Biochar Derived from Agricultural Residues for Wastewater Contaminant Removal

Abstract

The valorization of agricultural residues helps improve crop economic efficiency and alleviate environmental pressures. Owing to the merits of simplicity, high efficiency, low costs, and scalability, adsorption removal of contaminants using biochar has been widely investigated. The adsorption removal of organic and inorganic contaminants from wastewater using biochar derived from agricultural residue follows the principles of the circular economy and green chemistry, facilitating both environmental remediation and agricultural development. Due to the distinctive precursors—agricultural residues—biochar exhibits unique physicochemical properties, enabling it to interact differently with contaminants in real wastewater. Herein, this review addresses the knowledge gap in wastewater remediation using agricultural residue-based biochar. It compiles the principles of adsorption with agricultural waste-derived biochar, including general concepts, interactions between biochar and wastewater contaminants, and selective adsorption. The preparation, activation, modification, functionalization, and regeneration of such biochar, as well as their application to wastewater remediation, are comprehensively outlined. Furthermore, the economic evaluation and environmental impacts, as well as the future directions and challenges in this field, have also been presented.

1. Introduction

Wastewater from various industries poses a tremendous environmental problem and health risk to human beings and water ecosystems [1]. Abundant agricultural wastes are continuously produced globally. It has been reported that the dry biomass production is over 200 billion tons per year around the world, causing a huge agricultural burden [2]. The remediation of wastewater and the appropriate disposal of these agricultural wastes are of great significance to mitigate the aforementioned burden and address these issues.

Adsorption has been a widely applied physicochemical strategy for wastewater purification and the recovery of useful compounds from wastewater [3], owing to the various well-known merits, such as high cost efficiency, simple design and implementation, high efficiency, robustness, effectiveness, scalability, sustainability, technical flexibility, technological feasibility, and environmental friendliness, while being free of by-products and providing simultaneous removal of various contaminants and resilience against toxic contaminants [4,5,6]. On the other hand, the limitations of adsorption include the potential for secondary pollution, the production of solid wastes, and the requirement for additional treatment to degrade the adsorbed contaminants, among others [7,8,9,10]. Appropriate adsorbents are crucial to ensure excellent adsorption performance [3]. Currently, different adsorbents have been fabricated, such as biochar, activated carbon, zeolites, two-dimensional materials (e.g., graphene, carbon nanotubes, and MXene), resin, bentonite, covalent organic frameworks, metal–organic frameworks, and composites, among others [2,11,12]. Among them, carbon materials have attracted increasing attention in recent decades due to their abundant precursors and excellent physicochemical properties, such as numerous surface functional groups, high mechanical strength, and outstanding adsorption performance and reproducibility. As a type of carbon material, biochar is relatively simple and economical to produce and has been widely used for wastewater remediation. The main advantages of biochar compared to other advanced carbon materials, such as activated carbons, carbon nanotubes, and graphene oxides, include its high sustainability (e.g., waste-to-resource principles and renewable feedstock), low energy requirements and production costs, excellent scalability, and environmental co-benefits (e.g., large-scale applications in agriculture and carbon sequestration, as well as better compatibility with other materials) [7,13,14,15]. Agricultural wastes have abundant biomass (e.g., lignin, hemicellulose, and cellulose), which is accessible and affordable, and can provide sufficient carbon sources and thus can be used as green and non-toxic precursor sources of biochar. The preparation of biochar using agricultural wastes not only facilitates the valorization of these wastes but also reduces the environmental and economic burden associated with them (e.g., greenhouse gas emissions). Up to now, biochar has been prepared from date seeds [4], almond shells [16], coconut shells [5,17,18], wheat straw [19], rice husks [18,20,21,22,23], corn wastes [24,25,26,27,28], wood wastes [11,29], and rape stalk [30]. These abundant precursors give biochar various physicochemical properties and distinctive advantages compared to other types of biochar.

Table 1. Comparison of biochar derived from agricultural residues, woods, and manure.

Feature	Agricultural Residue Biochar	Woody Biochar	Manure/Waste Biochar
Carbon content	Moderate (45–70%)	High (70–90%)	Low (25–45%)
Ash content	High (10–40%)	Very low (<5%)	Very high (>40%)
Nutrient levels	Moderate (some P, K, Si)	Low (mostly carbon)	Very high (N, P, K, Ca, Mg)
Porosity	High (fine pores)	Very high (large pores)	Low (pores often clogged by ash)
Stability	Moderate to high	Very high (resists decay)	Low (decays faster in soil)
pH level	High (alkaline)	Moderate to high	Often very high (highly alkaline)

compares biochar derived from agricultural residues, wood, and manure. As shown, the unique advantages of agricultural residue-derived biochar include high silicon content, excellent stability, more surface oxygen-containing groups, a low liming effect, superior cation exchange capacity, and its use as a bifunctional soil conditioner and slow-release fertilizer (providing K and P for N fixation) [31,32,33]. Therefore, the use of agricultural waste-derived biochar for adsorption and decontamination is a sustainable and feasible alternative for wastewater remediation [34,35].

Unlike activated carbons, pristine biochar usually possesses a lower surface area and total pore volume and limited adsorption sites due to poor porosity [13,14,15,36], along with poor adsorption ability [37]. The dominant drawback of pristine biochar is poor dispersibility (e.g., easy to float and aggregate) [38]. Fortunately, the physicochemical properties (e.g., dispersibility; porosity; separability; mechanical strength; surface area; electronegativity; and the surface’s functional groups, pore size, interaction forces, etc.) and the adsorption properties (e.g., adsorption capacity, affinity, selectivity), adsorption kinetics, isotherm, and thermodynamics) of a specific biochar can be promoted via activation and modification to improve the porosity, adsorption sites, and surface chemical affinity and interactions [5]. The pristine biochar has been activated using strong acids, strong bases, and metal oxides and functionalized using various modifiers, via element doping (e.g., N, S, and heavy metals), and surface charge change to better catch target contaminants from wastewater [14,15,19,39,40,41,42,43].

Herein, based on the background above, this review presents the recent advances of agricultural waste-derived biochar for wastewater remediation via liquid–solid adsorption, mainly focusing on the adsorption mechanisms, biochar preparation and modification, main outcomes, economic evaluation, and environmental impacts. Moreover, the future direction and research needs are also recommended to guide and inspire the incoming work. This review follows the principle of the circular economy and waste valorization, providing a deep understanding of agricultural waste disposal and use, presenting the practical application of wastewater decontamination, and contributing to the sustainable development of agriculture and society globally, as well as resource integration and utilization locally.

2. Mechanism of Adsorption Using Agricultural Residue-Based Biochar

2.1. General Conceptions of Adsorption

The performance and properties of adsorption decontamination from wastewater depends on the physicochemical properties of biochar (e.g., ash content, hydrophobicity, porous property (micro- or mesoporous), point of zero charge (pH_PZC), element composites (e.g., C/O and C/H ratios), and the surface chemical functional groups), the properties of substrates (e.g., pK_a value, solubility, hydrophobicity, chargeability, electron distribution, and molecular or ion size), the interaction between the components in the bulk liquid and the surface of biochar, and so forth [13,14,15,36,40,44]. The specific absorption may be the synergistic result of these as-mentioned impacts.

2.2. Interactions Between Biochar and Contaminants in Wastewater

Wastewater contains a complex mixture of compounds with a wide range of molecular sizes. Effective adsorption requires that these compounds access the micropores of biochar. Larger molecules are often preferentially adsorbed over smaller ones; however, they can block micropores and hinder the diffusion of smaller molecules into narrower pores. The particle size of the biochar, such as powder versus granular, influences adsorption kinetics by limiting intraparticle mass transfer, but it does not change the overall adsorption capacity, which depends on the total specific surface area of the biochar [39]. Biochar with a high surface area and an abundant porous structure, including mesopores and micropores, provides ample adsorption sites for physicochemical interactions with wastewater contaminants [14,15,45,46]. Adsorbent affinity is a key determinant of capacity, irrespective of contaminant concentration [47]. For example, smaller ions typically show higher affinity for reactive sites and are removed more rapidly, yielding superior adsorption performance [48]. In addition, functional groups can contribute substantially when the biochar has a low specific surface area [49]. The dominant physicochemical interactions governing adsorption are critical for contaminant removal and are summarized in

Table 2. The mechanisms of the adsorption using agricultural residue-based biochar.

Mechanisms	Illustrations	Examples	Refs.
Precipitation	Contaminants can chemically precipitate via reaction with the liquid solute or biochar surface and are finally adsorbed on the biochar surface	(i) Between Al3+, Fe3+, Ca2+, Mg2+, Zn2+, Cu2+, Pb2+, and Cd2+ and OH- at alkaline conditions (ii) Between PO43− and Fe-doped biochar to form Fe3(PO4)2·(H2O)8	[37,42,48]
Complexation	Biochar surface’s functional groups can act as electron donors or acceptors and interact with metal ions or ammonium ions to produce complexes	(i) Between -OH groups and Fe2+ (ii) Between phosphate and ammonium ions (iii) Between -COOH and C=O or -OH with Cr6+ (iv) Between -OH and C=O or C-OH with Pb2+ (v) Between -NH2 and Cu2+ or Pb2+	[28,40,49]
H-bonding	H-bonding (theoretical bond energies of 4–17 kJ/mol) can be formed via the interaction between the functional groups on the biochar surface (e.g., -NH2 and -OH) with F-, N-, or O- containing molecules	(i) Between -OH and ammonium ion or -NO2 (ii) Between -OH and -OH or -NH2 (iii) Between -COOH and its conjugate acid	[5,50,51]
Electrostatic attraction and repulsion	Electrostatic interaction refers to the formation of ionic bonds between surface-charged biochar and ions or charged molecules. Electrostatic interaction is highly related to the pH of liquids, surface charge of the biochar, and pKa of the target substrates, which can be limited under high pH conditions	(i) Between cationic dye and -COO− of biochar (ii) Between the Si-N of biochar and anionic dyes (iii) Between Mn2+ and -OH, -COOH, or C=O of biochar (iv) Between PO43− and nitrate or nitrite of biochar (v) Between the same ions on the biochar surface and in liquids	[5,52,53,54]
π-π electron donor–acceptor interaction	π-π interactions (theoretical bond energies of 4–167 kJ/mol) are weak non-covalent bonds, referring to interactions between groups with π electron systems (e.g., -Ph, -C=C-, C=O, -COOH, -OH, and C-O) of the biochar surface and the target compounds	(i) Between -Ph of biochar and the enone structure of tetracycline (ii) Between -OH or -COOH of biochar surface and -Ph of phenolic compounds	[2,6,12]
Pore filling	Pore filling is a physisorption process, referring to the substrate being adsorbed and concentrated on biochar’s pore, depending on the properties of biochar (e.g., porosity) and the substrate (e.g., polarity)	(i) Extensively occurs during various porous biochar involved adsorption	[42,55]
Ion exchange	Ion exchange refers to the exchange of ions between the biochar surface and the charged substrate in liquid	(i) Between the SiO2 of biochar and ammonium-N (ii) Between Ca2+, Na+, or K+ of biochar and Hg2+ (iii) Between -COOH, -OH, and -FeOOH of biochar with Cr6+	[23,28,56]
Ligand exchange	Ligand exchange refers to the original ligand of a coordination compound of biochar being selectively substituted by other ligands in liquids, which is limited at high pH	(i) Between -OH of biochar and PO43− in liquids (ii) Between S- and O-containing groups of biochar with Cd2+ in liquids	[27,57,58]
Hydrophobic interaction	Hydrophobic interaction refers to the interaction between aromatized, graphitized layers or the hydrophobically modified surface of biochar and hydrophobic substances	(i) Extensively occurs between the hydrophobic surface of biochar and hydrophobic compounds (ii) Between oleic acid-modified activated biochar and naphthalene	[40,59]
Redox effects	Redox effects occur between biochar surfaces with oxidation or reduction capabilities and substrates in liquids	(i) [Adsorbent]-Fe2+ + CrO42− + 4OH− + 4H2O → 3 Fe(OH)3 +Cr(OH)3	[38]
Van der Waals forces	Van der Waals forces, a weaker electrostatic interaction than H-bonding, refer to non-directional and unsaturated interactions between the biochar surface and substrates in liquids	(i) Between the biochar surface and neutral creatinine, urea, or uric acids	[23]

[2,4,40,50,51]. The dominant interactions that contributed to the adsorption can be identified via a comprehensive analysis of various adsorption properties and the characterization of biochar before and after adsorption [6,24,45,46,47].

The assumed adsorption mechanisms of organic and inorganic substances on biochar are compiled in Figure 1.

2.3. Selective Adsorption

Achieving selective interaction of biochar with specific substrates (also known as selective adsorption) is crucial for targeted wastewater remediation. The selectivity of biochar is determined by the interaction between its surface properties and the molecular characteristics of the pollutant, as shown in Table 1. In particular, hydrophobic compounds are generally adsorbed more readily via hydrophobic interactions than hydrophilic compounds [60]. The -OH and -NH₂ groups on the biochar surface are reported to be the dominant groups responsible for the adsorption of anionic dyes [52]. Interactions between the aromatic rings of biochar and those of organic pollutants facilitate the selective adsorption of aromatic compounds [14,59]. π-π interaction and O-containing groups (e.g., -OH and C=O) on the surface of biochar composites can facilitate the adsorption of cations through surface polarization and electrostatic attraction [58,61]. Negatively charged biochar can adsorb ammonium ions via electrostatic attraction and H-bonding between O-containing groups, -OH, C-O, -COOH, and -COC- of biochar and ammonium ions, and phosphate via surface precipitation [23,24,62]. Cations Mg²⁺ and Ca²⁺ in biochar can trap PO₄³⁻ and H₂PO₄⁻ in liquids via precipitation, while anion Cl⁻ of biochar contributes to adsorbing PO₄³⁻ via electrostatic forces [24,63,64]. The specific interactions of SiO₂ in biochar can enhance the affinity towards PO₄³⁻ via electrostatic interactions and H-bonding (or ligand exchange) by its -OH group, especially at acidic pH levels [23]. Groups like -OH, -COOH, -Ar, Ar-NH₂ (aromatic amines), R1-CO-N-R2R3 (amides), and -C₅H₁₀N- (pyridine) of biochar are beneficial to the adsorption of ammonium, nitrate, and phosphate through ion exchange [17]. Biochar can interact with both ammonium- and nitrate-N via electrostatic attraction, ion exchange, and physisorption, with electrostatic attraction being particularly significant [51]. Positively charged biochar surfaces can also interact with cationic compounds via Coulombic interactions under acidic conditions [49]. Additionally, small organic molecules can be physically trapped within the micro- and mesopores of biochar via pore filling [42,55].

Selective adsorption by biochar can be influenced by its preparation and adsorption conditions. For example, low pyrolysis temperature (<400 °C) results in more oxygen-containing functional groups, making it better for inorganic or heavy metal adsorption through complexation. High temperature (>600 °C) increases surface area and aromaticity, making it more selective for organic pollutants via π-π stacking and pore filling. Solution pH alters the surface charge of biochar; if the pH is above the pH_PZC, the biochar becomes negatively charged, enhancing its selectivity for positively charged metal cations. Thus, the chemical and physical structure of biochar can be tailored through specific preparation strategies (e.g., molecular imprinting), production conditions, and post-treatment to improve its selective adsorption, as described in Section 3.3.

3. Preparation, Regeneration, and Characterization of Agricultural Residue-Based Biochar

3.1. General Conceptions and Properties of Biochar

Biochar, a porous C-rich material, can be prepared via high-temperature (200–1700 °C) pyrolysis of biomass (e.g., lignocellulose) under anaerobic, O₂-limited, or O₂-free atmospheres, such as inert gases and vacuum environments [12,18,37,39,48]. In general, biochar has a rough surface, irregular shape, condensed structure, relatively high pore volume and surface area (lower than activated carbon), density, greatly aromatized and graphitized architectures, and abundant surface functional groups (e.g., -OH, -COOH, -NH₂, and C=O) [19,27,40,65]. Biochar’s C contents can be identified as high (≥60%), middle (30–60%), and low (10–25%) [18,51,52]. Biochar has the strengths of rich C content, stable C structure, abundant precursors, sustainable production, renderability, high cost-effectiveness, convenient modification and surface functionality, high catalytic activity and thermal stability, high accessibility for dissolved pollutants, and can remove both inorganic metal ions and organic contaminants [12,38,42,65]. The limitations of biochar include poor affinity, low selectivity, and small co-adsorption capacity [51,65].

3.2. Preparation of Agricultural Residue-Based Biochar

Various approaches have been developed for the biochar preparation, including conventional high-temperature carbonization [29,42,66], microwave-assisted high-temperature carbonization [15,30,67], strong acid carbonization [68], conventional hydrothermal carbonization [14,15], and microwave-assisted hydrothermal carbonization [15] using different agricultural wastes as precursors at present. Different agricultural wastes have different elemental compositions and thus exhibit different properties [39]. In general, these agricultural wastes require pre-treatment to clean and dry the material and to reduce them to a small and uniform size, as shown in Figure 2 [4].

Conventional carbonization is often operated at high temperature under inert gas atmosphere or vacuum conditions in open or closed systems using a muffle furnace, tubular furnace, curable tube, heat pipe reactor, top-lit updraft two-barrel furnace, portable charring kiln, microwave oven, and so forth [12,19,42,48,51,70]. According to the preparation conditions, pyrolysis can be divided into slow (300–700 °C, several hours, 35–50 wt% of yields), fast (400–600 °C, <10 s, <30 wt% of yields), and flash (about 1000 °C, <3 s, <20 wt% of yields) [48]. The dynamic molecular structures of the biochar produced at different pyrolysis temperatures are presented in Figure 3.

Unlike pyrolysis under atmospheres, vacuum decomposition induces faster volatile removal, slighter pore blockage, lower loss of surface groups, and shorter combustion duration, followed by greater yields and properties of the biochar [1]. Hydrothermal carbonization is usually performed by mixing the precursor with less solvent (e.g., distilled, acidified, or alkaline water and or organic reagents), subsequently reacting under high temperature (80–250 °C) and pressure (2–6 MPa) [14,15]. Microwave heating can assist both the regular and hydrothermal carbonization owing to the distinct heating ways [13,14,15,59,72]. These approaches have been described in detail in our previous works [13,14,15,59,73]. Additionally, the molten salt method has also been used, in which micropores are formed via the partial oxidation of components under high temperature and air atmospheres, while larger pores are formed thanks to the etching and the template impacts of the molten inert salts [61]. Among the above-mentioned approaches, the conventional high-temperature carbonization is the most widely applied approach for biochar preparation [43,74]. The limitations and advantages of these approaches for the preparation of biochar are compiled in

Table 3. The advantages and limitations of developed strategies for biochar preparation [13,14,15,59,61,72].

Strategies	Advantages	Limitations
Conventional high-temperature heating (pyrolysis)	Most common and well-established method. Can use a variety of feedstocks. Higher temperatures (above 500 °C) yield highly stable carbon and porous biochar with high fixed carbon content. Products (bio-oil, syngas, and biochar) can be used for energy.	Longer processing times (especially slow pyrolysis). Heat transfer can be slow and non-uniform, particularly in large reactors. Requires a significant amount of energy, especially to drive off moisture from wet biomass.
High-temperature heating under vacuum	Lower decomposition temperature and shorter processing time compared to atmospheric pyrolysis. This strategy can potentially increase bio-oil yield and improve product quality by quickly removing volatiles.	Increased complexity and cost due to the need for vacuum equipment. This strategy may require specialized reactor design.
Microwave calcination (pyrolysis)	Rapid and uniform heating due to direct energy transfer to the biomass. Shorter processing time and higher energy efficiency compared to conventional heating. Can produce biochar with enhanced microporous structure and surface functional groups.	Potential for hotspots and temperature non-uniformity in large-scale applications. Reactor design and precise control are more complex.
Hydrothermal synthesis (hydrothermal carbonization)	Uses wet biomass directly (no pre-drying needed), which is an advantage for high-moisture feedstocks. Operates at lower temperatures (180–250 °C) under high pressure. Produced hydrochar is carbon-rich but often less stable than pyrolysis biochar.	Requires specialized equipment (autoclaves) to handle high pressure. Longer reaction times (hours to days). Hydrochar may have lower C-stability and surface area than high-temperature biochar.
Microwave hydrothermal synthesis	Combines the benefits of microwave heating with hydrothermal carbonization. Offers significantly reduced reaction time (hours to minutes) and homogeneous heating compared to conventional hydrothermal carbonization. Allows for rapid and precise process control.	Requires specialized microwave equipment designed for high-pressure hydrothermal conditions. Cost and scalability can be challenging.
Salt-melting method (molten salt pyrolysis)	Molten salts can act as pore-forming agents or activating agents to produce biochar with high specific surface areas and enhanced properties (e.g., magnetic biochar). The salt can often be recovered and reused, promoting sustainability.	Introduces a new chemical, i.e., salt, into the process, requiring a separation step to recover the salt and clean the biochar. Higher complexity and potential for secondary pollution if not properly managed.

. The optimal strategy for biochar preparation heavily depends on the properties of biomass (e.g., moisture content, composition, type of agricultural wastes) and the intended application of biochar (e.g., soil amendment, adsorbent, catalyst).

During the preparation processes, the biomass undergoes melting, softening, vesicle formation, and decomposition with gas emission [61]; the light organics of biomass precursors are highly reacted and disrupted via volatilization [18]; the hemicellulose and cellulose usually decompose at 200–300 °C and 300–400 °C, respectively (the mechanism of lignin, hemicellulose, and cellulose pyrolysis during biochar preparation are shown in Figure 4) [61]; the existing metal components may accelerate the pyrolysis of the biomass and improve the porosity of biochar [23,75]; the oxidation induces low ash components and high gas emission under air atmosphere [49]; the pH_PZC value can be increased (due to the elimination of acidic O-containing groups, like -COOH, -OH, and R-CH=O), and yields of C and biochar can be improved (due to low oxidation) under high temperatures and limited O₂ atmosphere [23,76]; the exterior of biomass usually suffers more rapid decomposition compared to the inside part, leading to visible cracks and the exposure of the inner Si-containing structure [77]; the aromatic groups can be formed via the condensation and pyrolysis of lignocellulosic [35]; and the C yield and ash fractions relate to the lignocellulose contents in the precursors [18]. Moreover, the porous structure of biochar can be blocked by tarry and alkaline metal components during the biomass preparation [40].

Precursor types, carbonization strategies, heating approaches (temperature, way, rate, and holding duration of heating), and carbonization atmosphere (e.g., air, N₂, Ar, vacuum, and adiabatic conditions) can influence the physicochemical and structural properties of biochar [19,48,49,70]. Various precursors include different metals and inorganic and organic components, impacting the pH_PZC value, morphology, and C/H and C/O ratios of biochar [76]. Wood-based biochar is usually reported to exhibit higher stability and better surface and adsorption properties than the others [45]. Alkali metals in the precursor help broaden the biochar lattice and the pore size [41]. Mg and Ca remaining in biochar after pyrolysis favor cation adsorption via ion exchange [63,64]. SiO₂ and its content in precursors, especially for rice husk, have been reported to promote the texture, architecture, and porosity of biochar [23,77]. SiO₂ can also react with H₂O to produce Si-OH on the biochar’s surface, which favors adjusting its surface charge at different pH values [52]. Biochar’s hydrophobicity, aromaticity, and graphitization improve with the augmented carbonization temperature [40]. On the other hand, these metal components may induce the reduction of biochar’s surface area and the obstruction of its pores [35]. Low carbonization temperatures (<400 °C) lead to low specific surface area (<10 m²/g) but favor protecting the functional groups, increasing the hydrophilicity of biochar (due to the preservation of O-containing groups) and the removal of polar or inorganic contaminants via surface functional group-induced precipitation, ion exchange, and electromagnetic gravity [17,48,76]. In contrast, raising the temperature (500–700 °C) greatly improved the surface area (>100 m²/g), pore volume, and porosity, owing to the rapid generation of volatile substances (e.g., H₂, CO, and CH₄), the sufficient discharge of them, decomposition of by-products in pores (e.g., tar oil), and the occurrence of aromatic condensation [23,41,78]. Nevertheless, high carbonization temperatures are not always beneficial. Excessive temperature (>700 °C) could decrease the surface functional group (e.g., C=O, -COOH, -OH) content via dehydration and deoxygenation, reduce the surface area and yield of biochar, increase the alkalinity of biochar (due to the removal of acidic groups), raise the pH_PZC of biochar (since the dehydration and decarboxylation and the presence of basic carboxylate and hydroxide groups) intensifies), disrupt the biochar’s structure, and limit the adsorption ability [23,45,51,61,78]. Moreover, the C content of biochar can be improved via H₂SO₄ and sonication treatment in sequence [68].

3.3. Improved Fabrication of Agricultural Residue-Based Biochar

The physicochemical processing, including activation, modification, and functionalization, favors the improvement of biochar’s surface and pore properties [40,51]. In general, biochar can be chemically and physically activated using KOH, K₂O, NaOH, NH₃·H₂O, MgCl₂, ZnCl₂, CaCl₂, HNO₃, H₂SO₄, H₃PO₄, HCl, CO₂, steam, etc. [14,15,19,39,40,41,42]. Compared to chemical activation (450–900 °C), physical activation commonly requires a higher temperature and longer duration [41]. Chemical treatment can increase the pH_PZC through protonating the functional group and removing carbonates and hydroxide in biochar [51]. Particularly, activation using NH₃·H₂O can trigger the dehydration, decarbonization, and dehydrogenation of substances containing C=O, changing the H/C and C/O fractions and introducing basic N-containing groups [65]. Washing with KOH or H₃PO₄ can increase the density of specific functional groups to target either acidic or basic pollutants [59,79]. Activation using CO₂ can induce the oxidation of biochar and introduce O-containing groups [75]. Nevertheless, the use of these activators could cause negative environmental effects and low cost-effectiveness. For instance, ZnCl₂ is known as a toxic activator, which may induce secondary pollution and pose potential environmental risks. Biochar has been functionalized using a variety of modifiers, like CS₂, sodium alginate, cellulose, chitosan, coal fly ash, inorganic elements (e.g., N and S), and transition metals (e.g., La) and their salts (e.g., Fe₃O₄) [5,22,28,38,58,75,80]. Specifically, incorporating sodium alginate in biochar can enhance metal cations adsorption via coordination interactions [81]. Coal fly ash is conductive to promoting the surface aromaticity and hydrophobicity and reducing C=O but protecting R1(R2)C=O groups [75]. S-doping is conducive to immobilizing the metals and introducing functional groups (e.g., S²⁻ and SO₃²⁻) [58]. N-doping can enhance the surface polarization and electrostatic attraction of metal cations [58,61]. Adding N or S into the carbon lattice can create specific sites that have a high affinity for mercury or gold [82,83]. Metal doping in biochar can decrease the electronegativity and alter the surface charge distribution and thus improve its affinity [27]. La-doping can enhance the specific affinity of PO₄³⁻ [57]. Fe-doping could promote various properties of biochar, such as surface texture, magnetism, surface charge density, mechanical strength, thermal stability, redox ability, etc. [38,48]. It has been reported that Fe-impregnated biochar can selectively pull out arsenic or phosphates and be easily removed from water using magnets [15,28,72]. Ca-doping contributes to broadening the pH suitability of biochar, as well as improving the biochar decomposition, improving the porosity, and supporting the Fe doping [48,84,85]. However, the modifiers may lead to some adverse effects on biochar, e.g., the blockage of pores, the decrease in the surface area, and the reduction of the economic efficiency, yield, and effectiveness of biochar (Figure 5) [58].

3.4. Regeneration of Agricultural Residue-Based Biochar

Regeneration of biochar are crucial for evaluating its stability, economic efficiency, sustainability, feasibility, and practicality [1]. In general, biochar used for wastewater remediation can be regenerated to restore its surface-active sites and adsorption capacity. Reusing biochar multiple times makes the process economically viable and environmentally sustainable. Reported approaches for biochar regeneration include thermal, chemical, and biological methods [87]. Among these, thermal regeneration is the most common, involving heating the spent biochar to high temperatures (200–500 °C) under conventional or microwave heating in an inert atmosphere (such as N2) to decompose or volatilize the adsorbed pollutants. Although this method is very effective at restoring pore structure, it can cause a slight loss of biochar mass (carbon burn-off) and may destroy some surface functional groups (such as -OH or -COOH) over multiple cycles. Chemical regeneration refers to the use of solvents (e.g., ethanol, acetone, or acids/bases) to wash the contaminants off the biochar surface or oxidizing agents (e.g., H₂O₂ and S₂O₈²⁻) to chemically degrade them, with the latter approach commonly requiring the activation of the oxidizing agents. For example, washing using H₃PO₄ can remove accumulated mineral deposits and metallic ions that block the pores of biochar. Bio-regeneration is an emerging, eco-friendly approach that uses colonized microorganisms on the biochar surface to digest organic pollutants trapped in the biochar’s pores. This method requires low energy consumption and operates at ambient temperatures, but it is much slower than thermal or chemical methods [29,42,71,88]. Typically, thermal, chemical, and biological regeneration are suitable for the removal of volatile substances, heavy organics, and biodegradable compounds from spent biochar, respectively.

Many authors have stated that biochar can be reused at least three to five times [5,87]. The decrease in adsorption capacities of biochar after several adsorption–desorption cycles has been frequently reported, which can be attributed to the inactivation of certain adsorption sites owing to the reduced surface area and pore obstruction after thermal processing, the disruption of surface functional groups and the loss of some functional elements (e.g., N, S, and mental) by solvents and under high temperature processing, incomplete desorption due to the strong bonding, and the interference of other compounds in wastewater [28,36,51,58,85]. The consumed biochar can be regenerated via high-temperature treatment (e.g., 150 °C [51]) and solvent elution (e.g., 30% EtOH, distilled water, 0.1 M EDTA, HCl, NaOH, HNO₃, and H₂SO₄ solutions), which are favorable for the valorization of waste biochar and the removal of the secondary pollution risk from the enriched contaminants in biochar [12,14,18,19,28,51,68,78].

4. Use of Agricultural Residue-Based Biochar for Wastewater Remediation

Agricultural waste biochar adsorption is a low-cost and effective wastewater remediation strategy that is able to remove different contaminants, including dyes, antibiotics, pesticides, pharmaceutical residues, heavy metal ions, nutrient substances, and organic carbon, as well as improve the wastewater quality (e.g., turbidity, chroma, smell, pH value, and toxicity) [35]. The adsorption capacity of biochar is commonly lower in wastewater than in clean water due to the competitive adsorption among the target compounds with other contaminants, caused by the complex composites of wastewater [47]. Thus, biochar has been combined with other materials, e.g., poly(N-hydroxyethylacrylamide) hydrogel, to achieve better decontamination performance of wastewater [3,62,89,90]. Adsorption decontamination facilitates decreasing the total solids (TS), total suspended solids (TSS), total volatile suspended solids (TVSS), total dissolved solids (TDS), total organic carbon (TOC), biochemical oxygen demand (BOD), and chemical oxygen demand (COD) of wastewater [15,37,39]. Meanwhile, the recovered macronutrients such as N, P, and K (in the forms of ammonium, nitrate, phosphate, and potassium salts) from certain types of eutrophication wastewaters like human urine, septic tank wastewater, blackwater, and biogas wastewater via adsorption can be reused to support the growth of crops [23,24,51]. Similarly, the recovery of phenolic compounds from olive mill wastewater and N and S from latex industrial wastewater can also be further valued [37,70]. Generally, the adsorption decontamination performance of wastewater by agricultural waste-based biochar depends on the preparation processes (e.g., the pyrolysis temperature, gas atmosphere, and heat approaches), the target substrates, the complexity of liquids, the operation conditions like dosage of adsorbents, volume of liquids, reaction temperature, content of contaminants, pH of the matrix, contact duration, adsorption mode (batch and column modes), co-existing ions, etc. [15,69].

In general, high initial concentrations favor adsorption due to the large concentration gradient and high driving force leading to great interphase mass transfer, intrapore molecule diffusion, and improved affinity of biochar towards the target compounds, followed by rapid adsorption with high capacity [1,11,57,78,91]. At the initiation of adsorption, the process proceeds rapidly, attributed to the sufficient and accessible adsorption sites, a high concentration difference driving force, and low diffusion resistance. With prolonged connection time, the adsorption will gradually slow down due to the limited adsorption sites and low mass transfer and molecular diffusion. Eventually, a dynamic adsorption–desorption equilibrium will be reached, where the total adsorption capacity becomes stable, and the adsorption site may continue the empty–occupy cycle, and the adsorbed compounds might influence the remaining ones in liquids via electrostatic repulsion, size exclusion, or other weak interactions [1,2,18,37,58,78]. High dosages of biochar do not always favor adsorption. Appropriate dosages can not only offer enough surface adsorption sites and ensure economical and fast operation but also avoid the biochar congestion [11,27,28,58]. The influence of pH on adsorption is complex. For biochar, the pH of adsorption systems can change the types and distribution of biochar’s surface charge via altering the zeta potential of biochar relative to its pH_PZC and the dissociation of chemical groups of biochar’s surfaces, thus affecting the electrical interactions with the target substrates in liquids [1,5,49]. It has been reported that the alternation of biochar’s surface charge can promote its adsorption selectivity [51]. For the target substances (e.g., dyes, TC, phosphorates, ammonium, and Fe ions), especially the amphoteric compounds, pH can change their existence form (molecule or ion) in liquids and thus impact their affinity with biochar [6,11,27,76]. Usually, acidic conditions (pH 5–6) facilitate metal sorption, whereas alkaline conditions facilitate the organics and PO₄³⁻ sorption [48,77]. Under very acidic conditions, competitive adsorption between target compounds (e.g., metal ions and NH₄⁺) and H⁺ and electrostatic repulsion will occur, limiting the adsorption [11,58,61]. In contrast, alkaline conditions are favorable for biochar adsorption by promoting their surface negative charge density and enhancing the induced electrostatic interaction [48,76]. Meanwhile, organics are highly ionized under alkaline conditions, allowing high mitigation and diffusion of them [77]. Alkaline conditions also help cation adsorption via electrostatic attraction, surface complexation, and precipitation but inhibit anion adsorption via the competitive adsorption of OH⁻ ion and electrostatic repulsion [18]. High temperature is usually conducive to endothermic adsorption, while low temperature is conducive to exothermic adsorption. However, high temperatures can decrease the adsorption interaction and intensify molecular motion, which may result in the desorption of substances [1,11]. The effect of temperature can be detailed via the thermodynamic investigations [13,14,59]. The co-existing ions (e.g., Cl⁻) change the ionic strength of the liquids, impacting the adsorption via the salting-out effect, molecular dissociation, competitive adsorption, and electrostatic screening effect [51,58]. Thus, the optimal adsorption conditions are crucial for the desired adsorption performance.

The optimized adsorption of various contaminants from wastewater using biochar derived from various agricultural wastes is summarized in

Table 4. The adsorption of contaminants from wastewater using biochar derived from agricultural residues.

Wastewater	Precursors of Biochar	Pollutants	Fabrication Conditions of Biochar	Adsorption Conditions	Surface Area, Pore Size, and Total Pore Volume	Functional Groups and Assumed Mechanisms	Adsorption Capacities	REs (%)	Refs.
Municipal wastewater	Date seed	Carbendazim	550 °C, 0.5 h, N2 atmosphere	3 g biochar/L, pH 7, 40 min, 200 mL, 1 mg/L, batch mode	307.5 m2/g, 3.80 nm, 0.278 cm3/g	-OH, -COOH, -Ph π–π electron donor–acceptor interactions, π–π stacking, dipole–dipole interactions, pore filling, electrostatic attraction, H-bonding	-	88.7	[4]
		Linuron						85.9
	Walnut shell	Quinoline	500 °C, 2 h, N2 atmosphere	0.01 g KOH-activated biochar, 50 mg/L, 25 °C, 50 mL, batch mode	969.8 m2/g, 2.34 nm, 0.4 cm3/g	C-O-C, C-O, C=OC-H, C-C, C=C-OH porous adsorption, π–π interaction, H-bonding, electrostatic attraction	78.2 mg/g	-	[47]
		TSS						81–85
		Ammonia						87–91
		Total K and N						59–69
		Total K						78–88
		As ion						79–87
		Cd ion						53–95
		Cr ion						83–88
		Pb ion						78–95
		Zn ion						90–95
		Cu ion						93–96
	Rice husks	Mn, Se, Fe ions	Biochar in 1 M NaOH (mBiochar/mNaOH, 2:1), 12 h, 25 °C	0.25 g NaOH-biochar/, biochar/HCl− biochar, 0.303 mg/L Mn, 0.116 mg/L Se, 0.390 mg/L Fe, 50 mL, 200 rpm, 10 h, batch mode	-	C≡C, C≡N, C=C, C-O, C-H, Si-O-Si electrostatic attraction, ion exchange, complexation, precipitation	-	76, 66, 66	[40]
			Biochar in 10% wt. HCl, 3 h, 500 °C at 10 °C/min, 200, 8 h, mL/min of N2 flow				-	30, 26, 59
			350 °C at 10 °C/min, 6 h, 200 mL/min of N2 flow				-	3, 39, 48
	Coffee husk	Ammonium	350 °C, 1 h	20 g biochar/L, 130 rpm, 6 h, 108 mg/L, pH 7.4, batch mode	0.43 m2/g, -, -	-OH, C-H, C=O, C=C, -COOH Complexation, ion exchange, H-bonding, electrostatic attraction	-	20	[76]
Industrial wastewater	Garlic peel	Methylene blue	150 °C at 5 °C/min, 2 h, vacuum atmosphere	0.005 g biochar, 20 mL, 50 mg/L, 1 h, 25 °C, batch mode	5.46 m2/g, 1.49 nm, 0.18 cm3/g	O-H, C=O, C-O, C=C-H Electrostatic attraction, H-bonding, π-π stacking	14.33 mg/g	-	[1]
	Citrus trees	Tetracycline	-	3.5 g biochar, 50 mL, pH 4, 90 mg/L, 20 °C, batch mode	364.9 m2/g, 1.08 nm, 0.2 cm3/g	O-H, C=C, C=O, C-H, C-Cl π-π interaction	-	95	[12]
	Rice husk	Pb ion	500 °C at 5 °C/min, 2 h, N2 atmosphere	Biochar, 35.7 mg/L, pH 6.68, 100 mL, batch mode	63 m2/g, -, 0.381 cm3/g	C-H, C=O, -OH, C-O Ion exchange, surface physisorption, electrostatic interaction, complexation	-	63.8	[22]
	Prosopis juliflora	Sulfamethoxazole	600 °C at 10 °C/min, 2 h, N2 atmosphere	1 g biochar/L, pH 5, 5.3 mg/L, 2 h, batch mode	875 m2/g, -, -	-OH, -COOH, -Ph, C-N, C-H, C-Cl, C-O electrostatic interactions, H-bonding, π-π stacking	-	76.7	[45]
		Ciprofloxacin		1 g biochar/L, pH 5, 8.3 mg/L, 2 h, batch mode				80.4
		COD		1 g biochar/L, pH 5, 2.5 g/L, 2 h, batch mode				79.4
		TOC		1 g biochar/L, pH 5, 1.05 g/L, 2 h				88.2
	Corn straw	Cr ion	500 °C, 2 h, Ar atmosphere	0.05 g Fe3O4/biochar, 32.8 mg/L, 3 h, pH 6, batch mode	508.4 m2/g, 4.6 nm, 0.55 cm3/g	Fe-O, Fe-OOH, C=O, O-H Surface physisorption, pore filling, and electrostatic interaction	-	72.6	[28]
	Potato peel	Cu ion	450 °C, 6 L/min of N2 flow	0.25 g chitosan-modified biochar, 4 h, batch mode	-	-NH2 -	1.117 mg/L	-	[80]
		Pb ion					0.506 mg/L
	Bamboo	Phosphate	900 °C at 8 °C/min, 2 h, N2 atmosphere	Iron/CaO-modified biochar, 1660 mg/L, 48 h, batch mode	146.5 m2/g, 2.78 nm, 0.1 cm3/g	- Chemical precipitation	-	~100	[48]
	Parthenium hysterophorus	Cr ion	500 °C, 2 h	Fe3O4/biochar, 85.13 mg/L, batch mode	237.4 m2/g, -, -	O-H, C-O-C, C-OH, Fe-O, Van der Waals forces, H- H-bonding, hydrophobic interactions	-	81.8	[38]
	Oil palm fronds	COD	300–438 °C at 13 °C/min, 3 h	15 g biochar/L, 4 h, 150 mL, batch mode	68.98 m2/g, 1.68 nm, -	O-H, C=C, C-H, C-O, S=O, Si-O-Si, S-S Ion exchange, H-bonding	-	41.2	[70]
		Suspended solids						87.6
		Sulfate						58.8
		Sulfide						56.8
	Auricularia auricula spent substrate	Cd ion	500 °C, 2 h, anoxic conditions	0.1 g CS2-modified biochar/L, 5.21 mg/L Cd2+, 1.11 mg/L Cu2+, 48.72 mg/L Zn2+, 25 °C, pH 5.59, 2 h, batch mode	2.54 m2/g, 13.4 nm, 0.009 cm3/g	C-S, -OH, S=C=S, C=O, -NH2 Complexation, precipitation	14.01 mg/g	-	[58]
		Cu ion					13.56 mg/g
		Zn ion					50.19 mg/g
	Jujube seeds	TSS	Jujube seeds/H2SO4, 1:3 for 4 h, sonication 20 min at 24 kHz	2 g biochar/L, pH 1 h, 30 °C, 20 mg/L, batch mode	48.32 m2/g, -, 0.16 cm3/g	-OH, C=C, C=O, C-OH, La-OP-, -CO= Ligand exchange, electrostatic attraction, complexation	-	10	[68]
		TDS		2 g biochar/L, pH 1 h, 30 °C, 2.8 g/L, batch mode				0.79
		Ni ion		2 g biochar/L, pH 1 h, 30 °C, 15 mg/L, batch mode				99.9
		Zn ion		2 g biochar/L, pH 1 h, 30 °C, 20 mg/L				~100
		Cu ion		2 g biochar/L, pH 1 h, 30 °C, 40 mg/L, batch mode				~100
		Cr ion		2 g biochar/L, pH 1 h, 30 °C, 70 mg/L, batch mode				~100
	Mandarin tree pruning	Dissolved organics	600 °C, N2 atmosphere	5 g biochar, 6800 mg/L, 100 mL, 25 °C, 160 rpm	-	- Precipitation, surface complexation, electrostatic interactions, π–π interactions	-	28	[37]
				1 g biochar, 17 g/L, 100 mL, 25 °C, 160 rpm, batch mode			140 mg/g	-
	Palm bunch	Methyl paraben	450 °C at 10 °C/min, 0.5 h, 400 mL/min of N2 flow	H2SO4-activated biochar, batch mode	60.3 m2/g, -, 0.54 cm3/g	C=C, -OH, C=O, S=O, C≡C Channel diffusion, H bonding, Van der Waals force, n-π/π-π interaction	-	80.3	[79]
		Carbamazepine						79.9
		Ibuprofen						70.2
		Triclosan						74.3
	Bagasse	Pb ion	300 °C, 2.5 h	5 g biochar, pH 5, 2.5 h, 25 °C, 2.393 mg/L, batch mode	12.38 m2/g	C=O, C=C, C-H, C-N, -COO-, -COOH, -Ph-OH Complexation, ion exchange	12.74 mg/g	75.4	[49]
Livestock wastewater	Corncob	Ammonia	450 °C for 1.5 h, 4 °C/min	0.3 g, 50 mL, 6.2 mg/L, pH 12, 1.5 h, batch mode	-	-	-	83.98	[92]
	Rice straw	COD	300 °C, 6 h	4 g biochar/L, pH 9, batch mode	35.4 m2/g, -, 0.36 cm3/g	- Polarity, hydrophobic/aromatic interaction, and molecular size	-	40	[77]
		BOD		4 g biochar/L, pH 9, batch mode				40
		COD		373 mg/L COD, 1.75 h, column mode				79
		BOD		240 mg/L BOD, 1.75 h, column mode				84
	Corn straw	TS	500 °C, 1 h, N2 atmosphere	Biochar or NaOH-activated biochar, 10.6 g/L TS, 0.3 g/L TVSS, 2985.6, 1908.2, 1270.3, 981.4, 85.7, 4138.6, 655.9, 0.6, 2.7, 1.1, 6.1, 0.5, and 0.2 mg/L for TC, TOC, TV, NH4+-N, TP, COD, K, Mg, Cu, Zn, Ca, Fe, and Mn, respectively, batch mode	-	- H-bonding, electrostatic attraction, ion exchange, hydrophobic interaction	-	50–42	[42]
		TVSS						67–67
		TC						53–72
		TOC						55–73
		TN						18–33
		NH4+-N						22–32
		TP						19–25
		COD						20–26
		K ion						39–67
		Mg ion						33–83
		Cu ion						59–41
		Zn ion						27–73
		Ca ion						30–54
		Fe ion						80–80
		Mn ion						100
	Corn stover	Phosphate	500 °C, 2 h, N2 atmosphere	0.2 g Ce-modified biochar, 100 mL, 24 h, 180 rpm, 25 °C, batch mode	14.1 m2/g, 7.05 nm, -	-CH2-, -CH-, Ce-O surface precipitation, ligand exchange, complexation, electrostatic attraction	27.96 mg/g	43.3	[27]
Textile wastewater	Coconut shells	Methylene blue	-	0.02 g Fe3O4/biochar/sodium alginate aerogel beads, 50 mL, 50 mg/L, 150 rpm, 25 °C, 24 h, pH 7, batch mode	152.5 m2/g, 2.60 nm, -	-OH, -COOH pro-filling, H-bonding, electrostatic interaction	-	-	[5]
	Eucalyptus bar	Anthraquinon	500 °C, 1.5 h, anaerobic condition	0.913 g biochar composite/L, 21 mg/L, pH 3.9, 1.95 h, batch mode	57.4 m2/g, 1.48 nm, 0.41 cm3/g	Si-OH, Si-N, -COOH, -OH, C-O-C, C=O π–π interaction, electrostatic attraction, surface functional groups, chemisorption, pore filling	-	-	[52]
	Wheat straw	COD	300–500 °C	2.5 g biochar/L, 25 mL, pH 7.62, 150 rpm, batch mode	-	C=C, C=O, C-H, C-O-C, -OH, -COOH Ion exchange, Surface physisorption, electrostatic interaction, complexation	-	62	[19]
	Giant reed	Basic blue 41	10 °C/min at 600 °C for 2 h, 5 L/min of N2 flow	4 g biochar/L, 5.7 mg/L, 1 h, batch mode	429.0 m2/g, -, 0.09 cm3/g	C-H, C-O, C-C, C-OH, C=O, C=C electrostatic interactions	5.14 mg/L	90.3	[39]
		Color		4 g biochar/L, 106 Pt–Co, 1 h, batch mode			83 Pt-Co	89.3
		Turbidity		4 g biochar/L, 48.55 NTU, 1 h, batch mode			33.7 NTU	69.4
		COD		4 g biochar/L, 928 mg/L, 1 h, batch mode			582 mg/L	62.7
Human urine	Corncob	N, P, K	600 °C, anaerobic condition	60 g biochar, 600 mL, 5 days, batch mode	1.7 m2/g, -, 0.0005 cm3/g	-OH, -COOH, C = C ion exchange, chemical interaction	1200, 242.8, 43.7 mg/L	-	[24]
	Rice husk	TC	-	0.1 g biochar/mL, 5 days, batch mode	4.63 m2/g, -, -	-OH, C-H, C-O, C=C H-bonding, ligand exchange, ion exchange, electrostatic interactions	-	60–80	[23]
		N, P, K					236.5, 256.7, 4.6 mg/L	50, 70, 80
Pharmaceutical wastewater	Groundnut shells, drumstick seeds, coconut fiber	BOD	Groundnut shell: 500 °C, 4 h; drumstick seeds: 600 °C, 2 h; coconut fiber: 700 °C, for 2 h	35 g biochar mixture (1:1:1), 443.6 mg/L, pH 7, 25 °C, 1.5 h, batch mode	-	-OH, -CH3, C-H, C=C, C-OH, C=O -	-	72.1	[69]
Hospital wastewater	Corn stalks	COD	400–500 °C	56.0 mg/L, batch mode	-	-	-	57.1	[26]
		BOD		46.8 mg/L, batch mode				56.8
Vehicle-wash wastewater	Eucalyptus wood	Anthracene	450 °C, 1 h, N2 atmosphere	0.4 g biochar, 40 ppm, 1 h, pH 5, 50 °C, batch mode	18.4 m2/g, 1.5 nm, 0.01 cm3/g	C-H, C=C, C=O Van der Waals dispersive contacts, electrostatic interactions, H-bonding	-	98.4	[11]
Slaughterhouse wastewater	Coconut husk	NO3-N and NO2-N	700 °C, 6 h, under N2 atmosphere	1.5 g biochar, 26 °C, pH 7.35, 50 mL, 120 rpm, 2 h, batch mode	6.84 nm	-OH, C=C, Si-O-Si ligand exchange, electrostatic attraction, complexation	0.2–13 mg/g	-	[68]
	Rice husk				1.97 nm		0.2–12 mg/g
	Coffee husk				1.63 nm		0.2–12 mg/g
Actual wastewater	Platanus balls	Phosphate	600 °C at 10 °C/min, 2 h, N2 atmosphere	1 g La-modified biochar, column mode	77.01 m2/g	LaO-, O-PO-, P-O electrostatic adsorption, ligand exchange, complexation	14,850 mg/g	-	[57]
Synthetic wastewater	Peanut shell	Atrazine	450 °C, 4 h	0.02 g biochar, 25 mL, 20 mg/L, 150 rpm, batch mode	61.8 m2/g, 1.96 nm, 0.03 cm3/g	-OH, NH2, C-O, C=O, C-H, C=C, C-C π-π interactions, H-bonding	2.8 mg/g	-	[6]
	Coconut shell	Ammonium, nitrate, phosphate	-	0.5 g biochar, 100 mL, 80 mg/L, 6 h, 80 rpm, batch mode	-	C=C, C-O-C, C=O ion exchange, chemical interaction	10.12, 7.51, 10.79 mg/g	-	[17]
	Lotus leaf	Be ion	600 °C, 3 h	0.05 g PO43−/NH4+ modified biochar, 50 mL, 35 °C, pH 5.5, 16 h, 175 rpm, batch mode	4.927 m2/g, 3.86 nm	Phosphoric acid, ammonia, -OH surface complexation and precipitation, pore filling,	40.38 mg/g	-	[65]
	Palm leaves	Tetracycline	500 °C, 2 h, 10 °C/min under N2 atmosphere	1 g biochar/L, 20 mL, 0.5 mg/L, 180 rpm, pH 5.7, 24 h, 25 °C, batch mode	31.5 m2/g, 5.38 nm, 0.03 cm3/g	-COOH, -OH, C=O, C-O, C=C, C-H H-bonding, π-π interaction, electrostatic interaction, pore filling	-	80	[2]
	Fronds and leaves of date palm	Phenol	600 °C at 8 °C/min, anaerobic condition	0.1 g biochar, pH 6, 20 h, 800 mg/L, 50 mL, 200 rpm, batch mode	245.8 m2/g, 4.6 nm, 0.12 cm3/g	O-H, C=C, C-H, Si-O, -COOH π-π interactions, H-bonding, pore filling, electrostatic interaction	241 mg/g	60.3	[78]
				0.1 g biochar, pH 6, 20 h, 52 mg/L, 50 mL, 200 rpm, batch mode			22.28 mg/g	85.7
	Wheat straw	Inorganic-N	450 °C at 5 °C/min, 5 h, 400 mL/min of N2 flow	10 g Mg-modified biochar/L, 24 h, 250 mL, 25 °C, 80 rpm, batch mode	23.4 m2/g, -, 0.062 cm3/g	C=C, -OH, -Ph -NH2, -COOH H-bonding, π-π/n-π interaction	4.44 mg/g	-	[51]
	Rotten sugarcane bagasse	Pb ion	600 °C, 2 h, air atmosphere	0.03 g biochar, 50 mg/L, pH 5, batch mode	391.9 m2/g, 20.9 nm, 0.532 cm3/g	-COOH, CHO, C=O, C-H, C=C, O-C=O Ion exchange, surface complexation/function group coordination, precipitation, π-π interaction	-	97.3	[61]
		Cu ion						99.8
		Cr ion						100
	Pomelo peel	Tetracycline	400 °C at 10 °C/min, 2 h	0.08 KOH-activated biochar/L, 10 mg/L, Ph 7, 21 °C, 75 h, batch mode	2457.4 m2/g, -, 1.14 cm3/g	C≡C, C≡N, C-C, C=C, C-H, C-O-C, C=O π-π electron donor–acceptor interaction, electrostatic interaction, pore filling	-	85.0	[41]
		Oxytetracycline						82.2
		Chlortetracycline						96.6

Note: REs, removal efficiencies; Refs., references; - means the relevant entry is inapplicable or the relevant works do not present this data.

.

As listed in Table 4, a variety of agricultural wastes have been used for the adsorption of different contaminants and nutrients from various wastewater. The calcined temperature and duration for biomass carbonization are in the range of 300–900 °C and 0.5–6.0 h, respectively, under a N₂, Ar, or vacuum atmosphere, depending on the agricultural wastes. The specific surface area, pore diameter, and total pore volume of different pristine biochars range from 1.65 to 428.98 m²/g, from 1.1 to 20.9 nm, and from 0.0005 to 0.5320 cm³/g, respectively. The activated biochar usually possesses better porosity and higher surface areas. The functional groups -OH, -COOH, C-H, C=C, and C=O are crucial for the effective adsorption of contaminants from various wastewater. The adsorption equilibrium duration lasts from 40 min to 5 days. Most biochar adsorbents demonstrate strong performance and reusability in real wastewater treatment. In municipal, industrial, livestock, textile, and synthetic effluents, the adsorption capacities of agricultural residue-based biochar are 78 mg/g, 14–140 mg/g (or 0.5–1.1 mg/L), 28 mg/g, 5–582 mg/L, and 3–241 mg/g, respectively. Municipal wastewater typically contains nutrients (N and P), pathogens, and emerging micropollutants (such as pharmaceuticals) in large volumes with relatively low concentrations of diverse pollutants. In this context, the main roles of biochar are physical trapping in pores and supporting biodegradation. Industrial wastewater comprises heavy metals, phenols, and complex organics with high toxicity at elevated concentrations and extreme pH levels. Here, ion exchange and surface complexation with oxygen-containing functional groups (-OH and -COOH) are crucial, and biochar often requires functionalization. Livestock wastewater contains high levels of ammonia, nitrates, and antibiotics, where biochar adsorption primarily occurs through electrostatic attraction for ammonium ions and pore filling for bulky antibiotic molecules. Textile wastewater is rich in residual dyes and surfactants, with high turbidity and recalcitrant aromatic structures; in this case, biochar with a higher surface area, more aromatic rings, and enhanced π–π interactions facilitates adsorption. It is worth noting that the abundant diversity of biochar, contaminants, and wastewater types, as well as insufficient data samples make comparisons of adsorption performance more challenging and require greater caution. Unlike stable synthetic wastewater, adsorption in real cases is complicated by varying components and conditions. Most studies focus on batch adsorption. Few authors have discussed the impact of actual wastewater instability on adsorption, and research on wastewater has primarily aimed to validate the potential feasibility of biochar for remediation. Comparative adsorption studies across different wastewater streams and investigations into reusability for wastewater remediation are even more rarely reported.

5. Economic Evaluation and Environmental Impacts

Owing to the high cost-effectiveness, adsorption ability, and large agricultural biomass reserves, biochar has been seen as an excellent alternative adsorbent for activated carbons [51,78]. Accordingly, biochar is ~6-fold cheaper compared to the majority of activated carbons (up to 9 USD/kg) [40]. Compared to activated carbon, the production cost of wheat straw-based biochar is reduced by 34-fold [51]. The energy requirement for preparing biochar was reported to be 15-fold lower compared to activated carbon [51]. Another work states that the cost of biochar (0.08 USD/kg) is 12 times lower than that of other C-based adsorbents [93]. Specifically, in the Philippines, the evaluated costs are 0.09 USD/kg for plant-based biochar and 1.1–1.7 USD/kg for activated carbons [2]. The costs of biochar prepared from coconut shells [94], pine wood [94], rotten sugarcane bagasse [61], and pomelo peel [41] are 0.8, 0.9, 2.4, and 9.8 USD/kg, respectively. It has been reported that the biochar market share will reach USD 0.45 billion by 2030 globally [48]. Furthermore, the evaluated cost for treating real antibiotic wastewater using biochar is in the range of 9–914 USD/m³ [12]. For treating livestock wastewater, the cost is expected to be as high as 61.6 thousand USD/year [77]. On the other hand, the recovered nutrients and bioactive compounds from wastewater can be valorized as fertilizer or bio-products to enhance the process’s economic viability and sustainability. Moreover, the spent biochar can be used for soil improvement, while the silicon element also helps plants resist pests and diseases, strengthens cell walls to prevent lodging (falling over), and improves drought tolerance. Overall, by using agricultural wastes, optimizing the pyrolysis process for energy efficiency, valorizing the absorbed substances and using biochar, and ensuring responsible management of the spent biochar, the environmental benefits of wastewater remediation can significantly outweigh the localized or production-related burdens.

The positive environmental impacts of the preparation of biochar using agricultural waste for wastewater remediation are as follows: (i) Preventing the decomposition or open burning of agricultural waste minimizes the release of greenhouse gases and other harmful air pollutants. (ii) Biochar is highly effective at adsorbing and immobilizing a wide range of water contaminants, including heavy metals, nutrients, and recalcitrant organic pollutants, favoring the production of cleaner effluent, safeguarding aquatic ecosystems and human health. (iii) The pyrolysis process converts the carbon in the agricultural biomass into a highly stable, recalcitrant form of C in the biochar, while the used biochar is often co-disposed or applied to soil (where applicable and safe). Around 50% of the original carbon in the biomass feedstock is retained in the stable biochar for centuries, acting as a carbon-negative technology. (vi) Even though the biochar preparation releases certain waste gases into the atmosphere, the total emission is much decreased compared to the direct incineration, and the good reusability and regeneration properties of biochar reduce the potential environmental impacts [41]. A sustainability assessment described that the preparation of wheat straw-based biochar decreased fossil depletion by 35% and human toxicity by 13% [51]. (v) By substituting traditional chemical adsorbents or treatment methods, biochar application can indirectly reduce emissions associated with the production of those alternatives. (vi) Mitigating the pollution risk of soil induced by agricultural wastes.

Nevertheless, the topic can pose several potential environmental issues, including the following: (i) Energy consumption. A study shows that low-temperature slow pyrolysis can be an energetically efficient strategy, with the energy produced per unit energy input ranging from 2 to 7 MJ/MJ [95]. The pyrolysis process used to create biochar requires energy (heat), which, if derived from non-renewable sources (e.g., grid electricity or fossil fuels), can contribute to the value of the global warming potential and other environmental burdens. (ii) Gas emissions. It has been reported that the energy consumption for biochar production was 0.40 and 0.45 kg CO₂(eq) per ton of wood waste during pre-treatment (e.g., chipping and drying) and pyrolysis, respectively. The total for the production process can be up to 0.9.5 kg CO₂(eq) per ton of wood waste [96]. Another work stated that the greenhouse gas emissions (76.6 kg CO₂(eq) per ton) for biochar production were far exceeded by the amount of biochar sequestered long-term (2430 kg CO₂(eq) per ton) [97]. Improper pyrolysis conditions (e.g., low temperature, incomplete combustion) can lead to the release of volatile organic compounds and polycyclic aromatic hydrocarbons, which are environmental and health concerns. (iii) Precursor transportation. The environmental impact of transporting bulky agricultural waste from its source to the biochar production facility can be significant, influencing overall CO₂ emissions. (iv) Long-term/post-application impacts. Although biochar initially immobilizes heavy metals, its aging in the environment can, under certain conditions (e.g., changes in Ph or redox potential), lead to the slow release of contaminants back into the environment; (v) Contaminants in biochar. Suppose the agricultural waste itself contains high levels of pollutants (e.g., heavy metals from contaminated soil or feed). In that case, these can become concentrated in the resulting biochar, limiting its safe use for subsequent soil application. These issues can be addressed by valorizing the syngas generated during pyrolysis (e.g., to power the process), using higher temperatures and well-controlled systems, locating facilities close to precursor sources, ensuring proper post-treatment disposal, and carefully selecting precursors, respectively. Fortunately, the negative impacts associated with biochar production, such as energy consumption and gas emissions, represent only a minor fraction of the overall life cycle assessment. The process often yields a net environmental benefit owing to carbon sequestration and energy substitution.

6. Future Directions and Challenges

The future perspective of biochar derived from agricultural wastes for wastewater remediation is highly promising, positioning it as a key sustainable technology within the circular economy. This topic converts agricultural residues (a waste management problem) into a value-added adsorbent and provides a cost-effective, eco-friendly alternative to conventional, energy-intensive wastewater treatment methods. The future studies in this field should move beyond simple, pristine biochar to highly engineered, designer materials for specific contaminants and applications. The most urgent research gaps include pilot-scale studies, the applicability of biochar to the complexity of real wastewater, standardization of biochar quality criteria, and regulatory barriers. The additional aspects are related to (i) the advanced biochar modification to fabricate nanocomposites and the development of innovative biochar (e.g., magnetic biochar) with great reusability, good recyclability, and easy separation from the adsorption systems; (ii) the investigation of the fixed bed adsorption and promotion of the biochar’s regeneration and recycling properties; (iii) the chemical and physical activation utilizing chemical and physical activators to optimize the porosity and surface chemistry for targeted contaminants; (iv) developing ‘intelligent’ or ‘smart’ biochar that can adjust their properties (like surface charge) in response to environmental conditions (e.g., Ph), allowing for better contaminant retention in dynamic systems; (v) removal of emerging contaminants such as pharmaceuticals, pesticides, and microplastics, where traditional treatments often fall short; (vi) recovering valuable resources like nutrients (N and P) and heavy metals from wastewater, turning a pollution problem into a resource opportunity; (vii) synergistic integrating biochar not just as a standalone adsorbent but within hybrid systems, for instance, microbial fuel cells (biochar can serve as an electrode material to enhance electricity generation and pollutant breakdown simultaneously), membrane bioreactors (using biochar as a component in membranes or as a pre-treatment to reduce fouling and improve effluent quality), and algal biochar systems (utilizing biochar to support microalgae growth for enhanced bioremediation, CO₂ capture, and even biofuel production); (viii) estimation of the feasibility of the long-term use of biochar in biofilters; (ix) simultaneous adsorption, decomposition, and mineralization via biodegradation using bacterial-loaded biochar, photocatalysis or sonocatalysis using biochar as catalysts, and activation of peroxymonosulfate or persulfate for wastewater purification [6,15,21,38,98]; (x) optimizing biochar production technologies through innovations in pyrolysis, gasification, and hydrothermal carbonization to improve energy efficiency, enhance scalability, and ensure consistent biochar quality from diverse agricultural feedstocks; (xi) integrating machine learning and artificial intelligence with biochar production and application to accurately predict material properties, optimize treatment systems, and manage regeneration/disposal; and (xii) establishing closed-loop systems where agricultural waste is converted to biochar, used in wastewater treatment, and then the spent biochar (rich in recovered nutrients/carbon) is safely reused as a soil amendment, further supporting sustainable agriculture.

Several challenges should be overcome for scaling up and commercialization: (i) Material variability. Establishing standardized protocols for biochar production, characterization, and quality control to ensure consistent performance. (ii) Regeneration and disposal. Developing cost-effective and environmentally friendly regeneration methods (e.g., chemical, thermal, biological) for spent biochar to ensure its long-term reusability and prevent secondary pollution. (iii) Scalability and cost. Reducing production and modification energy costs to make engineered biochar competitive with established commercial adsorbents like activated carbon on a large scale. (iv) Long-term ecotoxicity. Conducting extensive long-term field studies and life cycle assessments to fully understand the environmental stability and potential ecological risks of modified biochar.

7. Conclusions

Various agricultural wastes are valorized to produce biochar, which enables efficient decontamination and recovery of valuable compounds from a range of wastewaters. Such biochar has specific surface areas and pore diameters of 1.7–429.0 m²/g and 1.1–21.0 nm, respectively. Most adsorption capacities of biochar in real and synthetic wastewater streams are in the ranges of 13–140 mg/g (or 0.5–582.0 mg/L) and 3–241 mg/g, respectively. Pore trapping, ion exchange/surface complexation, electrostatic attraction/pore filling, and π-π interactions are commonly the primary mechanisms for the adsorption of N/P in municipal wastewater, heavy metals/phenols/complex organics in industrial wastewater, ammonium/ammonia/nitrates/antibiotics in livestock wastewater, and dyes/surfactants in textile wastewater by such biochar, respectively. Urgent challenges include material variability, standardization of biochar quality criteria, regeneration, scalability, and long-term ecotoxicity. Further validation and scale-up are essential for the practical implementation of agricultural waste-derived biochar in wastewater remediation.