Customized High-Value Agricultural Residue Conversion: Applications in Wastewater Treatment

Abstract

Wastewater treatment methods based on catalytic technology are receiving more and more attention in recent years. In order to achieve the high-value utilization of agricultural residues, the development of greener, more efficient, and economically favorable catalysts is the focus of this review. The main contents include: (i) discussions around the differences in surface morphology, chemical stability, electrical conductivity, porosity, and the specific surface area of the prepared biochar according to the biochemical composition of different agricultural residues; (ii) through the above comparison, the shortlisting and selection of suitable catalysts for the preparation of agricultural residues to improve the adsorption and catalytic performance; (iii) discussions around the interaction mode and catalytic mechanism between the surface properties of biochar and pollutants; (iv) based upon future needs, discussions around making full use of the distribution of the pyrolysis products of agricultural residue, and designing biochar-based catalysts to achieve the efficient degradation of pollutants in wastewater. The review aims to provide a waste disposal alternative for the alleviation of environmental pollution and the promotion of green chemistry as well as sustainable development.

1. Introduction

Water resources are one of the world’s most pressing environmental issues [1]. The provision of safe and clean water is an extremely challenging task all around the world [2]. In recent years, the problem of the pollution of natural resources has become increasingly serious and needs to be solved as a priority to safeguard the future of the earth and mankind. In particular, freshwater resources are under threat, as untreated or inadequately treated wastewater discharges are rapidly polluting freshwater resources [3]. Currently, traditional wastewater treatment technologies include coagulation/flocculation, ion exchange, floatation, reverse osmosis, oxidation, adsorption, membrane separation, ultrafiltration, precipitation, electroprecipitation, and advanced oxidation [4,5]. However, it is quite difficult for the traditional wastewater treatment and purification methods to meet the accurate or cost-effective discharge standards [6,7]. Therefore, it is necessary to develop new wastewater treatment technologies and methods to meet the growing demand for the sustainable utilization of water resources and protecting the environment.

Catalysis is one of the most important foundations of green chemistry. The use of various new catalysts in the deployment of treatment systems can be beneficial in terms of increased degradation rates, reduced energy consumption, and the safety of the reaction environment/conditions [8]. For various wastewater-containing refractory substances, wastewater treatment methods based on catalytic technology are receiving more and more attention [9,10]. It is important to note that the expensive catalysts used to achieve this technology can be effectively replaced by materials from low-cost sources, such as renewable alternatives or waste materials. Therefore, the development of “greener”, efficient, and economically favorable catalysts is considered to be a key factor in the treatment of refractory wastewater [11].

The use of biomass/agricultural residue as a source of catalyst/adsorbent is gaining attention due to its unique chemical composition, wide range of sources, low cost, and sustainability [12,13]. For example, nearly one-third of the food produced globally (about 1.3 billion tons per year) was wasted or discarded, accounting for around 44% of the total global solid waste [14]. If there are no effective measures to deal with the waste resources, it will have a significant impact on the entire ecosystem. Therefore, during the treatment of wastewater using catalysis, it is very meaningful to pay attention to the conversion and utilization of resources and energy as well as ecological benefits to achieve the sustainable management of solid wastes [15].

In order to turn waste into wealth, researchers have used agricultural residue to prepare various carbon-based materials, such as corn cobs [16], corn stalks [17], waste tea leaves [18], and sugarcane residues [19]. These carbon materials can be used as catalysts/adsorbents [20] because of their abundant oxygen-containing functional groups (OFGs), high specific surface area (SSA), and strong cation-exchange capacity (CEC) [21]. The preparation of carbon-based materials from agricultural residue has mainly been conducted in the form of thermochemical transformation (pyrolysis, thermal cracking, and hydrothermal carbonization) by adjusting various pyrolysis parameters (pyrolysis temperature, reaction time, and residence time) and/or by post-treatment (modification and activation) to improve its physicochemical properties (porosity and oxygen-containing functional groups (OFGs)), high specific surface area (SSA), and strong cation-exchange capacity (CEC)), thereby increasing its catalytic capability [22,23]. Advanced oxidation processes (AOPs) have been widely used to remove refractory organic pollutants in wastewater [24]. Many previous reports have illustrated the potential application of green catalysts for the synthesis of agricultural residue in the field of sewage remediation [25]. Nidheesh et al. discussed the degradation effect and potential mechanism of sulfate-based deep oxidation on textile wastewater [26]. Kumar et al. reviewed the application and mechanism of biochar-supported deep oxidation technology in the removal of organic pollutants from water and wastewater [27]. However, previous reports did not discuss and summarize the biochemical composition of agricultural residue and the characteristics of the resulting biochar as a green catalyst for the degradation of pollutants in aquatic environments.

Therefore, the main contents of this review include: (i) discussions around the differences in surface morphology, chemical stability, electrical conductivity, porosity, and the specific surface area of the prepared biochar according to the biochemical composition of different agricultural residues; (ii) through the above comparison, the shortlisting and selection of suitable catalysts for the preparation of agricultural residues to improve the adsorption and catalytic performance; (iii) discussions around the interaction mode and catalytic mechanism between the surface properties of biochar and pollutants; (iv) based upon future needs, discussions around making full use of the distribution of pyrolysis products of agricultural residue, and designing biochar-based catalysts to achieve the efficient degradation of pollutants in wastewater. In short, the publication of this manuscript will help in solving the problems of the utilization of waste resources and the remediation of environmental pollution, thus promoting the process of green chemistry and sustainable development.

2. Biochemical Composition of Agricultural Residue

Agricultural residue comes from a wide range of resources and mainly consists of the following four components: cellulose (38–50%), hemicellulose (23–32%), lignin (15–25%), and protein (1–30%) [28]. Within the temperature range of 200–260 °C, hemicellulose begins to react, followed by the decomposition of cellulose at 240–350 °C, and the decomposition of lignin at 280–500 °C [29]. The pyrolysis products of cellulose and hemicellulose are mainly polysaccharides, which have a high oxygen content and reactivity. Due to this reason, the pyrolysis reaction of polysaccharides is relatively fast [30]. In contrast, the pyrolysis process of lignin is complex. Its macromolecular structure is interrupted, and there are more reaction products, making the differential thermogravimetric curve (DTG) wider [29]. The reaction-pathway-level products of the pyrolysis processes of cellulose, hemicellulose, lignin, and protein are shown in Figure 1 [31].

Cellulose is a linear macromolecular polysaccharide that is linked through β-1,4-glycosidic bonds [31]. Under the high-temperature condition of 300 °C, cellulose decomposes and polymerizes to produce low-molecular-weight compounds, such as furfural, furan, glycolaldehyde, formic acid, H₂O, and CO₂ [32]. In addition, anhydrous sugars are also formed during pyrolysis, such as levoglucosan, which is formed by the cleavage of glycosidic bonds and dehydration reactions [32]. If pyrolysis at a high temperature (>300 °C) is continued, levoglucosan will be further hydrated to generate levoglucosone, which will be converted to more stable oxygenated compounds such as furfural, 5-hydroxymethylfurfural, and pyran through the cyclization reaction [31,32]. These reactions are accompanied by the generation of functional groups, such as carboxylic acid (–COOH), ester (–COO–), ketone (–C=O), hydroxyl (–OH), and ether (C–O–C) groups [31]. When the temperature exceeds 800 °C, the cellulose will be completely carbonized, enhancing the aroma of the biochar [33]. During this process, decarboxylation and decarbonylation reactions occur in parallel, resulting in the production of a series of compounds [33]. The pyrolysis pathway of cellulose and the obtained products are shown in Figure 1a.

The pyrolysis behavior of hemicellulose is similar to that of cellulose. However, the types and proportions of the products may be different for the two. The pyrolysis products usually include phenolic, aldehyde, ketone, and acidic compounds (Figure 1b) [34].

Lignin is a complex three-dimensional amorphous polymer composed of three phenylpropane units: p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) [35]. These units are linked together by carbon–carbon and ether bonds to form a long-chain structure [35]. The conversion temperature for the propyl chain of lignin is about 180 °C [36]. Among the chemical bonds connecting the monomer units, the ether bond on the alkyl chain is considered unstable and can be broken at 200 °C [36]. The chemical groups formed after the cleavage of these ether bonds can recombine into oxygenates, such as CO, CO₂, and H₂O [37]. When the temperature exceeds 300 °C, the carbon–carbon bonds inside the alkyl chain will decompose into small-molecular-weight compounds, such as methane, acetaldehyde, or acetic acid [36]. As the temperature increases, the methoxy group adjacent to the hydroxyl group will be broken and replaced by –OH, –CH₃, or –H [38]. When the temperature exceeds 450 °C, most of the original bonds among the monomer units are broken [36]. As the temperature further increases, the degree of carbonization of lignin will also increase, and noncondensable gases (such as CO and H₂) will be produced through the hydrogenation and substitution reactions of the ether bonds (Figure 1c) [36,37,38].

The decomposition products of protein during pyrolysis include aromatic hydrocarbons, such as benzene, phenol, m-cresol, toluene, xylene, nitriles, and alkenes, and nitrogen-containing heterocyclic compounds, such as graphitized-N and oxidized-N, which occur through decomposition, deoxygenation, and deamination reactions [39,40]. The pyridine-N, pyrrole-N, graphitized-N, and oxidized-N in the product stream can improve the catalytic performance of carbon-based materials by changing the electronic structure and chemical activity of carbon materials [41]. Protein-pyrolysis products have been recognized as the most important source of endogenous N-doping in carbon materials.

In summary, the properties and uses of biochar are influenced by its source materials and preparation methods [42]. Biochar prepared from lignin will have a higher ash content, and the particle-agglomeration reaction will lead to a decrease in pore volume [34,35]. However, biochar prepared from cellulose has high thermal stability and provides the basis for a carbon-fixing framework [30,31]. Although the content of functional groups on the surface of biochar prepared from cellulose is less, it has more pores and a higher surface area, which improve its adsorption efficiency [30,31]. As a raw material for endogenous N-doping, protein has unparalleled advantages in the preparation of biochar, which can improve the adsorption performance and catalytic activity of biochar [40]. These differences can be optimized by adjusting the preparation methods of biochar and the conditions of these preparatory methods to obtain the desired properties and uses [41].

3. Removal of Emerging Pollutants from Sewage

3.1. Photocatalysis

Biochar has unique physical and chemical properties that make it play an important role in the photolysis cycle. Biochar itself has semiconductive properties and can act as a conductive platform to facilitate the shuttle flow of electrons [43]. It can also effectively suppress the charge recombination effect. The introduction of biochar can also increase the porosity and specific surface area of a catalyst. In particular, according to the pyrolysis characteristics of the biochemical composition of agricultural residue, the prepared biochar has an adjustable and adaptable surface pore structure, making it suitable for the support of various light-sensitive components [44]. Lisowski et al. prepared TiO₂/carbon photocatalytic materials under low-temperature ultrasonic conditions and successfully applied them to the degradation of phenol in water [45]. In their study, a carbon skeleton was prepared using cork particles as a raw material. The introduction of the carbon skeleton improved the porosity, specific surface area, and binding capacity of the catalyst. However, more importantly, the use of the carbon skeleton reduced the band gap, enhanced the electron-shuttle flow, and suppressed the charge recombination effect [46]. In addition, ultra-low-temperature conditions were used to promote the growth of crystalline TiO₂ on the carbon skeleton. The rich pore structure of lignocellulosic biochar effectively prevented the leaching of titanium and the loss of the specific surface area. The research results showed that TiO₂/carbon photocatalytic material can be efficiently recycled multiple times for the degradation of phenol (Figure 2a) [45]. TiO₂/carbon also has advantages for the degradation of perfluorooctanoic acid. TiO₂/carbon was able to degrade more than 90% of Pfos after the ultraviolet irradiation (254 nm) for 4 h and mineralize 62% of fluoride ions in the pH range of 4 to 8. The decomposition of PFOA is initiated by the hollow oxidation, which activates the molecules, leading to a series of decarboxylation, C-F-bond-breaking, and chain-shortening reactions. After six adsorption/photodegradation cycles, the adsorption capacity and photocatalytic activity of the materials were not decreased significantly [47]. Hu et al. prepared a photosensitive material using bean dregs as the source of the biochar, and dispersed LaMnO₃ nanoparticles on the porous biochar [46]. The results showed that the composite had strong degradation activity to Direct Green BE (GBE). The reason is that biochar made from the soybean dregs using pyrolysis had an obvious mesoporous structure and a larger specific surface area, which helped in releasing all the excited e⁻ and delaying the recombination with h⁺ [46]. This pore structure effectively becomes the separation barrier of e^×−/h⁺ as a charge carrier, and can also effectively regulate the storage and transfer of charges. Furthermore, graphitic-N, pyridinic-N, and pyrrolic-N on the surface of biochar are considered to be efficient active catalytic sites due to natural endogenous N-doping (Figure 2b) [46]. In short, the use of okra biochar as a catalyst support has the obvious ability to stabilize the photoactive components without the need for postfunctionalization [46]. Hou et al. successfully improved the efficiency of the photocatalytic removal of Cr(VI) and phenol by preparing a soybean-straw biochar/TiO₂ composite catalyst [26]. The removal rates of Cr(VI) and phenol using TiO₂/D500 were 80.4% and 77.7%, respectively, which are higher than those of D500 and TiO₂ [26]. This difference is mainly due to the increased generation of photoexcited electrons due to the coupling of TiO₂ with D500 and the reduced recombination of e⁻-h⁺ pairs (Figure 2c) [26]. In addition, D500 stores electrons under illuminated conditions and releases electrons under dark conditions, further enhancing the activity of the catalyst. When D500 is recombined with TiO₂, the electrons on D500 can activate the catalytic redox activity of TiO₂, thereby effectively removing the pollutants under dark conditions. Meanwhile, TiO₂/D500 showed good reusability and stability characteristics [26]. Loading g-C₃N₄ on biochar can significantly improve the photocatalytic activity and quantum efficiency of g-C₃N₄. Meanwhile, it can also show good photoresponsiveness, whereas the decolorization effect on methylene blue is obvious. However, when g-C₃N₄ was used alone, this excellent photoresponsiveness was not observed [48]. Furthermore, Wang et al. used waste bamboo as a raw material and directly converted it into carbon spheres using a two-step hydrothermal carbonization (HTC) technology at 180–200 °C. Subsequent modification of g-C₃N₄ through protonation on carbon spheres (BS) was used to prepare a metal-free photocatalyst with a unique core–shell structure [49]. The results showed that these biochar spheres had the dual functions of reservoirs and sensitizers. Their core–shell heterostructure could improve the light absorption and the separation efficiency of the photogenerated electron–hole pairs of the photocatalysts [49]. In addition, biochar spheres have excellent charge-storage capacity, which can prolong the lifetime of electrons and improve the transport efficiency of carriers [49]. These characteristics render this metal-free photocatalyst with excellent catalytic activity in the degradation of organic pollutants. Fang et al. simulated the photogeneration of reactive oxygen species (ROSs) during the degradation of diethyl phthalate (DEP) using wheat-straw-derived biochar under ultraviolet light and simulated solar illumination [50]. Under ultraviolet and simulated sunlight irradiation, hydroxyl radicals (−OH) and singlet oxygen (¹O₂) were the main reactive oxygen species that destroyed the structure of the DEP molecules and promoted their mineralization (Figure 2d) [50]. The maximum rate of mineralization was 61.8% (after 8 h of ultraviolet exposure). In addition, the organic component was able to generate 86.3% of the singlet oxidation, whereas the carbon matrix mainly provided 74.6% of the formation of hydroxyl radicals [50].

3.2. Redox Catalysis

After converting agricultural residue into biochar, a structured and controllable preparation can be achieved by tuning the functional groups on its surface. By optimizing different functional groups, the surface properties of biochar, such as the hydrophilicity and nucleophilicity, can be altered [51]. This directional modification/preparation of biochar has abundant and evenly distributed active sites. These active sites can provide abundant catalytically active centers, thereby enhancing the reactivity and selectivity of biochar in catalytic reactions.

Biochar has great potential as a catalyst for the oxidation of organic pollutants in perhydrosulfate (PDS) activation [52]. Chen et al. used corn stover and melamine as raw materials to prepare nitrogen-doped biochar (N-biochar) (Figure 3a) [53]. Due to high-temperature pyrolysis and ball milling, the obtained N-biochar had the characteristics of a large specific surface area, abundant oxygen-containing groups, and obvious structural defects. In the N-biochar/PDS system, the degradation efficiency of 2,4-dichlorophenol (2,4-DCP) reached 98.4% within 40 min (Figure 3b) [53]. In another PDS-assisted degradation of sulfamethoxazole in malt-root and algae-residue biochars [54,55], it was found that the activation of PDS occurred on the surface moieties and complexes. For samples prepared at 900 °C, the rate constant of the degradation can be increased by 35% through the coupling effect of ultrasound or solar irradiation. Avramiotis et al. studied the degradation effect of sulfamethoxazole (SMX) using a rice-hull-organism/PS system [56]. The rice husk was continuously heated at 850 °C for 1 h, and the generated biochar achieved a 100% degradation effect on the SMX within 1 h. Li et al. converted rice husk and sawdust into biochars [57]. The catalytic degradation of these two biochars on acid orange 7 (AO7) were also evaluated. The results showed that the rate of removal of the sawdust biochar was higher than that of the rice-husk biochar. The specific surface areas and pore volumes of the two biochars were not significantly different from each other. However, the high content of lignin in the wood chips pyrolyzed to form a large number of oxygen-containing functional groups, especially carboxyl and phenolic hydroxyl groups, which are up to twenty-times higher than those of the rice-husk biochar (Figure 3c). Generally speaking, the content of lignin-derived carbon is 89 wt.%, which mainly exists in the form of C–C [58,59]. The surface of biochar includes functional groups, such as phosphoric acid, amine, peroxide, an ether bridge, carboxylic acid, carboxylic anhydride, pyran, the lactone group, the phenol group, and dioxin, making its electronegativity adjustable within a wide range [60]. Biochar prepared by the pyrolysis of lignin is a more effective activator of persulfate than the biochar derived from cellulose.

3.3. Electrocatalysis and Other Joint Technologies

Biomass biochar is a favorable electrode material [61,62]. Biochar prepared from agricultural residue has a porous structure and shows an encouraging electrical conductivity and capacity [63,64]. The application of fabricating electrochemical catalytic sensors adds more value to otherwise wasted crop residues.

The ultimate goal of a wastewater treatment system is to remove pollutants and recycle water while minimizing energy consumption and chemical use [65,66]. To this end, the combination of biofilter and microbial electrochemical technology has been proposed. A recent report pointed out that, in the METland^® pilot plant, the developed wood-derived biochar was an effective material for microbial beds in both the nonpolarized and polarized configurations, with good chemical oxygen demand (COD) removal [67]. In actual influent water, when the COD is 890 mg L⁻¹, the maximum COD removal efficiency (92%) and rate of degradation (185 gm⁻³d⁻¹) can be achieved at an anodic potential of up to 0.6 V [68]. The remarkable performance of biochar is mainly due to its abundant electroactive surface oxygen functional groups, which can reversibly exchange electrons through a battery mechanism (Figure 3d) [67,69]. The high efficiency and sustainability of biochar render it with broad application prospects in the large-scale application of METs [68].

Figure 3. (a) The process of preparing nitrogen-doped biochar with corn stalk and melamine as raw materials [53]; (b) Degradation pathway of 2,4-dichlorophenol in the N-BC/PDS system [53]; (c) The surface of biochar formed by lignin pyrolysis is rich in oxygen-containing functional groups [57]; (d) Wood-derived biochar is an effective material for the microbial bed in both nonpolarized and polarized configurations, and has a good removal effect on the COD [67].

Figure 3. (a) The process of preparing nitrogen-doped biochar with corn stalk and melamine as raw materials [53]; (b) Degradation pathway of 2,4-dichlorophenol in the N-BC/PDS system [53]; (c) The surface of biochar formed by lignin pyrolysis is rich in oxygen-containing functional groups [57]; (d) Wood-derived biochar is an effective material for the microbial bed in both nonpolarized and polarized configurations, and has a good removal effect on the COD [67].

The surface properties of biochar are adjustable. However, the complex physicochemical properties of its raw materials are still the main obstacle in further optimizing its performance. Nonetheless, the low cost of biomass feedstock makes it a viable alternative to industrial catalysts. In particular, biochar has been widely used in coupled designs, which can be used as a biological anode/cathode and semiconductor anode/cathode, or semiconductor–microbe composite electrodes, and can act as photoactive components and ultrasonic media [70]. Therefore, an in-depth understanding of the mechanisms of the strengths and weaknesses of a particular biochar can validate mission-fit principles and improve engineering strategies to more effectively exploit its advantages [71]. Biochar prepared from agricultural residues to remove pollutants from sewage is shown in

Table 1. Biochar prepared from agricultural residues removes pollutants from sewage [27].

Biomass	Pollutant	Operating Conditions	Removal Efficiency (%)	Reference
Soybean dregs	Green BE	Green BE = 20 mg/L, pH = 10, Pyrolysis Temp. = 800 °C	90%, 3 h	[45]
Soybean stalks	Cr(VI) and phenol	Cr(VI) = 50 mg/L, phenol = 100 mg/L, pH = 2, Pyrolysis Temp. = 500 °C	77%, 80.4%, 1 h	[26]
Bamboo	Tetracycline	tetracycline = 10 mg/L, Pyrolysis Temp. = 200 °C	90%, 1 h	[47]
Pine needles	Diethyl phthalate	diethyl phthalate = 20 mg/L, pH = 2, Pyrolysis Temp. = 200 °C	60%, 8 h	[48]
Wheat straws	Diethyl phthalate	diethyl phthalate = 20 mg/L, pH = 2, Pyrolysis Temp. = 200 °C	45%, 8 h	[48]
Corn stover	2,4-dichlorophenol	2,4-dichlorophenol = 100 mg/L, pH = 3–9	98.4, 40 min	[51]
Rice husk (RH)	Sulfamethoxazole (SMX)	PS = 500 mg/L, RH = 100 mg/L, pH = 4–6, Pyrolysis Temp. = 850 °C	96%, 120 min	[54]
Rice husk (RH)	Acid orange 7 (AO7)	AO7 = 50 mg/L, Pyrolysis Temp. = 700 °C, PS/AO7 = 40/1, pH = 3	100%, 2 h	[57]
Sawdust (SD)	AO7	AO7 = 50 mg/L, Pyrolysis Temp. = 700 °C, PS/AO7 = 40/1, pH = 3	100%, 2 h	[57]
Corn stalk biochar	Humic acid	CB = 0.5 g/L, PS = 1 mM, Pyrolysis Temp. = 900 °C, HA = 100 mL, pH = 5.0	84.3%, 180 min	[72]
Cereal residue	2,4-dichlorophenoxyacetic acid	[H2O2] = 5 mmol/L, [BC] = 3 g/L, [2,4-D] = 20 mg/L, pH = 3	94.8%, 60 min	[73]
Peanut shell biochar	Sulfamethoxazole	[H2O2] = 1 mM, [BC] = 0.5 mg/L, pH = 9, Pyrolysis Temp. = 700 °C	56.9%	[74]
Sunflower husk	Benzo[a]pyrene	[H2O2] = 1.25 M, [BC] = 5 % w/w, hematite = 2 mg/g, Pyrolysis Temp. = 500 °C	95%	[75]
Rice husk	Benzoic acid	[BA] = 1 mM, [BC] = 10 g/L, pH = 3, Pyrolysis Temp. = 550 °C	95.2%, 6 h	[76]
Rice husk	Acid red1	[AR1] = 50 mg/L, [H2O2] = 16 mM, [BC] = 5 g/L, pH = 3	97.6%, 2 h	[77]

.

4. Key Factors and Mechanism of Catalysis

Understanding the physicochemical properties and biochemical behavior of agricultural-residue biochar can help explain and predict its catalytic reaction process. The controlling factors of the surface properties of biochar include the intrinsic redox potential, charge-transfer ability, conjugate conductivity, heterojunction interface, and structure and type of functional groups [78,79]. When exploring their catalytic properties, electron storage and charge transfer are critical factors, and a good interaction mode between them is the key to expanding the active sites [80,81]. Key factors and the mechanism of catalysis are shown in

Table 2. Key factors and the mechanism of catalysis [68].

Biomass	Pollutant	Removal Performance	Driving Factor	References
Wheat straw	Hexachlorobenzene, Dehalococcoidaceae;	Up to 56%	Carbon-centered PFRs	[82]
Kapok-derived	Bisphenol A	91%	Direct h+ oxidation; large surface junctions; promoted separation and transfer of e−/h+ pairs	[83]
Pine needles, wheat straw	Diethyl phthalate	76% (2 h)	e− transfer; conjugated network-confined PFRs; photo-Fenton; secondary formation of ROSs.	[47]
Soybean dreg char@LaMnO3	Direct Green BE	80%; Reuse: 4 cycles	Separation of e−/h+ pairs; h+ oxidation; O2∙− attack	[44]
Platanus acerifolia	Sulfadimidine,	40% (in dark), 50% (daylight)	PFRs, H2O2 and secondary ROSs (mainly ∙OH)	[84]
Rice straw	Tetracycline	40% TOC removal in 1 h	g-MoS2-enhanced surface adsorption and graphitization, Mo6+-induced PFR formation; h+ and ∙OH are predominant active species	[78]
Pine needles	2,4,4′-Trichlorobiphenyl, PDS	70–100%	PFRs-activated SO4∙− formation via e− transfer; metal species activators	[81]
Rice straw	Aniline, PDS	94.1%	Generation of h+	[85]
Oak wood	DDT, DDD, DDE, sulfide		e− transfer; carbon–sulfide intermediates	[86]
Lignin and cellulose	p-Nitrophenol	60%	PFRs direct reaction; nonradical sites	[87]
Rice straw	TNT, RDX, DBP, DFP, DCP	99% (TNT); 70% (RDX); 65% (DBP, 24 h); 70% (DFP, 24 h)	Large SBET; redox potential of Fe(0)	[88]
Rice hull	Trichloroethylene, PDS	99.4%	e− transfer and redox iron species	[89]
Rice husk	Nonylphenol, PDS	96.2%; 73.4% mineralized; reuse: 5 cycles	e− transfer and redox iron species	[90]

.

4.1. Active Substance and Mechanism Analysis

This paragraph summarizes the recent work by Mian et al., who explored the characteristics of active species in different photocatalysts (such as TiO₂, ZnO, ZrO₂, CdS, and BiOX) supported on biochar [91,92,93]. On this basis, after assembly with biochar, the asymmetric electron density of the doped carbon sites enhanced the catalytic activity under visible light [94]. Oh et al. used rice straw and Fe(0) as raw materials to synthesize a new catalyst-biochar-coated zero-valent iron (g-MoS2/PGBC) for the removal of nitroexplosives and halogenated phenols in polluted water (Figure 4a) [94]. Compared with the direct reduction of Fe(0), the reduction and transformation of Fe(0) coated with biochar is better. The study found that, when methanol and formaldehyde were used to block the surface function of biochar, it could effectively prevent the electron transfer of biochar during the degradation of nitrotoluene/halophenol [94]. This indicated that the functional groups on the surface of biochar participated in the catalysis of the electron transfer process, thereby enhancing its degradation effect [94]. Biochar made from agricultural residue can be relatively easily constructed as a controllable charge trap to suppress the recombination of the charge for redox reactions involving H⁺ or e⁻. On the one hand, the e⁻ generated by light is injected into the conductive band of biochar, followed by the release to the surface to react with O₂ and produce O₂∙ [94]. On the other hand, H⁺ accelerates the redox process of pollutants by increasing the catalytic activity under visible light (Equations (1)–(3)) [94].

O₂ + 2H⁺ + 2e⁻ → H₂O₂

(1)

H₂O₂ + e⁻ → OH- + ∙OH

(2)

–OH + H₂O₂ → ∙OH + O˙

(3)

In addition, the adsorption capacity of g-MoS₂/PGBC on TC in a tap-water system is slightly lower than that of deionized water due to the influence of the pH value. After several cycles, the removal rate of TC by g-MoS₂/PGBC is still maintained at nearly 70% of that in the actual water recycling treatment. Fang et al. investigated the effect of the loading of metals (Fe³⁺, Cu²⁺, Ni²⁺, and Zn²⁺) and phenolic compounds (PCs-hydroquinone, catechol, and phenol) on the formation of persistent free radicals (PFRs) in biochar [95]. The results of this study demonstrated that the treatment of metals and phenolic compounds not only increases the concentration of persistent free radicals (PFRs) in biochar, but also changes the types of formed PFRs [96]. Therefore, this study suggests controlling the contents of metals and phenolic compounds in biomass to regulate the formation of PFRs in biochar. Moreover, biochar is also rich in dissolved organic matter (DOM), which can trigger the stronger evolution of hydroxyl radicals induced by singlet oxygen and surface-confined persistent free radicals (PFRs), thus exhibiting stronger reactivity [96,97]. Additionally, in biochar/UV systems, H₂O₂ participation pathways dominate. This phenomenon enables the coupling of biochar/irradiation with electro-Fenton, photo-Fenton, and/or radical-assisted processes [98,99]. Selecting an appropriate charge-separation mechanism provides useful design guidance for the optimization of catalytic reactions, thereby enhancing the reaction efficiency and controlling the reaction selectivity (Equations (4) and (5)) [100].

PFRs H⁺ PFRs

Diffusive O₂ uptake → O₂˙⁻ → H₂O₂→ ∙OH

(4)

PFRs H⁺ Metal species

Diffusive O₂ uptake → O₂˙⁻ → H₂O₂ → ∙OH

(5)

4.2. Persistent Free Radical

The thermal cracking process of organic compounds in biomass is a prerequisite for the existence of PFRs [101]. The detection of electron paramagnetic resonance (EPR) showed that O-centered PFRs were more inclined to transfer electrons to PDS than C-centered PFRs, and could further be activated to form ROSs [102]. The results showed that, during the process of the biochar-catalyzed persulfate degradation of PCBs, the concentration and type of PFRs were the main factors affecting the catalytic capability of biochar to activate persulfate [102]. In addition, the superoxide radical anion accounted for 20–30% of the generated sulfate radicals in the biochar/persulfate reaction system (Figure 4b) [96]. Among other functions, PFRs can absorb and degrade nearby H₂O₂ to generate hydroxyl radicals through one-electron reductive-annihilation reactions [89]. Huang et al. found that, in the biochar/H₂O₂ system, the degradation pathway of tetracycline involves the generation and reaction of free radicals. Through EPR capture experiments and linear sweep voltammetry measurements, the researchers identified the main active radical as the hydroxyl radical (∙OH). In addition, the study also shows that the electron transfer pathway may be the main pathway of H₂O₂ activation catalyzed by biochar [103,104]. Ding et al. investigated the photodegradation process of common plastic materials, such as polyethylene (PE) and polypropylene (PP), mediated by iron (hydrogen) oxides (goethite and hematite), and found that, under light, the iron (hydrogen) oxides react with O₂ to produce H₂O₂, which promotes the release of Fe²⁺, and then a light-driven Fenton reaction occurs, producing a large amount of ∙OH. This leads to the efficient simplicity for PE and PP [105]. Sun et al. showed that the cotton-straw biochar/H₂O₂ reaction system could effectively remove sulfonamide antibiotics from the synthetic urine matrix [106]. The experimental results showed that, in the synthetic urine matrix, the cotton-straw biochar/H₂O₂ reaction system could effectively degrade sulfa antibiotics such as sulfamethoxazole, sulfadiazine, sulfapyrimidine, and sulfadimethoxine. Carbonate radicals produced by biochar-catalyzed peroxymonocarbonate are the main contributors to the reaction system [106]. In the presence of biochar, H₂O₂ reacts with bicarbonate to form peroxymonocarbonate. When biochar donates an electron to peroxymonocarbonate, a powerful carbonate radical is generated (Equations (3) and (4)), which further oxidizes the substrate. This process is an interesting example of how biochar can mediate secondary redox reactions between reactants, substrates, and background entities (Equations (6) and (7)) [106].

HCO₃⁻ + H₂O₂ → HCO₄⁻ + H₂O

(6)

HCO₃⁻ + e⁻ → CO₃˙⁻ + OH⁻

(7)

Biochar prepared from agricultural residue has an irregular electronic structure, which will lead to differences in the nucleophilic and electrophilic interactions between catalysts and pollutants at different positions. Previous studies have shown that nitrogen doping at the edge of the carbon network of biochar can significantly affect its electronic structure [107]. Wang et al. produced nitrogen-doped biochar (N-biochar) using the pyrolysis of corncob biomass and urea that were used in different proportions [107]. The results showed that nitrogen-doped biochar (N-biochar) exhibited good catalytic performance in catalyzing the degradation of sulfadiazine (SDZ) by peroxodisulphonate (PDS). The study also revealed edge nitrogen structures, mainly the pyridinic and pyridinic nitrogen, as efficient activation sites for binding to PDS [107]. In addition, this study confirmed the existence of a nonradical mechanism (¹O₂) dominated by electron transfer in the N-biochar/PDS system. Moreover, N-biochar, as an electron mediator, plays an important role in promoting the electron transfer from SDZ to PDS (Figure 4c) [107]. After three cycles, the removal rate of SDZ decreased from 96.5% to 83.0% [107]. Typically, the reversible charge-transfer capacity and number of redox cycles are limited when nitrogen-doped biochar electrodes are fabricated at moderate temperatures [95]. In contrast, high-temperature treatment makes the biochar highly graphitized without producing nitrogen-doped biochar with semiconducting properties [108]. Ho et al. measured the electrical conductivity of algae-derived biochar and “graphite biochar”, and revealed the activation mechanism of nonradical PDS mediated through the electron-shuttle mechanism [55]. According to electrochemical impedance spectroscopy analysis, the electrical conductivity of algae-residue biochar treated at a temperature of 400 °C is significantly lower than that of high-temperature-treated biochar, indicating that the “graphite biochar” was prepared at 900 °C (Figure 4d) [55]. This study demonstrates that the conductivity of algae-derived biochars is closely related to their performance in nonradical processes. More specifically, the higher the conductivity and degree of graphitization, the better the electron-transfer-mediated activation of electron acceptors.

PFRs consist of various bulky aromatic structures and are contained within carbonaceous solids. Compared with short-lived ROSs, PFRs have a more complex structure, which allows them to have multiple nonselective activation pathways and a broad product distribution. PFRs can trigger powerful ring-opening and molecular-cleavage reactions by interacting with oxygen molecules in the environment [109,110]. SSA and the number of functional groups are considered as key parameters that control the formation and reactivity of PFRs. A larger specific surface area can provide more active sites, thereby increasing the chances of the generation of free radicals in the reaction.

4.3. Electrical Conductivity and Electron Reservoir

As mentioned in Section 4.2, the principle of the biochar redox catalyst is to use its internal redox center to promote the redox reaction through electron transfer. In addition, biochar has the ability to store electrons (Figure 4e) [95]. Generally speaking, biochar prepared at low temperatures can be used as an electron donor, and has a reducing capability, while biochar pyrolyzed at medium temperatures has a high electron-storage capacity and a reversible electron-supply-acceptance capability, as well as the ability to absorb and release electrons. However, due to the changes in functional groups and chemical structures, biochar treated at a high temperature has redox activity [111]. Prevoteau et al. measured the electron-accepting (EAC) and electron-donating (EDC) capabilities of pyrolyzed woody biochar at three different temperatures (400, 500, and 600 °C) using hydrodynamic electrochemical techniques [112]. They reported that the EDCs were as high as 7 mmol (e⁻)g_char⁻¹ at 400 °C, and 3.7 and 1.47 mmol (e⁻)g_char⁻¹ at 500 °C and 600 °C, respectively [112]. Moreover, biochar prepared under a relatively low temperature had a huge e⁻-donating capability. Wang et al. used a biochar (biochar)-induced Fe(III)-reduction persulfate (PS) activation system for the degradation of sulfamethoxazole (SMX) [113]. The results showed that the biochar prepared at 500 °C contained carbon-centered persistent free radicals as electron donors. Electrons are donated to Fe(III) from the c-centered PFRs, generating Fe(II) ions that decompose PDS into free radicals [113]. Scavenger-quenching experiments and electron spin resonance spectroscopy analysis confirmed the presence of sulfate radicals (SO₄˙⁻) and hydroxyl radicals (HO˙) in the biochar/Fe(III)/PS system, leading to the efficient degradation of SMX (Equations (8) and (9), Figure 4f) [113]. Charge transfer, surface functional groups, free radicals, and singlet oxidation often result in very mild and selective reactions to e− donor contaminants [114,115].

SO₄˙⁻ + S₂O₈²⁻ → S₂O₈˙⁻ + S₂O₄²⁻

(8)

HO˙ + S₂O₈²⁻ → S₂O₈˙⁻ + HO⁻

(9)

Fe(III) + S₂O₈²⁻ → Fe(II) + S₂O₈˙⁻

(10)

Figure 4. (a) Wood-derived biochar and metal-reducing bacterial GS–15 processes [94]; (b) Effects of biochar loaded with metals and phenolic compounds on the production of persistent free radicals [96]; (c) The existence of the nonfree radical mechanism dominated by electron transfer in the NBC/PDS system [107]; (d) Graphitic biochar with nonradical PDS activation mediated by the electron–shuttle mechanism [55]; (e) Biochar treated at different temperatures has different electron-donor–acceptor properties and REDOX activity [95]; (f) Biochar–induced Fe(III)–reduction persulfate system for the degradation of sulfamethoxazole [113].

Figure 4. (a) Wood-derived biochar and metal-reducing bacterial GS–15 processes [94]; (b) Effects of biochar loaded with metals and phenolic compounds on the production of persistent free radicals [96]; (c) The existence of the nonfree radical mechanism dominated by electron transfer in the NBC/PDS system [107]; (d) Graphitic biochar with nonradical PDS activation mediated by the electron–shuttle mechanism [55]; (e) Biochar treated at different temperatures has different electron-donor–acceptor properties and REDOX activity [95]; (f) Biochar–induced Fe(III)–reduction persulfate system for the degradation of sulfamethoxazole [113].

At high temperatures, the organic matter in biochar will pyrolyze and form a carbonaceous structure [116]. These carbonaceous structures have a high electrical conductivity and can transfer charges more efficiently, thus improving the conductivity of the electrode [117,118].

5. Conclusions

This review summarizes the research progress on the preparation of green catalysts using agricultural residues and discusses the applications of these catalysts in water and wastewater treatment. Biochar and its composites are considered to be environment-friendly and economical materials that can effectively remove various environmental pollutants through adsorption and catalysis. The following are the main findings of this study:

(1)

Biochar has the characteristics of being low-cost, having a simple preparation, a large specific surface area, a rich pore structure, and being rich in various functional groups. It is an important catalytic material for the removal of refractory organic pollutants;

(2)

The content of lignin is high in agricultural residue, whereas the prepared biochar will have a higher ash content. Moreover, the particle-agglomeration reaction will lead to a decrease in pore volume;

(3)

The content of cellulose is the highest in agricultural residue, whereas the prepared biochar has a high thermal stability and provides the basis of a carbon-fixation framework. Although the biochar prepared from cellulose has less functional groups on the surface, it has more pores and surface area, due to which it has a better adsorption effect;

(4)

The protein content is the highest in agricultural residue. As the raw material of endogenous N-doping, it introduces nitrogen vacancies and changes the electronic state of atoms, increasing the number of redox-active sites;

(5)

It has been shown that biochar prepared at 400 °C can be induced by PFR-based oxidation reactions, while increasing the temperature to 700–900 °C leads to singlet oxidation and nonradical pathways in defective graphitic regions through surface PDS complexes;

(6)

The higher the conductivity and degree of graphitization, the better the electron-transfer-mediated activation of electron acceptors;

(7)

Due to the changes in functional groups and chemical structures, biochar treated at a high temperature has redox activity.

6. Future Perspectives

Although there has been much research on the use of agricultural residue as a feedstock to produce biochar, and its application as a catalyst/catalyst support in water and wastewater treatment, there are still some gaps that need to be filled. Therefore, further research and investigation are needed. The main research areas to focus on include the following:

(1)

The surface properties of biochar prepared from agricultural residues are adjustable. However, the complex physicochemical properties of its raw materials are still the main obstacle to further optimize its performance. Therefore, an in-depth understanding of the mechanisms of the strengths and weaknesses of specific biochars can validate mission-appropriate principles and improve engineering strategies to more effectively exploit their advantages;

(2)

The nature of the active sites should be determined by analyzing the chemical composition of the waste sample. Moreover, its kinetic characteristics should also be analyzed under different conditions. The distribution of active sites to characterize their distribution in the waste needs to be studied. Through an in-depth study of the nature and distribution of active sites, utilization strategies for different wastes can be proposed;

(3)

Most of the studies are devoted to the catalytic removal of singular pollutants. However, actual wastewater is often a complex mixture of multiple pollutants. Therefore, it is necessary to improve the efficiency and effectiveness of wastewater treatment through the comprehensive design of catalysts, the study of reaction mechanisms, the development of multistage treatment systems, and the consideration of environmental factors to achieve more sustainable and environment-friendly wastewater treatment methods;

(4)

The stability and recyclability of catalysts are the key factors to determine the feasibility of their practical applications. At present, most of the research is limited to the laboratory scale, ignoring the problems faced in practical applications. However, a comprehensive assessment must be made considering all factors, including the feasibility of large-scale preparations, stability, and recovery and reuse.