Identification and Distribution of Vanadinite (Pb5(V5+O4)3Cl in Lead Pipe Corrosion By-Products

This study presents the first detailed look at vanadium (V) speciation in drinking water pipe corrosion scales. A pool of 34scale 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 bulk XANES (X-ray absorption near edge spectroscopy), mu-XANES, mu-XRD (X-ray diffraction), and mu-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(VO4)(3)Cl) 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 mu g.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 Delta G(f)degrees for vanadinite as approximately -3443 kJ,mol(-1), or less, leading to a log K-s0 value of approximately -86 for the reaction Pb-5(VO4)(3)CI(s) reversible arrow {Pb2+}(5) + {VO43-} + {Cl-} in which {}denotes activity.