Role of Orthophosphate As a Corrosion Inhibitor in Chloraminated Solutions Containing Tetravalent Lead Corrosion Product PbO₂

Addition of orthophosphate has been commonly employed to suppress lead levels in drinking water. Its detailed mechanism and time required for it to become effective, however, have not been fully elucidated. In this study, we investigated the mechanistic role of orthophosphate as a corrosion inhibitor in controlling lead release from tetravalent lead corrosion product PbO₂ in chloraminated solutions, a system representing distribution networks experiencing disinfectant changeover from free chlorine to monochloramine. In all experiments with orthophosphate addition of at least 1 mg/L as P, peaking of soluble Pb­(II) concentration within the first 24 h was observed before lead concentrations decreased and stabilized at levels lower than 15 μg/L. The variation of soluble Pb­(II) concentration could be attributed to the dynamics between the rate of PbO₂ reductive dissolution, primarily induced by monochloramine decomposition, and that of chloropyromorphite (Pb₅(PO₄)₃Cl) precipitation, which did not occur until a critical supersaturation ratio of about 2.36 was reached in the solution. Our findings provide insights to how orthophosphate reduces lead levels under drinking water conditions and highlight the potential risk of short-term elevated lead concentrations. Intensive monitoring following the disinfectant changeover may be required to determine the overall lead exposure when using orthophosphate as a corrosion inhibitor