Oxidative degradation of cylindrospermopsin and anatoxin-a by FeIII–B*/H2O2

Abstract

Cylindrospermopsin (CYL) and anatoxin-a (ANA) are alkaloid-like potent cyanotoxins produced during cyanobacterial blooms. The high toxicity and widespread distribution of these compounds has raised concerns over the safety of impacted drinking water sources. Current physical and chemical oxidation technologies sometimes fail in treating CYL and ANA contaminated water. The water-soluble iron catalyst, Fe^III–TAML (designated as Fe^III–B*), activates H₂O₂ to degrade a range of recalcitrant pollutants by a pathway that does not involve hydroxyl radicals. Fe^III–TAML/H₂O₂ catalytic oxidation systems have been shown to degrade microcystin-LR, a potent aquatic cyanotoxin, but it is not known whether they can be used to transform other cyanotoxins. In addition, the effect of natural organic matter (NOM), which is present in most water sources, has not been tested. Therefore, in this study we explored the degradation of CYL and ANA by the Fe^III–B*/H₂O₂ catalytic oxidation system in the presence of NOM. In contrast to the negative influence of NOM on most other water treatment technologies, oxidative removal of both cyanotoxins in this case was enhanced when NOM was present. Complete removal of CYL and ANA was observed with the rate constants increasing as the NOM concentration was raised from 0 ppm to 30 ppm. It is possible that the enhanced removal was due to facile coupling of NOM based radicals with the cyanotoxins and/or the initial cyanotoxin oxidation products. Fe^III–B* was immobilized by direct adsorption onto silica gel which had been surface modified by treatment with dimethyloctadecyl[3-(trimethoxysilyl)propyl]ammonium chloride. The resulting Fe^III–B*/Si-DMOAP/H₂O₂ catalytic oxidation system (2.5 × 10⁻⁷ mole Fe^III–B*/5.0 mM H₂O₂) reduced the concentration of CYL by 93% and ANA by 88% after 2 hours at pH 9.5. The mechanisms of cyanotoxin removal by the immobilized catalytic system were a combination of adsorption onto the solid and catalytic oxidative degradation. Under the same conditions, the total removal efficiency of cyanotoxin by the dissolved catalyst was similar to the immobilized catalyst. The main CYL oxidation products (m/z 448 and 420) result from oxidation of the C5, C6 double bond of the uracil ring, and the main ANA oxidation product (m/z 182) is C2, C3 epoxy-ANA. The oxidation products observed suggested reduced toxicity of CYL and ANA.