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.