Degradation of cyanotoxin cylindrospermopsin by TiO₂-assisted ozonation in water

Abstract

The stable tricyclic structure of the cylindrospermopsin (CYN), a cynotoxin, has presented several challenges to water treatment facilities, as conventional treatment methods have a limited ability to remove it from water. This study examines the effectiveness of titanium dioxide (TiO₂) in catalytic ozonation for degrading CYN. The chemical kinetics of the reactions of ozone (O₃) and hydroxyl radicals (OH^•) with CYN were determined. The results reveal that TiO₂ significantly increases the rate of degradation of CYN by increasing the rate of production of hydroxyl radicals (OH^•) by initiating the decomposition of O₃ on the surface of the catalyst. At a pH of 7 with 1.0 mg L⁻¹ O₃ and 500 mg L⁻¹ TiO₂; the pseudo–first-order ozone decomposition rate constant (k_D) increased from 3.04 × 10⁻³ to 16.53 × 10⁻³ s⁻¹ and the ratio of OH^• to O₃ concentrations (R_ct) increased from 1.87 × 10⁻⁸ to 126.4 × 10⁻⁸. The calculated second-order rate constant (k_overall) of the reaction of CYN with O₃ and OH^• was 3.22 M⁻¹s⁻¹ without TiO₂. However, the greatest improvement in k_overall in this study was observed using 500 mg TiO₂ L⁻¹, which increased k_overall by a factor of five. TiO₂-catalyzed ozonation is an efficient method of oxidation that reduces the toxic activity of CYN. The results of a Microtox test concerning the toxic activity of CYN during oxidation reveal that catalytic ozonation may either increase or reduce the toxicity of CYN toward test samples. The toxic effects of CYN on the samples are greatly influenced by the TiO₂ dosage and reaction time, possibly yielding by-products that may change the mutagenic properties of CYN. Three water samples from a eutrophic lake in Taiwan were examined to evaluate the effect of dissolved organic carbon (DOC) and alkalinity on the oxidation of CYN. DOC had the greatest effect on the oxidation of CYN in the ozonation of eutrophic water. Overall, the degree of CYN oxidation depended on the rate constant of the reaction with ozone and the consumption of ozone by the natural water matrix.

Keywords:

• Catalytic ozonation
• cyanobacteria
• Cylindrospermopsin
• titanium dioxide
• toxic activity

Acknowledgments

We appreciate the editorial assistance of Ted Knoy.

Funding

The authors would like to thank the National Science Council of the ROC, Taiwan, for financially supporting this research under contract No. NSC103-2221-E-241-003-MY2.