Kinetics of aqueous Pb reaction with apatites

Apatite has been used to remediate Pb contamination; apatite dissolution re leases phosphate, which combines with Pb to form highly insoluble Pb-phosph ate minerals. This research focused on the effects of aqueous Pb (initial [ Pb-aq] = 0.185 mM) on the kinetics of apatite dissolution. Synthetic microc rystalline hydroxylapatite (HAP) and natural chlorapatite (CAP) and fluorap atite (FAP) were used in batch experiments at 22 degreesC, with pH within t he range of 4.2-7.0, and in the presence of aqueous Cl. In these batch expe riments, apatites followed linear (zeroth-order) dissolution kinetics. Diss olution experiments were performed using 1 g apatite/L for all three apatit es. When dissolution rate constants (k(AP)) are adjusted for particle speci fic surface area (A(s)), k(CAP) > k(FAP) > k(HAP). In the presence of Pb-aq and Cl, all three apatites reacted to form pyromor phite (PY; Pb-10(PO4)(6)Cl-2). Rates of Pb-aq uptake by the apatites decrea sed in the same order as the apparent (not normalized for A(s)) dissolution rate constants of apatite (k(Ap)(o)): HAP > CAP > FAP, suggesting that Pb- aq uptake is controlled by the total amount of dissolved phosphate in the s ystem. While HAP and CAP removed more than 98% of Pb-aq during 2 weeks of t he experiment, FAP decreased the initial [Pb-aq] by similar to 30%. Pb upta ke rates calculated on a molar basis correlated with Ca release rates. Conc entration of dissolved phosphate during the reaction with Pb-aq was below t he detection limit of 10(-7) mol/L. Phosphate concentration was probably co ntrolled by solute equilibrium with precipitating PY, which has very low so lubility (log K-sp = -167). This indicates that the rate-controlling step w as apatite dissolution. The presence of Pb-aq increased apatite batch dissolution rates, most proba bly because formation of PY acted as a sink for dissolved phosphate, hence increasing the thermodynamic drive for dissolution. Although PY formed hete rogeneously on the surfaces of apatite particles, the PY did not prevent co ntinued apatite dissolution.