Oxidation of the cyanobacterial hepatotoxin microcystin-LR by chlorine dioxide: influence of natural organic matter.

Cyanobacteria (blue-green algae) are known producers of cytotoxic, hepatotoxic, and neurotoxic compounds with severe acute and chronic effects on vertebrates. Successful removal of these toxins in drinking water treatment is therefore of importance for public health. In the present work the oxidation of the cyanobacterial hepatotoxin microcystin-LR (MC-LR) by chlorine dioxide (ClO2) was studied at natural microcystin concentrations (10 microg L(-1)) and normal ClO2 dosages (1 mg L(-1)) in the absence and presence of natural organic matter (NOM). ClO2 was found to be rapidly consumed by fulvic and humic acids, leaving less residual ClO2 to oxidize MC-LR. Predicted decrease rates in MC-LR concentration correlated highly with experimental data both in pure water and in the presence of NOM. Rate constants determined at high ClO2 and MC-LR concentrations in pure water could be used to predict the oxidation of MC-LR at natural concentrations. Toxicity tests with a protein phosphatase inhibition assay on reaction solutions and high-performance liquid chromatography fractions revealed that PP1 enzyme inhibition emerged only from intact MC-LR, while the oxidation products, dihydroxy isomers of MC-LR, were nontoxic even at unnaturally high concentrations.

Oxidation of the cyanobacterial hepatotoxin microcystin-LR by chlorine dioxide: reaction kinetics, characterization, and toxicity of reaction products.

Cyanobacteria are known producers of cytotoxins, hepatotoxins, and neurotoxins. The main toxins are microcystins, cyclic heptapeptide hepatotoxins, produced by strains of several cyanobacterial genera frequently found in eutrophied freshwaters. Due to the acute and chronic toxicity of microcystins, successful removal of these toxins in drinking water treatment processes is of increasing concern. In the present work the kinetics of microcystin-LR (MC-LR) oxidation by chlorine dioxide (ClO2) was studied with UV-spectrometry and high performance liquid chromatography (HPLC). Characterization of reaction products was performed with mass spectrometric (MS) analysis, while the toxicity of reaction products was tested with a protein phosphatase inhibition assay (PPIA). The main reaction products formed, dihydroxy isomers of MC-LR as identified by MS, were nontoxic according to the PPIA. The overall rate constant k for the reaction between MC-LR and ClO2 at 293 K and pH 5.65 was modest, k = 1.24 M(-1) s(-1), suggesting that ClO2 is not a suitable oxidant for the degradation of microcystins in drinking water treatment processes.