mass and size (Hoang and Pham 2021b); (c) Scheme for the fabrication of lignin-based polyurethane–graphene oxide–octadecylamine foam, (d) Absorption capacity of lignin-based polyurethane–graphene oxide–octadecylamine foam for different organic pollutants compared to commercial polyurethane sorbent (Oribayo et al., 2017).

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Moreover, Oribayo et al. (2017), have described the production of a polyurethane foam derived from lignin, followed by the addition of surface modification to achieve a sorbent with high superhydrophobic and superoleophilic properties that could be applied in the remediation of oil spills. The porous structure of the lignin-based polyurethane was, in this case, chemically modified by the introduction of polydopamine-reduced graphene oxide and octadecylamine, thus turning the foam structure into a 3D-network with high oil-binding capacity and water-repellence. The modified polyurethane-graphene oxide-octadecylamine foam maintained its interconnected pore structure and attained a high-water contact angle of 152^°, as shown in Fig. 11(c). Experiments on oil uptake done with crude oil, engine oil, kerosene, and chloroform showed excellent absorption ability of between 26 and 68 times the weight of the foam. These values were much higher than values acquired using traditional nonwoven polypropylene sorbents, as represented in Fig. 11(d). The results reveal that lignin originating from rice straw and rice husk can be used as a potential source of a precursor in the manufacture of polyurethane materials that can be used in the oil sorption process. In a similar study, Pereira et al. (2024) prepared bio-based polyurethane foams where rice husk ash was included in varying concentrations (2, 8, and 20% of polyol mass) to improve the diesel oil adsorption in single-phase and biphasic systems. The composites with higher ash levels (8 and 20%) were found to have as large as 43% increase in the hydrocarbon removal effectiveness in a single-phase diesel oil setting when compared to neat polyurethane foam. The adsorption values were found to be 5.348 mg/g and 5.518 mg/g of the 8% and 20% ash foams, respectively. Nevertheless, in a two-phase, between 10 and 150 g/L, contact angle-dependent sorption behavior was observed with the composite in 2% ash, showing a high oil uptake. All in all, Table 3 gives a summary of using rice straw and rice husk-based materials in the untreated and modified states in terms of oil absorption.