Oxidation Of Cr-Iii To Cr-Vi During Chlorination Of Drinking Water

Drinking water treatment typically uses strong oxidants such as chlorine which are capable of converting Cr-III to Cr-VI. The rates and extent of Cr-III oxidation by chlorine are not well established. Cr-III oxidation experiments were therefore conducted in distilled deionized water and New York City tap water dosed initially with Cr-III and supplemented with sodium hypochlorite to increase free chlorine residual. Reaction progress was monitored using capillary electrophoresis which quenched reactions and allowed for quantification of Cr-VI. Three different forms of Cr-III were used as reactants: a Cr-III nitrate salt, Cr-III-EDTA, and Cr-III hydroxide. Rates of Cr-VI production for all three forms of Cr-III were rapid, on the order of hours. However, oxidation rates slowed and a plateau in Cr-VI concentrations was reached. This resulted in less than 100% conversion of Cr-III to Cr-VI even at relatively high chlorine doses (10 to 100 mg L-1 as Cl-2). The loss of free chlorine due to a non-Cr chlorine demand, the precipitation of Cr-III to Cr(OH)(3)(s), and the partial oxidation of Cr-III to intermediate oxidation states (i.e. Cr-IV and Cr-V) were examined and eliminated as possible explanations for this behavior. Consumption of chlorine via reaction with intermediate oxidation states of Cr is therefore offered as a possible explanation for the plateau in Cr-VI concentrations.