KINETICS OF AQUEOUS Pb REACTION WITH APATITES

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Abstract
Apatite has been used to remediate Pb contamination; apatite dissolution releases phosphate, which combines with Pb to form highly insoluble Pb-phosphate minerals. This research focused on the effects of aqueous Pb (initial [Pbaq] = 0.185 mM) on the kinetics of apatite dissolution. Synthetic microcrystalline hydroxylapatite (HAP) and natural chlorapatite (CAP) and fluorapatite (FAP) were used in batch experiments at 22°C, with pH within the range of 4.2-7.0, and in the presence of aqueous Cl. In these batch experiments, apatites followed linear (zeroth-order) dissolution kinetics. Dissolution experiments were performed using 1 g apatite/L for all three apatites. When dissolution rate constants (kAP) are adjusted for particle specific surface area (As), kCAP > kFAP > kHAP. In the presence of Pbaq and Cl, all three apatites reacted to form pyromorphite (PY; Pb10(PO4)6Cl2). Rates of Pbaq uptake by the apatites decreased in the same order as the apparent (not normalized for As) dissolution rate constants of apatite (kAp°): HAP > CAP > FAP, suggesting that Pbaq uptake is controlled by the total amount of dissolved phosphate in the system. While HAP and CAP removed more than 98% of Pbaq during 2 weeks of the experiment, FAP decreased the initial [Pbaq] by ∼30%. Pb uptake rates calculated on a molar basis correlated with Ca release rates. Concentration of dissolved phosphate during the reaction with Pbaq was below the detection limit of 10−7 mol/L. Phosphate concentration was probably controlled by solute equilibrium with precipitating PY, which has very low solubility (log Ksp = −167). This indicates that the rate-controlling step was apatite dissolution. The presence of Pbaq increased apatite batch dissolution rates, most probably because formation of PY acted as a sink for dissolved phosphate, hence increasing the thermodynamic drive for dissolution. Although PY formed heterogeneously on the surfaces of apatite particles, the PY did not prevent continued apatite dissolution.

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