Hexavalent chromium in drinking water: Chemistry, challenges and future outlook on Sn(II)- and photocatalyst-based treatment

Highlights

• Wide occurrence of Cr(VI) in US source drinking water.

• A strong dependence of occurrence on groundwater sources.

• Elucidate Redox and equilibrium chemistry of Cr(VI).

• Sn(II)-based and TiO₂-based reductive treatments hold extreme promise.

• Key challenges include residual waste, Cr(VI) re-generation and socioeconomic drivers.

Abstract

Chromium (Cr) typically exists in either trivalent and hexavalent oxidation states in drinking water, i.e., Cr(III) and Cr(VI), with Cr(VI) of particular concern in recent years due to its high toxicity and new regulatory standards. This Account presented a critical analysis of the sources and occurrence of Cr(VI) in drinking water in the United States, analyzed the equilibrium chemistry of Cr(VI) species, summarized important redox reaction relevant to the fate of Cr(VI) in drinking water, and critically reviewed emerging Cr(VI) treatment technologies. There is a wide occurrence of Cr(VI) in US source drinking water, with a strong dependence on groundwater sources, mainly due to naturally weathering of chromium-containing aquifers. Challenges regarding traditional Cr(VI) treatment include chemical cost, generation of secondary waste and inadvertent re-generation of Cr(VI) after treatment. To overcome these challenges, reductive Cr(VI) treatment technologies based on the application of stannous tin or electron-releasing titanium dioxide photocatalyst hold extreme promise in the future. To moving forward in the right direction, three key questions need further exploration for the technology implementation, including effective management of residual waste, minimizing the risks of Cr(VI) re-occurrence downstream of drinking water treatment plant, and promote the socioeconomic drivers for Cr(VI) control in the future.

Graphical abstract

Keywords

Chromium / Chemistry / Treatment / Future outlook

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Acknowledgements

This work was supported by the US National Science Foundation CAREER Program (CBET-1653931). We thank current and former members of the Liu group at UC Riverside who have contributed to this effort, as well as the emotional support from Jacob Y. Liu to complete this article during the COVID-19 pandemic.

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