Preparation and lead ion removal property of hydroxyapatite/polyacrylamide composite hydrogels

We report the synthesis of hydroxyapatite/polyacrylamide (HAp/PAAm) composite hydrogels with various HAp contents by free radical polymerization and their removal capability of Pb2+ ions in aqueous solutions with controlled initial Pb2+ ion concentrations and pH values of 2-5. The swelling ratio of the composite gels in aqueous solutions decreases with increasing the HAp content in the gels. The composite gel with higher HAp content exhibits the higher removal capacity of Pb2+ ions owing to the higher adsorption sites for Pb2+ ions. but shows the slower removal rate of Pb2+ ions due to the lower degree of swelling. The removal mechanism of Pb2+ ion is very sensitive to the pH value in aqueous solution, although the removed amount of Pb2+ ion is nearly same, regardless of pH values of 2-5. The removal mechanism, the dissolution of HAp in the composite gel and subsequent precipitation of hydroxypyromorphite (HPy), is dominant at lower pH 2-3, whereas the mechanism, the adsorption of Pb2+ ions on the composite gel and following cation exchange reaction between Pb2+ ions adsorbed and Ca2+ of HAp, is dominant at higher pH 4-5. The equilibrium removal process of Pb2+ ions by the composite gels at pH 5 is described well with the Langmuir isotherm model. The equilibrium removal capacities of the composite gels with 30, 50, and 70 wt.% HAp contents are evaluated to be 123, 178, and 209 mg/g, respectively. (c) 2008 Elsevier B.V. All rights reserved.