Adsorptive Removal of Heavy Metal from Acidic Wastewater with Biochar Produced from Anaerobically Digested Residues: Kinetics and Surface Complexation Modeling

In this study, the adsorptive characteristics of biochar generated from anaerobically digested garden wastes (AD-char) were investigated. Metal adsorption onto AD-chars reached equilibrium in 48 h; the adsorption capacity of Cu by AD-char was 182 μmol g, which was higher than that of Zn (35.3 μmol g) and Mn (60.7 μmol g). The metal adsorption was well described by the pseudo second-order kinetic and Langmuir isotherm models. pKa1, pKa2, and pkCu for AD-char, which described surface protonation reactions and complexation with Cu, were 5.75, -10.20, and -4.70, respectively, as optimized by the surface complexation model. Cu adsorption onto AD-char increased with increasing pH to < 8.6, which suggests that the presence of surface alkaline functional groups can be attributed to the metal adsorption capacity of biochar. This study concluded that converting anaerobically digested food and garden wastes into biochar could be an efficient method of treating municipal solid waste and producing metal adsorbents for environmental remediation.