Effects of ortho- and polyphosphates on lead speciation in drinking water

Abstract

Lead is a potent neurotoxin and drinking water represents an important route of exposure, especially where legacy lead pipe is widespread. Polyphosphates are often added to drinking water to sequester iron and calcium, but they may form coordination complexes with lead, increasing its solubility. This risk is not well characterized in practice: the prevalence of lead-polyphosphate complexes in drinking water systems is not known. We used size-exclusion chromatography (SEC) with multi-element (ICP-MS) detection to compare the speciation of lead below 0.45 μm in two low-alkalinity water systems dosing phosphate-based corrosion inhibitors at different ortho : poly ratios and doses. In one system dosing polyphosphate at 0.05 mg P L⁻¹ (3 : 1 ratio), it reverted almost completely to orthophosphate during distribution. In another system dosing polyphosphate at 0.20 mg P L⁻¹ (1 : 1 ratio), it was detectable at points of use. Under the influence of polyphosphate, lead and iron were present as dissolved species strongly associated with phosphorus. Moreover, experiments with sodium hexametaphosphate confirmed our ability to detect lead as a coordination complex using SEC. In the absence of polyphosphate, lead and iron were present as colloidal particles. Orthophosphate on its own also appeared to reduce lead solubility, as estimated using paired sample profiles collected at seventeen single-unit residences. Increasing orthophosphate from 0.5 to 1.0 mg PO₄ L⁻¹ accompanied decreases in total lead concentrations of 38% (95% CI: 15–51%) within eight months. Analysis of recovered lead pipe corrosion scale was consistent with the presence of an insoluble lead phosphate compound: hydroxypyromorphite (Pb₅(PO₄)₃OH).