Lead is a toxic heavy metal and the adverse effects of lead consumption are a current public health concern. Internal corrosion of lead-containing pipe, fittings, and solder in water distribution systems is currently the most significant source of lead to drinking water. While new construction does not use lead pipe, many older buildings retain the original lead service lines and internal plumbing. Lead concentrations in drinking water are affected by chemical reactions that occur within the water distribution system. The highest concentrations of lead are observed in water systems with relatively low pH and low alkalinity. Previous studies have investigated the equilibrium solubility of lead corrosion products, while this project focuses on the dissolution and transformation rates of lead carbonate corrosion products. Hydrocerussite, Pb_3(CO_3)_2(OH)_2, is a widely observed lead corrosion product and its dissolution in response to changes in water chemistry can greatly affect the dissolved lead concentration in water distribution systems. The dissolution rate of hydrocerussite was investigated as a function of pH, dissolved inorganic carbon, orthophosphate, and chloramine concentration. The dissolution rates of hydrocerussite were measured using completely-mixed continuous-flow reactors. The hydrocerussite was also characterized for surface area, molecular structure, morphology, and mineralogy before and after each experiment. The experimentally measured dissolution rates are used to generate a model for dissolution rates as a function of water chemistry. Such a model will allow water treatment facilities to determine lead concentrations as a function of measurable bulk water properties. Continuing work on this project will apply the model for dissolution rates to predicting lead release from pipes removed from distribution systems. Utilities are implementing lead remediation strategies, such as the addition of phosphate, to control lead concentrations in their distribution systems. This research monitors the transformation of hydrocerussite to lower solubility lead phosphates in realtime using Raman spectroscopy in a flow through reactor. The data will then used to develop conceptual models for the specific processes governing the transformation of hydrocerussite to lead phosphates. Important processes include: dissolution, nucleation and crystal growth, particle-particle interactions, or direct solid-state conversion. In the near future, mathematical models for each approach are being tested to provide insight into the transformation process.
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