Reactions of aqueous Pb(II) with apatites

Apatite dissolution with or without the presence of aqueous Pb +2 (initial [Pb] 185.6 · 10−6 M) was studied in batch experiments at 22°C within the pH range of 4.2–7, in the presence of Cl− anion. Synthetic hydroxylapatite (HAP), and natural chlorapatite (CAP) and fluorapatite (FAP) were used. A combination of atomic force microscopy, scanning election microscopy, optical microscopy, energy dispersive spectroscopy, infrared spectroscopy, atomic absorption spectroscopy, and X-ray diffraction were used for in-situ and ex-situ examinations of the interface and the reaction products. Under the conditions of batch experiments, the dissolution of apatites is linear (zeroth-order kinetics). FAP and CAP dissolution kinetics adjusted for the specific surface area (A_s) are faster than for HAP. pH changes are responsible for the changes in dissolution kinetics during very early stages.; The reaction of various apatites with Pb_aq in the presence of Cl− anion results always in formation of chlorpyromorphite (CPY). Heterogeneous nucleation of CPY on CAP or FAP surface takes place epitaxially. CPY crystals grow away from the substrate surface, which is the only source of phosphate necessary for crystal growth, this indicates that phosphate ions enter the solution and diffuse along growing CPY crystals. Kinetics of apatite reaction with Pb_aq corresponds with their apparent (not normalized for A_s) dissolution kinetics HAP > CAP > FAP. This suggests that it is related to the total amount of available phosphate in the system. While HAP and CAP are capable of removing more than 98% of Pb during 2 weeks of the experiment, FAP reaches the capacity limit after lowering the initial [Pb] by $\sim$30%. Concentration of dissolved phosphates during the reaction with Pb_aq is below the detection limit. It is probably controlled by CPY equilibrium indicating that the rate controlling step is apatite dissolution. These results indicate that in-situ immobilization of Pb_aq by apatite to contaminated soils may take place both on the apatite grains surface in contact with Pb_aq (mostly heterogeneous precipitation) as well as in the soil profile as a result of reaction between Pb+2 and dissolved phosphates released from leaching apatite (mostly homogeneous precipitation).