Mystery chemical in drinking water identified

For decades, scientists have known that a mysterious chemical forms when inorganic chloramines are used to disinfect water. But they didn’t know this chemical’s molecular formula or structure. Now the mystery molecule has been identified as the chloronitramide anion. Little is known about this anion or its toxicity.

Approximately 113 million people in the US consume drinking water disinfected with the inorganic chloramines NH₂Cl and NHCl₂, according to the US Environmental Protection Agency (EPA). This type of water treatment has grown in popularity as people seek to avoid dangerous disinfection by-products that form with chlorine treatment, which is more common. But chloramine treatment also creates disinfection by-products, and the identity of one of these had long eluded researchers.

“Despite being able to reproducibly form the unidentified product for over 30 years now, prior characterization efforts have been stymied by analytical limitations and an incomplete understanding of chloramine decomposition chemistry,” said the University of Arkansas’s Julian L. Fairey during a press conference. Fairey led the identification effort along with Kristopher McNeill of the Swiss Federal Institute of Technology (ETH), Zurich, and David G. Wahman of the EPA.

Using ion chromatography and electrospray ionization mass spectrometry, the researchers were able to figure out the compound’s formula—N₂O₂Cl. There were only two possible anions it could be, McNeill says, and they were able to synthesize the most likely one via an alternative route and confirm that it was the same mystery anion found in chloramine-treated water (Science 2024, DOI: 10.1126/science.adk6749).

The researchers also examined water samples from several locations for the chloronitramide anion. They couldn’t find the anion in water from Switzerland, where ozone is used for disinfection instead of chloramines. But they found the anion in all the samples from places where chloramine is used to treat water in the US. In some cases, the team found it at levels as high as 120 µg/L. The EPA limits most regulated disinfection by-products to 60–80 μg/L. The chloronitramide anion should be studied for toxicity, the researchers say.

Susan D. Richardson, an expert in drinking water disinfection by-products at the University of South Carolina, calls the work groundbreaking. “It will be important to quantify this new disinfection by-product in drinking water distribution systems to determine whether it increases or decomposes over time before it reaches consumers’ taps,” she says in an email. Richardson says that other halogenated nitro disinfection by-products are toxic, and she suspects that the chloronitramide anion is too. Important next steps will be to determine its toxicity, Richardson says. “It is great that the authors identified an easy solution for consumers to remove it: activated carbon.”