Effects of residual disinfectants on the redox speciation of lead(ii)/(iv) minerals in drinking water distribution systems

Abstract

This study investigated the reaction kinetics on the oxidative transformation of lead(II) minerals by free chlorine (HOCl) and free bromine (HOBr) in drinking water distribution systems. According to chemical equilibrium predictions, lead(II) carbonate minerals, cerussite PbCO_3(s) and hydrocerussite Pb₃(CO₃)₂(OH)_2(s), and lead(II) phosphate mineral, chloropyromorphite Pb₅(PO₄)₃Cl_(s) are formed in drinking water distribution systems in the absence and presence of phosphate, respectively. X-ray absorption near edge spectroscopy (XANES) data showed that at pH 7 and a 10 mM alkalinity, the majority of cerussite and hydrocerussite was oxidized to lead(IV) mineral PbO_2(s) within 120 minutes of reaction with chlorine (3 : 1 Cl₂ : Pb(II) molar ratio). In contrast, very little oxidation of chloropyromorphite occurred. Under similar conditions, oxidation of lead(II) carbonate and phosphate minerals by HOBr exhibited a reaction kinetics that was orders of magnitude faster than by HOCl. Their end oxidation products were identified as mainly plattnerite β-PbO_2(s) and trace amounts of scrutinyite α-PbO_2(s) based on X-ray diffraction (XRD) and extended X-ray absorption fine structure (EXAFS) spectroscopic analysis. A kinetic model was established based on the solid-phase experimental data. The model predicted that in real drinking water distribution systems, it takes 0.6–1.2 years to completely oxidize Pb(II) minerals in the surface layer of corrosion scales to PbO_2(s) by HOCl without phosphate, but only 0.1–0.2 years in the presence of bromide (Br⁻) due the catalytic effects of HOBr generation. The model also predicts that the addition of phosphate will significantly inhibit Pb(II) mineral oxidation by HOCl, but only be modestly effective in the presence of Br⁻. This study provides insightful understanding on the effect of residual disinfectant on the oxidation of lead corrosion scales and strategies to prevent lead release from drinking water distribution systems.