Cr(VI), known as hexavalent chromium, is a highly toxic and soluble compound that has been widely observed in groundwater. It enters groundwater through improper industrial waste disposal as well as geological weathering of naturally occurring chromium-containing aquifer minerals. The proposed low drinking water standard for Cr(VI) will post great challenges to public water systems to meet the regulation. Available water treatment technologies normally transform soluble Cr(VI) to particulate Cr(III) before removing the solids. Albeit that, these treatment processes are not efficient to achieve the low concentrations that will be required. Subsequently, in water distribution systems, residual Cr(III) can be potentially re-oxidized to Cr(VI) by disinfectants, resulting in reoccurrence of Cr(VI) in drinking water. This study seeks to understand the kinetics of Cr(III) oxidation and formation of Cr(VI) in typical drinking water conditions by the widely used disinfectant, i.e., chlorine. Batch experiments were carried out with chromium hydroxide as a model Cr(III) source and at varying pHs ranging from 6 to 8. It was found that Cr(VI) was produced with the consumption of disinfectant, suggesting that Cr(III) was oxidized to toxic Cr(VI) by disinfectants. The oxidation rate was greatly impacted by solution pH. Furthermore, Cr(III) oxidation in electrochemical system suggested the formation of chromium intermediates before producing Cr(VI). Results from this study will help understand the redox chemistry of chromium and prevent the occurrence of Cr(VI) in drinking water.

Impacts of redox chemical conditions on the occurrence and transformation of hexavalent chromium in drinking water

248th National Meeting (2014)

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