Kinetics and mechanisms of degradation of selected environmental pharmaceuticals by aqueous chlorination / Wan Nor Adira Wan Khalit

The occurrence of pharmaceuticals as pollutant in the environment has been widely reported across the world. It has been a growing concern due to it negative impacts of these pharmaceuticals on the ecosystem and living organism. During conventional water treatment processes, untreated pharmaceuticals are often exposed to chemical oxidation reaction during disinfection process. Among various disinfection methods, chlorination is one of the most commonly used methods. The reactive species in the chlorination process is hypochlorous acid (HOCl) which is known to react with organic pollutants. The main objectives of this study were to determine the second-order rate constants (kapp) for the reaction between selected pharmaceuticals with free available chlorine (FAC) and identification of the transformation by-products generated from the chlorination of selected pharmaceuticals. According to the identified transformation byproducts, the mechanism of the transformation pathway of selected pharmaceuticals in chlorination was elaborated. The efficiency of chlorination in the removal of selected pharmaceuticals in different matrices was also evaluated. The selected pharmaceuticals for this study were acebutolol and sotalol (β-blockers), mefenamic acid (nonsteroidal anti-inflammatory drugs), sulfacetamide and sulfanilamide (antibiotics). These pharmaceuticals have been frequently detected in the aquatic environment. In the kinetics study, kapp for the reaction between selected pharmaceuticals and FAC were determined at 25 ± 0.1 °C. The result indicated that the degradation of selected pharmaceuticals by free FAC was highly pH dependence at the selected pH range. At pH 6 to 8, it was found that kapp of Abt, Stl, Mfe, Sfa and Sfn was ranged from 0.03 to 0.19 M-1s-1, 0.93 to 0.65 M-1s-1, 16.4 to 4.35 M-1s-1, 4.50 to 4.50 M-1s-1 and 2.20 to 0.97 M-1s-1, respectively. The elimination of the selected pharmaceuticals in different water matrices (lake water, ground water and secondary effluent) showed that the efficiency of chlorination in the pharmaceuticals removal was retarded by high TOC and COD concentrations. Chlorination showed the highest efficiency in the removal of selected pharmaceuticals containing in ground water which has the lowest TOC and COD concentrations. However, in secondary effluent that containing higher concentrations of COD and TOC, the percentage removal of selected pharmaceuticals was relatively poor as compared with other water matrices. Characterization of the transformation by-products formed during the chlorination of selected pharmaceuticals was carried out using liquid chromatography coupled to quadrupole time-of-flight high resolution mass-spectrometry. The transformation by-products were determined after 24 h of FAC exposure. The result indicated that chlorination of pharmaceuticals could produce various transformation byproducts. Overall, 18 transformation by-products were identified for the selected pharmaceuticals. These transformation by-products were mainly formed through the hydroxylation, chlorination, oxidation and dealkylation reactions. In conclusion, this research showed that pharmaceuticals could react with HOCl during disinfection process. The reactivity of the selected pharmaceuticals towards the reaction with HOCl is highly pH dependent at the common pH range of natural water. Chlorination might remove some of the pharmaceuticals however its efficiency was depending on the characteristic of water matrices. Chlorination of pharmaceuticals was also found to form various transformation by-products