AWWA ACE59936

In order to understand the redox chemistry of chromium at low concentrations under conditions typically found in drinking water systems, five oxidants and three reductants were tested in four different water qualities: deionized water with 10^-3 M NaNO₃; synthetic water; reducing water; and, a natural water. All the tests were done at three pH levels (5, 7, and 9). The initial chromium concentration was adjusted to 100 µg/L in all the tests. Doses of the oxidants and reductants were determined according to levels usually found in drinking water plants. The results indicated that dissolved oxygen and chloramine had essentially no effect on the oxidation of Cr(III) to Cr(VI), while MnO₄^- was the most effective oxidant. To achieve the same conversion percentage as MnO₄^-, Cl₂ needed a longer reaction time, which indicated that even though Cl₂ may not be as efficient as MnO₄^- as a treatment method, its residual as a disinfectant in the drinking water distribution system may ultimately oxidize Cr(III) to Cr(VI) because of the long contact time, hence posing a potential health risk to the consumer at the tap. Because H₂O₂ was found to react with the colorimetric reagent (diphenylcarbohydrazide) and interfered the development of the color needed to quantify Cr(VI) concentration, its performance as an oxidant could not be evaluated, although it is expected to be effective. SnCl₂ was more effective as a reductant than SO₃^2- or S^2-. The test results also indicated that particulate species (CaCO₃ and Cr(III) precipitates) present in the water at higher pH greatly impacted the redox reactions. Includes 17 references, tables, figures.