Equilibrium and kinetic studies of heavy metal retention by hydroxyapatite

Abstract

Sorption capacities and kinetics of toxic heavy metals sorption by synthetic, wellcharacterized, hydroxyapatite (HAP) was studied using the batch technique. Initial concentration of Pb2+, Zn2+, Cd2+ and Sr2+ solutions, obtained from their nitrate salts, was 2. 10-3 mol/dm3 for kinetic experiments. The effect of initial metal concentration (10-4-10-2 mol/dm3 for Zn2+, Cd2+ and Sr2+ and 10-4-5. 10-2 mol/dm3 for Pb2+) on the amounts of retained heavy metals was examined in order to calculate sorption capacities. Initial pH values, in all batches were adjusted to 5.0 ±0.1. Suspensions of HAP and individual metal solutions were equilibrated on a horizontal shaker, at the solid to solution ratio of 1:200, at room temperature (20±1o C). The suspensions were filtrated and analyzed for residual heavy metal concentration, final pH and the amount of Ca2+ released into the solution at different time intervals (from 5 minutes up to 2 days) for kinetic and at 24 h for equilibrium studies. Sorption of all examined metals was followed by Ca2+ release from crystal lattice of HAP, indicating ion-exchange or dissolutionprecipitation mechanisms. The isotherm data were found to be well fitted by linear Langmuir equation. The sorption of Pb2+ was found to be the most rapid in respect to other cations (for equilibrium, a contact time of approximately 30 minutes was required for Pb2+ and 24 hours for Cd2+, Zn2+, and Sr2+). In order to determine the sorption mechanisms and rate constants, two most widely used kinetics models in sorption processes (pseudo-first and pseudo-second order equations) have been applied to experimental data. According to correlation coefficients and calculated values of equilibrium concentrations, the pseudo-second order model was found to be more appropriate, suggesting that the rate limiting step in heavy metal sorption on HAP involves valence forces through sharing or exchange of electrons between the sorbent and sorbates.