Hydroxylamine discovery complicates water purification efforts

Hydroxylamine, a reactive inorganic compound and potential carcinogen, has been identified as a key intermediate in the catalytic reduction of nitrogenous pollutants. The finding raises concerns for researchers developing water purification catalysts, as it suggests that some treatment processes might inadvertently introduce hydroxylamine into drinking water.

Nitrate and nitrite enter water systems through inorganic fertilisers and can lead to health conditions such as methaemoglobinaemia in children. Consequently, the European Union has mandated a concentration limit of 50mg/l and 0.5mg/l for nitrate and nitrite in drinking water, respectively.

While there has been extensive research into developing metal catalysts for removing nitrate and nitrite from water, incomplete mass balance calculations mean these studies had not fully elucidated the role of hydroxylamine as an intermediate before. ‘The issue with this reaction is that you have gas phase products and liquid phase products,’ comments Janek Betting, who led the study. ‘Typically, this reaction is done at relatively low nitrate and nitrite concentrations of less than 1mmol/l and a tiny nitrogen leak can mess up your mass balance.’

Now, Betting and colleagues Jimmy Faria Albanese and Leon Lefferts at the University of Twente in the Netherlands have studied the extent to which hydroxylamine accumulates during nitrate and nitrite reduction over metal catalysts. The researchers used benzaldehyde to trap the hydroxylamine, which formed during the reaction and quantified the resulting oxime with liquid chromatography. Crucially, they noted that hydroxylamine accumulated during the reduction reaction and accounted for up to 56% of the reaction mixture as the reaction progressed before being converted to nitrogen and ammonia, depending on the metal catalyst they used.

‘I was surprised the percent conversion to hydroxylamine was as high as it was for the different catalyst formulations,’ comments Charles Werth, an expert in civil and environmental engineering at the University of Texas at Austin in the US.

This hydroxylamine intermediate now presents a challenge for both catalysis researchers and chemical engineers. ‘I think investigating which catalyst formulations give rise to fast hydroxylamine kinetics is an important next step,’ continues Werth. ‘The water quality conditions may also affect how the hydroxylamine intermediate persists … its production might be affected in the presence of other constituents in water. When we think about scale-up, it’s going to be important to design a reactor with hydroxylamine explicitly in mind to ensure that it isn’t discharged into treated water.’