Degradation of the neonicotinoid insecticide imidacloprid by ozonation and the E-peroxone process: Mechanisms, transformation products, and toxicity

Abstract

In this study, degradation of the neonicotinoid insecticide imidacloprid (IMI) by ozonation and the E-peroxone process was investigated. Results show that IMI is an O3-refractory compound with second-order rate constants for the reaction with O3 and •OH (kO3,IMI and k•OH,IMI) being 3.9 and 3.95 × 109 M−1 s−1 at pH 7, respectively. Due to its low O3 reactivity, IMI was only abated by < 20 % in a selected groundwater during ozonation with typical ozone doses applied in water treatment (1–5 mg/L). In comparison, the E-peroxone process considerably enhanced IMI removal efficiency to > 80 % under similar reaction conditions. During both ozonation and the E-peroxone process, IMI was mainly abated by •OH oxidation. The reaction mechanism of IMI with •OH was investigated with quantum computations, which show that H-abstraction at H23 site was the most favorable pathway. Sixteen transformation products (TPs) generated from IMI degradation were identified using various chromatographic and mass-spectrometric techniques. The acute and chronic toxicities of IMI and its TPs to different aquatic organisms were evaluated with Microtox bioassay and the ECOSAR program, which indicate that some TPs (e.g., TP2–TP5 and TP7–TP9) are even more toxic than IMI to aquatic organisms (e.g., daphnia). The results of this study suggest that IMI along with its TPs should be sufficiently abated during water treatment, and the E-peroxone process provides an efficient option to upgrade conventional ozonation for enhanced IMI abatement.