AWWA ACE59936

In order to understand the redox chemistry of chromium at low concentrations underconditions typically found in drinking water systems, five oxidants and three reductants weretested 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). Theinitial chromium concentration was adjusted to 100 µg/L in all the tests. Doses of the oxidantsand reductants were determined according to levels usually found in drinking water plants. Theresults indicated that dissolved oxygen and chloramine had essentially no effect on the oxidationof Cr(III) to Cr(VI), while MnO₄^- was the most effective oxidant. To achieve the sameconversion percentage as MnO₄^-, Cl₂ needed a longer reaction time, which indicated that eventhough Cl₂ may not be as efficient as MnO₄^- as a treatment method, its residual as a disinfectantin the drinking water distribution system may ultimately oxidize Cr(III) to Cr(VI) because of thelong contact time, hence posing a potential health risk to the consumer at the tap. Because H₂O₂was found to react with the colorimetric reagent (diphenylcarbohydrazide) and interfered thedevelopment of the color needed to quantify Cr(VI) concentration, its performance as an oxidantcould not be evaluated, although it is expected to be effective. SnCl₂ was more effective as areductant than SO₃^2- or S^2-. The test results also indicated that particulate species (CaCO₃ andCr(III) precipitates) present in the water at higher pH greatly impacted the redox reactions. Includes 17 references, tables, figures.

Product Details

Edition:

Vol. - No.

Published:

06/17/2004

Number of Pages:

20

File Size:

1 file , 420 KB