Formation of Pb(IV) oxides a in chlorinated water

Recent research has shown that Pb(IV) oxides play a significant geochemical role in drinking water distribution systems. However, most of the guidance for lead control in drinking water is based on the presumption that Pb(II) solids control lead solubility. Therefore, a better understanding of the chemistry of Pb(IV) in water is needed. Long-term lead precipitation experiments were conducted in chlorinated water (1-3 mg/L Cl-2) at pH 6.5, 8, and 10, with and without sulfate. Results showed that two Pb(IV) dioxide polymorphs - plattnerite (beta-PbO2) and scrutinyite (alpha-PbO2) - formed overtime, as long as a high suspension redox potential was maintained with free chlorine. Neither mineral formed spontaneously, and the rate of formation increased with increasing pH. Hydrocerrusite and/or cerrusite initially precipitated out and overtime either disappeared or coexisted with PbO2. Water pH dictated mineralogical presence. High pH favored hydrocerrusite and scrutinyite; low pH favored cerrusite and plattnerite. Along with a transformation of Pb(II) to Pb(IV) came a change in particle color from white to a dark shade of red to dark grey (differing with pH) and a decrease in lead solubility. If free chlorine was permitted to dissipate, the aging processes (i.e., mineralogy, color, and solubility) were reversible.