Release of Pb(II) from monochloramine-mediated reduction of lead oxide (PbO2).

A contributing factor causing the sudden release of excessive lead into drinking water is believed to involve the change in redox conditions occurring when monochloramine (NH2Cl) replaces free chlorine as a disinfectant. Studies suggest that NH2Cl cannot effectively oxidize Pb(II) to form PbO2, a Pb(IV) mineral scale formed from the oxidation of metallic lead and Pb(II) species by free chlorine. Unexpectedly, we observed that NH2Cl is actually capable of reducing PbO2 to form Pb(II). We systematically investigated this reaction by varying important water chemistry factors such as solution pH, total carbonate concentration, and the Cl/N molar ratio to control chloramine speciation and its rate of decomposition via a complex set of autodecomposition reactions. The amount of Pb(II) formed was found to be proportional to the amount of NH2Cl that autodecomposed regardless of the rate of this reaction. This implies thatthe rate of Pb(II) release is proportional to the absolute rate of NH2CI decomposition. It is proposed that the species responsible for the reduction of PbO2 is likely a reactive intermediate produced during the decay of NH2Cl. This finding is the first to report that NH2Cl can act as a reductant.