Optimizing Biochar for Heavy Metal Remediation: A Meta-Analysis of Modification Methods and Pyrolysis Conditions

Abstract

Modified biochars have emerged as effective adsorbents for remediating heavy metal-contaminated environments, yet variability in modification methods, feedstocks, and pyrolysis conditions has led to inconsistent findings. This study provides a quantitative meta-analysis of 173 peer-reviewed publications to systematically evaluate how modification strategies, feedstock types, and pyrolysis temperatures influence the adsorption of cadmium (Cd), lead (Pb), and copper (Cu). Six modification approaches were assessed (metal oxides, bases, strong acids, weak acids, hydrogen peroxide, and physical treatments), pyrolysis temperatures were grouped into three ranges (<400 °C, 400–550 °C, and >550 °C), and feedstocks were categorized as wood-, straw-, herbaceous-, and manure-based. Effect sizes were calculated to identify the most effective combinations of modification, feedstock, and thermal regime, providing a robust, data-driven framework for predicting biochar performance. Results show that metal oxide-treated biochars consistently exhibited the highest adsorption, while physical modifications were least effective. Moderate pyrolysis temperatures (400–550 °C) and wood-derived biochars also significantly enhanced adsorption across all three metals. These findings provide actionable guidance for designing tailored biochars, resolving inconsistencies in the literature, and supporting future studies aimed at optimizing biochar for heavy metal remediation and sustainable environmental applications.

1. Introduction

Human activities such as mining, industrial operations, and agriculture are major sources of heavy metal pollution—recognized as one of the most serious environmental threats due to their toxicity, persistence, bioaccumulation potential, and diverse origins [1,2]. Globally, more than 70 countries are affected by heavy metal contamination [3]. Among these metals, cadmium (Cd) is particularly hazardous and is widely found in rivers, lakes, agricultural soils, and industrial areas [4]. Human activities such as fossil fuel extraction, vehicle emissions, smelting engineering, agricultural operations such as over-fertilization, and wastewater irrigation are the primary contributors of Cd in the environment as compared to natural forces, including volcanic eruptions, windblown dust, forest fires, waves, and tides [5].

Lead (Pb) and Cd are two of the most toxic heavy metals that are harmful to human health, even at low concentrations [6]. While Pb can originate from natural processes such as volcanic emissions and river erosion, its bioavailability has drastically increased due to human activities like mining, fossil fuel combustion, metal manufacturing, and waste recycling [7,8]. Over one billion humans are estimated to be exposed to high amounts of Pb, resulting in more than 600,000 children being diagnosed with intellectual disabilities each year [9]. Pb pollution accounts for approximately 1% of the global disease burden, making it a critical public health concern [10,11].

Unlike Cd and Pb, copper (Cu) is an essential micronutrient but becomes highly toxic once concentrations exceed safe thresholds of around 40 ng mL⁻¹ [12]. Excess Cu can harm ecosystems, contaminate groundwater, and enter the food chain [13]. Previous research shows that soil contaminated with Cu has the potential to pollute both surface and groundwater [14]. Cu concentrations have increased significantly in many places as a result of anthropogenic activities, notably industrial and agricultural practices [15]. Excessive amounts of heavy metals are a serious problem, and it is getting harder to figure out how to remove or eliminate the pollution caused by these harmful elements [16,17]. Heavy metals can be removed from soil and water through phytoremediation, adsorption, flotation, electrocoagulation, packed bed filtration, ion exchange, and reverse osmosis [18]. Due to the high cost, theoretical scarcity, or incompleteness, some remediation procedures cannot be applied widely [19]. Currently, adsorption is the best method for handling this issue and is commonly used since it is easy to use, affordable, and remarkably effective [20].

Numerous studies have shown that biochar is an efficient way to adsorb heavy metals from the soil, improve soil quality, reduce heavy metals’ bioavailability, increase crop productivity, and prevent plants from absorbing and accumulating heavy metals [21,22,23]. Biochar is a carbon-rich material that is produced by pyrolysis (300–700 °C) of organic wastes under low- or no-oxygen conditions [24], although some studies report pyrolysis temperatures up to 900 °C [25]. Because of its great efficiency and environmental friendliness, biochar adsorption technology has received closer attention recently. It can effectively reduce the mobility and bioavailability of heavy metals in soil and water due to a large specific surface area, high cation exchange capacity, rich functional groups, and high chemical and biological stability [9,26].

Previous studies showed that pristine biochar is often less effective than modified types due to fewer functional groups, lower porosity, and surface area [19,27]. Therefore, modification methods have been widely used to improve the adsorption performance. Several modification methods for enhancing the efficiency of biochar were recommended. Chemical, physical, and metal impregnation are considered common categories for biochar modification [28]. Previous investigations have shown that metal impregnation facilitates biochar with higher sorption capacities rather chemical and physical treatments [29]. On the other hand, chemical modification made biochar contain a greater quantity of oxygen functional groups on its surface, while physical modification enhanced its specific surface area [30,31,32].

Several recent review papers have summarized the role of biochar in heavy metal immobilization [19,33,34,35,36]. However, most of these studies are narrative reviews focusing on specific metals, modification techniques, or single factors such as feedstock type or pyrolysis temperature. In contrast, the current work provides a quantitative synthesis using a meta-analysis, integrating 43 peer-reviewed studies to statistically assess how the modification method, pyrolysis temperature, and feedstock type jointly influence Cd, Pb, and Cu adsorption. To our knowledge, this is the first comprehensive meta-analysis that simultaneously compares these three controlling factors across multiple modification categories, providing quantitative effect sizes and identifying consistent patterns and trade-offs in heavy metal removal efficiency.

With a review of the literature, there could be various contradictory conclusions and recommendations about using pristine and modified biochar on heavy metal absorption and immobilization. Some research on biochar modification has focused on a certain aspect: for example, studying the effect of pristine and modified biochar on one heavy metal [37,38], one feedstock or pyrolysis temperature of biochar [39], or one modification method [40,41]. These studies are not able to provide a comprehensive and systematic understanding of the remediation of heavy metal-contaminated environments. As a result, there is a serious concern that a comprehensive meta-analysis of the research regarding the effectiveness of different modification methods, feedstocks, and pyrolysis temperature of biochar on the absorption of different heavy metals is necessary. The objective of this study was to perform a meta-analysis to better understand how different biochar modification strategies, feedstock types, and pyrolysis temperatures influence heavy metal adsorption. To achieve this, we systematically reviewed and analyzed published research that met defined inclusion criteria. The analysis evaluated the effects of (i) six modification approaches (including metal oxides, physical treatments, bases, strong acids, weak acids, and hydrogen peroxide), (ii) three pyrolysis temperature ranges (<400 °C, 400–550 °C, and >550 °C), and (iii) four major feedstock categories (wood-, straw-, herbaceous-, and manure-based) on the sorption of cadmium (Cd), lead (Pb), and copper (Cu), as well as their corresponding distribution coefficients (K_d) on biochar surfaces. This study addresses a key gap in the current literature by providing a comparative synthesis of biochar modification efficiency, offering a practical foundation for tailoring biochar applications in environmental remediation and sustainable agriculture.

2. Methods

2.1. Literature Survey and Eligibility Criteria

A structured literature search was conducted using the Web of Science, Scopus, and Crossref databases. This process followed the general principles of the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) framework to ensure transparency and reproducibility in study selection. To address the study’s main focus on the catalytic behavior of biochar in the sorption of Cd, Pb, and Cu, a set of relevant keywords was used, including biochar, pyrolysis, pyrolysis temperature, carbonization, biochar as a catalyst, feedstock, wood-based biochar, straw-based biochar, manure-based biochar, herbaceous biochar, modified biochar, activated biochar, engineered biochar, acid-modified biochar, base-modified biochar, physical modification, metal oxide modification, chemical activation, mineral modification, biochar sorption, and heavy metal stabilization, among others. To enable consistent and meaningful comparisons in the meta-analysis, studies were selected based on the following criteria: (1) inclusion of both control (pristine biochar) and treatment (modified biochar) results; (2) presence of replication for both treatment and control groups; (3) publication in a peer-reviewed journal; (4) availability in English; and (5) publication between 2011 and 2024. Titles, abstracts, and full texts were screened sequentially to identify studies meeting these inclusion criteria.

The majority of the quantitative data were extracted directly from figures using GetData Graph Digitizer 2.24. (Software, ©2002–2023, C. Fedorov, Russia). To maximize data availability, each study was also reviewed for the inclusion of Supplementary Materials, which were used when available to supplement the dataset.

2.2. Collection of Data and Heavy Metals’ Calculations

A total of 43 studies met the inclusion criteria, yielding 1400 paired observations (effect sizes). Of these, 20, 19, and 19 studies provided data for Cd, Pb, and Cu, respectively.

For the classification of data, we defined three effective factors in the catalytic behavior of biochar: (i) modification techniques, (ii) pyrolysis temperature, and (iii) type of feedstock. Then, different modification techniques were classified in seven classes based on the materials and manners, which were used in the biochar modification process as follows: (1) metal oxide (i.e., FeOx, MnOx, etc.), (2) physical (i.e., aged biochar with clay, quartz, freeze–thaw, dry–wet cycling, etc.), (3) base (i.e., NaOH, KOH, etc.), (4) strong acid (i.e., HNO₃, H₂SO₄, etc.), (5) weak acid (i.e., NaHSO₄, HF, etc.), (6) H₂O₂ < 15% (i.e., H₂O₂ with low concentration), and (7) H₂O₂ >15% (i.e., H₂O₂ with high concentration). Also, the second effective factor, which is the pyrolysis temperature of biochar, was classified into three classes as follows: (1) <400 °C, (2) 400–550 °C, and (3) >500 °C. Finally, the type of feedstock, as the third considered effective factor, was classified into four classes as follows: (1) wood-based (e.g., wood residues, bamboo, nut shells, etc.), (2) straw-based (e.g., wheat straw, maize straw, hull, etc.), (3) herbaceous-based (e.g., miscanthus, switchgrass, etc.), and (4) manure-based (e.g., sewage sludge, manure, etc.).

To explore the effectiveness of biochar and different modification techniques on the sorption of our targeted heavy metals (Cd, Pb, and Cu), we looked for the absorption capacity of different biochars in an equilibrated solution (Q_e). Therefore, we studied isotherm sorption of metal ions (mg g⁻¹) onto biochar, which contained a wide range of equilibrium concentration of metal ions (C_e) as follows: 0–970 mg L⁻¹ for Cd, 0–1200 mg L⁻¹ for Pb, and 0–900 mg L⁻¹ for Cu. After collecting Q_e and C_e, the distribution coefficient (K_d) of each metal was calculated based on Equation (1), as follows:

$$K_d={\displaystyle \frac{Q_e}{C_e}}$$

(1)

Also, the selectivity coefficient of each metal was calculated based on Equation (2):

$$\alpha ={\displaystyle \frac{K_d \mathit{metal}A }{K_d \mathit{metal}B }}$$

(2)

2.3. Meta-Analyses

A meta-analysis was used to quantify the degree and significance of variation in response to specific influencing factors. The magnitude of this variation is referred to as the effect size. In this study, effect sizes were calculated using the natural logarithm of the response ratio (RR) [42] using Equation (3):

$$\mathrm{l}\mathrm{n} (RR)=\mathrm{l}\mathrm{n} ({\displaystyle \frac{X_T}{X_C}})$$

(3)

where X_C and X_T represent the mean values for the treatment (modified biochar) and control (pristine biochar), respectively. The response ratio was converted to a percentage change using the formula (e^ln(RR) − 1) × 100 [43]. Standard deviation (SD) and sample size (n) were documented for both control and treatment groups to calculate 95% confidence intervals (CIs) around each effect size. In cases where studies reported standard error (SE) or coefficient of variation (CV), SD values were estimated using the following relationships: SD = SE × √n or SD = CV × mean. Statistical significance was determined at the 95% confidence level; effect sizes were considered significant when their CIs did not overlap with zero (p ≤ 0.05).

2.4. Statistical Analyses

Effect sizes and corresponding 95% confidence intervals for each categorical variable were calculated using SPSS software (version 28; IBM Corp., Armonk, NY, USA). A random-effects model was applied to account for variability among studies, as supported by heterogeneity assessments. Categories with fewer than three comparisons were excluded. Meta-analytical robustness was evaluated using funnel plots and Rosenthal’s Fail-Safe N method, based on 999 resampling iterations. Egger’s regression test was applied to assess publication bias. Where Egger’s test was significant (p < 0.05), Fail-Safe N values were compared with 5(n + 1) to determine robustness (Table S1). Between-group heterogeneity was tested using Qb statistics and p-values from the random-effects model, examining the influence of the modification method, pyrolysis temperature, and feedstock type on heavy metal sorption (Table S2). The results of all tests are presented in the Supplementary Materials.

3. Results and Discussions

3.1. Effect of Biochar Modification Techniques on Heavy Metal Adsorption

The adsorption of heavy metals varied significantly across different biochar modification techniques (Figure 1). Metal oxide-modified biochar demonstrated the highest adsorption efficiencies for Cd, Pb, and Cu, with effect sizes of 63.8%, 66.7%, and 58.1%, respectively. In contrast, physical modification showed the lowest performance among all techniques, with negative effect sizes of −13.1% for Cd and −9.4% for Cu. Although physical modification resulted in a slight improvement in Pb adsorption (+4.9%), this increase was not statistically significant. Base modification exhibited a moderate and consistent enhancement across all three metals, with effect sizes of 21.1% (Cd), 22.5% (Pb), and 42.1% (Cu). For strong acid modifications, a notable improvement was observed in Cd (+37.8%) and Cu (+31.7%) adsorption, whereas Pb showed no significant response to this treatment. Both weak acid and H₂O₂ modifications followed a similar pattern, improving the adsorption of all three metals. However, H₂O₂ treatment produced slightly higher effect sizes than weak acid modification, indicating superior performance in enhancing heavy metal adsorption.

The results of this meta-analysis clearly indicate that biochar modification plays a crucial role in enhancing heavy metal adsorption, with metal oxide modifications consistently delivering superior performance for Cd, Pb, and Cu. This enhanced efficiency is largely attributed to the physicochemical enhancements introduced by metal oxides—such as increased surface area, greater porosity, and the formation of additional active sites that promote stronger interactions with metal ions through mechanisms like complexation and electrostatic attraction [16]. For example, incorporating iron oxides into biochar is known to improve both magnetic properties and metal ion adsorption capacity, as the addition of iron particles facilitates the rapid binding of heavy metals [44,45]. Similarly, Mn-based biochars exhibit strong immobilization capacity due to the amphoteric nature of manganese oxide surface groups, which can engage metal ions through multiple processes, including redox reactions, complexation, adsorption, and co-precipitation. These interactions significantly alter the surface characteristics of Mn-modified biochar, resulting in improved heavy metal removal efficiencies [46,47].

This finding was further supported by the analysis of metal affinity, as reflected in the distribution coefficient (K_d). A higher K_d value indicates a stronger affinity of the metal for the sorbent surface. The K_d results (Figure 2) revealed that biochar modification significantly increased the adsorption affinity of all three metals (Cd, Pb, and Cu), reinforcing the conclusion that surface modifications enhance the number and effectiveness of adsorption sites. This improvement is likely due to increases in specific surface area and the enrichment of oxygen-containing functional groups (OFGs) on the biochar surface. In contrast to other modification techniques, physical modification led to a lower K_d compared to pristine biochar, aligning with the patterns observed in the forest plot analysis. Among all treatments, metal oxide-modified biochar exhibited the highest K_d values, with ranges of 0–50 g L⁻¹ for Cd, 0–240 g L⁻¹ for Pb, and 0–150 g L⁻¹ for Cu, outperforming all other modification strategies. The well-obvious elevated distribution coefficient (K_d) values observed for metal oxide-modified biochars further confirm their enhanced affinity for heavy metals, reinforcing their potential as highly effective sorbents for remediation applications. K_d results show that Pb and Cu have a higher affinity compared to Cd in metal oxide-modified biochar for adsorption on the surface of biochar. This can be rationalized through fundamental metal ion properties. One important factor is the ionic radius: smaller radii generally allow metal ions to approach the biochar surface more closely, enhancing interaction with functional groups. The ionic radii follow the order of Cu²⁺ (0.73 Å) < Pb²⁺ (0.77 Å) < Cd²⁺ (0.95 Å). In fact, its larger ionic radii may limit its effective interaction with the biochar surface, especially with metal oxide active sites that often involve ligand exchange or inner-sphere complexation [48,49,50].

Physical modifications, which primarily aim to alter the biochar’s surface structure without chemical alteration, showed limited or even negative effects on metal adsorption. The decrease in adsorption capacity and K_d values for Cd and Cu after physical treatments suggests that these methods may reduce the availability or accessibility of functional groups critical for metal binding or cause pore blockage [51]. This highlights that physical modification alone may be insufficient to improve biochar’s remediation performance, especially compared to chemical-based modifications. Base modifications showed moderate and consistent improvements across all metals, likely due to the increase in oxygen-containing functional groups (such as hydroxyl and carboxyl groups) that enhance cation exchange capacity and metal chelation [52]. Similarly, strong and weak acid modifications improved Cd and Cu adsorption, though Pb was less responsive, which might reflect differences in metal speciation or affinity for acidic functional groups. The relatively higher performance of H₂O₂ treatments compared to weak acids suggests that oxidative functionalization plays a significant role in increasing surface reactivity and adsorption sites [27]. These results underscore the critical role of biochar surface chemistry in heavy metal adsorption, with chemical modifications that enrich oxygen functional groups and introduce metal oxide moieties, markedly enhancing sorption capacity. This aligns with previous research emphasizing that modification strategies that increase functional group density and surface heterogeneity improve biochar’s capacity to immobilize heavy metals in contaminated soils and waters [49,53,54].

3.2. Effect of Pyrolysis Temperature on Heavy Metal Sorption

The forest plot from the meta-analysis revealed that biochars produced at moderate pyrolysis temperatures (400–550 °C) exhibited the highest adsorption capacities for Cd, Pb, and Cu (Figure 3). Specifically, biochars pyrolyzed at <400 °C showed positive effect sizes of 41.8% (Cd), 25.1% (Pb), and 66.7% (Cu). In comparison, high-temperature pyrolysis (>550 °C) resulted in 46.2% (Cd), 43.9% (Pb), and 79.5% (Cu) effect sizes. However, moderate temperatures (400–550 °C) yielded the greatest improvements, with 57.2% for Cd, 64.9% for Pb, and 92.3% for Cu.

Distribution coefficient (K_d) values provided additional evidence supporting the above observations (Figure 4). There was no significant difference in K_d values between pristine and modified biochars at pyrolysis temperatures below 400 °C, with a narrow K_d range of 0–10 g L⁻¹. However, biochars produced at 400–550 °C exhibited notably higher adsorption affinities—particularly for Cd and Pb—with K_d ranges of 0–70 g L⁻¹ for Cd and 0–140 g L⁻¹ for Pb. At pyrolysis temperatures above 550 °C, Cu showed the highest metal affinity, with a K_d range of 0–150 g L⁻¹, significantly surpassing that of pristine biochar and the other metals under the same conditions.

The results indicate that pyrolysis temperature plays a critical role in determining biochar’s adsorption capacity for heavy metals, with moderate temperatures (400–550 °C) producing biochars with the most effective sorption properties for Cd, Pb, and Cu. This is consistent with the understanding that pyrolysis temperature governs the physicochemical characteristics of biochar, including surface area, porosity, and the abundance and nature of functional groups [55,56]. At lower pyrolysis temperatures (<400 °C), biochars tend to retain more labile organic compounds and, somehow, oxygen-containing functional groups such as carboxyl, hydroxyl, and carbonyl groups, which can enhance metal sorption through complexation and ion exchange. However, the relatively lower surface area and porosity at these temperatures [51] might limit adsorption capacity, as reflected by the smaller effect sizes and narrow K_d range observed. In contrast, biochars produced at moderate temperatures (400–550 °C) seem to achieve an optimal balance, combining an increased surface area and porosity with sufficient retention of reactive oxygen functional groups. This combination likely facilitates both physical adsorption and chemical interactions with metal ions, explaining the highest observed effect sizes and K_d values in the moderate temperature range. The significantly improved adsorption affinities for Cd and Pb at moderate pyrolysis temperatures further support this hypothesis. At higher pyrolysis temperatures (>550 °C), the biochar structure becomes more aromatic and graphitic [57], with increased surface area and microporosity, but with reduced abundance of oxygen-containing functional groups [58,59,60]. This shift can reduce specific chemical binding sites. The results suggest that for Cu, which showed the highest affinity at these elevated temperatures, physical adsorption mechanisms and possibly specific interactions with graphitic surfaces may dominate. This metal-specific response highlights the complex interplay between biochar properties and metal chemistry. These findings align with previous studies demonstrating that biochar’s sorption capacity is highly sensitive to pyrolysis conditions [61]. Moderate pyrolysis temperatures appear to optimize the balance between surface area and functional group availability, resulting in superior heavy metal adsorption.

3.3. Effect of Feedstock on Heavy Metal Sorption

The influence of feedstock type on biochar’s heavy metal adsorption is a key determinant of performance. Statistical analysis revealed that wood-based biochars exhibited the highest effect sizes for Cd (65.3%), Pb (42.4%), and Cu (61.7%), all with 95% confidence intervals excluding zero, indicating significant enhancement over other feedstocks (Figure 5). Straw-based biochars ranked second, with effect sizes of 21.3% (Cd), 30.5% (Pb), and 39.8% (Cu), while manure-based biochars outperformed herbaceous-based biochars for Cd and Pb but remained less effective overall.

These differences can be mechanistically explained by the inherent physicochemical properties of the feedstocks. Wood feedstocks typically produce biochars with higher carbon content, more developed porous structures, and greater surface area after pyrolysis, providing abundant adsorption sites and promoting both physical adsorption and chemical interactions with metal ions [27,62]. These characteristics provide abundant adsorption sites and facilitate both physical adsorption and chemical interactions with metal ions [63]. Straw-based biochars, containing a mixture of cellulose, hemicellulose, and lignin, generally have moderate surface area and functional group diversity, which accounts for their intermediate adsorption performance [51]. Manure-based biochars often have higher ash content and mineral constituents, which can alter surface chemistry and reduce available binding sites for metals, explaining their lower effect sizes for Cu and moderate values for Cd and Pb [64]. Additionally, the lower adsorption affinities observed for herbaceous-based biochars may be linked to their relatively lower surface area and less favorable functional group composition after pyrolysis [65]. The distribution coefficients (K_d) corroborate these trends, with wood- and straw-based biochars exhibiting the highest K_d ranges (0–150 g L⁻¹), while modified herbaceous- and manure-based biochars show more limited improvement (Figure 6). These findings emphasize that feedstock selection directly influences the physicochemical properties of biochar, which in turn govern sorption mechanisms, including surface adsorption, ion exchange, and complexation with functional groups. Overall, the combination of feedstock type and targeted modification strategies can synergistically enhance biochar performance, and the statistically significant differences among feedstocks underscore the importance of choosing appropriate biomass sources to optimize heavy metal remediation.

3.4. Biochar Preference in Heavy Metal Absorption

As presented in Figure 7, the selectivity coefficient (α) for Cd and Pb, as well as Pb and Cd in mixed solutions, did not show a significant difference between pristine and modified biochar. The selectivity coefficient (α) represents the preference of biochar for one metal ion over another under competitive adsorption conditions. In this study, α values were affected by the relative concentrations of coexisting Cu and Cd. Notably, α values greater than 1 in the case of modified biochars indicate a higher affinity for Cu, suggesting that biochars, particularly those modified, tend to preferentially adsorb Cu rather than Cd when both are present in solution.

The selectivity analysis reveals that, under mixed-metal conditions, biochar—whether pristine or modified—does not exhibit a statistically significant difference in preference between Cd and Pb adsorption. This suggests that the modification processes do not substantially alter the competitive adsorption behavior between these two metals. However, the influence of coexisting metal concentrations, particularly Cu and Cd, appears to be a key factor in determining adsorption selectivity.

Modified biochars consistently showed higher selectivity coefficients favoring Cu adsorption over Cd, indicating a preferential affinity toward Cu ions. This preference could be attributed to Cu’s distinct chemical characteristics, such as its lower ionic radius, higher charge density, and stronger complexation tendencies with oxygen-containing functional groups on biochar surfaces [49,66].

The higher affinity for Cu may also reflect enhanced surface chemistry and active site availability introduced by modification treatments. These findings underscore the importance of considering metal speciation and solution chemistry when applying biochar for multi-metal remediation. Although modification improves overall adsorption capacity, it does not uniformly shift selectivity among metals, except in the case of Cu, which is preferentially adsorbed. This has practical implications for treating contaminated environments where metal mixtures are common, suggesting that biochar amendments might be particularly effective for immobilizing Cu in mixed-metal systems.

3.5. Study Limitations and Prospects

While this meta-analysis provides important insights into the catalytic behavior of biochar in heavy metal sorption, several limitations should be acknowledged.

First, data heterogeneity across studies presents a challenge. Variations in experimental design, biochar characterization methods, and reporting standards—such as inconsistent or missing values for surface area, pH, or elemental composition—may introduce bias or obscure finer mechanistic interpretations. Second, the analysis relied on digitized data from figures, which may result in slight inaccuracies compared to the original raw datasets. Despite careful extraction, this process inherently limits precision, especially for closely clustered data points. Third, most studies included were conducted under laboratory conditions using synthetic single-metal or binary-metal solutions. These conditions may not fully represent the complexity of real-world soil or wastewater environments, where competing ions, organic matter, and pH variability can influence sorption dynamics. Fourth, long-term stability and desorption potential of adsorbed metals on modified biochars were not assessed due to the lack of temporal data. Therefore, the durability and environmental safety of biochar amendments under field conditions remain uncertain. Lastly, the limited number of studies for certain categories (e.g., specific feedstocks or rare modification methods) restricted deeper subgroup comparisons. In particular, data for Cu sorption using manure-based biochars were unavailable, preventing complete cross-metal comparisons.

Future research should aim to (i) standardize biochar characterization and reporting practices, (ii) evaluate the performance of modified biochars under field-scale and multi-metal conditions, (iii) investigate the long-term stability and ecological effects of heavy metal-laden biochar, and (iv) incorporate life cycle assessments (LCAs) to evaluate the sustainability of various biochar production and modification strategies.

Such advancements will strengthen the application of biochar in environmental remediation and help transition from experimental trials to practical field-scale solutions.

4. Conclusions

This meta-analysis provides a comprehensive evaluation of factors influencing the catalytic sorption behavior of biochar for heavy metals Cd, Pb, and Cu. Our findings demonstrate that biochar modification techniques significantly enhance heavy metal adsorption, with metal oxide modification showing the highest effectiveness across all three metals. Pyrolysis temperature is a critical determinant of sorption capacity, with biochars produced at moderate temperatures (400–550 °C) exhibiting superior adsorption performance. Feedstock type also plays a key role, as wood-based biochars consistently outperformed those derived from straw, manure, and herbaceous sources. In multi-metal systems, biochar displays a preferential affinity for Cu over Cd and Pb, particularly following modification, highlighting the importance of considering metal selectivity in remediation strategies. These insights into modification methods, pyrolysis conditions, and feedstock choices offer valuable guidance for optimizing biochar design tailored to specific heavy metal contaminants. Overall, the study underscores biochar’s potential as a versatile and effective adsorbent for heavy metal immobilization, contributing to sustainable approaches for soil and water remediation. Future research should focus on the mechanistic understanding of selective adsorption in mixed-metal environments and long-term field validations.