Ozonation is increasingly integrated into wastewater treatment trains to abate micropollutants, which are known to induce adverse effects to aquatic organisms in receiving water bodies. While ozonation has been found to enhance the biological quality of wastewater as reflected by the reduced toxicity in a range of bioassays, concerns remain with regard to the potential formation of toxic ozonation transformation products. Except for some specific byproducts such as bromate and nitrosamines, little is so far known about the identity of toxic ozonation transformation products and their formation pathways. Such an understanding is essential to ensure the optimization of treatment processes and for the potential implementation of source control measures limiting the introduction of problematic precursors into wastewaters. However, the complexity of wastewater samples along with the typical occurrence of ozonation transformation products at trace concentration levels constitute main challenges when the identification of novel transformation products is sought under realistic conditions. Non-target analysis based on high-resolution mass spectrometry (HRMS) offers a useful tool to address these challenges and discover unknown transformation products. The present study aimed at screening for transformation products which might be relevant with regard to the induction of mutagenicity in ozonated wastewaters. Samples were collected at different treatment steps: secondary treatment, ozonation, and biological filtration effluents. Solid-phase extraction was performed for enrichment followed by liquid chromatography HRMS/MS. Prioritization of signals was achieved by comparing the occurrence and intensity in the categorized samples (secondary treatment vs. ozonation vs. biological filtration). Compound identification efforts focused on prioritized compounds classified as ozonation transformation products and were based on isotope patterns, retention time, and fragmentation spectra. The applied workflow allowed the identification of nitro compounds as novel transformation products formed from multiple micropollutants (e.g., diuron and carbendazim) during ozonation. Considering the potential toxicity of nitro compounds in general, this pathway might be of toxicological relevance. This study highlights how applying a suitable sample preparation method and a robust non-target analysis workflow can contribute to enhancing our understanding of oxidative wastewater treatment.

Non-targeted analysis based on high-resolution mass spectrometry reveals novel toxicologically relevant transformation pathway during ozonation of wastewater

ACS Spring 2022

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