Kinetics and Mechanisms of Cr(VI) Formation via the Oxidation of Cr(III) Solid Phases by Chlorine in Drinking Water

Michelle Chebeir,Haizhou Liu

doi:10.1021/acs.est.5b05739

Michelle Chebeir, Haizhou Liu

Open Access

https://doi.org/10.1021/acs.est.5b05739

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Abstract

Hexavalent chromium Cr(VI), typically existing as the oxyanion form of CrO4(2-), is being considered for more stringent drinking water standards by regulatory agencies. Cr(VI) can be inadvertently produced via the oxidation of trivalent chromium Cr(III) solids. This study investigated the kinetics and mechanisms of Cr(III) solids oxidation by chlorine in drinking water and associated Cr(VI) formation. Batch experiments were carried out with three Cr(III) solids of environmental relevance, i.e., chromium hydroxide Cr(OH)3(s), chromium oxide Cr2O3(s), and copper chromite Cu2Cr2O5(s). Impacts of water chemical parameters including pH (6.0-8.5) and bromide concentration (0-5 mg/L) were examined. Results showed that the rapid oxidation of Cr(III) solid phases by chlorine was accompanied by Cr(VI) formation and an unexpected production of dissolved oxygen. Analysis of reaction stoichiometry indicated the existence of Cr intermediate species that promoted the autocatalytic decay of chlorine. An increase in pH modestly enhanced Cr(VI) formation due to changes of reactive Cr(III) surface hydroxo species. Bromide, a trace chemical constituent in source waters, exhibited a catalytic effect on Cr(VI) formation due to an electron shuttle mechanism between Cr(III) and chlorine and the bypass of Cr intermediate formation. The kinetics data obtained from this study suggest that the oxidation of Cr(III) solids by chlorine in water distribution systems can contribute to Cr(VI) occurrence in tap water, especially in the presence of a trace level of bromide.

Highlights

The presence of hexavalent chromium Cr(VI) in drinking water is an increasing concern in recent years because it can cause adverse human health effects, including lung cancer, stomach cancer, and dermatitis.[1−6] Cr(VI) typically exists as an oxyanion in drinking water and is released from both anthropogenic and natural sources
Anthropogenic sources include industrial waste discharges from tanneries and electroplating, metallurgical smelting of chromite ore, and corrosion of metal alloys.[7−9] Naturally occurring Cr(VI) typically originates from the geological weathering of Crcontaining aquifer minerals.[10−16] As an emerging contaminant, a new drinking water standard of Cr(VI) was set at 10 μg/L in California in 2014,17 and a new U.S EPA regulation may be proposed in the future.[18]
On the basis of the most recent sampling data from U.S EPA’s third round Unregulated Contaminant Monitoring Rule (UCMR3), it was determined that 63% of the participating public water systems nationwide detected Cr(VI) at the end point of the drinking treatment plant.[18]

Summary

Introduction

The presence of hexavalent chromium Cr(VI) in drinking water is an increasing concern in recent years because it can cause adverse human health effects, including lung cancer, stomach cancer, and dermatitis.[1−6] Cr(VI) typically exists as an oxyanion (i.e., chromate CrO42−) in drinking water and is released from both anthropogenic and natural sources. Anthropogenic sources include industrial waste discharges from tanneries and electroplating, metallurgical smelting of chromite ore, and corrosion of metal alloys.[7−9] Naturally occurring Cr(VI) typically originates from the geological weathering of Crcontaining aquifer minerals.[10−16] As an emerging contaminant, a new drinking water standard of Cr(VI) was set at 10 μg/L in California in 2014,17 and a new U.S EPA regulation may be proposed in the future.[18] On the basis of the most recent sampling data from U.S EPA’s third round Unregulated Contaminant Monitoring Rule (UCMR3), it was determined that 63% of the participating public water systems nationwide detected Cr(VI) at the end point of the drinking treatment plant.[18]. Trivalent chromium Cr(III), which is much less toxic than Cr(VI) and even a micronutrient, mostly exists as a solid with limited solubility at circumneutral pH.[19,20] Typical Cr(III) solids relevant to drinking water systems include chromium hydroxide

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