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.