Imidacloprid elimination by O₃ and O₃/UV: kinetics study, matrix effect, and mechanism insight

The removal of imidacloprid (IMI) from water by ozonation (O₃) and photo-ozonation (O₃/UV) was comparatively studied, paying particular attention to the kinetics, matrix effect, and mechanistic aspects of the processes. The IMI removal by O₃ was considerably enhanced at alkaline pHs, leading to almost complete removal under 20 min with a pseudo-first-order rate constant of 0.2374 min^-1 at pH 8.25. Three different matrices, Milli-Q water, full-scale vacuum rotating membrane bioreactor plant effluent (VRMBR WW), and laboratory-scale instantaneous fed-batch reactor bioreactor effluent (Bio WW) spiked with IMI, were tested. The ozonation, coupled with UV, improved IMI removal remarkably regardless of the wastewater matrix, and there occurred a six times decrease in ozonation time requirement for 99% IMI elimination at pH 7.25. The IMI degradation mechanism proved that IMI is an ozone-resistant pollutant and is mainly degraded by OH• via an indirect mechanism. The second-order rate constants for IMI degradation with OH• were calculated as 2.23 × 10¹¹ and 9.08 × 10¹¹ M^-1 s^-1 for the O₃ alone and O₃/UV processes, respectively. The IMI degradation pathway analysis showed that IMI lost NO₂, HNO₂, and then Cl^- from its structure, and the O₃/UV process yielded fewer by-products than O₃.