Dissolved lead (Pb) in drinking water can be controlled by the solubility of Pb-containing corrosion products that form on the inner surfaces of lead pipes. Among these corrosion products, Pb(IV) oxide (PbO₂) has the lowest solubility when free chlorine is available to maintain Pb in the +IV oxidation state. However, depletion of free chlorine or a switch from free chlorine to monochloramine can cause reductive dissolution of PbO₂. Soluble cations in drinking water can interact with PbO₂ in ways that alter its characteristics. We evaluated the effects of Cu(II), Zn(II), and Pb(II) on the rate of PbO₂ reductive dissolution. Cu(II) and Zn(II) inhibited PbO₂ reductive dissolution for contact times up to least 12 h, and they then enhanced PbO₂ reductive dissolution for reaction times of 48 h and longer. The Zn and Cu extended X-ray absorption fine structure spectra for PbO₂ solids collected after reacting with Cu(II) and Zn(II) indicate that the Zn and Cu adsorb to PbO₂ as bidentate edge-sharing complexes. The adsorption of Cu(II) and Zn(II) probably decreases the abundance of protonated sites on the PbO₂ surface, which would otherwise be available for reductive dissolution. The enhanced dissolution for longer reaction times may be caused by competitive adsorption of Cu(II) and Zn(II) with Pb(II), which could limit the re-adsorption of Pb(II) onto PbO₂. The different effects of Cu(II) and Zn(II) on PbO₂ dissolution at different times highlight the importance of stagnation time in determining lead concentrations in drinking water. We further studied the interactions between aqueous Pb(II) and PbO₂ using ²⁰⁷Pb(II) as an isotope tracer that was initially enriched in the aqueous solution. We measured the evolution of the ²⁰⁷Pb/²⁰⁸Pb ratios in the solution and collected solids for characterization. We observed no net sorption of Pb(II) in the systems and saw no changes in the size, surface oxidation states, or crystalline phases of the solids. However, measurement of the aqueous isotopic composition indicated that extensive atom exchange between the solid and solution had occurred within 24 h. The combination of isotope exchange results and solid characterization indicates that PbO₂ had undergone redox-driven recrystallization. Such recrystallization is likely to occur in water that stagnates in lead pipes that contain both PbO₂ and soluble Pb(II). This recrystallization process may alter the reactivity of PbO₂ and makes it less vulnerable to reductive dissolution.

Weiyi Pan

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