In drinking water distribution system, chromium (Cr) may be extensively accumulated in corrosion scales of iron pipe, and the chromium(III) solids may advertently be oxidized by chlorine to form hexavalent chromium Cr(VI), typically existing as the oxyanion form of CrO<sub>4</sub><sup>2-</sup>. Since Cr(VI) is highly toxic and carcinogenic, its formation in drinking water is undesirable. This study investigated the kinetics of hexavalent chromium Cr(VI) formation by chlorine as a residual disinfectant in drinking water via the oxidation of two corrosion scales collected in the United States. The oxidation states and species of Cr and Fe in corrosion scales were characterized by using X-Ray Absorption Spectroscopy (XAS) to further understand the releasing mechanism. Oxidation experiments showed that the oxidation of iron corrosion scales by chlorine significantly increased the release of Cr(VI) in water for both corrosion scales. Approximately 25% of the total Cr in solids the corrosion scales were oxidatively released as Cr(VI), which may be explained by different Cr speciations in the solids. The XAS results further confirmed that both Cr(0) and Cr(III) existed in the solid. The presence of bromide significantly accelerated the oxidation of the solids by chlorine due to the catalytic effect of bromide. Furthermore, organic matter and ferrous iron in solids may are the dominant reductants to consume chlorine, competing with the oxidation of Cr. These findings suggested that chlorine as disinfectant may increase the risk of Cr(VI) occurrence in tap water. The abundance of Fe(II) and organic matter in solids, as well as the bromide levels in water can control the dissolved Cr(VI) formation.

Effects of chlorine on hexavalent chromium occurrence in drinking water via oxidation of iron corrosion scales

2019 Fall National Meeting

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