Sulfate radical induced catalytic degradation of metolachlor: Efficiency and mechanism

Abstract

Efficient removal of residual metolachlor (MET), a frequently used herbicide from aquatic system is highly desirable but remains a challenge. In the present study, the emerging sulfate radical based advanced oxidation processes (SR-AOPs) is developed for the degradation of MET from aqueous solution. At the conditions of [MET] = 10 mg L−1, [Peroxymonosulfate (PMS)] = 3 mM, [Catalyst] = 0.2 g L−1, pH = 6.5, the degradation process could be accomplished within 40 min and the derived pseudo-first-order kinetic rate constant (kapp) was 0.11 min−1. The effects of reaction conditions including pH, oxidant dosage, temperature, concomitant anions and humic acid on the degradation process were comprehensively investigated. Sulfate radical was identified to be the main contributor to the degradation process through radical scavenging and electron paramagnetic resonance (EPR) analysis. The second-order rate constant for reaction of SO4− with MET was determined to be 2.17 × 109 M−1s−1 by competition kinetic method. The degradation showed a moderate pH-tolerance. Specifically, alkaline/neutral conditions were beneficial for the MET degradation whilst acidic condition exhibited a detrimental effect on the degradation. Through LC-Q-TOF-MS analysis, eleven aromatic derivatives were detected and their molecular structures were tentatively proposed. A tentative mechanism was proposed based on the detected derivatives. The toxicity test further revealed that the degradation products performed a less toxicity than MET. The results pave an avenue toward the applications of SR-AOPs for MET elimination from aquatic environment.