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Review

Agricultural Wastes as Renewable Biomass to Remediate Water Pollution

by
Awal Noor
1,* and
Sher Ali Khan
2,*
1
Department of Basic Sciences, Preparatory Year Deanship, King Faisal University, Al-Hassa 31982, Saudi Arabia
2
Department of Chemistry, Government Postgraduate Jahanzeb College, Saidu Sharif 19130, Pakistan
*
Authors to whom correspondence should be addressed.
Sustainability 2023, 15(5), 4246; https://doi.org/10.3390/su15054246
Submission received: 11 January 2023 / Revised: 2 February 2023 / Accepted: 21 February 2023 / Published: 27 February 2023
(This article belongs to the Special Issue Sustainable Material and Technology in Wastewater Treatment: Contaminant Removal, Adsorption and Nutrient Recovery)
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Abstract

:
Increases in agricultural waste, population, and industrialization are leading to serious environmental problems, in particular drinking water contamination. Continuous efforts have been made to remediate water pollution through different approaches, either by decreasing the interring of pollutants or treatment of already contaminated water. The development of an efficient, cheaper, and renewable adsorbent is the focus of the current research. Agricultural wastes are cheap materials for this purpose and have attracted much attention of researchers. These agricultural wastes are either field residues such as stems, stalks, and leaves, or process residues such as husks, roots, and bagasse, as they have the same chemical composition (cellulose, hemicelluloses, and lignocelluloses). These wastes are processed using different methods to yield an efficient adsorbent. Chemical modification is used to prepare novel efficient adsorbents using agricultural wastes, rather than incineration of these materials. This review summarizes the research outcomes in terms of chemical modification and application of agricultural wastes used for the eradication of organic and inorganic pollutants from water.

1. Introduction

In recent decades agriculture has improved and crop yields have increased in order to fulfill the nutritional and health requirements of a world population facing hunger and malnutrition. The use of biotechnology and adopting modern methods of agriculture, as well as discovery of new pesticides, has overcome the previously existing major problem of crop loss. It is estimated that agricultural production has been increased more than threefold in the last 50 years. In 2018 Asia became the major producer of cereal food in the world [1]. This growth in production also leads to pollution and degradation of soil, water, and air and thus, has serious negative impacts on the environment. Agricultural waste is increasing rapidly all over the world, and its management for the benefit of human beings is an important issue [2].
Alongside a rapid increase in population, urbanization and industrialization have resulted in increased water pollution due to sewages, industrial emissions, chemicals, domestic wastes, pesticides, and pharmaceuticals. Water pollution is a global problem and it has been estimated that 14,000 people are dying every day due to water pollution [3]. A recent report from the United Nations in 2020 shows that despite continuous efforts, in particular after the announcement of the “Water Action Decade” (2018–2028), only 74% of the world population has access to safely managed drinking water services. The aim of the Sustainable Development Goal (SDG) 6 is to ensure the access of the remaining two billion (almost) people to clean water. Additionally, SDG 14 (Life Below Water) aims to prevent and minimize marine pollution and ocean acidification, in order to protect marine and coastal ecosystems [4]. Water pollutants may be organic, inorganic, biological, or radioactive wastes that make the water unfit for drinking. Inorganic pollutants are heavy metals such as Cr, Ni, Cu, Zn, Cd, Hg, Pb, U, and Pu, and metalloids such as Se and As [5,6]. The main problem associated with these metals is their bioavailability and non-biodegradable nature [7]. They may be present in certain oxidation states, and due to oxidation/reduction they can form different complexes which are toxic for living things. Some of the metals, like Fe and Mn, are more sensitive to redox reactions [8] and can achieve different oxidation states depending on the conditions, and may form different oxide particles in water. These oxides absorb heavy metals and metalloids, and through reductive dissolution these adsorbed pollutants are released in water [9]. Microorganisms also play a role in the precipitation and dissolution of these reactive particles in water, and thus the process turns from physical to a chemical and biological process [10]. The metabolic process of microorganisms plays a major role in heavy metal dissolution due to biogeochemical cycles. These metals then show activity for biosorption and chelation [11]. The most common organic pollutants are phenolic compounds, polycyclic aromatic hydrocarbons, dyes, agricultural chemicals (organic pesticides and herbicides), and oil spills (crude oil) from the petroleum industry. These compounds have toxic effects on human health, including reproductive system disorder, endocrine disturbance, obesity, and cancer [12,13,14,15]. Some of the azo dyes have been reported to cause allergies and other human maladies [16,17]. Prevention or remediation of water pollution is the main aim and objective of modern environmentalists. Different methods are used to eliminate water pollutants, such as photo catalytic degradation, using oxidizing agents, ultra-filtration, combined photo-Fenton and ultrasound, coagulation, flocculation, distillation, extraction, precipitation, ozonation, chlorination, aerobic degradation, and adsorption [18,19]. However, it has been found that adsorption is one of the best methods for water pollution remediation because it is more environment friendly, economical, and efficient. The development of more efficient adsorbents is among the top priorities of modern research. Different adsorbents that have been introduced for water purification include mineral oxides, activated carbon, polymers, resins, and modified sol gel [20]. It was found that activated carbon has the ability to remove organic pollutants from water [21]. Activated carbon may be obtained through different sources, i.e., olive stone, vermiculate plants, bamboo dust, saw dust, ground nut shell, banana peel, rice husk and straw, and corn cob. Most of these sources are agricultural waste products [22]. Different agricultural wastes have been used to develop the best adsorbents for the removal of water pollutants [23]. These wastes are cheap, require less energy, show high removal efficiency, and are easily available to synthesize efficient adsorbents to remediate water pollution. Carboxymethylated lignin has been effectively applied for the adsorption of lead [24]. All these results show that the development of efficient adsorbents needs detailed insight into the chemical composition of these readily available raw materials. Herein, we have discussed the composition of agricultural wastes and have summarized the strategies of chemical modification that have been used for water purification.

2. Composition of Agricultural Wastes

Agricultural residues are mainly composed of polysaccharides (cellulose and hemicelluloses), lignin, and smaller amounts of pectin substances [25]. The contents of the main components vary in the ranges shown in Table 1, where polysaccharides represent the major portion of both wood types.
Cellulose is long-chain polymer of β-d-glucose in the pyranose form, in which the pyranose units are linked by 1–4 glycoside linkages to form cellobiose repeating units of the polymeric chain (Figure 1). The degree of polymerization of each cellulose chain is determined by the number of monomers [27,28]. In plants, it is synthesized by cellulose synthase enzymes. It is considered a major sink of atmospheric carbon (CO2) [29].
There are different polymorphs of cellulose, of which the most important crystalline forms are cellulose I, cellulose II, cellulose III, and cellulose IV [30]. These different polymorphs are applied for the synthesis of nanocrystals [31], which are applied in paints, coatings, cosmetics, special paper, biomedical materials, textiles, the automotive industry, pharmaceuticals, electronics, and the electrical industry [32,33,34,35]. The transformation of these polymorphs is shown in Figure 2.
Hemicellulose is another polysaccharide found in plant structures, in which the main structural components are homo- and heteropolymers. It mainly comprises anhydrous β-(1 → 4)-d-xylopyranose, mannopyranose, glucopyranose, and galactopyranose main chains with a number of constituents. It is found in association with cellulose in the secondary cell wall of plants. The principal component of softwood hemicellulose is glucomannan (Figure 3) [36].
Lignin is an aromatic polymer in which the monomeric units are linked by ether and carbon-carbon linkages. It consists of three main building units: p-coumaryl alcohol (p-hydroxyphenylpropenol), coniferyl alcohol (guaiacylpropenol), and sinaphyl alcohol (syringylpropenol) [37]. Coniferyl alcohol is the principal constituent of softwoods, while guaiacyl and syringyl units constitute the lignin of hardwoods [38]. The units of lignin are shown in Figure 4 and the bonding between the different units of lignin is shown in Figure 5. Lignin degradation is an important step for obtaining pulp with the best porosity for further modification; therefore, different agents are used for its degradation [39].
For the application of agricultural residues in water treatment, derivatization is needed, for which extraction of different components is important. Therefore, cellulose, hemicellulose, and lignin are extracted for better results [40,41].
Sustainability 15 04246 g005 550
Figure 5. Structural segment of softwood lignin proposed by Adler [42]. Some bond lengths are extended for clarity.
Figure 5. Structural segment of softwood lignin proposed by Adler [42]. Some bond lengths are extended for clarity.
Sustainability 15 04246 g005

3. Strategies of Chemical Modifications

3.1. Extraction of Cellulose from Wood

Dissolving pulp is an important strategy for the derivatization of agricultural waste to increase its efficiency for the removal of contaminants from water. It contains high cellulose (95–98%) and relatively low hemicelluloses (1–5%) and lignin (0.05%) contents [43]. The term dissolving pulp refers to obtaining a maximum percentage of cellulose from raw materials. The raw material chips are delignified to separate pulp fibers. Different reagents such as alkali sulfite, anthraquinone, polysulfide, and hydrogen sulfide are used for this purpose [44]. In kraft pulping, wood chips are treated with a solution of hydroxide and hydrogen sulfide at elevated temperatures to attain better strength [45]. Lignin is degraded and dissolved to obtain lignin free pulp. A method for complete degradation and removal of pulp is to treat it with the dilute acid 0.1 M HCl at 120 °C, or with concentrated HCl (20–30%) at 40 °C [46]. This process is combined with kraft pulping to produce quality dissolving pulp. The same was used in wood saccharification to preserve pulp quality in other applications [47,48]. Peeling is carried out by treatment of dissolved pulp with alkali to obtain a low degree of polymerization [49]. Franzon and Samuelsson studied un-mercerized cotton and concluded that on average, 65 glycoside units were peeled off before a stopping reaction occurred [50,51]. For the complete removal of lignin and hemicelluloses, the pulp was treated with bleaching agents such as chlorine, sodium chlorite, chlorine dioxide, oxygen, sodium dithionate, and hydrogen peroxide. Enzymes were also used for delignification, though the recycling of enzymes is a challenging and expensive process [52].

3.2. Derivatization of Cellulose for Water Treatment

Cellulose derivatization is an important step to improve its efficiency of adsorption in water treatment. β (1 → 4)-d-glucopyranose has three reactive hydroxyl groups, i.e., two secondary (2-OH and 3-OH) and one primary (6-OH) hydroxyl group. The primary hydroxyl group (6-OH) is more reactive and suitable for derivatization than the secondary OH groups [53,54]. The morphology of cellulose also affects the reactivity. The hydroxyl group situated in the amorphous region is more reactive than in the crystalline region. A suitable solvent for cellulose reactions is a mixture of dimethyl sulfoxide and paraformaldehyde, while cupric ethylenediamine and cadmium ethylenediamine are also important solvents [55,56]. Figure 6 shows the positions in the cellulose structure which are suitable for chemical modifications, and Figure 7 summarizes the different strategies of modification of agricultural wastes [57,58].

3.2.1. Graft Copolymer from Agricultural Wastes as an Adsorbent

Modification of different agricultural raw materials also involves the introduction of nucleophilic groups such as -NH2, -SONH2,-COOH, -CN, -SH, etc., on pulp to increase the efficiency of metal adsorption. The inclusion of such groups on the main stem of cellulose leads to good complexation of toxic transition metals. This strategy can be very effective for selective adsorption of metals.
Grafting reactions provide a potential derivatization of substrate polymer. The pulp is treated with an initiator or radiation (gamma rays, plasma, UV-rays, or microwaves) to produce free radicals for the graft copolymerization reaction [59]. Graft copolymerization of vinyl monomers onto cellulose/cellulose derivatives and lignocelluloses fibers has always been the focus of research activities. In some processes, the grafting of multifunctional monomers leading to cross linking, such as allyl acrylate, is reported [60]. The technical problem in graft copolymerization is the formation of homo polymers, which needs to be resolved. The general scheme and different methods for derivatization are mentioned in Figure 8 and Figure 9.
Graft copolymerization of cellulose is a classic subject [61], and a part of this review covers research developments in the field of water treatment. Radiation induced graft copolymerization of pulp was applied using acrylic acid, and was successfully applied for the removal of metals such as Cr+3, Fe+3, Cd+2, and Pb+2. pH plays a crucial role in these applied purification methods. For instance, it was shown that metal uptake is enhanced with an increase in pH, and was found to be a suitable strategy to remediate metals from industrial wastewater at high pH [62]. -COOCH3 and NH2 functionalities were introduced on okra fibers using methacrylate and acrylamide monomers, and the resulting product was successfully used for the removal of Cu+2 ions from water, with an uptake efficiency of 33.05 mg/g [63]. The amine type adsorbent was prepared from hyacinth fibers using gamma radiation with glycidyl methacrylate as a monomeric unit, and was used for the successful removal of Pb2+, Cu2+, and Cr3+ from water solutions [64]. A modified sunflower stem was used for the removal of manganese and phenol. In the modification process, ion-exchange resin was fabricated through the hydrolysis of graft copolymer of pretreated sunflower stem with acrylamide while using Fenton’s reagent as an initiator [65]. In another study, Jute fibers were grafted with n-butyl methacrylate and phosphoric acid using gamma radiation. It was found that only 2.035 g jute fiber could be used to adsorb 4.30 ppm of Cu2+ when the adsorbent was treated for 90 min [66]. Kumar et al. grafted acrylamide and acrylonitrile on psyllium using ceric ammonium nitrate as an initiator, and used it as an adsorbent for the removal of mercury ions [67].
Acrylonitrile was successfully grafted on cellulosic material obtained from sisal fiber using ceric ammonium nitrate as an initiator, and the resulting cellulose grafted acrylonitrile copolymer was used as an efficient adsorbent for the removal of Cr6+ from industrial waste [68]. Rahman et al. synthesized cellulose-graft-polyacrylonitrile copolymer by a free radical initiating process, and the oximation reaction led to the conversion of the nitrile functionalities of the grafted copolymers into the amidoxime ligand. It was found to be not only an efficient absorbent for Cu2+ (up to 326 mg/g), but also for a number of other transition metal ions. For instance, Fe3+, Co3+, Mn2+, Cr3+, Ni2+, and Zn2+ could also be efficiently absorbed (273.6, 271.6, 241.7, 228.2, 204.2, and 224.3 mg/g, respectively) at pH 6 [69]. In another study, Luffa cylindrica was chemically grafted with acrylic acid and was applied for the removal of methylene blue (MB) and metal ions (such as Mg2+, Ni2+, Zn2+) from aqueous solution using a batch process. The grafted copolymer was efficient enough to absorb up to 62.15 mg/g of dye in 175 min. Compared to other metals, a maximum removal of 45.8% could be achieved for Mg2+ ions [70]. Wang and co-workers introduced an amidoxime group on cellulose fibers through modification of polyacrylonitrile grafted cellulose for the removal of U6+ from sea water. The amidoximated cellulose fibers showed promising results by showing an adsorption efficiency of 52.88 mg/g at pH 5 (static adsorption process) and 1.22 mg/g at pH 8 after filtrating 10.0 L simulated seawater, despite the fact that the co-existing K+, Na+, Ca2+, and Mg2+ ions were 1000 times more concentrated than U6+ in seawater [71]. Modified cellulose fibers are also used for the adsorption of rare earth metals such as La3+, Ce3+, Pr3+, Gd3+, and Nd3+ with the observed values of 262, 255, 244, 241, and 233 mg/g, respectively, at pH 7 [72]. Abdel-Halim and co-workers extracted cellulosic components from palm tree wastes and grafted it with acrylic acid. It was successfully applied for the removal of MB and lead ions from waste water. The results were quite interesting, as 100% dye, as well as lead, removal was achieved in 60 and 35 min, respectively (25 mg/L dye concentration) [73,74].
Gamma radiation was previously used for grafting itaconic acid on cellulosic fiber, and was applied for cationic dye removal with an intake of MB up to 38 mg/g [75]. The cellulose fibers obtained from Luffa cylindrica could be grafted with methyl acrylate/acryl amide via microwave-induced radiation without the use of an initiator, and the grafted copolymers were used for the removal of hazardous Congo red dye from an aqueous system with an adsorption efficiency of 17.39 mg/g [76]. The fibers of Hibiscus cannabinus were grafted with vinyl monomer acrylic acids, and a mixture of acrylic acid and acrylamide was used for the removal of dyes from water [77]. The grafted poly [2-(methacryloyloxy) ethyl]trimethylammonium chloride was covalently attached to cotton cellulose substrate via gamma radiation and was applied for the adsorption of Acid Blue 25 (AB25) and Acid Blue 74 (AB74), with adsorption efficiencies of ∼540.0 mg/g and ∼340.0 mg/g, respectively [78]. The same source of cellulose was used for radiation induced graft polymerization with poly (vinylbenzyltrimethyl ammonium) chloride, and was used for the adsorption of different dyes such as AB25, Acid Yellow 99 (AY99), and (AB74). The adsorption capacity was found to be ∼540 mg/g, ∼474 mg/g, and ∼122 mg/g, respectively [79]. Similarly, cotton cellulose was functionalized using gamma irradiation-induced grafting of glycidyl methacrylate to obtain a hydrophobic cellulose derivative with epoxy groups, and was applied for pesticides adsorption [80]. In another study, cotton linter-based fibers were first grafted with dimethylaminoethyl methacrylate using radiation grafting polymerization, followed by further quaternization (QCL = Quaternary cotton linter) or protonation (PCL = Protonated cotton linter), and these two kinds of cotton linter-based adsorbents were used for the removal of humic acid from water. The maximum capacity for PCL and QCL reached 250 mg/g and 333 mg/g at pH 6, respectively [81]. Nasef and co-workers grafted cotton fabrics with N-methyld-glucamine and used it for boron adsorption from water [82,83]. Jute fibers have been used as a cellulose source for grafting with acrylic acid applying gamma radiation, and were used for dye adsorption [84].
Oil spills from the petroleum industry is another big environmental issue. The hydroxyl groups of cellulose can be modified through acetylation and silanation, that can then be used as oil sorbents. Such acetylated cellulose fibers obtained from corn straw were successfully used to remediate spilled oil, and the sorption capacity was found to be 42.53, 52.65, and 57.64 g/g for pump oil, diesel oil, and crude oil, respectively (by immersion at 25 °C for 1 h) [85]. Feng and co-workers, for the first time, converted paper waste into biocompatible cellulose aerogels by doping it with methyltrimethoxysilane and applied it for crude oil adsorption, with not only excellent results that were 40–95 times its weight, but also showed stable hydrophobicity for over five months [86]. Silanized cellulose prepared by sol-gel reaction between microcrystalline cellulose and hexadecyltrimethoxysilane was used for oil–water separation, with an efficiency of 99.93% that remained almost constant (99.77%) even after recycling 10 times [87]. Oil palm empty fruit bunches and cocoa pods were acetylated under mild conditions and have been investigated for the removal of crude oil from aqueous solutions [88]. In another finding, ultrasonic waves were used for grafting butyl acrylate on cellulose fibers to obtain more regular and homogenous network polymers for oil adsorption [89]. Recently, saw dust pulp was grafted with butyl acrylate and lauryl methacrylate using 2,2′-azobisisobutyronitrile as an initiator and divinyl benzene as a cross-linker. The product, having hydrophobicity and oleophilicity characteristics, was investigated for the removal of crude oil from sea water. It was found that by this treatment the capacity was more than 20 g/g [90].

3.2.2. Doped Pulp as Adsorbent

One of the aims of current research studies is to introduce sulfur containing groups on the cellulose stem for a wide range of applications in the use of water purification due to the high affinity of the sulfur containing functional groups toward a large number of the heavy metal ions. The lone pair electrons of the S atom are responsible for complex formation with the empty d orbitals of heavy metal ions. In this regard, natural cotton cellulose was modified with thiourea for the selective adsorption of Hg+2 from water with promising results of 110.3 mg/g [91,92]. Continuing their studies, the same research group modified cellulose cotton fibers through graft copolymerization of polyacrylonitrile, and then by insertion of phenyl thiosemicarbazide moieties for the adsorption of precious metals (Au3+, Pd2+, and Ag+) from their aqueous solutions with adsorption capacities of 198.31, 87.43, and 71.14 mg/g, respectively [93]. Neto et al. treated coconut bagasse with a thiourea/ammonia solution to increase the adsorption capacity for the removal of cadmium, and the adsorption capacity was found to be 35.97 mg/g [94]. In another study, N/S doped magnetic carbon aerogel was fabricated using sugarcane bagasse-based cellulose and was successfully applied for the removal of bisphenol from aqueous solution. The maximum removal efficiency of 98–99% was achieved at 343 K at pH 7 [95]. Lignin-based pitch obtained from black liquor was investigated as a precursor for the synthesis of activated carbon and was used for the removal of benzene [96]. Wang and co-workers carried out xanthation of cellulose and studied the removal of Pb+2 [97].

3.2.3. Carboxylated Cellulose as Adsorbent

Another useful group having the best complexation for heavy metals and some organic cationic species is the carboxylic (-COOH) group, and has been the focus of various research studies. It binds to metals in a bidentate fashion and is considered a stronger chelator compared to monodentate thiourea, alcohols, and epoxides. A number of different methods have been used for the introduction of this group on cellulose extracted from agricultural wastes. Singha and his coauthors extracted pulp from okra and introduced the carboxylic group on it using succinic anhydride in the presence of pyridine by heating. The adsorbent achieved was then used for the removal of Cu+2, Zn+2, Cd+2, and Pb+2 ions with adsorption efficiencies of 72.72, 57.11, 121.51, and 273.97 mg/g, respectively [98]. Li et al. used pomelo peel to prepare carboxylated cellulose through single step hydrolysis with a mixture of hydrochloric acid and citric acid [99]. The cellulose extracted from almond shell, almond stem, and fig stem could be converted to carboxmethyl cellulose [100]. Tang and co-workers converted the agro-forestry residues of corn stalks, Cinnamomum camphora leaves, and Cinnamomum camphora saw dust into highly effective adsorbents using solvent-free conditions with maleic anhydride. The product was then applied for the removal of MB. The adsorption capacities were found to be 870, 741, and 787 mg/g, respectively [101].

3.2.4. Activated Carbon from Agricultural Wastes as Adsorbent

Activated carbon is considered as one of the best adsorbents for organic pollutants due to it high surface area. Researchers are using woody and non-woody materials for the production of activated carbon; however, non-woody materials are preferred over woody materials [102]. Activated carbon, usually via pyrolysis, is a cost effective adsorbent obtained from agricultural wastes with high efficiency in removing heavy metals from wastewater [103]. Activated carbon is prepared either using physical methods or chemicals.

Activated Carbon through Physical Methods

This method is chemical free and environment friendly. It involves a two-step process in which pyrolysis is carried out in a neutral atmosphere, followed by activation in aerobic conditions (steam, carbon dioxide, and nitrogen) in a temperature range of 800–1100 °C. The method is inexpensive and produces activated carbon with a porous structure and good physical power. The only disadvantages are a long activation time and high energy consumption [104].
Different agricultural wastes are used for this purpose to obtain activated carbon for the removal of inorganic and organic pollutants from aqueous solutions. Almond shell and orange peel were used to obtain activated carbon for the removal of 2-picoline from water. The observed removal efficiency for activated carbon obtained from almond shell and orange peel was 6.66 × 10−4 and 2.51 × 10−5 g/mg.min, respectively [105]. Activated carbon obtained from orange peels was also used for the removal of a mixture of five NSAIDs (Non-steroidal anti-inflammatory drugs), namely diclofenac, ibuprofen, ketoprofen, salicylic acid, and paracetamol [106]. The usually applied thermal heating method for obtaining activated carbon from agricultural wastes was replaced by the microwave radiation method to save time and energy, as well as to improve the adsorption capacity while using optimum conditions [107]. Using this approach, an enhancement in the activation process was observed within 1–4 h and 500–1200 °C, after whichh molecular break down led to pyrolysis and the efficiency of the activated carbon for the removal of pollutants from polluted water was affected [108]. Blachino and co-workers used two different methods to prepare activated carbon from strawberry seeds and pistachio shells. For chemical activation, acetic acid was used, while carbon dioxide and water vapor were used for physical activation. The adsorption studies revealed that the adsorption capacity for the removal of 4-chloro-2-methlyphenoxyacetic acid of the activated carbon obtained from strawberry seed was quite low, but for pistachio shells it reached to 1.43–1.56 mmol/g [109].

Activated Carbon through Chemical Methods

In chemical activation, the agricultural waste containing cellulose is treated chemically at high temperatures of 400–900 °C [104]. This method is also called wet oxidation. It leads to dehydration, and thus the cellulose degradation results in porous carbon materials with increased surface area for adsorption. Compared to physical methods, here the carbonization and activation take place in a single step. The common chemical activators used are phosphoric acid, zinc chloride, potassium carbonate, sodium hydroxide, potassium hydroxide, sulpuric acid, ferric chloride, and diammoniun hydrogen phosphate [110,111,112,113,114,115].
Isoda and co-workers used chemical activators to prepare activated carbon from rice husk with a high surface area (up to 1593 m2 g−1), mesopore volume (up to 1.22 cm3 g−1), and surface acid groups to show good adsorption capacity [116]. Simple methods could be used to produce activated carbon from corncob, rice husk, and wheat straw using sodium chloride as the chemical activator. The adsorption study was carried out for MB removal. The results revealed that the activated carbon obtained from wheat straw had the highest adsorption capacity (90.9 mg/g), while activated carbon obtained from rice husk and corncob showed nearly identical adsorption capacities (72.4 and 72.5 mg/g, respectively) [117]. In another study corncob and bagasse were used as sources for the preparation of activated carbon and were applied for the removal of chromium metal from water [118]. Nitrate ions are another significant environmental problem and are of serious concern, as it causes eutrophication. The removal of such ions was done by using activated carbon from agricultural wastes [119]. Rice straw is a source for the production of bioethanol. Cellulosic waste was used for the preparation of activated carbon via pyrolysis and activation by KOH. The resulting activated carbon was applied for the removal of dyes (MB: 217 mg/g) and heavy metals (Cu2+: 169 mg/g) from aqueous solution [120]. Activated carbon obtained from saw dust and peanut shell was applied for the removal of metals from aqueous solution with good adsorption efficiency (Pb2+: 14.01 mg/g; Cu2+: 13.1 mg/g; Cd2+: 5.5 mg/g) [121]. Coir pith was used as a source for activated carbon with zero valent iron. The results showed that using FeSO4 as an activator increased the adsorption of trichloroethylene seven times more than the activated carbon without FeSO4 [122]. Chemically activated carbon was obtained from chickpea husk using KOH and K2CO3 as chemical activators. The activated carbon was used for the removal of heavy metals. The adsorption capacities were observed to be 135.8, 59.6, and 56.2 mg/g for Pb2+, Cr6+, and Cu2+, respectively [123]. Activated carbon prepared from sugar beet bagasse has been successfully applied to adsorb Cr6+ from aqueous solution [124]. Crude oil contaminated water was treated with activated carbon from plantain peels biomass. The efficiency was high at pH 5 with 90.9% crude oil removal [125]. Coconut shell, palm kernel shell, and wood sawdust have been also used for the preparation of activated carbon, and have been applied for the removal of dihydrogen monosulphide from simulated petroleum refinery wastewater [126]. Research studies carried out on activated carbon from barley husk, corncob, and Agave salmiana leaves by chemical activation with phosphoric acid have also shown promising results for the removal of MB [127].

3.2.5. Absorbent Derived from Sol-Gel Synthesis

The sol-gel process is another important method used for preparing efficient adsorbents for water treatment. Agricultural wastes have been used by different research groups to obtain sol-gel for water treatment purposes. The agricultural wastes from sugarcane bagasse, bamboo culm, bamboo leaf, corncob, banana leaf, and cigarette butt were used for the production of silica using the sol-gel process. It was found that sugarcane bagasse contained 92.5% silica followed by bamboo leaf (62.72%), corncob (27%), bamboo culm (20%), banana peel (0.35%), and cigarette ash (0.1%) [128]. Silica nanoparticles obtained by the sol-gel method have a high surface area (about 340 m2/g), which could be modified with doping of metal-loving ligands and thus can increase the metal impurities adsorption efficiency [129]. Bambusa vulgaris leaves were used for the preparation of silica nanoparticles through the sol-gel method. The product was used as an adsorbent for the removal of Cd and Cr metals from aqueous solution with promising adsorbing capacities of 133 and 172 mg/g, respectively (pH 7; 30 min) [130]. The sol-gel method was applied on grape stalk to obtain an environmentally friendly hybrid sol-gel silica adsorbent that was then used for the removal of azo-dyes from water. The adsorption capacity was found to be 205.3 mg/g [131].
Generally, adsorbents prepared through the sol-gel method show better adsorption efficiency toward organic pollutants than inorganic pollutants. For instance, an adsorbent prepared from grape bagasse could adsorb Basic Blue 41 (BB41) dye up to 268.1 mg/g, more than the activated carbon attained from sugar scum which could adsorb 166.16 mg/g [132,133]. Grape stalk-based adsorbent derived through the sol-gel technique showed a higher efficiency of 205.3 mg/g for the removal of cationic azo-dye BB41 than grafted cellulose (165.7 mg/g) [131,134]. Sugarcane-based composite prepared through the same technique showed a higher adsorption efficiency of 362.3 mg/g for MB than activated carbon (148.8 mg/g) [135,136]. On the hand, the sol-gel adsorbent prepared from Bambusa was found to be a less efficient adsorbent of Cd (133 mg/g) compared to biochar-based adsorbent (216 mg/g) [130,137].

3.2.6. Hydrogel from Cellulose as Adsorbent

Hydrogel is a three-dimensional network of hydrophilic polymer. It can retain large quantities of water and nutrients for a long time. Its structure comprises hydrophilic groups such as -OH, -COOH, -NH2, -CONH2, and -SO3H, or a hydrated polymer network under aqueous conditions. Based on target applications, hydrogels are prepared using different chemicals and techniques. Cellulose is poorly soluble in water as well as in a majority of organic solvents. However, it can be dissolved in specific solvent systems (such as NaOH/urea, NaOH/thiourea, etc.), and then a stoichiometric amount of the cross linker is added to the solution [138]. The cross linking can be achieved by applying chemical, physical, or polymerization techniques [139,140,141,142].
Agricultural wastes are rich in cellulose, which is converted to nanocellulose to produce superabsorbent hydrogels [143,144]. Gamma radiation was applied to prepare modern hydrogels from orange peel and N-vinyl-2-pyrrolidone using free radical polymerization, and were used for the removal of Congo red and methyl orange dyes from water solutions [145]. Low-cost green hydrogel from shrimp shells and rice husks was prepared and applied for the removal of ciprofloxacin and enrofloxacine, with a removal efficiency of 106.038 and 100.433 mg/g, respectively [146]. Rice bran with sodium alginate and chitosan was used to obtain two new types of hydrogel beads to remove dyes from water solutions. The process was low-cost, environmentally friendly, and the absorbents were recyclable. The rice bran/chitosan hydrogel beads showed maximum adsorption for reactive blue 4 at pH 3, a dosage of 40 mg, at 50 °C and for 7 h of adsorption. On the other hand, the rice bran/alginate hydrogel beads adsorbed crystal violet efficiently at pH 5, with a dosage of 30 mg, at 30 °C and for 6 h of adsorption time [147]. The carboxymethyl cellulose-based hydrogel was prepared from sugarcane bagasse via microwave-assisted irradiation for selective adsorption [147,148]. A reusable hydrogel blended with poly (vinyl alcohol) and glutaraldehyde cross linkers selectively adsorbed Cu2+ ions from water with an efficiency of 2.3 mg/g (92.4% removal) under ambient temperatures at pH 5.0 [149]. A melanin-based hydrogel free from cross linker was prepared from chestnut shell pigment and acrylic acid via radical polymerization, and was successfully applied for Cu2+ removal from water solutions with an adsorption capacity of 200.3 mg/g [150]. Alginate-based hydrogel was prepared from olive pomace and lignin for the removal of 3,4-dichloroaniline with a removal efficiency over 90% [123]. Lemon peel has been used as a source for direct regeneration of hydrogels that could be applied for the removal of MB [151].
The recovery, as well as reusability, of hydrogel adsorbents is the most important feature in practical applications for the removal of organic and inorganic pollutants from wastewater. Chu and co-workers successfully regenerated and reused the hydrogels for removing short- and long-chain perfluoroalkyl acids by treating it with 70% methanol containing 1% NaCl [152]. A carboxymethylcellulose-based hydrogel was reported to be recyclable/reusable without significant drop in efficiency for the adsorption of MB [153]. In another study, a sugarcane-based carboxymethyl cellulose hydrogel was applied for copper adsorption with a promising potential of reuse after a regeneration process [149]. A 2,2,6,6-tetramethylpiperidinyl-1-oxyl radical (TEMPO) oxidized cellulose hydrogel was used repeatedly for the removal of Zn, Fe, Cd, and Cs [154]. The reusable and biodegradable characteristics of cellulose-based hydrogels make these adsorbents environmentally friendly [155].

4. Conclusions

Minimization of agricultural wastes and their use to improve the quality of life are among the major challenges of present times. The incineration of agricultural wastes is leading to serious environmental problems, such as greenhouse gases, smog, and water pollution. Water pollution is further triggered by household washing powders and industrial activities. Agricultural wastes can be utilized on a commercial basis for water treatment. The resulting adsorbents are mostly environmentally friendly due to their biodegradable, and in most cases reusable, features. However, the chemical modifications may also be hazardous. Valuable work has been done by many researchers in this field. In our view, more agricultural materials could be explored and utilized for the preparation of inexpensive and modified agricultural materials, applying graft copolymerization, carboxylated dissolving pulp, ligand doped pulp, activated carbon, or through the sol-gel method. The adsorbents have shown significant contributions to the removal of contaminants from aqueous solutions. The adsorption efficiencies may vary depending on the nature of the adsorbents and could be affected by surface area, functional groups, pH, initial concentration, and temperature. More efforts are needed to introduce a simple method for the modification of agricultural wastes to develop water purification filters, which can be easily processed by the common man indigenously. It is necessary that the adsorbents should be applied for real samples instead of synthetic aqueous solutions of choice. Such materials must be cheaper, easily prepared, renewable, and environmentally friendly for waste water treatment domestically and industrially.

Funding

The authors extend their appreciation to the Deputyship for Research and Innovation, Ministry of Education in Saudi Arabia for funding this research work through the project number INSTV008.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Cellobiose unit of cellulose polymer.
Figure 1. Cellobiose unit of cellulose polymer.
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Figure 2. Transformation of cellulose into its various polymorphs.
Figure 2. Transformation of cellulose into its various polymorphs.
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Figure 3. Glucomannan from angiosperms.
Figure 3. Glucomannan from angiosperms.
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Figure 4. Lignin building blocks.
Figure 4. Lignin building blocks.
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Figure 6. Positions in cellulose structure for chemical modifications.
Figure 6. Positions in cellulose structure for chemical modifications.
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Figure 7. Modification of agricultural wastes to different adsorbent.
Figure 7. Modification of agricultural wastes to different adsorbent.
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Figure 8. Synthesis of graft copolymers from (agricultural wastes) pulp.
Figure 8. Synthesis of graft copolymers from (agricultural wastes) pulp.
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Figure 9. Methods for the synthesis of graft copolymers from dissolving pulp, where nX stands for monomer units and Cell-OX stands for grafted cellulose.
Figure 9. Methods for the synthesis of graft copolymers from dissolving pulp, where nX stands for monomer units and Cell-OX stands for grafted cellulose.
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Table
Table 1. Contents of the main components, % of dry wood [26].
Table 1. Contents of the main components, % of dry wood [26].
CelluloseGlucomannanXylanOther PolysaccharidesLignin
Soft wood33–4214–2005–113–927–32
Hard wood38–5101–4.014–302–421–31
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Noor, A.; Khan, S.A. Agricultural Wastes as Renewable Biomass to Remediate Water Pollution. Sustainability 2023, 15, 4246. https://doi.org/10.3390/su15054246

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Noor A, Khan SA. Agricultural Wastes as Renewable Biomass to Remediate Water Pollution. Sustainability. 2023; 15(5):4246. https://doi.org/10.3390/su15054246

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Noor, Awal, and Sher Ali Khan. 2023. "Agricultural Wastes as Renewable Biomass to Remediate Water Pollution" Sustainability 15, no. 5: 4246. https://doi.org/10.3390/su15054246

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