Effect of moderate pre-oxidation on the removal of Microcystis aeruginosa by KMnO4-Fe(II) process: significance of the in-situ formed Fe(III)

This study developed a novel KMnO₄-Fe(II) process to remove the cells of Microcystis aeruginosa, and the mechanisms involved in have been investigated. At KMnO₄ doses of 0-10.0 μM, the KMnO₄-Fe(II) process showed 23.4-53.3% higher efficiency than the KMnO₄-Fe(III) process did. This was first attributed to the moderate pre-oxidation of M. aeruginosa by KMnO₄, achieved by dosing Fe(II) after a period of pre-oxidation, to cease the further release of intracellular organic matter (IOM) and the degradation of dissolved organic matter (DOM). The extensive exposure of M. aeruginosa to KMnO₄ in KMnO₄-Fe(III) process led to high levels and insufficient molecular weight of DOM, inhibiting the subsequent Fe(III) coagulation. Additionally, Fe(II) contributed to lower levels of the in-situ formed MnO₂, the reduction product of KMnO₄ which adversely affected algae removal by Fe(III) coagulation. However, the in-situ formed Fe(III), which was derived from the oxidation of Fe(II) by KMnO₄, in-situ MnO₂, and dissolved oxygen, dominated the remarkably high efficiency of KMnO₄-Fe(II) process with respect to the removal of M. aeruginosa. On one hand, in-situ formed Fe(III) had more reactive surface area than pre-formed Fe(III). On the other hand, the continuous introduction of fresh Fe(III) coagulant showed higher efficiency than one-off dosage of coagulant to destabilize M. aeruginosa cells and to increase the flocs size. Moreover, the MnO₂ precipitated on algae cell surfaces and contributed to the formation of in-situ formed Fe(III), which may act as bridges to enhance the removal of M. aeruginosa.