Mechanistic studies on the degradation of cyanobacterial toxins and other nitrogen containing compounds with hydroxyl and sulfate radical based Advanced Oxidation Technologies

Analytical techniques and instrumentation have progressed significantly in recent years allowing the detection of xenobiotic compounds in water resources at ng/L levels, thereby providing explanations to “mysterious” events of human and animal poisoning worldwide. Among them are the Poisonous Lake of Australia, 1878, the Caruaru Syndrome, 1996 and the Palm Island Mystery, 1979, all related to the same group of naturally occurring toxins, the cyanotoxins. The recalcitrant nature of these compounds towards natural degradation combined with the inadequacy of current water treatment practices for complete toxicity removal of cyanotoxin- contaminated water has propelled research towards finding appropriate treatment technologies. This dissertation explored the potential use of emerging hydroxyl (HO^•, TiO₂ photocatalysis) and sulfate (SO₄^•-) radical-based advanced oxidation technologies (AOTs) for the treatment of the cyanotoxin microcystin-LR (MC-LR) and the chemically related uremic toxin, creatinine.

Herein, an array of TiO₂ photocatalytic films activated under UV-A were tested. The study also aimed to determined how the structural properties of the catalyst (porosity, crystallinity, thickness) and water quality (pH, and initial toxin concentration) affected the degradation rates of MC-LR. The findings of this study can assist in adopting practices that elevate the performance of the photocatalytic films and provide insightful input for the design of a large scale reactor. Detoxification studies based on the inhibition of the PP1 enzyme indicated complete loss of MC-LR toxicity following treatment, suggesting that TiO₂ photocatalysis may be an effective technology for detoxification of cyanotoxin-contaminated water.

SO₄^•--AOTs were also tested for the degradation of MC-LR and compared to more commonly used HO•-AOTs. Even though SO₄^•- are strong oxidants with redox potentials comparable to HO^•, few studies have utilized SO₄^•- for water and wastewater treatment. To the best of our knowledge, this is the first study on the degradation of any cyanotoxin with SO₄^•-.

In this dissertation, emphasis was on transformations of MC-LR during treatment with HO^• and SO₄^•-. The reaction intermediates and pathways of MC-LR were reported in three separate studies: two TiO₂ photocatalysts and SO₄^•--AOTs. From these studies, new sites of MC-LR where HO^• and SO₄^•- can initiate toxin degradation were unveiled. The determining factor (position or susceptibility) that lead to the preferable site of degradation was also investigated.

MC-LR has a complex structure comprised of various functional groups (double and peptide bonds, carboxylic and amidic groups) and heteroatoms (nitrogen). The intermediates of nitrogen containing-compounds induced by HO^• oxidation are of great importance in environmental chemistry because they follow different oxidation pathways than carbon-based compounds and are not studied as well. Creatinine possesses the same basic guanidine group as MC-LR and was chosen for a fundamental study on the formation of intermediates with HO^• at different pH values.

The conclusions from studies conducted as part of this dissertation have broader impacts on the water industry and public health sector because they can assist in developing detection, toxicity assessment, and effective treatment techniques for cyanotoxins. The intermediates identified herein can be used as markers of their presence in our water resources.