Ecotoxicity Assessment of Photoinduced Imidacloprid Degradation Using HPLC-HRMS, QSAR and Ecotoxicity Equivalents

BackgroundImidacloprid is among the most widely used insecticides and today is found in surface and ground water worldwide. Together with four other neonicotinoids, it has been registered in the EU watchlist for monitoring. To prevent imidacloprid from entering water bodies, Advanced Oxidation Processes have been intensely researched. Photoirradiation proved one of the most efficient methods to degrade and eliminate anthropogenic micropollutants from waters. Their ecotoxicity assessment of photoinduced degradation and transformation products especially in the absence of reference standards is still heavily explored.ResultsIn this study, UVA and UVC irradiation in combination with titanium dioxide P25 as photocatalyst were investigated for their degrading and eliminating effects and effectiveness on imidacloprid. Humic acid was used as natural organic matter additive. High-performance liquid chromatography coupled with high-resolution and higher order mass spectrometry allowed to identify and monitor imidacloprid and its degradation intermediates yielding seven new structures and concentration-time (c-t) profiles. The correlation of structures and the application of radical scavengers and photocatalyst helped distinguish between direct photoinduced and indirect hydroxyl-radical induced degradation mechanisms. Only imidacloprid-urea and desnitro-imidacloprid resulted from direct degradation, all other products from the indirect mechanism. The ecotoxicity of all identified compounds was assessed by quantitative structure activity relationship (QSAR) analysis. Ecotoxicity equivalents (ETEs) were introduced allowing a classified ranking of the products and an assessment of the overall hazardous potential of the irradiated solution at a given moment. Generally, the number of hydroxyl substituents was inversely correlated to ecotoxicity due to a single product. From the c-t curves, time-dependent ETE profiles were established.ConclusionsStructure elucidation and c-t profiles from liquid chromatography-high resolution mass spectrometry allowed to distinguish between direct and indirect degradation mechanisms. Structure specific ecotoxicity assessment could be achieved through QSAR analysis. Ecotoxicity hazard was ranked based on ETEs. The time-dependent ETE profile proved suitable to reflect the effect of irradiation duration and allow to estimate the irradiation time required to eliminate ecotoxicity, which may be relevant for potential applications in wastewater treatment plants.