Removal of lead ions in aqueous solution by hydroxyapatite/polyurethane composite foams

We have prepared hydroxyapatite/polyurehthane (HAp/PU) composite foams with two different HAp contents of 20 and 50 wt.% and investigated their removal capability of Pb 2+ ions from aqueous solutions with various initial Pb 2+ ion concentrations and pH values of 2–6. HAp/PU composite foams synthesized exhibited well-developed open pore structures which provide paths for the aqueous solution and adsorption sites for Pb 2+ ions. With increasing the HAp content in the composites, the removal capability of Pb 2+ ions by the composite foams increases owing to the higher adsorption capacity, whereas the removal rate is slower due to the less uniform dispersity of HAp in composite foams. The removal rate of Pb 2+ ions is also slower with increasing the initial Pb 2+ ion concentration in aqueous solutions. The removal mechanism of Pb 2+ ion by the composites is varied, depending on the pH value of aqueous solution: the dissolution of HAp and precipitation of hydroypyromorphite is dominant at lower pH 2–3, the adsorption of Pb 2+ ions on the HAp/PU composite surface and ion exchange reaction between Ca 2+ of HAp and Pb 2+ in aqueous solution is dominant at higher pH 5–6, and two removal mechanisms compete at pH 4. The equilibrium removal process of Pb 2+ ions by the HAp/PU composite foam at pH 5 was described well with the Langmuir isotherm model, resulting in the maximum adsorption capacity of 150 mg/g for the composite foam with 50 wt.% HAp content.