UC Riverside

Current reductive water treatment processes typically transform soluble Cr(VI), a toxic carcinogen to particulate Cr(III), a micronutrient. These processes do not completely and effectively remove residual Cr(III) particles. Furthermore, it is assumed that Cr(III) entering a drinking water distribution remains chemically inert. However, this is not the case. Nationwide surveys have observed an increase of Cr(VI) levels from the entry point to the point of distribution. Cr can accumulate in corrosion scales and act as an unrecognized in situ source for the inadvertent generation of Cr(VI), especially in the presence of a residual disinfectant.

The goal of this dissertation is to elucidate the transformation of Cr(III) solids to Cr(VI) in drinking water conditions. First, extensive data mining was done on the data reported as part of the U.S. EPA’s Unregulated Contaminants Monitoring Rule. It was observed that Cr(VI) levels in drinking water averaged from non-detectable to 97 µg/L at the entry point to drinking water distribution systems with the majority of sample detections ranging between 0.03 to 5 µg/L. Additionally, it was calculated that 20% of Cr(III) solids can be potentially oxidized to Cr(VI), especially in the presence of a residual disinfectant. Second, the kinetics of Cr(III) solids oxidation and subsequent formation of Cr(VI) by chlorine were investigated. Batch experiments were carried out with chromium hydroxide, chromium oxide, copper chromite, and iron chromium hydroxide as model Cr(III) sources. It was found that Cr(VI) was produced with the consumption of chlorine, suggesting that Cr(III) was oxidized to toxic Cr(VI) at environmentally relevant time scales. Third, the impact of pH (6-8.5), bromide concentration (0.1-5 mg/L), alkalinity (1-5 mM) and iron content (0.84-8.4 mM). The results highlighted that the presence of low level of bromide catalyzed the formation rates of Cr(VI) by one order of magnitude whereas increasing iron decreases Cr(VI) formation rates. Fourth, a lack of stoichiometric conversion between the molar amount of chlorine consumed and Cr(VI) generated was observed, suggesting the formation of Cr intermediates. The outcome of this study aimed to inform Cr(III) control strategies to minimize Cr(VI) formation in drinking water distribution systems.