Mechanisms of the oxidation of lead (II) solid phases by chlorine and its relationships with colloidal lead release

Lead (Pb) is one of the most dangerous contaminants commonly found in drinking water. Its release is caused by the corrosion of lead-bearing materials (LBM) widely used in distribution systems, including lead pipe, lead/tin solder and lead-containing brass. This research investigated the formation of lead dioxide PbO2 via the oxidation of Pb(II) solid phases by chlorine, an important process controlling lead release in drinking water distribution systems where LBM are present.;The mechanisms of PbO2 formation via Pb(II) oxidation were studied both in chemically and electrochemically controlled conditions. This study demonstrated that reactivities of Pb(II) solid phases that are precursors of PbO2 in chlorine oxidations were very different. While lead phosphate solids tended to be largely inert towards chlorine attack, hydrocerussite Pb3(CO3)2(OH)2 and cerussite PbCO 3 were oxidized by chlorine in an auto-catalytic reaction that was accelerated by products of the reaction. The non-monotonic increase of zeta-potentials of hydrocerussite and cerussite during the reaction suggested effects of Pb(III) intermediates involved in PbO2 formation.;Electrochemical (EC) techniques further demonstrated that PbO2 formation via Pb(II) oxidation involves two types of Pb(III) intermediates. EC data also showed that their presence is closely associated with hydroxyl radical generation catalyzed by PbO2 nuclei. This radical mechanism of PbO2 formation is also critical in non EC-controlled systems, as evidenced by decrease of the rate of lead hydroxide Pb(OH)2 oxidation by chlorine in the presence of radical scavengers. The existence of autocatalysis observed in the case of hydrocerussite and cerussite oxidation by chlorine is likely due to acceleration of hydroxyl radical generation (possibly mediated via the generation of chlorine radicals) by PbO2 nuclei formed on the reactive surface.;Contribution of colloidal Pb(II) to operationally defined "soluble" Pb(II) in the case of lead release from hydrocerussite and cerussite was determined to be highly important. Chlorine oxidation tends to decrease concentrations of Pb(II) colloids as PbO2 phase is formed. However, PbO2 particles can undergo colloidal dispersion in the presence of NOM which adsorbs on their surface and substantially increases negative surface charge of this solid phase. Addition of orthophosphate greatly decreased colloidal lead release from both Pb(II) and Pb(IV) phases.