Kinetics of the reductive dissolution of lead(IV) oxide by iodide

Lead(IV) oxide (PbO₂) is a corrosion product found in lead service lines used to convey drinking water. The presence of reductants can accelerate PbO₂ dissolution and enhance lead release to drinking water. The dissolution rate rather than the equilibrium solubility of PbO ₂ can control the dissolved lead concentrations in water distributed through pipes containing PbO₂. Iodide, a known reductant for PbO ₂, was selected as a model reductant for investigating the kinetics and mechanisms of the reductive dissolution of PbO₂. The dissolution rate of plattnerite (β-PbO₂) was determined as a function of pH, iodide concentration, and dissolved inorganic carbon (DIC) concentration using continuously stirred tank reactors. The dissolution rate of plattnerite increased with decreasing pH and increasing iodide concentrations. The presence of 10 mg C/L DIC accelerated plattnerite dissolution, but further increases in DIC concentration did not affect the dissolution rate. The reductive dissolution of PbO₂ can be interpreted as a coupled process involving chemical reduction of Pb^IV to Pb^II at the PbO₂ surface followed by detachment of Pb^II to solution. The data suggest that chemical reduction is the rate-limiting step for PbO₂ dissolution in the presence of iodide.