Chromium isotope fractionations resulting from electroplating, chromating and anodizing: implications for groundwater pollution studies

Available online 27 March 2017A number of shallow aquifers in industrial regions have been polluted by toxic Cr(VI). At some sites, spontaneous reduction of dissolved Cr(VI) to insoluble Cr(III) has been observed. Precipitation of non-toxic Cr(III) is accompanied by a Cr isotope fractionation, with the residual Cr(VI) becoming enriched in the heavier isotope 53Cr, and depleted in the lighter isotope 52Cr. Thus far, δ53Cr values of the contamination source have been poorly constrained. These values are needed to quantify the extent of Cr(VI) reduction in the aquifers. We present δ53Cr values of solutions generated during Cr-electroplating, chromating and anodizing at nine industrial sites. The source chemical, CrO3, had a mean δ53Cr of 0.0‰. A small-to-negligible Cr isotope fractionation was observed between the solutions of the plating baths and the source chemical. Across all sample types, the mean δ53Cr(VI) value was 0.2‰. The mean δ53Cr(VI) value of contaminated groundwater in the same region, studied previously, was significantly higher (2.9‰), indicating Cr(VI) reduction. Based on low δ53Cr(VI) values of plating baths and rinsewaters as potential contamination sources, and their low variability, we suggest that most aquifer δ53Cr(VI) values higher than 1.0‰ are a result of in-situ Cr(VI) reduction.Martin Novak, Vladislav Chrastny, Ondrej Sebek, Eva Martinkova, Eva Prechova, Jan Curik, Frantisek Veselovsky, Marketa Stepanova, Barbora Dousova, Frantisek Buzek, Juraj Farkas, Alexandre Andronikov, Nikoleta Cimova, Marie Houskov