Quantitative contribution of Cd²⁺ adsorption mechanisms by chicken-manure-derived biochars

This study investigated the efficiency and mechanisms of Cd²⁺ removal by chicken-manure biochar produced at different temperatures. Adsorption kinetics, isotherms, thermodynamic, and desorption were examined, and the biochars before and after adsorption were analyzed by SEM-EDS, FTIR, Boehm titration, and XRD. Kinetics of adsorption were better described by pseudo-second-order kinetic model than pseudo-first-order kinetic and intraparticle diffusion model under different initial Cd²⁺ concentrations of 20, 50, and 100 mg L^-1. Equilibrium adsorption was better modeled by Freundlich and Temkin isotherm equations than Langmuir equation at different temperatures of 25, 35, and 45 °C. Thermodynamic parameters confirmed the spontaneous and endothermic nature of the adsorption of Cd²⁺ at all of temperatures. Moreover, functional group complexation, precipitation, and cation exchange jointly contributed to Cd²⁺ adsorption on the biochars, whose relationship with the properties of biochar were also analyzed. The new precipitate as Cd₅(PO₄)₃OH was found during the adsorption. Complexation and precipitation were predominant mechanisms for all biochars (together accounting for 92.4-98.8%), while cation exchange made a relatively minor contribution to total Cd²⁺ removal (accounting for 1.2-7.6%). The relative distribution of each mechanism on the biochars was determined, which deepen our understanding of the Cd²⁺ adsorption process. These results are useful for future practical applications of biochar to removal heavy metals from water.