Identification and Distribution of Vanadinite (Pb_5(V~(5+) O_4)_3CI) in Lead Pipe Corrosion By-Products

摘要

This study presents the first detailed look at vanadium (V) speciation in drinking water pipe corrosion scales. A pool of 34 scale layers from 15 lead or lead-lined pipes representing eight different municipal drinking water distribution systems in the Northeastern and Midwestern portions of the United States were examined. Diverse synchrotron-based techniques, including bulkXANES (X-ray absorption near edge spectroscopy), μ-XANES, μ-XRD (X-ray diffraction), and μ-XRF (X-ray fluorescence) mapping were employed along with traditional powder XRD, SEM-EDXA (scanning electron microscopy-energy dispersive X-ray analysis), and ICP-OES (inductively coupled plasma-optical emission spectrometry) to evaluate vanadium speciation and distribution in these deposits. Vanadinite (Pb_5(VO_4)_3CI) was positively identified, and occurred most frequently in the surface layers. Low V_(tot) in these waters is likely the limiting factor in the abundance of vanadinite in the pipe scales, along with the existence of divalent lead. The occurrence of V in these samples as a discrete mineral is important because it is formed in the presence of very low concentrations of V in the finished water, it provides a mechanism to concentrate μg·L~(-1) amounts of V from the water to near-percent levels in the pipe scales, and the robustness of V accumulation and release in response to water chemistry changes is likely different than it would be with a sorption accumulation mechanism. Extrapolation from limited existing water chemistry data in this study provides an estimate of △G_f° for vanadinite as approximately -3443 kJ·mol~(-1), or less, leading to a log K_(s0) value of approximately -86 for the reactionrnin which denotes activity.