Removal of Cd2+ from aqueous solutions by hydroxyapatite

A study on the removal of cadmium ions from aqueous solutions by synthetic hydroxyapatite (Ca10(PO4)6(OH)2, HAP) with different morphologies was conducted in batch conditions. The influence of different sorption parameters, such as equilibration time, initial concentration of metal ion, the amount of HAP, pH value of solution and the specific surface area on the sorption amount of Cd2+ were studied and discussed. Our experimental results are in agreement with pseudo-second-order kinetic equation. The Freundlich and Langmuir adsorption isotherms, often used to describe the sorption of solutes from a liquid phase, can be applied to our experimental results. The Langmuir adsorption isotherm constant corresponding to adsorption capacity, Xm, was found to be 260.42 mg/g. The sorption amount of Cd2+ increased as the pH value enhanced from 5 to 8. The ion-exchange mechanism and surface complex mechanism may both exist in the adsorption process. Nearly linear dependence between Cd2+ sorption amount and specific surface area within a certain range has been confirmed. Cd2+ desorption amount decreased with the increasing pH value of the extracting solution.