Radical-Based Destruction of Nitramines in Water: Kinetics and Effciencies of Hydroxyl Radical and Hydrated Electron Reactions

Abstract

In support of the potential use of advanced oxidation and reduction process technologies for the removal of carcinogenic nitro-containing compounds in water reaction rate constants for the hydroxyl radical and hydrated electron with a series of low molecular weight nitramines (R ¹R ²-NNO ₂) have been determined using a combination of electron pulse radiolysis and transient absorption spectroscopy. The hydroxyl radical reaction rate constant was fast, ranging from 0.54–4.35 × 10 ⁹ M ^–1 s ^–1, and seen to increase with increasing complexity of the nitramine alkyl substituents suggesting that oxidation primarily occurs by hydrogen atom abstraction from the alkyl chains. In contrast, the rate constant for hydrated electron reaction was effectively independent of compound structure, ( k _av = (1.87 ± 0.25) × 10 ¹⁰ M ^–1 s ^–1) indicating that the reduction predominately occurred at the common nitramine moiety. Concomitant steady-state irradiation and product measurements under aerated conditions also showed a radical reaction efficiency dependence on compound structure, with the overall radical-based degradation becoming constant for nitramines containing more than four methylene groups. The quantitative evaluation of these efficiency data suggest that some (~40%) hydrated electron reduction also results in quantitative nitramine destruction, in contrast to previously reported electron paramagnetic measurements on these compounds that proposed that this reduction only produced a transient anion adduct that would transfer its excess electron to regenerate the parent molecule.