The Influence of Water Chemistry on Dissolution Rates of Lead(II) Carbonate Solids found in Water Distribution Systems

摘要

Lead is a toxic heavy metal and the adverse effects of lead consumption are acurrent public health concern. Internal corrosion of lead-containing pipe, fittings, andsolder in water distribution systems is currently the most significant source of lead todrinking water. While new construction does not use lead pipe, many older buildingsretain the original lead service lines and internal plumbing. Lead concentrations indrinking water are affected by chemical reactions that occur within the water distributionsystem. The highest concentrations of lead are observed in water systems with relativelylow pH and low alkalinity. Previous studies have investigated the equilibrium solubilityof lead corrosion products, while this project focuses on the dissolution andtransformation rates of lead carbonate corrosion products. Hydrocerussite,Pb_3(CO_3)_2(OH)_2, is a widely observed lead corrosion product and its dissolution inresponse to changes in water chemistry can greatly affect the dissolved lead concentrationin water distribution systems. The dissolution rate of hydrocerussite was investigated asa function of pH, dissolved inorganic carbon, orthophosphate, and chloramineconcentration.The dissolution rates of hydrocerussite were measured using completely-mixedcontinuous-flow reactors. The hydrocerussite was also characterized for surface area,molecular structure, morphology, and mineralogy before and after each experiment. Theexperimentally measured dissolution rates are used to generate a model for dissolutionrates as a function of water chemistry. Such a model will allow water treatment facilitiesto determine lead concentrations as a function of measurable bulk water properties.Continuing work on this project will apply the model for dissolution rates to predictinglead release from pipes removed from distribution systems.Utilities are implementing lead remediation strategies, such as the addition ofphosphate, to control lead concentrations in their distribution systems. This researchmonitors the transformation of hydrocerussite to lower solubility lead phosphates in realtimeusing Raman spectroscopy in a flow through reactor. The data will then used todevelop conceptual models for the specific processes governing the transformation ofhydrocerussite to lead phosphates. Important processes include: dissolution, nucleationand crystal growth, particle-particle interactions, or direct solid-state conversion. In thenear future, mathematical models for each approach are being tested to provide insightinto the transformation process.