Theoretical insights into the oxidation of quinmerac herbicide initiated by HO• radical in aqueous media: Mechanism, kinetics, and ecotoxicity

Abstract

In this study, the oxidation of quinmerac herbicide initiated by HO• radical was investigated using quantum chemistry and kinetic calculations. Three oxidation processes, namely radical adduct formation (RAF), formal hydrogen transfer (FHT), and single electron transfer (SET), were studied in aqueous media at the M06–2X/6–311 + +G(3df,3pd)//M06–2X/6–31 +G(d,p) level of theory. The results illustrated that the RAF reactions were more likely to occur during the oxidation process, followed by FHT, and SET was found to be a negligible pathway. Branching ratio calculations showed that the HO•-addition to C7 of the benzyl ring was the predominant pathway, followed by the H-abstraction at the methyl group. The rate constant was evaluated as a function of temperature (283−333 K) and pH (0−14). The highest reaction rates were obtained at low pH (0−1) and high temperature (333 K) conditions (i.e., 9.36 × 108 M−1s−1). Quinmerac can exist in the environment from a few seconds to several years, depending on conditions. The ecotoxicity prediction illustrated that quinmerac and its degradation products are not harmful and non-bioaccumulative substances to aquatic organisms. However, they have developmental toxicity but no mutagenic toxicity.