Removal of lead from aqueous solution by hydroxyapatite/magnetite composite adsorbent

A novel composite adsorbent, hydroxyapatite/magnetite (HAp/Fe3O4), has been prepared for the purpose of removing lead ions from aqueous solution. Varying factors that may affect the adsorption efficiency, including adsorbent dosage, solution pH and coexistent substances such as humic acid and competing cations (Ca2+, Mg2+, K+ and Na+), have been investigated. It is remarkable that using the developed composite absorbent, more than 99% of Pb2+ can be removed under desirable experimental conditions. The maximum adsorption capacity for HAp/Fe3O4 is 598.8 mg/g, which is much greater than that for the previously reported materials. It is found that depending on its concentrations, humic acid affects the efficiency of lead removal, but the coexisting cations, e.g., K+, Ca2+, Na+ and Mg2+, have small influence. Mechanistic studies show that the adsorption data fits the Langmuir and Freundlich isotherm models reasonably well and the adsorption processes obey the pseudo-second-order kinetics model. Based on the experiment results, it is concluded that dissolution/precipitation and the surface complexation are mutually responsible for lead adsorption on HAp/Fe3O4, which should be attributed to the multiple functionalities of the material.