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

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.