Treatment of cylindrospermopsin by hydroxyl and sulfate radicals: Does degradation equal detoxification?

Drinking water treatment ultimately aims to provide safe and harmless drinking water. Therefore, the suitability of a treatment process should not only be assessed based on reducing the concentration os a pollutant concentration but, more importantly, on reducing its toxicity. Hence, the main objective of this study was to answer whether the degradation of a highly toxic compound of global concern for drinking water equals its detoxification. We, therefore, investigated the treatment of cylindrospermopsin (CYN) by •OH and SO₄^-• produced in Fenton and Fenton-like reactions. Although SO₄^-• radicals removed the toxin more effectively, both radical species substantially degraded CYN. The underlying degradation mechanisms were similar for both radical species and involved hydroxylation, dehydrogenation, decarboxylation, sulfate group removal, ring cleavage, and further fragmentation. The hydroxymethyl uracil and tricyclic guanidine moieties were the primary targets. Furthermore, the residual toxicity, assessed by a 3-dimensional human in vitro liver model, was substantially reduced during the treatment by both radical species. Although the results indicated that some of the formed degradation products might still be toxic, the overall reduction of the toxicity together with the proposed degradation pathways allowed us to conclude: "Yes, degradation of CYN equals its detoxification!".