Removal of cyanotoxins by potassium permanganate: Incorporating competition from natural water constituents

Abstract

In recent years, harmful algal blooms capable of producing toxins including microcystins, cylindrospermopsin, and saxitoxin have increased in occurrence and severity. These toxins can enter drinking water treatment plants and, if not effectively removed, pose a serious threat to human health. The work here investigated the efficacy of permanganate oxidation as a treatment strategy, with a focus on incorporating competition by cyanobacterial cells and dissolved organic matter (DOM). We report rate constants of 272 ± 23 M−1 s-1 for the reaction between permanganate and microcystin-LR, 0.26 ± 0.05 M−1 s−1 for the reaction between permanganate and cylindrospermopsin, and, using chemical analogs, estimate a maximum rate constant of 2.7 ± 0.2 M−1 s−1 for the reaction between permanganate and saxitoxin. We conclude that permanganate only shows potential to remove microcystins. No pH (6−10) or alkalinity (0–50 mM) dependence was observed for the rate of reaction between microcystin-LR and permanganate; however, a temperature dependence was observed and can be characterized by an activation energy of 16 ± 5 kJ mol−1. The competition posed by cyanobacterial cells was quantified by an apparent second order rate constant of 2.5 ± 0.3 × 10-6 L μg chl-a−1 s−1. From this apparent second order rate constant, it was concluded that cyanobacterial cells are not efficient scavengers of permanganate within typical contact times but this second order rate constant can be used to accurately predict microcystin degradation in algal-impacted waters. The competition posed by DOM depended on both the amount of DOM present (as measured by TOC) and its electron donating capacity (as predicted by SUVA-254 or E2/E3 ratio). DOM was concluded to scavenge permanganate efficiently and we forward that this should be considered in permanganate dosing calculations.