Sonochemical degradation of DEET in aqueous medium: Complex by-products from synergistic effect of sono-Fenton – New insights from a HRMS study

Abstract

A detailed investigation on the sonochemical degradation of DEET (N,N-diethyl-m-toluamide), a widely used broad spectrum insect repellant and an ubiquitous contaminant of emerging concern (CEC) in aquatic environments, was conducted in aqueous medium. The degradation followed a pseudo-first-order kinetics. The effects of pH, inorganic anions, organic matter and additives such as surfactants and ferrous ion (Fe2+) were evaluated. The degradation was favored under alkaline conditions, while comparable rates were found at acidic and near-neutral conditions. Presence of inorganic ions, organic matter and surfactants showed a positive effect on the degradation. Significant TOC reduction (≈96%) was achieved at 90 min of sonolysis which is the highest figure compared to all other reported AOP based degradation of DEET. The addition of Fe2+ (5 × 10−5 M) exhibited a positive synergy and leads to remarkable enhancement in the degradation efficiency up to 99%. The degradation products of both the sonolysis and sono-Fenton were analyzed by using LC-Q-ToF-MS. Fourteen sonochemical and six sono-Fenton degradation products were identified. Based on the identified intermediates, possible degradation pathways are proposed. Sonolysis mainly proceeds through hydroxylation (mono and poly) along with oxidation, demethylation, aromatic ring cleavage and hydrolysis. Whereas, majority of the sono-Fenton products are observed to be the breakdown products after dimerization followed by radical reactions. Studies characterizing such products are rather limited. This study provides new insights on complex breakdown products of DEET dimers formed during sono-Fenton, which has high relevance in its degradation protocol in natural environment as well as waste water treatment.