Degradation of Cyanotoxin-Nodularin in Drinking Water by Catalytic Ozonation Using a Ag-TiO 2 Hybrid Catalyst

Abstract

Abstract

Catalytic ozonation was used to remove cyanotoxin-nodularin (NOD) from drinking water in a pilot plant. This investigation evaluates the effectiveness of the catalytic ozonation process in degradation of NOD using a hybrid silver-titanium dioxide catalyst coated on diatomite (Ag-TiO₂/DM). Degradation kinetic parameters were determined using NOD removal to evaluate the effectiveness of the catalytic ozonation method. Mutagenicity of water was evaluated using an Ames test during the catalytic ozonation processes in the presence of a hybrid Ag-TiO₂ catalyst. Results reveal that the Ag-TiO₂ catalyst significantly enhances the rate of degradation of NOD by increasing the rate of production of hydroxyl radicals (OH^•), owing to initiation of decomposition on the surface of the catalyst. Calculated pseudo-first-order rate constant (k_overall) of the reaction of NOD with O₃ and OH^• was 9.65 × 10⁻⁴/s without catalyst. However, the greatest improvement in k_overall in this study was observed using 500 mg/L Ag-TiO₂/DM, which increased k_overall by a factor of eight. The Ames test reveals that NOD exhibits positive mutagenic activity in the Salmonella/microsome assay with the strains TA98 and TA100 in the absence and presence of S9. However, based on mutagenic activity of NOD during oxidation, ozonation may reduce the toxic response of test samples. Compared with ozonation-only process, Ag-TiO₂/DM exhibited better ozonation capability with reducing mutagenic activities of NOD. Like the Ames test results, the Microtox test results reveal that catalytically ozonated samples are significantly less toxic and exhibit markedly lower residual toxicity than the original samples. Toxic effects of NOD on ozonated samples are greatly influenced by Ag-TiO₂/DM dosage and reaction time, possibly yielding by-products that may change mutagenic properties of NOD.