CaO2-based tablet for effective and green membrane cleaning without additional catalysts

Natural organic matter humic acid (HA) coupled with the inorganic metal oxides FeOx + MnOx will cause severe irreversible membrane fouling. Chemical cleaning is indispensable for the removal of irreversible membrane foulants and sustainable membrane filtration, while conventional cleaning methods have limitations with low cleaning efficiency, high reagents consumption and potential membrane damage. For the first time, calcium peroxide (CaO2), a solid alternative for liquid hydrogen peroxide (H2O2), is proposed for the membrane cleaning. For membranes fouled by HA, it is noteworthy that 40 mg/32.15 cm2 CaO2 tablet without any other catalyst can restore the terminal membrane flux dramatically from 51% to 82% in only 5 min, which is far more than 3 wt% H2O2 alone (only from 51% to 59%). Moreover, Fenton-like reaction induced by in-situ membrane foulants FeOx + MnOx further enhances the membrane flux from 53% to 97%. Based on HA oxidation tests and reactive species identification in CaO2 tablets cleaning process for the case of HA-fouled membranes and HA + FeOx + MnOx-fouled membranes, singlet oxygen (O21) in CaO2 system dominated the strong oxidation of electron-rich compounds HA, which plays a key role in membrane flux recovery. With involvement of FeOx + MnOx, signal intensities of free radicals such as hydroxyl radicals (OH•) are enhanced greatly, which further achieves more outstanding membrane cleaning performance. These results underline that the novel CaO2 system provides a promising candidate for more portable and cost-effective membrane cleaning method in a nonradical pathway.