'Phantom chemical' in drinking water finally identified

Many public water systems in the United States, as well as Australia, use inorganic chloramines to disinfect drinking water, but their decomposition products have long been a mystery. Now, researchers have reported in the journal Science the discovery of a previously unknown compound in chloraminated drinking water — a compound whose existence, if not its identity, has been known for 30 years.

For over a century, chemical disinfection of public water supplies has reduced waterborne diseases like cholera and typhoid fever by killing pathogens in drinking water; inorganic chloramines, like monochloramine (NH₂Cl) and dichloramine (NHCl₂), have been widely used for this purpose. However, for decades, chloramine decomposition has been suspected of producing elusive chemical by-products, including potential nitrogen-containing compounds with unknown toxicity.

According to Associate Professor Julian Fairey, from the University of Arkansas, researchers have known about one of these by-products for decades, but have been unable to identify it. By combining classic synthesis methods with advanced analytical techniques like high-resolution mass spectrometry and nuclear magnetic resonance (NMR) spectroscopy, Fairey and colleagues have now isolated and identified chloronitramide anion, chemically expressed as Cl–N–NO₂^-, as an end product of inorganic chloramine decomposition.

“It’s a very stable chemical with a low molecular weight,” said Fairey, who is first co-author on the new paper. “It’s a very difficult chemical to find. The hardest part was identifying it and proving it was the structure we were saying it was.”

Image credit: Oliver Jones.

After synthesising the compound in the lab, which had never been done before, samples were sent for analysis to Fairey’s colleague Juliana Laszakovits, a postdoctoral researcher at ETH Zurich and co-first author on the paper. The researchers also measured chloronitramide anion content in a range of chloraminated water systems in the US, detecting levels as high as ~100 μg/L — which surpasses the typical regulatory limits for many disinfection by-products (60–80 μg/L). Notably, the compound was absent in water systems that used alternative disinfectants.

While direct toxicological studies of chloronitramide anion have not yet been conducted, its prevalence and similarity to other toxic compounds is believed to warrant further study to assess any public health risk. Indeed, computational analyses suggest that it may not be benign, emphasising the need for immediate toxicological assessment and quantification in source waters, finished drinking waters and wastewater effluents.

“It’s well recognised that when we disinfect drinking water, there is some toxicity that’s created — chronic toxicity, really,” Fairey said. “A certain number of people may get cancer from drinking water over several decades. But we haven’t identified what chemicals are driving that toxicity. A major goal of our work is to identify these chemicals and the reaction pathways through which they form.”

Whether chloronitramide anion will be linked to any cancers or has other adverse health risks will be assessed in future work by academics and regulatory agencies, such as the US Environmental Protection Agency. At the very least, toxicity studies on this compound can now be completed.

“Even if it is not toxic, finding it can help us understand the pathways for how other compounds are formed, including toxins,” Fairey said. “If we know how something is formed, we can potentially control it.”

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