1. Introduction
The continuous increase in the variety and quantity of environmental contaminants released into soil, water, and air has raised growing concern worldwide [1, 2]. Contaminants such as microplastics and pesticides are known to persist in the environment, particularly in the agricultural environment, for extended periods. Once introduced, these contaminants can accumulate in ecosystems and eventually pose adverse effects on public health [3–5]. Pesticides are recognized as persistent environmental contaminants, and their global use has steadily increased to meet the growing demand for agricultural productivity [6]. Despite their effectiveness in crop protection and yield improvement, a substantial proportion of applied pesticides (approximately 30–50%) remains in agricultural soils [7]. This residual accumulation poses potential risks to soil health and agricultural ecosystems. Previous studies reported that up to 16 out of 46 applied pesticides could still be detected in soils even after 20 years [8], highlighting the persistence and limited degradability of pesticides in the natural environment. Residual pesticides can also alter the physicochemical properties of soil, and negatively affect soil microbial communities, including activities of various soil enzymes [8, 9].
Soil serves as a major sink for plastic contamination, containing 4 to 23 times higher plastic loads than marine environments [9, 10]. Plastic wastes can undergo various weathering processes, breaking down into microplastics (<5 mm in size) [11, 12]. Polyethylene (PE), polypropylene (PP), polystyrene (PS), polyethylene terephthalate (PET), and polyvinyl chloride (PVC) are commonly detected plastic polymers in soils, with reported maximum concentration reaching up to 7% (w/w) in site-specific heavily contaminated soils (e.g., industrial-area soils) [13, 14]. Microplastics in soil can pose multiple ecological risks. For example, microplastics can act as carriers for other environmental contaminants owing to their reactive surface functional groups (e.g., –COOH, –OH, –C=O, –NH2, –SO3H) [15]. They can also interfere with the uptake of water and nutrients by plants, resulting in reduced crop yields [16]. Furthermore, microplastics may induce oxidative stress and disrupt metabolic processes in plants [17]. In addition to microplastic particles, leachable additives such as diethyl phthalate released from microplastics can intensify soil contamination and deteriorate overall soil health [18].
Soil contaminants can be remediated using a variety of physicochemical methods (e.g., soil washing, oxidation/reduction processes, stabilization, and photocatalytic degradation) and biological methods (e.g., phytoremediation and bioremediation) [19, 20]. Among various approaches, biochar has gained significant attention as a promising material for contaminant removal, with broad applicability in areas such as climate change mitigation, improvement of agricultural environments, and energy production [21]. Biochar is a carbon-rich material produced via the thermochemical conversion of organic feedstocks under oxygen-limited conditions [22]. Its high carbon content and porous structure with large surface area facilitate the adsorption and immobilization of contaminants through various mechanisms such as pore filling, complexation, ion exchange, precipitation, π-interaction, and electrostatic attraction [23, 24]. Additionally, biochar can be produced from a wide range of feedstocks (agricultural residues, forestry resources, industrial by-products, and municipal wastes) making it widely available and cost-effective [25]. Moreover, biochar systems can reduce net greenhouse gas emissions in waste-to-resource conversion processes through long-term carbon storage in soils and fossil-fuel offsets from utilizing pyrolysis co-products (e.g., syngas and bio-oil) [26]. When applied to soil, biochar can also improve soil quality, nutrient availability, and microbial activity, leading to enhanced crop yields [27, 28].
An important consideration in soil remediation is the preservation of soil health while removing contaminants. Increasing emphasis has been placed on minimizing adverse impacts on soil physicochemical and biological properties during remediation to maintain a sustainable soil environment, particularly in agricultural environments [29]. In this context, biochar has emerged as an effective remediation alternative, as it can efficiently remove contaminants without compromising soil health [30]. Recent studies on biochar have primarily focused on its potential application as a soil amendment and adsorbent for various individual contaminants [31]. Most of these studies have investigated single-contaminant systems, while multiple contaminants often coexist in real-world soil contamination scenarios [31]. Therefore, the objective of this review is to provide a comprehensive analysis of current research on the use of biochar for the remediation of contaminated soils, with a particular focus on pesticides and microplastics. Specifically, it examines how factors such as biochar production conditions, feedstock characteristics, and surface modification methods influence contaminant removal efficiency and identifies knowledge gaps. Based on this analysis, the review also proposes future research needs for the effective application of biochar in soils contaminated with mixed contaminants.
2. Literature Review Methodology
This review comprehensively examines the effects of biochar on the remediation of pesticides and microplastics in soils by collecting relevant articles from the Web of Science (WOS) database. The search results were restricted to the document type “Article” (original research articles), and review papers and other document types were excluded. For studies related to microplastics, the literature search was conducted using the keyword combinations of “biochar*” AND “microplastic*” AND “soil*” AND (“sorp*” OR “adsorp*” OR “remov*” OR “immobiliz*”), which resulted in the identification of 137 articles published between 2018 and 2025. During the screening stage, abstracts were reviewed to select studies that simultaneously addressed biochar and microplastics in soil environments with a focus on common polymers such as PE, PS, polylactic acid, PP, PVC, and PET and reported outcomes related to sorption, adsorption, removal, remediation, or immobilization. Consequently, a total of 35 studies were finally included in this review. For pesticide-related studies, the keyword “pesticide*” was used in place of “microplastic*” in the WOS database, yielding 416 articles published between 2009 and 2025. Following the same abstract-based screening procedure applied to the microplastic literature, studies investigating the adsorption and mitigation of pesticides, including herbicides, insecticides, fungicides, and pesticide residues, in soils using biochar were selected. As a result, 171 studies were ultimately included in this review.
In addition, based on the retrieved data, a keyword-based thematic mapping analysis was conducted using the Bibliometrix to identify major research themes and trends within this field. The thematic map visualizes clusters derived from the keyword co-occurrence network based on relevance (centrality) and development (density), thereby illustrating the relative position of each theme within the research field and its degree of internal development [32]. Motor themes are characterized by high centrality and high density, representing well-developed topics with strong connections across the field and a mature internal structure [32]. Basic themes exhibit high centrality but low density, suggesting that they form the foundational core of the field while remaining relatively less developed internally [32]. Niche themes show high density but low centrality, reflecting specialized and internally cohesive topics that are less connected to the broader research landscape [32]. Emerging or declining themes have both low centrality and low density and are interpreted as candidate topics that are either newly developing or losing attention [32].
3. Research Trends in Biochar-Based Contaminant Remediation
Research on pesticide adsorption using biochar has expanded steadily since 2009, with a particularly sharp rise observed after 2017, reaching a cumulative total of 171 studies by 2025 (Fig. 1). In contrast, studies on microplastic remediation using biochar only began to emerge in 2020. Although the number of studies has increased in recent years, the total number of studies remains relatively small, reaching only 35 by 2025 (Fig. 1). These trends indicate that biochar-based microplastic remediation is a relatively new and still developing research field that requires further exploration. The themes related to biochar-based pesticide research were mainly clustered within the motor and emerging or declining theme quadrants (Fig. 2(a)). Specifically, the identified keyword clusters represent major topical foci, including biochar sorption-driven changes in pesticide bioavailability (“biochar-sorption-bioavailability”), biochar-associated pesticide degradation and integrated remediation strategies (“pesticides-degradation-remediation”), carbon-based adsorption studies using atrazine as a model herbicide (“adsorption-carbon-atrazine”), amendment effects on herbicide leaching and transport (“leaching-herbicides-amendments”), pesticide environmental fate and risk (“dissipation-impact-sorption/desorption”). The “biochar-sorption-bioavailability” cluster was positioned within the motor themes quadrant, indicating that research integrating biochar-mediated sorption and bioavailability represents a well-developed core topic with strong connectivity across the field. The “adsorption-carbon-atrazine” cluster was located in the right-hand region with high centrality, suggesting that studies linking carbon-based adsorption processes with atrazine, a representative herbicide, have been actively conducted. The “pesticides-degradation-remediation” cluster appeared in the upper region near the central axis, showing relatively high internal density for themes related to pesticide degradation and remediation. The clusters positioned within the emerging or declining theme quadrant represent research topics that are important but insufficiently integrated into the broader research framework. The presence of the “dissipation-impact-sorption/desorption”, “leaching-herbicides-amendments”, and “soil-water-removal” clusters in this quadrant suggests that research topics related to dissipation, sorption/desorption, herbicide leaching, and soil-water removal have been investigated through short-term or site-specific studies, limiting their consolidation into a cohesive theme. Thus, these topics need further integration with long-term behavior and co-contamination scenarios to become mature research themes.
Themes related to biochar-based microplastic research are distributed across all four thematic quadrants and structured around basic themes related to adsorption-based removal (Fig. 2(b)). Specifically, the identified keyword clusters represent major topical foci, including foundational biochar-microplastic interactions in soils (“microplastic-biochar-soil”), links between biochar production conditions, particularly pyrolysis temperature, and removal performance (“removal-pyrolysis temperature-contaminants”), physicochemical attachment and immobilization mechanisms (“adsorption-sorption-immobilization”), ionic-strength-dependent interactions in modified biochar systems (“ionic strength-modified biochar-particles”), co-contaminant remediation and competitive interactions involving heavy metals (“heavy metals-remediation-competitive adsorption”), dissolved organic matter-driven aggregation and colloid-facilitated transport “humic acid=aggregation-colloid transport”), and themes connecting pollution to bioavailability/biodegradation processes (“pollution-bioavailability-biodegradation”) and to the fate of co-occurring organics such as polycyclic aromatic hydrocarbons (PAHs) (“degradation-organic compounds-PAHs”). Core themes such as “microplastics-biochar-soil”, “removal-pyrolysis temperature-contaminants”, and “adsorption-sorption-immobilization” are positioned within the basic theme quadrant, indicating a developing stage of research related to these themes. The “ionic strength-modified biochar-particles” cluster within the motor theme quadrant indicates an active and internally cohesive research stream related to modified biochar systems considering ionic strength-dependent interactions. These clusters together suggest that biochar-based microplastic remediation is an emerging research area that is still consolidating fundamental mechanisms, with research on material modification strategies leading research paradigm. The “heavy metals-remediation-competitive adsorption” cluster, located in the emerging or declining theme quadrant, reflects growing interest in biochar-based microplastic remediation under co-contamination scenarios where heavy metals coexist with microplastics. Meanwhile, niche themes focusing on colloid transport, bioavailability, biodegradation, and PAHs indicate increasing specialization into diverse topics that are weakly connected to the broader research framework, highlighting potential future integration.
4. Factors Influencing Biochar Properties
Biochar, produced through pyrolysis under oxygen-limited conditions, typically contains 40–75% carbon along with small amounts of minerals and volatile organic compounds. Its large surface area and abundant functional groups contribute to a strong adsorption capacity for environmental contaminants [33]. The adsorption efficiency of biochar is largely determined by its physicochemical characteristics, including surface area, porosity, functional groups, pH, cation exchange capacity (CEC), and proximate composition (i.e., contents of fixed carbon, volatile matter, and ash) [34]. For instance, greater surface area and porosity favor physical adsorption, while surface functional groups and CEC play critical roles in chemical interactions with contaminants [34]. Additionally, a higher carbon content enhances the structural stability and hydrophobicity of biochar, thereby improving its affinity for hydrophobic organic contaminants [34]. Importantly, these physicochemical properties are influenced by various factors such as the type of feedstock, pyrolysis conditions, and subsequent surface modification [35]. Understanding how these factors shape the properties of biochar is therefore essential for optimizing its performance in removing pesticides and microplastics from soil.
4.1. Effect of Types of Feedstocks on Biochar Properties
Various types of feedstocks, including wood waste, crop residues, animal manure, and other organic solid wastes, have been widely used for biochar production. The physicochemical properties of biochar largely depend on the intrinsic characteristics of the feedstock. For example, lignin-rich feedstocks such as coconut shells and softwoods produce biochars with low volatile matter and high carbon content [34, 36]. In contrast, cellulose- and hemicellulose-rich feedstocks, including herbaceous plants and agricultural residues, tend to yield biochars with higher ash and volatile matter contents, greater porosity, and alkaline pH values [34]. Wood-derived biochars typically possess high levels of lignin, cellulose, and carbon, as well as large specific surface area, but they generally contain fewer plant-available nutrient [37]. Conversely, manure-derived biochars exhibit higher nutrient contents but lower carbon content due to their abundant inorganic fractions and lower lignin content [37]. Different feedstocks also contain different moisture contents, influencing biochar quality. Feedstocks with high moisture content (>50%) generally produce biochars with reduced carbon content but greater surface area and porosity, whereas feedstocks with low moisture content (<10%) yield biochars with higher carbon content but relatively lower surface area and porosity [38]. These differences in biochar properties directly affect contaminant removal efficiency. For example, hazelnut shell-derived biochar exhibited higher adsorption efficiencies for atrazine and terbuthylazine than rice husk-derived biochar, owing to its aromatic carbon structure and well-developed microporosity that promote π-π interactions and pore-filling [38]. In contrast, the high silica-based ash content of rice husk-derived biochar limited carbon-based adsorption sites, resulting in lower affinity for these pesticides [39].
4.2. Effect of Pyrolysis Conditions on Biochar Properties
Apart from the types of feedstocks, pyrolysis conditions are critical parameters influencing biochar properties [38]. Pyrolysis thermally decomposes biomass under oxygen-limited conditions, typically at 250–900°C, producing solid (biochar), liquid (bio-oil), and gaseous (syngas) products [40]. Depending on operating conditions such as heating rate, temperature, residence time, and pressure, the process can be divided into fast pyrolysis and slow pyrolysis [40, 41]. Fast pyrolysis is generally conducted at high temperatures (>500°C) with rapid heating rate (~100°C/min) and short residence times (0.5–2 sec). This process maximizes the production of bio-oil and syngas but yields relatively little biochar (10–20%) [40]. Biochar produced under these conditions tends to be more hydrophobic due to enhanced aromaticity and exhibits strong adsorption potential for organic contaminants. In contrast, slow pyrolysis operates at lower temperatures (300–700°C), with slower heating rates (~57°C/min) and extended residence time (>1 h). This process results in higher biochar yields (35–50%) [40]. Biochars derived from slow pyrolysis typically possess higher porosity and surface area, along with abundant oxygen-containing functional groups, which make them effective for soil improvement, carbon sequestration, and contaminant remediation [40]. Pyrolysis temperature also plays a pivotal role in determining biochar properties, and biochars can be broadly categorized as low-temperature (200–400°C), medium-temperature (400–600°C), and high-temperature (>600°C) products [42]. Low-temperature biochars retain abundant oxygen-containing functional groups (e.g., carboxyl, phenolic groups), resulting in high CEC, although they generally exhibit acidic pH and higher volatile matter content. Medium-temperature biochars develop greater pore structures, higher surface areas, and increased carbon content. High-temperature biochars exhibit the most extensive carbonization, with elevated surface area and porosity, alkaline pH, and reduced O/C and H/C ratios, but fewer oxygen containing functional groups [42, 43].
4.3. Effect of Surface Modification on Biochar Properties
The pristine form of biochar derived from biomass often exhibits limited efficiency in removing specific classes of contaminants, which has prompted a growing body of research aimed at developing surface modification strategies to overcome these inherent limitations [44]. Broadly, these approaches can be categorized into three types – physical, chemical, and biological modifications – each of which operates through distinct mechanisms to improve physicochemical characteristics of biochar and consequently its adsorption performance. Physical modification generally focuses on enhancing textural properties such as surface area and pore structure, thereby providing greater accessibility and more active sites for contaminant interaction. Commonly employed techniques include steam activation, heat treatment, and ball milling [45, 46]. Steam activation involves exposing biochar to high-temperature steam, which introduces additional porosity and oxygen containing functional groups; although this process significantly improves hydrophilicity, the extent of chemical functionality imparted is often less pronounced compared to treatment with strong oxidizing agents [45]. Heat treatment, by contrast, involves heating biochar at 800–900°C followed by additional processing in a hydrogen (H2), nitrogen (N2), or argon (Ar) atmosphere, which not only introduces new functional groups but also strengthens the aromatic carbon framework, ultimately increasing the hydrophobicity of the biochar [45]. Ball milling further extends the potential of physical modification by mechanically reducing biochar to nanoscale dimensions, thereby dramatically increasing its surface area and improving pore accessibility, which in turn enhances its performance in contaminant removal [46]. On the other hand, chemical modification seeks to directly tailor the surface chemistry of biochar by introducing functional groups such as oxygen-, nitrogen-, and sulfur-containing moieties, or by incorporating metal oxides and nanoparticles [45, 47]. Among the most widely applied approaches are acidic and alkaline treatments, impregnation methods, and magnetic modification. Acidic treatments using strong acids such as nitric acid (HNO3), sulfuric acid (H2SO4), or phosphoric acid (H3PO4) are effective in generating abundant acidic functional groups, such as carboxyl and hydroxyl groups, which enhance hydrophilicity and improve the adsorption of contaminants [47]. Alkaline treatments, typically employing potassium hydroxide (KOH) or sodium hydroxide (NaOH), increase the surface basicity of biochar, thereby improving its ability to adsorb a wide range of organic contaminants [47]. Impregnation methods involve coating biochar surfaces with metal oxides (e.g., Fe, Mn, Zn, Mg) or nanomaterials to increase adsorption sites and impart catalytic activity. Magnetic modification introduces magnetic substances such as Fe3O4 or γ-Fe2O3, which not only enhance the interaction with contaminants but also impart magnetic properties, facilitating the easy separation and recovery of biochar from aqueous media [45]. Biological modification, although relatively less explored compared to physical and chemical routes, represents a promising strategy by employing microorganisms or enzymes to alter surface chemistry in ways that are otherwise difficult to achieve through conventional methods. For example, biochar inoculated with phosphate-solubilizing bacteria has been shown to significantly enhance its ability to immobilize lead (Pb) [45]. Taken together, these surface modification techniques, whether implemented individually or in combination, can markedly strengthen the contaminant removal performance of biochar and substantially broaden its scope of application in environmental remediation and catalytic processes. The growing body of literature in this field underscores not only the versatility of biochar as a remediation material but also the importance of carefully matching modification strategies to specific target contaminants and environmental contexts.
5. Adsorption of Pesticides by Biochar
Previous studies employing biochar as an adsorbent for reducing residual pesticides in soils are summarized in Table 1. In soils, pesticide removal by biochar is governed by multiple adsorption mechanisms, including pore filling, hydrogen bonding, hydrophobic interaction, π-π interaction, electrostatic attraction depending on biochar and pesticide properties (Fig. 3) [48]. Column leaching experiments have been commonly used to evaluate pesticide-biochar interactions, and most results demonstrated decreased pesticide leaching and mobility following biochar application (Table 1). For instance, the addition of wheat residue biochar to soils contaminated with four different pesticides reduced pesticide leaching by 93–98%, primarily due to the adsorption of pesticides onto the biochar surfaces, which limited their mobility [49]. Interestingly, among the four pesticides, leaching of pyraclostrobin increased twofold, likely because its association with dissolved organic matter released from the biochar enhanced its mobility [49]. This finding highlights that pesticide-biochar interactions in soil are influenced not only by the properties of biochar but also by the chemical characteristics of pesticides themselves. In addition, biochar aging can significantly affect pesticide adsorption and contaminant mobility [50]. Aging refers to post-application weathering in soils (e.g., incubation or field exposure), during which biochar surfaces and pores interact with soil organic matter and mineral phases [50]. For example, a wood-derived biochar aged by incubating a soil–biochar mixture for 3.5 months exhibited lower adsorption capacities for two herbicides (diuron and glyphosate) than the corresponding fresh biochar-amended soil [51]. This decrease has been attributed to interactions with soil organic matter and mineral constituents during aging, which can coat biochar surfaces and partially block pores, thereby reducing the accessibility of adsorption sites. In addition, aging-related increases in pH can further influence the interactions among glyphosate, soil, and biochar surfaces. Similarly, another study reported that a fresh plant-derived biochar achieved high leaching reduction (80–100%) for sulfamethoxazole and ethofumesate, whereas after 90 days of aging the reduction decreased to 40–44% [52]. These findings suggest that aging-induced changes in the structure and surface properties of biochar can attenuate its sorption capacity over time, resulting in relatively greater leaching.
Biomass pretreatment has been employed as a strategy to enhance the adsorption capacity of biochar by increasing its specific surface area and porosity [53]. In a column leaching experiment with diazinon-contaminated soil, the application of 5% larch- and oak-derived biochars resulted in diazinon removal rates of 23% and 22%, respectively [53]. However, when these biochars were sulfonated prior to pyrolysis, the removal rates increased substantially to 77% and 89%, respectively [53]. Beyond pretreatment, surface modification has also been widely applied to improve the adsorption performance of biochar [54]. For example, in soil contaminated with the herbicide metolachlor, unmodified walnut shell biochar achieved a removal efficiency of 58.3%, whereas Fe- and illite-modified biochars exhibited higher removal efficiencies ranging from 73.3% to 92.4% [54]. These improvements are generally attributed to increased pore volume and enriched surface functional groups generated during the modification process [54]. However, not all surface modifications necessarily enhance adsorption. For instance, unmodified rice straw biochar reduced simazine leaching by 70.9%, which was comparable to or even greater than that achieved with iron-modified biochars prepared using ferric chloride (FeCl3) and iron oxy-sulfide (FeOS), which showed efficiencies of 48–73.3% [55]. This reduction in performance may be partly explained by the loss of organic carbon and changes in oxygen-containing functional groups during the iron modification process, which lowered adsorption capacity [55]. Moreover, the antimicrobial effects of FeOS may have disrupted the soil microbial community, reducing the potential for pesticide degradation [55]. Collectively, these findings indicate that both the feedstock characteristics and the physicochemical properties of the target contaminants must be carefully considered when applying pretreatment or surface modification to biochar for enhanced adsorption performance.
In addition to leaching experiments, the effectiveness of biochar in reducing pesticide contaminants has also been evaluated by assessing pesticide uptake by plants [56]. For example, in soils contaminated with chlorpyrifos and fipronil, the application of cotton-derived biochar followed by the cultivation of Chinese chives (Allium tuberosum) reduced the uptake of both pesticides compared to the control (i.e., no biochar application) and simultaneously promoted plant growth [56]. Biochar produced at 850°C exhibited a higher surface area and porosity than that produced at 450°C, resulting in greater effectiveness in reducing pesticide uptake [56]. This indicates that biochar can mitigate pesticide uptake in plants while enhancing growth performance. Moreover, when iron-modified biochar was applied, pesticide uptake was reduced even further [57]. This improvement is likely due to the release of iron ions into the soil, which promoted the formation of an iron oxide layer on root surfaces, serving as a physical barrier that hindered pesticide absorption [57]. Additionally, iron-modified biochar was shown to increase soil microbial diversity, thereby facilitating biological degradation of pesticides [57]. Higher application rates of biochar were also associated with further reduction in pesticide uptake, highlighting the combined role of enhanced adsorption capacity and increased microbial activity [56, 57].
6. Adsorption of Microplastics by Biochar
Biochar, owing to its high porosity, large surface area, and diverse surface functionalities, has recently been investigated as a promising amendment for reducing microplastic contamination in soil, and relevant studies are summarized in Table 2 [12, 62]. The interaction between biochar and microplastics primarily occurs through physicochemical adsorption mechanisms, including electrostatic interactions, van der Waals forces, hydrogen bonding and hydrophobic interactions, which collectively enhance the ability of biochar to immobilize microplastics (Fig. 4) [27, 63]. For instance, in column experiments with microplastic- contaminated soils, the application of banana peel-derived biochar achieved nearly complete (up to 100%) removal of microplastics, and this was attributed to its highly porous structure and strong hydrophobic interactions [64]. Mechanistically, microplastic retention by biochar has been explained by three dominant processes operating within its porous network: physical blockage, confinement, and entanglement [65]. However, the immobilization effect may not remain constant under field conditions. Repeated wetting-drying cycles driven by rainfall and evaporation can alter soil crack development and preferential flow pathways, thereby influencing microplastic mobility and retention efficiency [66]. In soil column experiments, wetting-drying cycles increased microplastic leaching, whereas the presence of biochar mitigated these effects by enhancing water retention and suppressing crack development, ultimately improving microplastic retention [66].
The efficiency of biochar in reducing microplastics is strongly influenced by its production conditions and application parameters. Pyrolysis temperature, in particular, plays a decisive role in shaping biochar properties and consequently its adsorption performance. In general, biochars produced at higher pyrolysis temperatures exhibit enhanced microplastic removal due to increased surface area and porosity [65, 67]. For example, jujube waste-derived biochar showed a 20-fold increase in the removal of nylon and PS microplastics at 300°C, and a 30-fold increase at 700°C compared with the control [67]. Similarly, biochars derived from lignin, cellulose, and wood chips demonstrated greater removal of PS microplastics at 700°C than at 400°C, highlighting the positive correlation between thermal processing and adsorption efficiency [68]. Nevertheless, this relationship is not strictly proportional. For example, wheat straw biochar exhibited only a modest increase in removal efficiency (from 86.8% to 89.7%) as the pyrolysis temperature increased from 500°C to 700°C, while cow dung biochar showed a decline from 92.4% to 88.1% under the same conditions [69]. These contrasting findings underscore that feedstock characteristics interact with pyrolysis temperature to determine microplastic removal capacity. In addition to pyrolysis conditions, biochar application rate also exerts a significant influence. Increasing the application rate of wood-derived biochar has been shown to suppress microplastic mobility more effectively, consistent with greater adsorption site availability [68]. Microplastic particle size further governs removal efficiency, with smaller particles more likely to be retained with the biochar matrix. Column studies demonstrated that smaller microplastic fractions led to lower eluent concentrations in biochar-amended soils, and this was explained by the greater propensity of small particles to become trapped within pores or adsorbed onto biochar surfaces due to their higher specific surface area [70].
Beyond physical adsorption, biochar can also contribute to the degradation of microplastics while simultaneously influencing soil microbial dynamics and plant growth [71–73]. For instance, chicken manure- and wood waste-derived biochars accelerated the degradation of polylactic acid (PLA) microplastics in contaminated soils by promoting alkaline conditions conducive to aminolysis, ammonolysis, oxidative reactions, and microbial activity [74]. Similarly, the addition of date palm-derived biochar to acrylic plastic-contaminated soils increased microbial activity and improved the physiological performance of Vicia faba, including increased chlorophyll content, elevated enzyme activity, and greater dry biomass production [75]. Overall, these findings highlight the multifaceted role of biochar in mitigating microplastic contamination, functioning not only as an effective adsorbent but also as a catalyst for degradation processes and a stimulator of beneficial soil-plant-microbe interactions.
7. Challenges and Future Perspectives
Biochar has emerged as an effective amendment for reducing a wide range of contaminants in soil. This review has examined its application in the remediation of pesticides and microplastics, both of which are critical contaminants of concern in agricultural systems. Although numerous studies have demonstrated the potential of biochar and compared its performance under diverse experimental conditions, several important research gaps remain. Addressing these gaps will be essential to move from laboratory findings to practical and sustainable field applications.
First, further research is required to refine and optimize biochar surface modification techniques. Various physical, chemical, and biological modification approaches have been applied to enhance the adsorption capacity of biochar. However, although many studies have reported performance changes after modification, relatively few have explained these changes using consistent metrics and methods that link contaminant mitigation performance to changes in key physicochemical properties (e.g., specific surface area, pore structure, and surface functional groups). As a result, it remains difficult to generalize which modification strategies are most effective under specific conditions [79]. In addition, most studies have not sufficiently addressed the fate of modified biochar after contaminant adsorption, including its regeneration, safe handling, and potential for reuse or recycling [79, 80]. For example, in the case of metal or magnetically modified biochar, concerns have been raised regarding the leaching of modification components after soil application and the associated risk of secondary pollution; however, standardized approaches to evaluate and compare these potential risks using unified criteria are still lacking [80]. Therefore, future studies should quantitatively link changes in biochar characteristic to remediation performance using comparable indicators. Also, an integrated assessment framework that evaluates post-adsorption regeneration, safe handling, and reusability together with secondary risks (e.g., leaching and secondary pollution) associated with modification components needs to be established.
Second, most existing studies have focused on batch or column experiments conducted at the laboratory scale, under controlled conditions that do not fully reflect the complexity of natural soils (Table 1 and Table 2). In real agricultural systems, soil texture, organic matter content, climate variability, and microbial communities strongly influence the performance of biochar [81]. Thus, laboratory-scale findings focused on short-term removal efficiency are insufficient to assess real-world applicability underscoring a pressing need to expand field-based trials across diverse soil types and environmental conditions [81]. In addition, it is necessary to systematically consider the aging processes that biochar undergoes over time after being applied to soil [50]. For example, soil organic matter may coat biochar surfaces and changes in soil redox conditions can alter the surface chemical properties and the availability of reactive adsorption sites [82, 83]. Because these processes may affect long-term adsorption performance, the potential remobilization of contaminants, and overall environmental stability, future studies should place greater emphasis on durability assessments that go beyond short-term laboratory tests and are linked to long-term field experiments [50, 84]. In addition, the long-term stability and field performance of biochar in actual farmland soils remain insufficiently constrained. While multi-year field trials and meta-analyses (1–10 years) indicated that biochar can sustain improvements in soil physical/hydraulic properties and carbon-related functions, the persistence and magnitude of benefits vary with soil type, biochar quality, and application strategy (e.g., one-off vs repeated applications) [85]. Therefore, more long-term field scale studies are needed to evaluate durability under realistic management and climate variability and to identify conditions under which remediation benefits persist.
Third, research on biochar-based mitigation of microplastics should be expanded in a broader and more systematic manner, particularly in terms of the range of target microplastics and the consistency of experimental design. Current research has focused predominantly on PS, which does not adequately represent the heterogeneity of plastics commonly detected in the environment, such as PE, PP, and PET (Table 2) [86]. To enhance practical applicability, studies should encompass a wider spectrum of microplastic polymer types including PE, PP, and biodegradable plastics (e.g., PLA). Furthermore, existing column studies have often employed heterogenous experimental settings for key variables, including biochar application methods, microplastic particle sizes and characteristics, and microplastic concentrations (Table 2), which complicates direct comparison among studies [86, 87]. Therefore, systematic investigations based on standardized experimental protocols where these core variables are stepwise and independently controlled are required to quantify conditionperformance relationships and to develop predictive models for microplastic immobilization under different environmental scenarios.
Finally, future studies should move toward assessing the performance of biochar in environments contaminated with several contaminants. In real-world settings, microplastics frequently co-exist with pesticides, and other organic contaminants, and these co-contaminants may influence each other’s mobility and bioavailability. For instance, in soils containing microplastics, the adsorption and desorption behavior of pesticides can be altered and such changes may in turn influence soil microbial communities and degradation processes, potentially affecting bioaccumulation in plants [88]. Apart from pesticide degradation, microplastics also affect availability and cycling of nutrients (e.g., phosphorus) in soil [89]. Moreover, because pesticide adsorption behavior can vary depending on the aging state and particle characteristics of microplastics, conclusions derived from single-contaminant systems may not be directly applicable to co-contaminated scenarios [90]. Nevertheless, based on our WOS search and screening (2009–2025), no direct experimental studies were identified that quantitatively evaluate the removal/mitigation performance of biochar in soils co-contaminated with both microplastics and pesticides, highlighting a critical evidence gap. However, a few studies have reported biochar effects in soil-plant systems where microplastics co-occur with antibiotics [91]. For example, in a soil-vegetable system where PE and PLA microplastics co-existed with an antibiotic, biochar application reduced the extractable concentration of the contaminants in soil and its accumulation in plants; however, the presence of microplastics was also observed to hinder the sorption and immobilization performance of biochar [91]. Similar interaction-driven shifts in biochar performance have also been reported for other co-contamination combinations (e.g., microplastic co-existing with heavy metals), where competitive effects can alter adsorption behavior [92, 93]. Because such findings were derived from a specific combination of polymer types and contaminants under constrained experimental conditions, they are unlikely to fully represent the wide range of interactions that can occur between diverse microplastics and pesticide classes in real agricultural environments. Therefore, future work should adopt systematic experimental designs that better reflect realistic co-contamination scenarios, quantitatively evaluate the performance of biochar, and establish an integrated assessment framework that accounts for interaction effects among co-contaminants.
In addition, large-scale implementation of biochar requires not only experimental validation of remediation efficacy but also techno-economic and cost-benefit assessments [94, 95]. For example, an analysis of biochar production and agricultural use systems across six countries using life cycle assessment (LCA) combined with economic evaluation showed that environmental benefits were generally observed, whereas economic benefits could vary substantially depending on the country and production technology [94]. Another study comparing the use of biochar for soil improvement versus its use as a fuel reported that the soil-improvement pathway was more favorable in both LCA and cost-benefit analysis [95]. Therefore, future studies should evaluate not only soil remediation performance but also cost-benefit conditions for practical deployment by incorporating feedstock supply and logistics, production scale, application costs, and co-benefits such as carbon sequestration and improved agricultural productivity.
8. Conclusions
Biochar is a sustainable and environmentally friendly material with considerable potential for mitigating soil contaminants, particularly pesticides and microplastics. Its distinctive physicochemical characteristics such as high porosity, large surface area, and modifiable surface chemistry enable multiple mechanisms of contaminant removal, including adsorption, entrapment, and enhanced microbial degradation. Numerous studies have demonstrated that biochar can effectively reduce pesticide leaching, lower plant uptake of agrochemicals, and suppress the mobility of microplastics in soil, while simultaneously promoting soil health and plant growth. Importantly, factors such as feedstock type, pyrolysis temperature, and surface modifications have been shown to significantly affect adsorption efficiency and long-term stability of biochar, highlighting the importance of tailored design for specific remediation targets. Despite these promising findings, several gaps remain before biochar can be applied widely in agricultural and environmental systems. Future research should prioritize the optimization of surface modification strategies and a clearer understanding of their effects on the physicochemical properties, environmental persistence, and reusability of biochar. In addition, large-scale field studies are essential to validate laboratory findings and to evaluate biochar performance under complex conditions of real soils. With regard to microplastics, research must extend to include a broader diversity of plastic types and particle sizes that more accurately reflect environmental contamination. Furthermore, greater attention should be given to the behavior of biochar in multi-contaminant environments, where interactions among pesticides, microplastics, and other contaminants may alter its adsorption mechanisms and remediation efficiency.
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