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

[Display omitted] •Mn(VII)/S(IV) process was proposed to improve the performance of UF in algae-laden water.•Calcium ions promoted the aggregation of in-situ MnO2 and algal pollutants.•Mn(VII)/S(IV) system produced active substances that facilitate the removal of algae organics.•The coupling process changed the fouling mechanism and greatly alleviated membrane fouling. 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 (OH and SO4−), 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.