Dissolution, formation, and transformation of the lead corrosion product lead dioxide: Rates and mechanisms of reactions that control lead release in drinking water distribution systems.

摘要

As one of the major lead corrosion products in lead service lines, lead(IV) oxide (PbO2) can react with water through reductive dissolution, resulting in elevated lead concentrations in tap water. Limited data are available on the rates and mechanisms of PbO2 dissolution. Information regarding the impact of water chemistry on the rates and mechanisms of PbO2 dissolution can provide potential strategies to control the release of lead from corrosion products to drinking water. The present study investigated effects of water chemistry on the rates and mechanisms of PbO2 dissolution, the equilibrium solubility of PbO2, and the rates of lead release from lead pipes with corrosion scales.;The dissolution rate of pure plattnerite (beta-PbO2) was investigated as a function of pH and the concentrations of carbonate, orthophosphate, free chlorine, and monochloramine in continuously stirred tank reactors (CSTR). Complementary batch experiments were conducted to compare the effects of water chemistry on the PbO2 dissolution rate for different solid-water contact times. The equilibrium solubility of plattnerite in the presence of free chlorine was then investigated. Lead release from pipe scales was determined under different water chemistry conditions at flow or no-flow conditions to optimize the water chemistry for mitigating lead release. For these experiments new lead pipes were conditioned in the presence of free chlorine and carbonate to form corrosion scales.;Dissolution experiments provided direct evidence that the PbO2 dissolution rate increases when the disinfectant is switched from free chlorine to monochloramine, which is consistent with the high lead concentrations observed in Washington D.C. from 2001 to 2004 following such a switch. Lower pH and the presence of carbonate accelerated PbO2 dissolution. Addition of orthophosphate as a potential corrosion inhibitor had multiple effects on PbO2 dissolution rates. A detailed mechanism and rate model were proposed for PbO2 reductive dissolution. Batch experiments showed that the residence time also played an important role in controlling dissolved lead concentrations. Pipe scales developed under drinking water conditions contained PbO2 and hydrocerussite (Pb3(CO 3)2(OH)2). In experiments with these pipe scales, when compared with stagnant conditions water flow significantly accelerated the release of both dissolved and total lead from pipe scales. Among various water chemistry conditions, the dissolved lead was lowest from corrosion scales in contact with solutions containing orthophosphate. Two models were proposed to predict lead release from pipe scales.