Reactions At The Mineral Water Interface Affecting Lead Mobility In Soil And Sediments

Phosphate addition to lead contaminated soil can be a cost-effective method for in situ remediation. This project focused on the effects of phosphate on lead reactions at the mineral-water interface. First, the pH dependence of the equilibrium solubility and dissolution rates of chloropyromorphite (CPY, Pb5(PO4)3Cl) were determined. Synthetic CPY was more soluble than predicted for pure CPY, which may be due to the low crystallinity of the synthetic material and the presence of a trace amount of lead hydroxide. A general rate law was developed that can predict the CPY dissolution rate as a function of pH and solution saturation. Second, lead and phosphate co-sorption on goethite-coated and uncoated quartz sand was investigated and interpreted with a surface complexation model. Lead immobilization on goethite-coated sand was enhanced by phosphate, and the enhancement was predicted by the combination of single sorbate surface complexation models. Although the model predicted that adsorption was the dominant uptake mechanism, uptake may be controlled by the relative rates of precipitation and adsorption reactions. Slow sorption was observed during co-sorption of lead and phosphate on goethite-coated sand from 1 day to 1 month; slow sorption indicates slow chemical reactions, including ternary surface complexation or surface precipitation. Finally, phosphate-induced lead immobilization was evaluated using the biogenic nanocrystalline apatite (Ca5(PO4)3OH (s)) in fish bone. Dissolved lead concentrations dropped significantly upon reaction with fish bones. The mechanism of lead immobilization (precipitation vs. adsorption) was affected by the ratio of the total lead concentration to the surface area of the solid phosphate phase.