Activation methods increase biochar's potential for heavy-metal adsorption and environmental remediation: A global meta-analysis

Removal of heavy metals (HMs) by adsorption on biochar's surface has shown promising results in the remediation of contaminated soil and water. The adsorption capacity of biochar can be altered by pre- or post-pyrolysis activation; however, the effect of activation methods on biochar's adsorption capacity varies widely. Here, we conducted a meta-analysis to identify the most effective methods for activation to enhance HM removal by biochar using 321 paired observations from 50 published articles. Activation of biochar significantly improves the adsorption capacity and removal efficiency of HMs by 136 and 80 %, respectively. This study also attempts to find suitable feedstocks, pyrolysis conditions, and physicochemical properties of biochar for maximizing the effect of activation of biochar for HMs adsorption. Activation of agricultural wastes and under pyrolysis temperatures of 350-550 °C produces biochars that are the most effective for HM adsorption. Activation of biochars with a moderate particle size (0.25-0.80 mm), low N/C (<0.01) and H/C ratios (<0.03), and high surface area (> 100 m2 g-1) and pore volume (> 0.1 cm3 g-1) are the most desirable characteristics for enhancing HM adsorption. We conclude that pre-pyrolysis activation with metal salts/oxides was the most effective method of enhancing biochar's potential for adsorption and removal of a wide range of HMs. The results obtained from this study can be helpful in choosing appropriate methods of activations and the suitable choice of feedstocks and pyrolysis conditions. This will maximize HM adsorption on biochar surfaces, ultimately benefiting the remediation of contaminated environments.