Unveiling the mechanism of imidacloprid removal by ferrate(VI): Kinetics, role of oxidation and adsorption, reaction pathway and toxicity assessment

Abstract

Imidacloprid (IMI), an emerging pollutant, has high toxicity to non-target organisms. This paper presents the kinetics of IMI removal by ferrate(VI) at different pH (6.0–9.0), molar ratios ([ferrate(VI)]:[IMI]) and added Fe(III) ions. The apparent second-order rate constant (kapp) decreased with increase in pH from pH 6.0 to 9.0 (i.e., (1.2 ± 0.1) × 102 M−1 s−1 to (8.3 ± 0.3) M−1 s−1). The species-specific rate constants were obtained as k (HFeO4−) = 1.3 × 102 M−1 s−1 and k (FeO42−) = 6.9 M−1 s−1. The decreases in the concentration of HFeO4− with increase in pH caused the observed pH dependence in kapp. At pH 7.0, the removal of IMI increased with the molar ratio from 1.0 to 10.0 with complete removal at the highest ratio. The variation in pH from 6.0 to 9.0 had no obvious effect on removal of IMI. Experiments indicate that IMI removal is mainly by ferrate(VI) oxidation and to a lesser extent by Fe(III) adsorption. Mineralization of IMI was also observed (20–26%). The addition of Fe(III) ions to ferrate(VI)-IMI at pH 7.0 and 8.0 resulted in enhanced removal of IMI, but the presence of Ca2+, SO42−, HCO3−, and humic acid (HA) has negative effects. The presence of coexisting substances in river water slightly decreased IMI removal by ferrate(VI) by less than 10%. Identification of products and frontier electron density (FED) calculations demonstrated involvement of opening of the five-membered heterocyclic moiety of IMI by ferrate(VI). Toxicity assessment with NIH 3T3 fibroblasts and ECOSAR analysis indicated lower toxicity of oxidized products than parent IMI.