Determination of PbO2 Formation Kinetics from the Chlorination of Pb(II) Carbonate Solids via Direct PbO2 Measurement

Lead dioxide (PbO2), a new form of lead corrosion product discovered in the distribution system, is formed via the chlorination of Pb(II) solids and plays an important role in regulating lead concentration in drinking water. The kinetics of its formation, however, has not been quantitatively determined primarily because of the difficulties in accurately measuring PbO2 concentration. In this study, we apply an iodometric method for direct PbO2 measurement to determine its formation kinetics from the chlorination of cerussite (PbCO3) and hydrocerussite (Pb-3(CO3)(2)(OH)(2)). The obtained rate equations suggest that for both Pb(II) carbonate solids, the formation of PbO2 is first-order with respect to the available Pb(II) solid surface area, free chlorine concentration, and OH concentration. Dissolved inorganic carbon concentration (DIC) was found to inhibit PbO2 formation because of the formation of carbonate lead surface complexes that protect the surface Pb(II) sites from oxidation. The rate of PbO2 formation from the chlorination of hydrocerussite was faster than that of cerussite under the same Pb(II) solid loading. However, after normalization of the surface area, the rate constants obtained for both Pb(II) solids are similar. The kinetics of PbO2 formation is elucidated for the first time in this study.