Aqueous Pb sorption by synthetic and natural apatite: Kinetics, equilibrium and thermodynamic studies

Natural apatites represent a cost effective soil amendment, which can be used for in situ reduction of lead bioavailability and mobility. In our previous work, we selected Lisina apatite (LA) as a promising natural mineral for lead immobilization based on theoretical predictions. This study investigated the adsorption equilibrium and kinetics of aqueous Pb sorption onto Lisina apatite and synthetic hydroxyapatite (HAP) at different temperatures. XRD analysis indicated that LA consists of three minerals: fluorapatite, quartz and muscovite. After reaction with a Pb solution, only FA peaks were changed, confirming that fluorapatite present in LA is responsible for Pb sorption. Sorption experiments confirmed that both LA and HAP are effective in Pb removal. Langmuir, Freundlich, Temkin, Koble–Corrigan and Redlich–Peterson isotherm models were applied to experimental data. The kinetics of the sorption process on both minerals was well characterized by the pseudo-second order reaction rate. Results indicate that Pb immobilization by HAP is a two-step process: the first rapid phase, possibly surface complexation and secondary dissolution of HAP and precipitation of pyromorphite. The sorption of Pb by Lisina apatite is also a two-step process, but the reaction mechanisms are more complicated and need further research as the results indicate that different mechanisms dominate at different temperatures. The values of thermodynamic equilibrium constants and Gibbs free energy were also calculated. The values of ΔG0 obtained confirm the feasibility of both HAP and LA as effective sorbents of the Pb ion.