Chromium Redox Chemistry in Drinking Water Systems

In order to understand the redox chemistry of chromium at low concentrations (100 (mu)g/L) under conditions typically found in drinking water systems, three reductants and four oxidants were tested in three different waters at pH 5, 7, and 9. In the absence of any oxidant or reductant, Cr(VI) was stable at all three pHs, while Cr(III) precipitated out of solution at pH 9 and greatly impacted the reduction reactions. Stannous chloride was more effective than sodium sulfite or sodium sulfide for reducing Cr(VI) to Cr(III). Sulfide is not likely to be used as a reductant due to the long reaction time (120 h) to achieve the same reduction as SnCl2, while sulfite may be effective at higher doses. The oxidation of Cr(III) by dissolved oxygen and chloramine was very slow, while Cl2 and KMnO4 were effective oxidants under many conditions. A Cl2 residual in a drinking water distribution system may oxidize any soluble Cr(III) to Cr(VI) because of the long contact time, so Cr treatment strategies will need to remove both Cr(III) and Cr(VI).