Oxidation of CrIII to CrVI during chlorination of drinking water

Abstract

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 Cl₂). 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.