Biochar Derived from Agricultural Residues for Wastewater Contaminants Removal

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, and sustainability, technically flexible, technologically feasible, and environmentally friendly, free of by-products, 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 waste, 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, 2-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 carbon, carbon nanotubes, and graphene oxide, 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) [7,13,14,15,16,17,18]. 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 [19], coconut shells [5,20,21], wheat straw [22], rice husks [21,23,24,25,26], corn wastes [27,28,29,30,31], wood wastes [11,32], and rape stalk [33]. The agricultural waste-derived biochar adsorption decontamination is a sustainable and feasible alternative to 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 [14,15,16,36], followed by poor adsorption ability [37]. Accordingly, the dominant drawbacks of pristine biochar are poor dispersibility (e.g., easy to float and aggregate) [38]. Fortunately, the physicochemical properties (e.g., dispersibility, porosity, separability, mechanical strength, surface area, ionic radius, surface area, electronegativity, and the surface’s functional groups, pore size, functional groups, 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 [15,16,22,39,40,41,42,43]. It is worth mentioning that the activation, modification, and functional may reduce the cost-effectiveness of biochar preparation.

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, the main outcomes, the 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

Adsorption refers to the process of the transfer of substrates from liquid matrices to the surface and the subsequent inner porous structure of solid adsorbents through the capillary effect [5,27,44]. The adsorption of contaminants from liquids can occur through the adsorption of individual contaminants (e.g., single solute solution), selective adsorption of target compounds in complex matrices, competitive adsorption between several compounds (e.g., naphthalene and phenanthrene in aqueous solution), and simultaneous adsorption of multiple contaminants (e.g., ammonium-N and nitrate-N in wastewater) [30,45,46]. Generally, both chemical and physical adsorption appear during adsorption, but with different contributions. Chemisorption relates to the precursor and chemical composition of biochar (e.g., the type and amounts of surface functional groups (e.g., –OH, C=O, and –COOH), the element components), but physisorption depends on the biochar's surfaces and architecture, and weak interaction forces (e.g., Van der Waals forces) [21]. Thus, the chemisorption is usually a single layer and stronger, in which the adsorbed molecules or ions are difficult to escape from the adsorption site. In contrast, the physisorption is commonly multiple adsorptions and relevant weak, thus, allowing the mitigation of the adsorbed compounds into the inner pores [14,15,45,47]. 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 meso- porous), point of zero charge (pH_pzc), elements 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 [14,15,16,36,40,45]. 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. Particle size of the biochar, such as powder versus granular, influences adsorption kinetics by limiting intraparticle mass transfer, but 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 [14,15,45,47]. Adsorbent affinity is a key determinant of capacity, irrespective of contaminant concentration [46]. For example, smaller ions typically show higher affinity for reactive sites and are removed more rapidly, yielding superior adsorption performance [49]. In addition, functional groups can contribute substantially when the biochar has a low specific surface area [48]. The dominant physicochemical interactions governing adsorption are critical for contaminant removal and are summarised in Table 1 [2,4,40,50,51]. In particular, hydrophobic compounds are generally adsorbed more readily via hydrophobic interactions than hydrophilic compounds [44]. Groups -OH and -NH₂ on the biochar surface are reported to be the dominant groups responsible for the adsorption of anionic dye [48]. The π-π interaction and O-containing groups (e.g., -OH and C=O) on the surface of biochar composites could favor the adsorption of cations through the surface polarization and electrostatic attraction [49,50]. 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 from urine [26,27,51]. Cations Mg²⁺ and Ca²⁺ in biochar can trap PO₄³⁻ and H₂PO₄⁻ in liquids via precipitation, while anion Cl- of biochar favors adsorbing PO₄³⁻ via electrostatic forces [27,52,53]. The specific interactions that SiO₂ in biochar can enhance the affinity towards PO₄³⁻ via electrostatic interactions and H-bonding (or ligand exchange) by its -OH group, especially at acid pH [26]. Groups like -OH, -COOH, -Ar, Ar-NH₂ (aromatic amines), R1-CO-N-R2R3 (amides), and -C₅H₁₀N- (pyridine) of biochar favor the adsorption of ammonium, nitrate, and phosphate through ion exchange [20]. Biochar can interact with both ammonium- and nitrate-N via electrostatic attraction, ion exchange, and physisorption, and the electrostatic attraction is particularly significant [46]. Additionally, positively charged biochar surfaces can also interact with cationic compounds via Coulombic interactions under acidic conditions [54]. The dominant interactions that contributed to the adsorption can be identified via a combined analysis of various adsorption properties and the characterization of biochar before and after adsorption [6,27,55,56,57].

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

2.3. Description and Illustration of Adsorption in Wastewater

The static adsorption behaviors and properties can be deconstructed using kinetic, thermodynamic, as well as isotherm models. The kinetic study favors clarifying the rate-control step (e.g., chemical adsorption and mass transfer) during adsorption [4,11]. The most used kinetic models are the pseudo-first order (PFO), the pseudo-second order (PSO), and the Elovich kinetic models [14,65]. In case of high-concentration substances, the PFO model is more suitable to describe the adsorption processes than the PSO model [66]. Adsorption isotherms describe the adsorption layer characteristics and the relationship between the substrates and biochar (e.g., the highest adsorption capacity, physi- or chemi-sorption, and adsorbent surface properties) [14,65]. The frequently applied adsorption isotherm models are the Langmuir, the Freundlich, the Temkin model, the Redlich-Peterson, and the Dubinin-Radushkevich isotherm models [1,2,11,54]. Moreover, Vaishali et al. studied the competitive adsorption of methyl paraben, carbamazepine, ibuprofen, and triclosan using Multicomponent Freundlich and Langmuir isotherm models [46]. Adsorption mechanisms (e.g., rate-limiting steps) can commonly be clarified via the Weber and Morris model (also known as the intraparticle diffusion model) and the Boyd model (also known as the film diffusion model) [12]. Moreover, the adsorption thermodynamic properties can be determined using the Arrhenius formula and the Van't Hoff equation with considering the effect of adsorption temperature [11,14,15]. The difficulty of the adsorption can be evaluated using Gibbs free energy (ΔG°), enthalpy (ΔH°), and entropy (ΔS°) [50]. On the other hand, the column adsorption (also known as dynamic adsorption) processes can be described using the Thomas and the Adams–Bohart breakthrough curve models. The higher the column height and the lower the flow rates, the more adsorption sites and the sufficient the axial diffusion and mass transfer, followed by the longer the breakthrough time [50,63].

The equations and their relevant characterizations for describing individual and competitive adsorption capacities of biochar and various models are compiled in Table 2. The applicability, merits, and limitations of these models in adsorption have been well detailed [67,68,69,70,71]. It is worth noting that these models can only speculate on the adsorption mechanism, which should be verified and analysed in combination with practical adsorption investigations and biochar characterizations.

3. Preparation and Characterization of Agricultural Residue-Based Biochar

3.1. General Conceptions and Properties of Biochar

Biochar, 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 gas and vacuum environments [12,21,37,39,58]. 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) [22,30,40,75]. Biochar’s C contents can be identified as high (≥60%), middle (30%-60%), and low (10%-25%) [21,46,48]. 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 [12,38,42,75]. The limitations of biochar include poor affinity, low selectivity, and small co-adsorption capacity [46,75].

3.2. Preparation of Agricultural Residue-Based Biochar

Various approaches have been developed for the biochar preparation, including conventional high-temperature carbonization [32,42,76], microwave-assisted high-temperature carbonization [16,33,77], strong acid carbonization [78], conventional hydrothermal carbonization [15,16], and microwave-assisted hydrothermal carbonization [16] 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 small and uniform the size, as shown in Figure 2 [4].

Conventional carbonization is often operated at high temperature under inert gas atmosphere or vacuum conditions at 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 [12,22,42,46,58,79]. According to the preparation conditions, pyrolysis can be divided into slow pyrolysis (300-700°C, several hours, 35-50 wt% of yields), fast pyrolysis (400-600°C, <10 s, <30 wt% of yields), and flash pyrolysis (about 1000°C, <3 s, <20 wt% of yields) [58]. 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) [15,16]. Microwave heating can assist both the regular and hydrothermal carbonization owing to the distinct heating ways [14,15,16,47,81]. These approaches have been described in detail in our previous works [14,15,16,47]. Additionally, the molten salt method has also been used, in which micro-pores 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 [49]. Among the above-mentioned approaches, the conventional high-temperature carbonization has been widely applied for biochar preparation [43,82]. The limitations and merits of these approaches for the preparation of biochar are compiled in Table 3. The optimal strategy heavily depends on the properties of biomass (e.g., moisture content, composition, type of agricultural wastes) and the intended application of the biochar (e.g., soil amendment, adsorbent, catalyst).

During the preparation processes, the biomass undergoes melting, softening, vesicle formation, and decomposition with gas emission [49]; the light organics of biomass precursors are highly reacted and disrupted via volatilization [21]; 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) [49]; the existing metal components may accelerate the pyrolysis of the biomass and improve the porosity of biochar [26,83]; the oxidation induces low ash components and high gas emission under air atmosphere [54]; 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 [26,84]; 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 [85]; 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 [21]. Moreover, the porous structure of biochar can be blocked by tarry and alkaline metal components during the biomass preparation [40].

Precursor type, 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 [22,54,58,79]. Various precursors include different metals, inorganic, and organic components, impacting the pH_pzc value, morphology, and C/H and C/O ratios of biochar [84]. Wood-based biochar is usually reported to exhibit higher stability and better surface and adsorption properties than the others [55]. 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 [52,53]. SiO₂ and its content in precursors, especially for rice husk, have been reported to promote the texture, architecture, and porosity of biochar [26,85]. 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 [48]. 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 [20,58,84]. In contrast, raising the temperature (500-700°C) highly 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 [26,41,74]. 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), rise 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 subsequent adsorption ability [26,46,49,55,74]. The C content of biochar can be improved via H₂SO₄ and sonication treatment in sequence [78].

3.3. Improved Fabrication of Agricultural Residue-Based Biochar

The physicochemical processing, including activation, modification, and functional of biochar, favors the improvement of its surface and pore properties [40,46]. 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. [15,16,22,39,40,41,42]. 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 [46]. 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 [75]. Activation using CO₂ can induce the oxidation of biochar and introduce O-containing groups [83]. 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 metal (e.g., La) and their salts (e.g., Fe₃O₄) [5,25,31,38,50,83,86]. Specifically, incorporating sodium alginate in biochar can enhance metal cations adsorption via coordination interactions [87]. Coal fly ash is conductive to promoting the surface aromaticity and hydrophobicity, and reducing C=O but protecting R1(R2)C=O groups [83]. S-doping is conducive to immobilizing the metals and introducing functional groups (e.g., S²⁻ and SO₃²⁻) [50]. N-doping can enhance the surface polarization and electrostatic attraction of metal cations [49,50]. Metal-doping into biochar can decrease the electronegativity and alter the surface charge distribution, and thus improve its affinity [30]. La-doping in biochar can enhance the specific affinity of PO₄³⁻ [63]. Fe-doping could promote various properties of biochar, such as surface texture, magnetism, surface charge density, mechanical strength, thermal stability, redox ability, etc. [38,58]. 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 [58,88]. However, the modifiers may lead to some adverse effects on biochar, e.g., the blockage of pores, the decrease of the surface area, and the reduction of the economic efficiency, yield, and effectiveness of biochar (Figure 5) [50]. In addition, recyclability and regeneration of biochar are crucial for evaluating its stability, economic efficiency, sustainability, feasibility, and practicality [1]. Most of the developed biochar or their composites exhibit excellent reusability, with low reduction of the adsorption capacity after at least three adsorption-desorption cycles [5]. 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 [31,36,46,50,89]. The consumed biochar can be regenerated via high-temperature treatment (e.g., 150°C [46]) and solvent elution (e.g., 30% EtOH, distilled water, 0.1 M EDTA, HCl, NaOH, HNO₃, and H₂SO₄ solutions), which favors the valorization of waste biochar and the removal of the secondary pollution risk from the enriched contaminants in biochar [12,15,21,22,31,46,74,78].

3.4. Characterization of Biochar

Usually, the physicochemical properties of biochar can be clarified using Fourier-transform infrared spectroscopy (FT-IR), Brunauer–Emmett–Teller (BET) surface analysis, scanning electron microscopy (SEM) characterization, energy-dispersive X-ray spectroscopy (EDS, also known as elemental analysis), zeta potential measurement, X-ray diffraction (XRD), X-ray photoelectron spectroscopy analysis, and Thermogravimetric Analysis (TGA) [14,16,47]. FT-IR analysis of biochar before and after adsorption favors clarify the chemical interaction and functional groups that contribute to the effective adsorption, as well as the adsorption mechanism. BET analysis offers the porosity information of biochar, including specific surface area, pore diameter distribution, micropore, mesopore, and total pore volumes. The pore size in biochar can be divided into micro- (<2 nm), meso- (2-50 nm), and macro-pores (>50 nm) based on the International Union of Pure and Applied Chemistry (IUPAC) [14,15,84,91,92]. Mesopores of biochar are of great importance for sufficient active sites, ensuring better diffusion and mass transfer [74]. SEM images present the morphology and texture of the biochar surface, such as porosity, granularity, roughness, construction (e.g., multi-layer, spherical, tube, and core-shell structure), etc. EDS analysis can deconstruction the element composites of the biochar surface, especially useful when combined with the SEM analysis. Zeta potential analysis facilitates the illustration of the pH effects and the adsorption mechanism (e.g., electrostatic interaction between the surface of biochar and substrate molecules) [6]. The pH_PZC value relates to the pH of liquids (pH_L) where biochar’s surface is free of charge. Biochar’s surface is negatively charged as pH_L >pH_PZC, and vice versa [84]. Combining various characterizations with the adsorption properties helps to interpret the adsorption mechanisms comprehensively. XRD can identify the crystal phase composition of biochar. Crystallinity can affect biochar's reactivity, porosity, and stability, thereby impacting its performance in various environmental and agricultural applications [30,55]. XPS analysis can determine the atomic composition, contents, valence, bonding patterns, and surface properties in the biochar [1,6,26]. TGA analysis was conducted to evaluate the thermal stability and degradation behavior of the biochar over a range of temperatures. It provided insights into temperature-dependent changes in weight and helped understand the decomposition patterns of the biochar under different conditions [55].

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

Agricultural waste-biochar adsorption is a low-cost and effective wastewater remediation strategy, able to remove different contaminants, including dye, 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 [57]. Thus, biochar has been combined with other materials, e.g., poly(N-hydroxyethylacrylamide) hydrogel, to achieve better decontamination performance of wastewater [3,51,93,94]. Adsorption decontamination favors 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 [16,37,39]. 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 [26,27,46]. Similarly, the recovery of phenolic compounds from olive mill wastewater and N and S from latex industrial wastewater can also be further valued [37,79]. 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 properties like dosage of adsorbents, volume of liquids, reaction temperature, content of contaminants, pH of the matrix, contact duration, adsorption mode (batch and column modes), etc. [16,72].

In wastewater, various conditions, such as initial contaminant concentration, connection time, dosage of biochar, pH, and temperature of the adsorption system, and co-existing ions, can influence the adsorption performance of the biochar. Generally, 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 [1,11,63,74,95]. 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 remained ones in liquids via electrostatic repulsion, size exclusion, or other weak interactions [1,2,21,37,50,74]. 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 [11,30,31,50]. 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, and thus affecting the electrical interactions with the target substrates in liquids [1,5,54]. It has been reported that the alternation of biochar's surface charge can promote its adsorption selectivity [46]. 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 [6,11,30,84]. Usually, acidic conditions (pH 5-6) favor metal sorption, whereas alkaline conditions facilitate the organics and PO₄³⁻ sorption [58,85]. 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 [11,49,50]. Under alkaline conditions, organics are highly ionized, allowing high mitigation and diffusion of them [85]. Alkaline conditions favor biochar adsorption by promoting their surface negative charge density and enhancing the induced electrostatic interaction [58,84]. Alkaline conditions also favor cation adsorption via electrostatic attraction, surface complexation, and precipitation, but inhibit anion adsorption via the competitive adsorption of OH^- ion and electrostatic repulsion [21]. High temperature is 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 [14,15,47]. 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 [46,50]. 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.

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 N₂, Ar, or vacuum atmosphere, depending on the agricultural wastes. The specific surface area, pore diameter, and total pore volume of different pristine biochar 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. 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. It is worth mentioning that due to the abundant diversity of the biochar, contaminations, and the wastewater, making a comparison of the adsorption should be more cautious and difficult.

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 [46,74]. 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 [46]. The energy requirement for preparing biochar was reported to be 15-fold lower compared to activated carbon [46]. Another work states that the cost of biochar (0.08 USD/kg) is 12 times lower than other C-based adsorbents[98]. 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 [99], pine wood [99], rotten sugarcane bagasse [49], 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 0.45 billion in US dollars by 2030 globally[58]. 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 [85]. By using agricultural wastes, optimizing the pyrolysis process for energy efficiency, 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: 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 produce 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 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% [46]; 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: 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 2–7 MJ/MJ [100]. 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 Global Warming Potential and other environmental burdens; ii) gas emissions. It has been reported that the energy consumptions for biochar production were 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 [101]. 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) [102]. 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 valorising 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. For example, 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) the 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 [6,16,24,38,103]; 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; 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; and xiii) The scale-up of the obtained lab-outcomes in pilot and industrial levels.

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

This review addresses the knowledge gap on the adsorption-based removal of wastewater contaminants using biochar produced from agricultural residues. It compiles the principles of adsorption with agricultural waste-derived biochar, including the general concepts, the interactions between biochar and wastewater contaminants, and schematic descriptions of adsorption processes. It also summarises the preparation, activation, modification, functionalisation, and characterisation of such biochars, and their application to wastewater remediation. In addition, economic assessments, environmental impacts, future directions, and key challenges are presented. Diverse agricultural wastes are valorised to produce biochar that enables efficient decontamination and the recovery of valuable compounds from a range of wastewaters. Most biochar adsorbents show strong performance and reusability in real wastewater treatment. Notably, many studies are titled as investigations of biochar adsorption in wastewater, yet do not actually involve wastewater matrices or treatment scenarios. Further validation and scale-up are essential for the practical implementation of agricultural waste-derived biochar in wastewater remediation.