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 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 bulk XANES (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 (Pb5(VO4)3Cl) was positively identified, and occurred most frequently in the surface layers. Low Vtot 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 ΔG f ° for vanadinite as approximately −3443 kJ·mol−1, or less, leading to a log Ks0 value of approximately −86 for the reaction Pb 5 ( VO 4 ) 3 CI ( s ) ⇄ { Pb 2 + } 5 + { VO 4 3− } 3 + { Cl − } , in which { } denotes activity .