Lead Phosphate Solubility in Water and Soil Suspensions

The widespread occurrence of soil Pb contamination requires a good understanding of the factors controlling Pb solubility. Pb solubility will often be controlled by phosphates, which are used to reduce environmental risks of Pb-contaminated soils. The solubility equilibria of two different synthetic lead phosphate minerals were determined electrochemically across a wide range of pH and Pb and phosphate concentrations. The resulting chemical equilibrium data could not be explained by standard solubility equations, although a simple empirical regression explained the data very well. The same lead phosphate mineral was then added to two different soils which were equilibrated across a pH range 4-8. Differential pulse anodic stripping voltammetry was used to determine the ASV-labile Pb in the extracted soil solutions, and the free Pb2+ ion activity was calculated. The free Pb2+ activity in the soil solutions followed the expected pH relationship, highest solubility at low pH and gradual decrease with increasing pH. In contrast, total dissolved Pb was high at low pH, decreased until pH of ∼6, and then increased again at higher pH. This behavior is attributed to the strong Pb-complexing capacity of the soil organic matter which is solubilized at neutral pH and above.