Degradation of 17α-Etinylestradiol by Nano-sized Zero Valent Iron

Abstract

The bioaccumulation of endocrine disrupting chemicals in water bodies is a problem affecting human health and the ecosystem. Estrogenic compounds are well known as endocrine disrupting chemicals in humans and aquatic life, and include 17α-ethinylestradiol (EE2), a widely used pharmaceutical. Human waste and animal agriculture are the major sources of this compound worldwide. The presence of EE2 in water bodies is due to the failure of wastewater treatment plants in treating and removing this micro-pollutant. The goal of the research described in this thesis was to investigate oxidation of EE2 by nano-sized zero valent iron (nZVI). It was decided to use nano-sized rather than macro-sized zero-valent iron due to its reactivity in the degradation of a wide range of organic compounds. Commercial nZVI was utilised throughout this study. The first phase of this study sought to gain a better understanding of the degradation mechanisms of EE2 by nZVI at pH 3, 5 and 7 in nitrogen-purged, non-purged and oxygen-purged conditions. In nitrogen-purged conditions, the kobs values at pH 3 and pH 5 were 0.985 and 1.110 min−1 respectively, while a significant decrease in removal rate (0.013 min−1) was observed at pH 7. The results for radical scavenging and transformation products revealed that direct reduction of EE2 by nZVI was responsible for the removal of EE2 in nitrogen-purged conditions. In non-purged conditions, the most effective degradation occurred at pH 3 with kobs =1.546 min-1, followed by pH 5 (kobs = 0.145 min-1) and pH 7 (kobs = 0.064 min-1). It was also found that different reactive species acted as key factors at each pH (OH• radicals for pH 3, O2•- radicals for pH 5, and Fe(IV) for pH 7). In air-purged conditions, kobs values at each pH were all lower than the corresponding values in the non-purged systems. However, the same radicals were present at each pH due to inhibition of reactive radical production and rapid surface oxidation of nZVI by the excess amounts of O2 in the systems. The results obtained from this study highlight that variations in oxygen content and pH can significantly influence the main drivers for the removal of organic contaminants by nZVI induced-Fenton reaction. The second phase of this study was built on the understanding of the degradation mechanism that would take place in real life in the presence of natural organic matter (NOM). The NOM used in this study was sourced from Suwannee River water. The results suggest that the degradation rate of EE2 in the presence of NOM decreased when increased NOM concentrations were observed at pH 3 and pH 5. A possible explanation is that the OH• radicals formed during the oxidation were scavenged by NOM. A different trend was observed in the presence of Bisphenol-A (BPA) and NOM between pH 3 and pH 5. EE2 degradation was inhibited at pH 3 and significantly enhanced at pH 5 in the presence of 5mg L-1 NOM and 25mg L-1 NOM. It is proposed that the presence of BPA, which has high hydrophobicity, and humic substances, which possess an apparent electron-donating capacity, may be involved in the formation of the O2•- radical, thereby leading to the enhanced removals observed at low NOM concentration. This observation was supported by the excitation emission matrix, which showed the scavenging of most of the humic-like substances. In the presence of excess amounts of NOM, it was adsorbed onto Fe0 surfaces and blocked the reactive sites, resulting in the reduced degradation observed.