Role of reactive manganese and oxygen species in the KMnO₄/Na₂SO₃ process for purification of algal-rich water and membrane fouling alleviation

Ultrafiltration (UF) is a highly efficient technique for algal-rich water purification, but it is heavily contaminated due to the complex water characteristics. To solve this problem, potassium permanganate (KMnO₄) oxidation enhanced with sodium sulfite (Na₂SO₃) was proposed as a pretreatment means. The results showed that the end-normalized flux was elevated from 0.10 to 0.91, and the reversible fouling resistance was reduced by 99.95%. The membrane fouling mechanism also changed obviously, without the generation of cake filtration. Regarding the properties of algal-rich water, the zeta potential was decreased from −29.50 to −5.87 mV after KMnO₄/Na₂SO₃ pretreatment, suggesting that the electrostatic repulsion was significantly reduced. Meanwhile, the fluorescent components in algal-rich water were significantly eliminated, and the removal of dissolved organic carbon was increased to 67.46%. In the KMnO₄/Na₂SO₃ process, reactive manganese species (i.e., Mn(V), Mn(III) and MnO₂) and reactive oxygen species (i.e., SO₄^•− and ^•OH) played major roles in purifying algal-rich water. Specifically, SO₄^•−, ^•OH, Mn(V) and Mn(III) could effectively oxidize algal pollutants. Simultaneously, the in-situ adsorption and coagulation of MnO₂ could accelerate the formation of flocs by decreasing the electrostatic repulsion between cells, and protect the algal cells from being excessive oxidized. Overall, the KMnO₄/Na₂SO₃ process showed significant potential for membrane fouling alleviation in purifying algal-rich water.