Influence of water matrix components and peroxide sources on the transformation products and toxicity of tebuthiuron...

Coupling of UV-C irradiation to different peroxides (H₂O₂, S₂O₈^2- and HSO₅^-) has great potential to degrade persistent organic compounds due to the formation of HO^• or SO₄^•- species. However, an in-depth comparison between the performance of different UV-C/peroxide processes as a function of (i) target compound degradation, (ii) generated transformation products and (iii) lethal/sub lethal toxicity effects has not yet been performed. To this end a comparison study was carried out to evaluate the effectiveness of different UV-C/peroxide processes using the herbicide tebuthiuron (100 or 500 μg L^-1) as a model pollutant. TBH degradation experiments were performed at lab-scale in real municipal wastewater treatment plant effluent and distilled water. Faster degradation occurred by increasing peroxide concentration from 735 to 2206 μmol L^-1 in the municipal wastewater treatment plant effluent, mainly for S₂O₈^2-. Experiments performed in the presence of peroxide trapping agents - HO^• and SO₄^•- (methoxibenzene) or HO^• (2-propanol) - revealed that oxidation in the UV-C/S₂O₈^2- system occurs mainly through SO₄^•-. Lower toxicity for the MWWTP effluent was obtained after oxidative treatments using hydrogen peroxide or monopersulfate as oxidants which react mainly through HO^• radicals. Two mechanistic pathways were proposed for tebuthiuron degradation: (i) hydrogen abstraction by HO^• (H₂O₂ and HSO₅^-) and (ii) electron transfer by SO₄^•- (S₂O₈^2-). In addition, one unprecedented transformation product was identified. In conclusion, results emphasize the relevance of comparing the degradation of toxic compounds in the presence of different peroxide sources and matrices and simultaneouly evaluating responses chemical and biological endpoints.