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Impact of DIC, Sulfate and Chloride on Pb(II) Solubility
Citation:
LYTLE, D. A., M. Nadagouda, J. Liggett, AND J. Conover. Impact of DIC, Sulfate and Chloride on Pb(II) Solubility. Presented at AWWA Water Quality Technology Conference, Savannah, GA, November 14 - 18, 2010.
Impact/Purpose:
To inform the public.
Description:
Corrosion of lead and the subsequent release of lead into drinking water distribution systems pose a concern to public health. Consequently, the U.S. Environmental Protection Agency’s (U.S. EPA) Lead and Copper rule established an action level for lead at the consumer’s tap of 0.015 mg/L in a one liter first draw sample. Since the rule’s passage, the understanding of relationships between water quality and the solubility of lead-containing minerals found in drinking water distribution systems has grown extensively. Of particular recent interest has been the potential role of chloride and sulfate on lead corrosion and lead levels at consumer’s taps. Some research has suggested that changes in the ratio of chloride to sulfate brought about by water source or treatment approach changes can directly contribute to elevated lead levels at consumer’s taps. The objective of this work was to systematically examine the impact of sulfate, chloride, dissolved inorganic carbon (DIC) concentration and pH on the solubility and properties of lead (II) solids formed during bench-scale precipitation experiments. Specifically, solids and surface analysis approaches (XRD, SEM, TEM, etc…) were used to compare the properties of lead solids. Experiments were conducted over a pH range between 3 to 10, in water containing 10 and 50 mg C/L DIC, and four different chloride to sulfate ratios (chloride and sulfate levels ranging from 0 to 1000 mg/L). Results showed that sulfate and chloride did slightly impact lead solubility in a relatively narrow pH range of approximately 6.8 to 7.2 in low DIC water, and had no impact at greater pH values. The impact of sulfate and chloride in high DIC water at neutral pH was not evident. At low pH values (< 6.5 to 6.8), sulfate and chloride reduced lead solubility in high and low DIC waters and the ratio of chloride to sulfate impacted the extent of reduction. A variety of lead minerals formed over the range of water conditions tested including: hydrocerussite, [Pb3(CO3)2(OH)2], cerussite, PbCO3, and Pb(II)-sulfate. Although sulfate and chloride did not impact lead solubility and mineralogy over a broad pH range, the parameters greatly impacted crystal morphology. The details of the impact of water chemistry on morphological differences, mineral stability regions, and lead solubility will be discussed in detail. Lastly, the practical significance of the findings will be discussed.