Journal article

Selective oxidation of key functional groups in cyanotoxins during drinking water ozonation

Chemical kinetics were determined for the reactions of ozone and hydroxyl radicals with the three cyanotoxins microcystin-LR (MC-LR), cylindrospermopsin (CYN) and anatoxin-a (ANTX). The second-order rate constants (k(O3)) at pH 8 were 4.1 +/- 0.1 x 10(5) M-1 s(-1) for MC-LR, similar to 3.4 x 10(5) M-1 s(-1) for CYN, and similar to 6.4 x 10(4) M-1 s(-1) for ANTX. The reaction of ozone with MC-LR exhibits a k(O3) similar to that of the conjugated diene in sorbic acid (9.6 +/- 0.3 x 10(5) M-1 s(-1)) at pH 8. The pH dependence and value of k(O3) for CYN at pH > 8 (similar to 2.5 +/- 0.1 x 10(6) M-1 s(-1)) are similar to deprotonated amines of 6-methyluracil. The k(O3) of ANTX at pH > 9 (similar to 8.7 +/- 2.2 x 10(5) M-1 s(-1)) agrees with that of neutral diethylamine, and the value at pH < 8 (2.8 +/- 0.2 x 10(4) M-1 s(-1)) corresponds to an olefin. Second-order rate constants for reaction with OH radicals ((OH)-O-center dot), k(OH) for cyanotoxins were measured at pH 7 to be 1.1 +/- 0.01 x 10(10) M-1 s(-1) for MC-LR, 5.5 +/- 0.01 x 10(9) M-1 s(-1) for CYN, and 3.0 +/- 0.02 x 10(9) M-1 s(-1) for ANTX. Natural waters from Switzerland and Finland were examined for the influence of variations of dissolved organic matter, SUVA254, and alkalinity on cyanotoxin oxidation. For a Swiss water (1.6 mg/L DOC), 0.2, 0.4, and 0.8 mg/L ozone doses were required for 95% oxidation of MC-LR, CYN, and ANTX, respectively. For the Finnish water (13.1 mg/L DOC), > 2 mg/L ozone dose was required for each toxin. The contribution of hydroxyl radicals to toxin oxidation during ozonation of natural water was greatest for ANTX > CYN > MC-LR. Overall, the order of reactivity of cyanotoxins during ozonation of natural waters corresponds to the relative magnitudes of the second-order rate constants for their reaction with ozone and (OH)-O-center dot. Ozone primarily attacks the structural moieties responsible for the toxic effects of MC-LR, CYN, and ANTX, suggesting that ozone selectively detoxifies these cyanotoxins.


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