Sulfite-activated permanganate pre-oxidation coupled ultrafiltration for treating algae-laden water: Role of calcium ions and in-situ formed MnO₂

Membrane fouling caused by algal-derived foulants severely impedes the widespread application of ultrafiltration (UF). Herein, two sulfites (Na₂SO₃ and CaSO₃, S(IV)) were used to activate potassium permanganate (Mn(VII)) as pretreatment to enhance the performance of UF in treating algae-laden water. The results showed that the Mn(VII)/S(IV) system produced MnO₂ with abundant hydroxyl groups, which was advantageous for adsorbing algal pollutants. Compared with Mn(VII)/Na₂SO₃, Mn(VII)/CaSO₃ process further increased the aggregation tendency of algal cells, which was due to the Ca²⁺ bridging effect that promoted the aggregation of MnO₂ and algal foulants. S(IV) can activate Mn(VII) to produce free radical (^[rad]OH and SO₄^[rad]−), and enhance removal of dissolved organic carbon (DOC) and fluorescent organics in coordination with the adsorption of in-situ MnO₂. Moreover, the presence of Ca²⁺ ensured compliance with residual manganese in the treated water. Extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory suggested that the Mn(VII)/S(IV) strategy weakened the attractive forces between pollutants and the membrane. FTIR analysis further confirmed that the combined process reduced the accumulation of protein organics on the membrane surface. As a result, the reversible fouling was reduced by 65.3 % and 92.2 %. The in-situ formed manganese oxide altered the structure of the cake layer and delayed the formation of the cake filtration mechanism. Mn(VII)/CaSO₃ pretreatment produced a looser cake layer, resulting in a shift of the fouling mechanism towards a single standard blocking. In summary, the novel integrated process can ensure the efficient operation of UF in the purification of algae-laden water.