Oxidation of Cylindrospermopsin and Anatoxin-a by the FeIII-B*/H2O2 Catalyst System

Abstract

Cyanotoxins released by cyanobacterial blooms into natural water reservoirs are considered hazardous contaminants, as they pose direct threats to the aquatic ecosystem and have serious effects on animals and humans. There are limitations in the current technologies for cyanotoxin degradation and removal, including the reduced degradation efficiency by natural organic matter (NOM). This thesis describes an investigation into oxidation of the cyanotoxins cylindrospermopsin (CYL) and anatoxin-a (ANA) by a green catalyst system, FeIII-B* (iron (III) tetra-amido-macrocyclic ligand)/H₂O₂. Studies on cyanotoxin oxidative degradation by FeIII-B*/H₂O₂ were conducted in three steps. The first step investigated cyanotoxin degradation by FeIII-B*/H₂O₂ and built the degradation pathway. Next, the influence of NOM on cyanotoxin removal by the FeIII-B*/H₂O₂ mediated oxidative system was explored. The third component of the research studied the potential estrogenicity of these cyanotoxins and the effects of FeIIIB*/ H₂O₂ oxidation on estrogenicity. The FeIII-B*/H₂O₂ catalyst system provides effective catalytic oxidative degradation of both cyanotoxins CYL and ANA, compared with the selective oxidation by general oxidants. Cyanotoxin degradation by homogeneous FeIII-B*/H₂O₂ (5 μM/ 5 mM) was observed to be pH-dependent at pH ranging from 8.5 to 11.5. The maximum removal of CYL (0.24 μM) and ANA (7.1 μM) occurred at pH 8.5 and pH 9.5 respectively, while the highest rate constant was observed at pH 11.5. Significant cyanotoxin removal by heterogeneous FeIII-B*, produced by immobilising dissolved FeIII-B* (2.5×10-7 mole) onto functionalised silica gel (240 mg), was observed when activated by H₂O₂ at 5 mM. Intermediate products identified by high resolution mass spectrometry enabled the formulation of cyanotoxin degradation pathways. Additionally, the toxicity of cyanotoxin oxidized by FeIII-B*/H₂O₂ is expected to reduce, due to the destruction of the CYL uracil ring and a conformational change in the ANA molecule. NOM increased CYL (0.24 μM) and ANA (7.1 μM) removal when mediated by FeIII-B*/H₂O₂ (5 μM/ 5 mM) at pH 9.5, resulting in 100 % removal (NOM ≥ 10 ppm). The results for excitation-emission matrix (EEM) and UV-vis show that NOM components with aromatic ring and carboxylate group were oxidized by FeIII-B*/H₂O₂. For cyanotoxins treated with model NOM compounds (i.e., guaiacol and glycolic acid), increased removal was related to their cross-coupling with specific functional groups of NOM constituents. The cyanotoxins and their degradation products suggest a further lowering toxicity due to their cross-coupling with NOM. Estrogenicity tests were conducted using the yeast estrogen screen (YES) assay. In the YES assay, CYL and ANA can act as agonists to induce the production of enzyme β-galactosidase. The ring structures of CYL and ANA display the binding affinity to estrogen receptors. The competition assay with cyanotoxins and E2 showed that CYL and ANA can perform as endocrine disruptors to modulate E2-induced estrogenicity and induce non-monotonic doseresponse behaviours. In comparison to the estrogenicity of the un-oxidized cyanotoxins, the estrogenicity of CYL after oxidation was significantly different, while the estrogenicity of ANA after oxidation did not change significantly. The reduced estrogenic activity of CYL is likely due to its reduced concentration and the formation of less active degradation products. The insignificantly changed estrogenic activity of ANA is associated with its ring-structural degradation products. This thesis has advanced the understanding of cyanotoxin oxidative degradation by the FeIII-B*/ H₂O₂ catalyst system, and also mimicked the cyanotoxin removal in the real system by NOM addition. This thesis has also added to our knowledge of cyanotoxins as endocrine disrupting chemicals.