Comparison of hydroxyapatite sorption properties towards cadmium, lead, zinc and strontium ions

Apatites are well known matrix for heavy metal immobilization and remediation of ground water and contaminated soils. In this work, capacities of different synthetic hydroxyapatite (HAP) samples for Pb, Cd, Sr and Zn sorption were analyzed, from single metal solutions and their equimolar mixture. Initial concentration of each metal in single metal solutions and total metal concentration in the mixture was 10(-2) mol/dm(3). Initial pH was adjusted to 5.0 +/- 0.1. After 24 h equilibration of HAP samples with heavy metal solutions (solid/liquid ratio 1:200), the concentration of metals and of Ca2+ released from crystal lattice, as well as the final pH values were measured. The selectivity of all samples, regardless of the differences in their compositions, crystallinities, specific surface areas, points of zero charge and sorption capacities, was as follows: Pb gt Cd gt Zn gt Sr. The same order was obtained for both single metal solutions and their mixture. In all cases, the sorption was followed by a decrease in final pH values, which are lower than the pH(PZC) values obtained by equilibration of HAP samples with an inert electrolyte (KNO3). A linear relationship between the amount of metals sorbed and Ca2+ released from HAP was observed. Also, a correlation between sorption capacities and physicochemical properties of HAP samples was found.