Role of phosphate in mitigating lead release from corrosion products

Lead concentrations in drinking water distribution systems are influenced by the dissolution, precipitation, and transformation of lead corrosion products that are present as scales in distribution system pipe. Changes in treatment processes that influence the distribution system water chemistry can induce the dissolution or transformation of the lead-containing corrosion products. The addition of orthophosphate as a corrosion inhibitor has been demonstrated in previous work to limit the dissolved concentrations of lead to below the action level established by the Lead and Copper Rule. In this research project, the role of orthophosphate in controlling the dissolution rates of the lead carbonate hydrocerussite and the lead phosphate hydroxylpyromorphite were evaluated in controlled laboratory-scale experiments using continuous-flow stirred tank reactors. Dissolution rates are interpreted within a reaction-based approach that considers the overall thermodynamic driving force for dissolution as part of a fundamental dissolution rate equation. The solid phases remaining at the conclusion of selected experiments were characterized by scanning electron microscopy and X-ray diffraction to assess transformations in the solid phases that may have been induced by orthophosphate. Low intrinsic solubility and dissolution rates were observed for hydroxylpyromorphite. These observations suggest that the formation of hydroxylpyromorphite can decrease dissolved lead concentrations in distribution system waters. The presence of orthophosphate during the dissolution of hydrocerussite dramatically decreased the rate of lead release to solution.