Prediction of transformation products during ozonation of micropollutant-containing waters: kinetics and mechanisms

Ozonation, which is widely used for drinking water disinfection, has recently been applied to mitigate potentially harmful effects of micropollutants (e.g., pharmaceuticals, personal care products, pesticides, etc.) present in municipal wastewater effluents. Generally, ozonation is efficient for the abatement of biological effects caused by micropollutants. However, limited empirical information is available about the transformation products formed during ozonation of micropollutants due to analy-tical limitations and a large number of micropollutants present in wastewater effluents. In this thesis, a computer-based prediction platform for kinetics and mechanisms for the reactions of ozone with micropollutants was developed to provide information about (i) the reactivity of micropollutants with ozone expressed as second-order rate constants (kO3, M-1s-1) and (ii) potential transformation products formed from the reactions of ozone with micropollutants. Regarding (i), kO3 for micropollutants were predictable using linear relationships between experimental kO3 in log units for compounds of certain chemical groups (e.g., phenols, olefins, amines, etc.) and the corresponding molecular orbital energies (e.g., highest occupied molecular orbital (HOMO) or natural bond orbital (NBO)) obtained from quantum chemical computations (mostly R2 = 0.75 - 0.95 for 14 compound groups consisting of 284 model compounds in total). Overall, the developed kO3 prediction models could predict kO3 on average within a factor of  5 of an experimental kO3 for model compounds used for the development of the kO3 prediction models as well as tetrachlorobutadienes, which were externally validated. In contrast, poor kO3 predictions (>10 fold) were observed for some model compounds excluded from the correlations as outliers as well as cetirizine, two pentachlorobutadiene congeners, and hexachlorobutadiene, which were used for external validation. (ii) A prediction tool for potential transformation products was developed based on numerous reaction pathways proposed in literature, which were encoded into 340 individual reaction rules using appropriate chemoinformatics tools. The predicted pathways and the transformation products for some micropollutants (i.e., carbamazepine and tramadol) were shown to be consistent with experimental observations. However, in the future, both kO3 and the pathway prediction modules need to be further validated with more compounds with experimental data and to be improved/updated accordingly. The developed prediction platform is expected to be useful for various groups of end-users in research and practice such as environmental engineers, chemists, or toxicologists. In addition, the treatability of 9 polychlorobutadienes, which are groundwater contaminants, with ozone, UV photolysis at 254nm, and their advanced oxidation processes (i.e., O3/H2O2 and UV/H2O2) was investigated. The abatement efficiencies for poly-chlorobutadienes during ozonation or O3/H2O2 in a natural groundwater could be well explained based on the experimental kO3 and kOH-values. UV treatment was shown to be effective for the abatement of polychlorobutadienes. However, the potential formation of photoisomers from UV irradiation of chlorobutadienes with either E or Z configurations needs to be taken into account because this isomerization will not necessarily lead to a loss of the biological effects of these compounds.


  • Thesis submitted - Forthcoming publication

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