Chromium(VI), known as hexavalent chromium, is a highly toxic and soluble compound that has been widely observed in groundwater nationwide. 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 chromium(VI) is estimated to post great challenges to Californian water systems to upgrade treatment to meet the regulation. Available water treatment technologies normally transform soluble chromium(VI) to particulate chromium(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 piping systems, residual chromium(III) can be potentially re-oxidized to Cr(VI) by disinfectants, resulting in reoccurrence of Cr(VI) in drinking water. This study is aimed to understand the process of Cr(III) oxidation and formation of Cr(VI) in typical drinking water conditions by two widely used disinfectants<i>, i.e</i>., chlorine and chloramine. Bench-scale experiments were carried out with different trivalent chromium particles as model Cr(III) sources and at varying pHs between 6 and 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 higher with chlorine than with chloramine by one order of magnitude. Furthermore, data suggested that trivalent chromium was not completely transformed to hexavalent chromium. A significant amount of chromium likely existed in stable tetravalent and/or pentavalent forms.

Transformation of hexavalent chromium via redox pathways in drinking water: Implications on Cr(VI) control and treatment

249th National Meeting (2015)

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