Author: Miguel Pelaez

Publisher:

Published: 2012

Total Pages: 203

ISBN-13:

The prevalent and increasing occurrence of cyanobacteria and their toxins, known as cyanotoxins, in drinking water sources have become a potential health risk to humans. Physical treatment methods in conventional drinking water treatment have the capacity to remove cyanotoxins but are limited to a merely physical separation, where further treatment is required. Cyanotoxins are susceptible to chemical oxidation and recently advanced oxidation technologies (AOTs) and nanotechnologies (AONs), such as titanium dioxide (TiO2) photocatalysis, have been proven an effective alternative technology to chemically transform cyanotoxins in water. However, conventional TiO2 is restricted to UV light photoactivation for the generation of highly reactive oxygen species (i.e., hydroxyl radicals) representing an economical and technological limitation for the use of renewable energy sources such as solar light, since UV radiation accounts only for 5% of the total solar spectrum compared to the visible spectrum (~45%). This dissertation explored the development of nanostructured nitrogen and fluorine co-doped TiO2 (NF-TiO2) that can be activated under visible and solar light for the photocatalytic degradation of cyanotoxins in water. This work aimed to develop highly efficient NF-TiO2 nanoparticles and films to evaluate the environmental fate of microcystins, the most widespread and highly persistent group of cyanotoxins found in surface waters, and cylindrospermopsin which has emerged as the most significant toxin in freshwater sources. Specific attention was given to 1) the fundamental aspects on the synthesis method that influenced the physicochemical properties of NF-TiO2, such as the incorporation of nitrogen and fluorine in the structure of TiO2 and the synergistic effects induced by both dopants, 2) the surface interaction between the cyanotoxins and NF-TiO2 in different water matrix, 3) the reactivity and degradation kinetics of microcystins and cylindrospermopsin with NF-TiO2 and 4) the mechanism of radical formation with NF-TiO2 under visible and solar light. The existence of interstitial nitrogen and substitutional fluorine in the NF-TiO2 lattice was determined and the formation of localized intra-gap states was established implying that fluorine promotes nitrogen incorporation in TiO2. A shift in the absorbance capacity of NF-TiO2 in the visible range was also observed. Anatase/brookite heterojunctions, which promote photocatalytic efficiency, were found in NF-TiO2. High initial degradation rates for microcystin-LR (MC-LR) were obtained with NF-TiO2 nanoparticles and films in synthetic water under visible light. The effect of pH indicated that attractive forces at acidic conditions between the oppositely charged NF-TiO2 and MC-LR contributed to higher MC-LR initial degradation rates. The presence of alkalinity and natural organic matter had a scavenging effect since the initial MC-LR degradation rates decreased. Modifications to NF-TiO2 with Evonik Aeroxide P25-TiO2 (P25) nanoparticles lead to composite NF-TiO2-P25 with improved photocatalytic activity towards MC-LR, MC-RR, MC-YR, MC-LA and cylindrospermopsin under visible and UV-vis light. The general reactivity was MC-LA>MC-LR>MC-YR>MC-RR. Finally, results using selected scavengers indicate that the main mechanism of NF-TiO2 radical formation under visible light irradiation differed from UV-mediated TiO2 photocatalysis since no evidence of hydroxyl radical production from the surface holes was observed. It was suggested that under visible light, surface oxygen reduction of NF-TiO2 occurred to form superoxide radical anion as main radical specie. The affinity of the scavenger with NF-TiO2 and MC-LR in terms of pH was established as an important parameter to determine the radicals formed in this study.