[Competitive adsorption kinetics of aqueous Pb2+ and Cu2+ on nano-HAP surfaces]

Competitive adsorption kinetics of aqueous Pb2+ and Cu2+ on Nano-HAP surfaces were investigated by performing the adsorption kinetic experiments and comparing the change of zeta potentials and XRD-map before and after metal adsorption. The results showed that the adsorption quantity of Pb2+ on Nano-HAP was higher than that of Cu2+ in the single system. However, the competitive adsorption of Pb2+ and Cu2+ occurred in the co-existence system of Pb2+ and Cu2+. The adsorption quantity of Cu2+ on Nano-HAP increased, while that of Pb2+ decreased. The X-ray diffraction (XRD) and adsorption quantities of Ca2+ surface analysis indicated that dissolution-precipitation is the primary immobilization mechanism for Pb2+, while surface complexation and electrostatic adsorption account for Cu2+ sequestration. The adsorption quantities of Pb2+ and Cu2+ on Nano-HAP had marked linear relationship with Ca2+ release from Nano-HAP (R2 0.861-0.954). The adsorption kinetics were fitted with the equations of first-order, second-order, parabola, Elovich, double constant equation and LJ function, respectively, in which the second-order and power function kinetics equation fitted the results best. According to above results, the adsorption kinetics of Pb2+ and Cu2+ on Nano-HAP is a complex adsorption processes with both chemical reaction and physical adsorption.