How Water Pipes Release Toxic Cr(VI)

Toxic metals can be released from drinking water distribution pipes when the water’s composition changes. Chromium, for example, is found in cast iron alloys, which are widely used as plumbing materials. As pipes corrode, a buildup of deposits, or scale, forms inside the pipes. Trace chemicals in water can react with scale and potentially form compounds that contain hexavalent chromium, or Cr(VI). Hexavalent chromium compounds are harmful and can, e.g., cause cancers or liver damage.

Haizhou Liu, University of California at Riverside, USA, and colleagues have investigated how Cr(VI) is released into drinking water, which might lead to new ways to prevent its formation. The researchers collected two sections of cast iron pipe from two drinking water distribution systems in the United States: one from a system with naturally high Cr(VI) levels, and the other from a system with undetectable levels of Cr(VI). The team scraped off scales from both pipes and analyzed their composition.

The dominant element in both corrosion scales was iron. The levels of total Cr were about 18 times higher in the first pipe than in the second. However, a substantial amount of Cr(0) existed in both corrosion scales, coexisting with Cr(III). By producing “fresh” corrosion scales in chromium-free water, the team found that the main source of Cr in the corrosion scales is the pipes’ cast iron alloy.

When the researchers added chlorine- or bromine-containing disinfectants (HOCl or HOBr) to the scale, they quickly reacted with Cr(0) to form Cr(VI) species. Cr(0) had a much higher reactivity with regards to HOCl than Cr(III) solids. This direct oxidation of Cr(0) by chlorine compounds is a previously unknown pathway of Cr(VI) formation in drinking water. According to the researchers, using less reactive disinfectants to treat drinking water could be a way to help mitigate Cr(VI) levels, and cast iron pipes with chromium alloys should be used with caution.