Reaction pathways, kinetics and toxicity assessment during the photocatalytic degradation of glyphosate and myclobutanil pesticides: Influence of the aqueous matrix

Assessment of reaction pathways, kinetics and water toxicity during the photocatalytic degradation of glyphosate and myclobutanil pesticides has been performed in different aqueous matrices of increasing complexity, from the single pesticides to the mix of their commercial formulations. Using Aeroxide (R) TiO2-P25 as reference UV-A (Ultraviolet A) light photocatalyst, the ability of photocatalysis to degrade glyphosate and myclobutanil pesticides in water was evidenced independently of the aqueous matrix complexity, complete mineralization into CO2, phosphate and chloride ions being achieved. Further, an unusual volcano-like TOC evolution profile resulting from the proposed glyphosate degradation pathway was observed whatever the aqueous matrix. Increasing the water matrix complexity from single pesticides to the commercial formulation mix reduced the degradation kinetics and consequently extended the time necessary for complete mineralization but, did not influence the overall pesticide fate profiles. This behavior was associated to the competitive adsorption of the organic matter onto the catalyst and to the presence of ions and inorganic matter. The co-presence of glyphosate and to lesser extent of Roundup (R) formulation additives strongly impacted the myclobutanil fate profile, due to preferential adsorption/degradation of glyphosate. By contrast, despite the impact in the degradation pathway, the inorganic additives of the Systhane (R) formulation influenced less both the myclobutanil removal duration and the TOC removal than the glyphosate pesticide and its organic additives from Roundup (R) do. The treatment allowed for most of the cases a strong reduction of acute toxicity to aquatic invertebrate test organisms (D. magna) whatever the water matrix complexity, while the ecotoxicity was reduced by half for the complex formulation mix.