A novel magnesium-calcium hydroxyapatite adsorbent was prepared by the Sol-gel method with different proportions of Mg/(Ca+Mg) using Mg2+ as doped ions, and the removal characteristics and process mechanism of Pb2+ on the magnesium-calcium hydroxyapatite in an aqueous solutions were studied. The results show that the surface of the adsorbent is composed mainly of a hydroxyphosphonite compound[Pb10(PO4)6(OH)2], The morphological characteristics of the magnesium-calcium hydroxyapatite adsorbent surface was investigated as crystal structure changes from short rods to needle structures according to scanning electron microscopy (SEM). Testing at a temperature of 25℃ and pH of 5 showed that the adsorption of Pb2+ by magnesium-calcium hydroxyapatite reached equilibrium within 720 min. The adsorption capacity was determined to be 813.17 mg·g-1 at a dosage of 0.6 g·L-1. The thermodynamic test results of ΔGθ<0, ΔSθ>0, and ΔHθ>0 indicated that the adsorption process of Pb2+ by magnesium-calcium hydroxyapatite is a spontaneous process with endothermic reaction and entropy increments, and higher temperatures were considered be favorable for adsorption at a range of 25-45℃. The adsorption could be effectively described by a pseudo-second-order kinetic equation. The equilibrium data were found to follow the Langmuir adsorption model. Material characterization and adsorption tests showed that surface complexation and dissolution-precipitation were the main mechanisms for the removal of Pb2+ by magnesium-calcium hydroxyapatite in an aqueous solution.
Keywords: adsorption kinetics; hydroxyapatite; isotherm adsorption; lead ions (Pb2+); removal mechanism.