Ferrate (Fe(VI)) is a novel water treatment oxidant that can be used for micropollutant abatement. However, there is still knowledge gap concerning the interaction between Fe(VI) and dissolved organic matter structures, notably with primary amines. Nitrogen-containing moieties are particularly of interest in water treatment due to toxicological concerns on nitrogenous disinfection by-products such as haloacetonitriles or halonitromethanes. The kinetics and products of the reaction between Fe(VI) and several aliphatic primary amines was investigated. The rate constants ranged from 2.7 to 68 M<sup>-1</sup> s<sup>-1</sup> depending on the amine side group and conditions. The effect of the amine structure on the kinetic ranked benzylamine &gt; phenethylamine &gt; phenylpropylamine &gt; propylamine/methylamine while the effect of pH ranked pH 9 &gt; pH 7 &gt; pH 8, which contrasted with the common enhancing effect of low pH on Fe(VI) reactivity. Nitriles were found to be the main products, except for methylamine which led to cyanate. Other N-containing products included ammonia, nitrite/nitrate and nitroalkanes. Buffering conditions impacted both kinetic and product speciation, with faster kinetics and lower nitrile yields in borate compared to phosphate and carbonate. This difference is likely due to the action of high valent iron intermediates, such as Fe(V) and Fe(IV), which is notably inhibited by phosphate. The impact of Fe(V) and Fe(IV) on Fe(VI) oxidation in real waters therefore needs to be considered. An in vitro evaluation of the oxidative stress response and cytotoxicity of 13 cyano-compounds revealed that non-halogenated nitriles acted as baseline toxicants and did not cause oxidative stress. They are therefore unlikely to be of concern. Compared to the ozonation of primary amines, which leads to high yields of nitroalkanes and to halonitroalkanes during subsequent chlorination, the formation of non-halogenated nitriles from Fe(VI), which remain unreactive during chlorination, is preferable.

Ferrate(VI) oxidation of primary amines in water: Kinetics, reaction products and implication for N-DBP formation control

ACS Spring 2022

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