Effects of metal oxides loaded with biochar on adsorption of cationic and anionic metals: A meta-analysis

Biochar stands as a globally recognized, highly effective agent for remediating heavy metal pollution. However, competitive adsorption efficiency of metal oxide-loaded biochar for heavy metals remains poorly understood, owing to insufficient comparative assessment across biochar properties and modification methods. This critical meta-analysis study addressed this significant gap by comprehensively evaluating the efficacy of metal oxide-loaded biochar in adsorbing both cationic (e.g., Cd²⁺, Pb²⁺) and anionic (e.g., AsO₄³⁻, CrO₄²⁻) heavy metals from contaminated environments. The results confirmed that metal oxide modification dramatically enhanced biochar's adsorption capacity. For cationic metals, Cu- and Mn-loaded biochar were most effective (effect sizes: 1.51 and 1.22), primarily governed by ion exchange and surface complexation. Conversely, for anionic species, Zr- and Fe-modified biochar showed superior performance (effect sizes: 2.29 and 1.43), driven by electrostatic interactions and inner-sphere complexation. Key factors influencing adsorption included acidic pH (enhancing protonation and electrostatic attraction), elevated temperatures (improving ion diffusion), and moderate contaminant concentrations (optimizing active site utilization). Notably, high heterogeneity (I² >90 %) across studies highlighted variability in biochar properties and experimental conditions. Crucially, regression analysis pinpointed biochar pH as the most critical predictor, with lower pH favoring adsorption. This study delivered valuable insights into the mechanisms and optimization of metal oxide-loaded biochar, providing substantial evidence and performance indicators for its targeted development.