Effects of oxidants and reductants on the dissolution rates of the lead corrosion product PbO₂

Lead(IV) oxide (PbO₂) is an important corrosion product in lead service lines, and its stability can affect lead release to drinking water. The dissolution rate rather than the equilibrium solubility of PbO₂ can control the dissolved lead concentration in distribution systems with PbO ₂ present. The dissolution of PbO₂ is affected by the specific water chemistry in a distribution system. Understanding the kinetics of PbO₂ dissolution could provide a quantitative view to understand lead release to drinking water from lead service lines. The dissolution rates of plattnerite (β-PbO₂) were investigated as a function of pH, the presence of free chlorine, and iodide concentration using continuously stirred tank reactors (CSTRs). Each experiment was conducted for 24 hours to ensure that the system reached steady state. The pH, free chlorine concentration, and dissolved lead concentration were monitored periodically in each set of the experiments, and the steady-state concentrations of dissolved lead were used to calculate the dissolution rate. The solids were characterized at the end of selected experiments by X-ray diffraction and scanning electron microscopy to determine whether a secondary solid phase had formed. In the absence of free chlorine and iodide, the plattnerite dissolution rate increased with decreasing pH values until reaching a plateau below pH 3.5. The steady-state lead concentrations were above the lead action level (0.015 mg/L) only when the pH was below 6.7. The presence of free chlorine inhibited the dissolution of PbO₂, which might be due to the elevated oxidation reduction potential (ORP) of the system that inhibited the chemical reduction of Pb(IV) in PbO₂ to more soluble Pb(II) species. The presence of the chemical reductant iodide enhanced the dissolution of PbO₂, and the dissolution rate increased with increasing iodide concentration. At the highest iodide concentration studied (100 μM), the reductive dissolution was sufficiently fast that the lead(II) carbonate cerrusite (PbCO₃) formed as secondary solid from the transformation of PbO₂. The present study showed that the dissolution rate of PbO₂ increased with the order of free chlorine, water, and iodide, which is consistent with a trend of decreasing ORP of the system, indicating that chemical reduction was important in controlling the dissolution of PbO₂. 2011