Apatite amendment to lead (Pb)-contaminated soils has been investigated in the last decade as a method of Pb remediation. Phosphate released by apatite dissolution combines with Pb to form highly insoluble and supposedly nonbioavailable pyromorphite (PY). As a first approach to determining whether Pb present in PY might be susceptible to remobilization by microorganisms, we investigated the dissolution of PY in the presence of the trihydroxamate microbial siderophore desferrioxamine-B (DFO-B) as a function of pH (from 2 to 11), temperature, DFO-B concentration, and different solution/solid ratios in batch experiments.
Desferrioxamine-B strongly complexes Pb in solution approximately above pH 6, and we observed enhancement of the net release of Pb from PY at pH 6 to 11. Dissolution in the presence of DFO-B was stoichiometric. Modeling using PHREEQC indicated that the increase of PY solubility can be accounted for by an increase in concentration of Pb-DFO-B complexes. Based on the enthalpy of dissolution reaction (ΔHr = −199.35 kJ/mol), dissolution of PY is exothermic, and PY solubility decreases with temperature. However, in the presence of DFO-B, the equilibrium-dissolved concentrations of Pb and P, along with the rate of net Pb and P release, increase with increasing temperature from 4°C to 50°C, suggesting that Pb-DFO-B complexation constants likely increase with temperature.
Our results indicate that Pb may be mobilized from PY by microbial siderophores over the pH range of many soils, and that the potential effects of microorganisms on Pb remobilization thus need to be considered in Pb remediation strategies that rely on PY formation.