LEAD PHOSPHATE SOLUBILITY IN WATER AND SOIL SUSPENSIONS

The widespread occurrence of soil Ph contamination requires a good und erstanding of the factors controlling Pb solubility. Pb solubility wil l often be controlled by phosphates, which are used to reduce environm ental risks of Pb-contaminated soils. The solubility equilibria of two different synthetic lead phosphate minerals were determined electroch emically across a wide range of pH and Pb and phosphate concentrations . The resulting chemical equilibrium data could not be explained by st andard solubility equations, although a simple empirical regression ex plained the data very well. The same lead phosphate mineral was then a dded to two different soils which were equilibrated across a pH range 4-8. Differential pulse anodic stripping voltammetry was used to deter mine the ASV-labile Pb in the extracted soil solutions, and the free P b2+ ion activity was calculated. The free Pb2+ activity in the soil so lutions followed the expected pH relationship, highest solubility at l ow pH and gradual decrease with increasing pH. In contrast, total diss olved Ph was high at low pH, decreased until pH of similar to 6, and t hen increased again at higher pH. This behavior is attributed to the s trong Pb-complexing capacity of the soil organic matter which is solub ilized at neutral pH and above.