Investigation of transformation reactions of phenolic compounds by oxidative processes in aqueous systems
Ozone (O3) and chlorine dioxide (ClO2) have been used as disinfectant for a long time and its use in trace organic compound abatement in water treatment has recently been suggested. Phenolic moieties in trace organic compounds (TrOCs) and dissolved organic matter are the main reaction sites for both oxidative agents. The mechanistic understanding of phenolic compound transformation may help to predict oxidant depletion, secondary oxidant and transformation products (TPs) formation. Therefore, the reaction of phenolic compounds with ClO2 and O3 were investigated. A new concept for the indirect determination of hypochlorous acid (HOCl) in the reaction of phenol with ClO2 by 2- and 4-bromophenol was developed. The reaction formed 62 ± 4 % chlorite and 42 ± 3 % HOCl per ClO2 consumed. The addition of ClO2 to real wastewater (5 × 10‑5 M ClO2) resulted in transformation of 40 % atenolol and 47 % metoprolol (TrOCs) while the presence of the selective HOCl scavenger, glycine, largely diminished their transformation. This indicates that both compounds were transformed by the reaction with HOCl (e.g., k (atenolol + HOCl) = 3.5 × 104 M‑1 s‑1), which is formed in reactions of ClO2 with the wastewater matrix. During the investigation of the ozonation of phenolic compounds, the amount of O3 to degrade one mole of compound (stoichiometric ratio) increased with increasing pH for phenol, 4-methylphenol and 4-methoxyphenol. In the case of 4-chlorophenol, stoichiometric ratio of O3 decreased with increasing pH, which was explained by the formation of organic TPs. However, the increase of stoichiometric ration of O3 in the case of phenol and activated phenols could be explained by the formation of highly reactive superoxide, which strongly contributes to O3 depletion. The results of the reverse order dosage experiments contradict previous studies suggesting that formed organic products are responsible for enhanced O3 amount. In the additional investigation by compound-specific stable isotope analysis (CSIA), a dependence of the reaction rate constant to carbon isotope fractionation was observed. The fractionation strongly depends on the phenol speciation. With decreasing pH values and reaction rates <105 M-1 s-1, the isotope enrichment factor ε increases (ε is between -5.2 and -1.0 ‰). For faster reactions (>105 M-1 s-1), the carbon isotope enrichment was not significant anymore (ε is between -1.0 and 0 ‰). Based on these data we propose a concept to correlate isotope enrichment factors with kinetic data for aromatic compounds. In conclusion, this work confirmed the postulated mechanism of ClO2 with phenol and found an influence of HOCl on compound transformation. Additionally, the high stoichiometry of O3 in the reaction with phenol could not be explained. However, prior assumptions could be refuted and new starting points (e.g. investigation of superoxide) could be found. Finally, the correlation of isotope enrichment factors with reaction rate constants supported the use of CSIA in the investigation of oxidative reactions. This thesis shows fundamental aspects for further studies of compounds with other functional groups in ClO2 application and for the use of CSIA in oxidation reactions.