Hydrophilicity modification of polypropylene microfiltration membrane by ozonation

► The surfaces of polypropylene microfiltration membranes were modified by ozonation, especially ozonation conducted in aqueous phase, to increase their hydrophilicity. ► A novel method, using homogeneous catalyst in aqueous ozonation, was proved to be effective. ► The improvement on membrane fouling after modification was confirmed by subsequent filtration tests. ► This study is a valuable addition to the membrane technology. To improve surface hydrophilicity and to reduce fouling, commercial polypropylene microfiltration membranes were ozonated to generate peroxides as grafting sites for hydrophilic monomers. Ozonation was conducted in aqueous and gaseous phases, respectively. In both phases, the amount of peroxides increased with the ozonation time. A novel way to enhance the generation of peroxides was tested, i.e., adding homogeneous catalyst, CuSO4, to aqueous ozonation. Results showed that with an optimum dose of 0.05g/L of CuSO4, the peroxides generated were 18.2% more than that by the non-catalyzed ozonation in aqueous phase. It was also confirmed by scavenger tests that during the aqueous ozonation both molecular ozone and free radicals contributed to the oxidation of the membranes, the latter was formed from the self-decomposition of ozone in water. Graft polymerization was also conducted after the generation of peroxides. A hydrophilic monomer, acrylic amide, was graft polymerized onto the membrane surface. The successful grafting of acrylic amide was confirmed by the formation of new peaks corresponding to amide groups in FTIR spectra. Results of contact angle measurements and filtration tests indicated that aqueous ozonation was more effective than its gaseous counterpart in terms of hydrophilicity improvement. In addition, the XRD analysis revealed that the ratio of the membrane surface crystallinity to amorphousity was changed by both ozonation and graft polymerization. Results of SEM scanning also showed changes in membrane surfaces after modification.