Spongy magnetic hydroxyapatite for the enhanced Pb2+ removal and its dynamic sorption mechanism

Nano-magnetic hydroxyapatite (Fe3O4/nHAP) has superior properties for the removal of lead (Pb) from aqueous solutions. In this study, we synthesized several spongy Fe3O4/nHAP composite materials using the facile chemical co-precipitation method with the assist of ultrasound. Appropriate Fe3O4 addition increased the specific surface area of Fe3O4/nHAP with a porous structure, contributing to the more sorption sites exposure to lead ions (Pb2+) for fast kinetic and larger sorption capacity of 1500 mg g-1. The characteristic results manifest that complexation and dissolution-precipitation were the two main sorption mechanisms. Through the Pb/Ca molar ratio, solution pH, Pb fraction changes and the additional XRD results, the main sorption mechanisms turned from complexation to dissolution-precipitation and the adsorbed-Pb transformed from unstable Pb-complexes to stable Pb-phosphate during the removal process. Complexation effect dominated the Pb2+ removal in higher Pb2+ concentration solutions while dissolution-precipitation controlled in low Pb2+ concentration solutions. Pb5(PO4)3(OH) was the main product in the solutions of pH ranging from 2.76 to 5.25, and PbHPO4 precipitates was formed when the pH was lower than 2.76. Regeneration experiment and fraction analysis demonstrated the stability of Fe3O4/nHAP after Pb2+ sorption and desorption. Our findings provide a facile preparation method of Fe3O4/nHAP with their specific properties for the Pb2+ removal from aqueous solutions. Comprehensive mechanisms for the affinity of Pb2+ and Fe3O4/nHAP can propose strategies in the further removal or stabilization of heavy metals in practical environmental remediation.