Lin Yi-Pin, Valentine Richard L
Department of Civil and Environmental Engineering, University of Iowa, Iowa City, Iowa 52242-1527, USA.
Environ Sci Technol. 2008 Dec 15;42(24):9137-43. doi: 10.1021/es801037n.
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.
导致饮用水中过量铅突然释放的一个促成因素被认为与用一氯胺（NH₂Cl）替代游离氯作为消毒剂时发生的氧化还原条件变化有关。研究表明，NH₂Cl不能有效地将Pb（II）氧化形成PbO₂，PbO₂是由游离氯将金属铅和Pb（II）物种氧化形成的一种Pb（IV）矿物垢。出乎意料的是，我们观察到NH₂Cl实际上能够将PbO₂还原形成Pb（II）。我们通过改变重要的水化学因素，如溶液pH值、总碳酸盐浓度和Cl/N摩尔比，来系统地研究这个反应，以控制氯胺的形态及其通过一系列复杂的自分解反应的分解速率。发现形成的Pb（II）量与自分解的NH₂Cl量成正比，而与该反应的速率无关。这意味着Pb（II）的释放速率与NH₂Cl的绝对分解速率成正比。有人提出，负责还原PbO₂的物种可能是NH₂Cl衰变过程中产生的一种反应性中间体。这一发现首次报道了NH₂Cl可以作为一种还原剂。
