Characterization of magnetic chitosan-modified biochar and its adsorption of Zn2+ in electroplating rinsing wastewater

This study introduced a novel chitosan-modified magnetic biochar (CMBC) for adsorbing Zn2+ from electroplating wastewater, aiming to enhance the adsorption capacity of biochar and facilitate its separation from aqueous phase. Results showed that the incorporation of Fe3O4 and chitosan into biochar reduced its specific surface area and pore volume by 93.8 % and 79.6 %, respectively, but increased the abundance of amino, carboxyl and hydroxyl groups on the carbon's surface. Zn2+ adsorption onto CMBC composites (up to 36.0 mg Zn2+/g at 125 mg/L Zn2+) tended to be monolayer and nonlinear, as well as spontaneous, endothermic and entropy-increasing, with chemisorption (surface complexation and ion exchange) being the predominant mechanism, in which negatively charged groups such as amino, hydroxyl, carboxyl and ester played key roles. pH affected Zn2+ adsorption onto CMBC by influencing its surface charge and degree of protonation/deprotonation of its functional groups, and 6.0 pH was most recommended. Balancing adsorption capacity and removal efficiencies, the economical CMBC dosage should be tailored to initial Zn2+ levels, e.g. it was 3.0 g CMBC/L for 100 mg/L Zn2+. Coexisting metal ions with smaller hydrated ionic radii and/or higher relative atomic masses competed with Zn2+ for CMBC adsorption. The desorption rate of Zn2+ from saturated CMBC was 73.7 % when using 1.5 % HCl over 60 minutes, and after five adsorption-desorption cycles, CMBC's adsorption capacity retained 74.8 % of its initial value. Together with superparamagnetic properties and 15.73 emu/g saturation magnetization, CMBC offered satisfactory regeneration capabilities and recyclability. Overall, CMBC demonstrated substantial potential in removing Zn2+ from electroplating wastewater.