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Comparison between Synthesized Lead Particles and Lead Solids Formed on Surfaces in Real Drinking Water Distribution Systems
Citation:
NADAGOUDA, M. N., D. A. LYTLE, AND M. R. SCHOCK. Comparison between Synthesized Lead Particles and Lead Solids Formed on Surfaces in Real Drinking Water Distribution Systems. In Proceedings, 2008 AWWA Water Quality Technology Conference, CINCINNATI, OH, November 16 - 20, 2008. American Water Works Association, Denver, CO, ., (2008).
Impact/Purpose:
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Description:
The objective of this work is to compare the properties of lead solids formed during bench-scale precipitation experiments to solids found on lead pipe removed from real drinking water distribution systems and metal coupons used in pilot scale corrosion testing. Specifically, solids and surface analysis approaches, including XRD, SEM, TEM, and EDS are used to compare the properties of lead solids and corrosion by-products. Under simulated conditions it was found that the morphological and crystalline features are very much comparable with lead coupons obtained from different sites. From XRD and EDS analysis, it has been found that the coupons surfaces contained various minerals such as hydrocerussite[Pb3(CO3)2(OH)2], cerussite(PbCO3), Pb(IV) oxides plattnerite, ß-PbO2, scrutinyite, α-PbO2¬¬ and other trace elements such as manganese, zinc and calcium. Similar phases and crystal structures were observed under bench-scale precipitation experiments. XRD patterns collected before and after aging of lead solids precipitated on bench-scale experimental pipes (14 days) clearly confirmed the transformation of Pb(II) to Pb(IV) solids. Fresh lead precipitate was indexed to only hydrocerussite. After aging, all of the peaks for hydrocerussite disappeared and new peaks were observed. These new peaks were indexed to cerussite, PbCO3, and the Pb(IV) oxides plattnerite, ß-PbO2, and scrutinyite, α-PbO2, The optically observed change in solid color of the solids from white to reddish brown indicates the transformation of phase, and was in agreement with the XRD findings. These results provide insight into the understanding of how corrosion by-product scales form on lead pipes, and into the dissolution of lead in drinking water distribution systems. Findings suggest that low to moderate alkalinity decreases corrosion rates, while higher alkalinities have a tendency to increase the corrosion rates of lead. Separate experiments, those that were carried out at different pH with and without orthophosphate inhibitor under laboratory conditions, were intended to represent real drinking water distribution systems. With an increase in pH from 6.5 to 8.5, the formation of a uniformed thickness in phosphate coating observed, in contrast to different crystal growth under low pH conditions. The X-ray diffraction patterns indicated that the surface contained a mixture of many phases.
