Potassium permanganate as a promising pre-oxidant to treat low-viability cyanobacteria and associated removal of cyanotoxins and extracellular organic matters.

Cell-viability of cyanobacteria declines from development to decay stage during a successive bloom. Potassium permanganate (KMnO4) has demonstrated to be a superior pre-oxidant to treat high-viability cyanobacteria compared to other common oxidants (e.g., chlorine), but whether it is feasible to treat low-viability cyanobacteria is unknown. Here, effects of KMnO4 on membrane integrity, cyanotoxin fate and extracellular organic matters (EOMs) removal of high- and low-viability cyanobacteria were compared. Results showed that cell-viability of cyanobacteria could affect oxidant decay (kdecay), membrane damage (kloss), and cyanotoxins release (ki) and degradation (ke) during KMnO4 oxidation, similar to chlorination. However, unlike chlorination, initial low dosages of KMnO4 (0.5 and 1 mg L-1) minimized membrane damage for low-viability cyanobacteria (< 27%), and continuously decrease extracellular cyanotoxins, extracellular organic matters (EOMs), and aromatic compounds to some degrees (P<0.05). High dosages of KMnO4 (> 2 mg L-1) caused severe membrane destruction (> 89%) for low-viability cyanobacteria, leading to a fast increase of extracellular cyanotoxins within 1 h. However, total/extracellular cyanotoxins were oxidized to below the safety guideline of 1 μg L-1 after being dosed with sufficient oxidant exposure. EOMs and aromatic compounds were also reduced by 5-18% (P<0.05). Additionally, KMnO4-assisted coagulation significantly improved the removal of low-viability cyanobacteria (2-5 fold). Consequently, KMnO4 could be a promising pre-oxidant to treat low-viability cyanobacteria at decay stage of a successive bloom.