Hexavalent chromium is a carcinogen that was made famous by the 2000 film “Erin Brockovich,” which dramatized a case of industrial pollution that contaminated water supplies in California. A new study shows that Cr(VI) can slip into drinking water when commonly used chlorine disinfectants corrode cast iron water distribution pipes. The newly discovered mechanism suggests other pathways for controlling levels of toxic chromium in drinking water (Environ. Sci. Technol. 2020, DOI: 10.1021/acs.est.0c03922).
Chromium is added to many products and occurs naturally, but its toxicity depends on its valence state. Cr(III) is found in soils and rocks and is an essential nutrient. Cr(VI) is used in dyes, paints, and plastics, but it’s toxic and carcinogenic. More than 200 million people in the US drink tap water with Cr(VI) concentrations above 0.02 ppb, according to the Environmental Working Group. This public health goal, set by the State of California, is the level of Cr(VI) thought to ensure that fewer than one in a million exposed people will get cancer over their lifetimes. The US Environmental Protection Agency does not regulate Cr(VI) in drinking water.
Until now, most scientists thought that any chromium found in drinking water must have originated outside the distribution system, entering from natural sources in groundwater and surface water or from industrial pollution. But once environmental engineer Haizhou Liu and his team at the University of California, Riverside saw hints that reactions inside water pipes might increase levels of Cr(VI) at the tap, the group decided to take a closer look.
The scientists obtained sections of cast iron pipes from two drinking water systems in two US West Coast states. Cast iron is the most widely used pipe material, Liu says, and it contains significant amounts of Cr(0) added to the alloy as an anticorrosive. Even with this additive, years of use creates crusty corrosion scale on the insides of pipes; the team scraped off some of this material and used X-ray spectroscopy to determine the levels of chromium in its various valence states.
Liu knew that pipe scales could help chemically reduce any Cr(VI) found in source water to Cr(III), so he expected to find Cr(III) in the samples, which they did. “But I was surprised to see Cr(0) in the scales,” he says.
To determine if the chromium in pipe scales could generate Cr(VI), the researchers incubated the samples in a solution of hypochlorous acid, the oxidizing sanitizer commonly used to treat drinking water. The disinfectant-treated scales rapidly cranked out Cr(VI) whereas scales in water with no disinfectant produced none.
Follow-up experiments showed that pipe scales and pure samples of Cr(0) produced Cr(VI) 10 times as fast when exposed to the sanitizer as samples of Cr(III). “This tells us that . . . the oxidation of Cr(0) in the scales by chlorine disinfectant accounts for most of the formation of Cr(VI) released into the solution,” Liu says. The concentrations of Cr(VI) generated by the pipe scales in the study were comparable to actual average Cr(VI) concentrations measured in US drinking water, the researchers estimate.
“Now that we know that cast iron pipes are a potential source of Cr(VI), utilities need to think proactively to use less reactive disinfectants and limit the amount of Cr(0) allowed in new drinking water pipes,” says Lynn Katz, an environmental engineer at the University of Texas who was not part of the study.