Oxidation of microcystins by permanganate: Reaction kinetics and implications for water treatment

A few genera of cyanobacteria produce toxins which contaminate drinking water resources. Microcystins (MC), widely reported cyanotoxins, cause acute and chronic toxicity effects in living beings including humans and warrant removal from drinking water. In the present study, unknown second-order rate constants for the reactions of microcystin-LR (MC-LR), -RR and -YR with potassium permanganate were determined at pH 6.2-8.2 and temperature 10-25 °C. The reaction of permanganate with MCs is second-order overall and first-order with respect to both permanganate and toxin. The second-order rate constant for the reaction of MC-LR with permanganate at pH 7 and 20 °C was 357.2±17.5 M^-1s^-1. The influence of pH on the oxidation process was not appreciable and the activation energy was 28.8 kJ mol^-1. Slightly higher reactivity with permanganate was found for MC-RR (418.0 M^-1s^-1) and MC-YR (405.9 M^-1s^-1). According to the results obtained, permanganate likely attacks the Adda moiety of the MC molecule. The oxidation of MCs in a natural surface water was also investigated. A permanganate dose of 1-1.25 mg L^-1 was enough to reduce MCs concentration below the guideline value of 1 μg L^-1. Permanganate oxidation is therefore a feasible option for microcystin removal during preoxidation processes. However, the oxidant dose must be carefully optimized in order to remove extracellular MCs without causing cell lysis (due to chemical stress) and further release of MCs.