Polypropylene Membrane Surface Modification Using Photoinduced RAFT Method And Its Application In Potocatalytic Membrane Reactor

Membranes, especially polypropylene porous membranes, were widely used in many fields such as ultrafiltration and microfiltration because of the low cost and easy processing. However, some disadvantages (hydrophobicity, poor anti-fouling characteristics and so on) limit its wider application. Surface modification of membranes becomes more and more important in the membrane science to endow membranes with desired properties. The modified PPMM with great permeability and antifouling properties can adsorb the photocatalyst, so it can be used in photocatalytic membrane reactors (PMRs).Polypropylene macroporous membrane surface modification via the three-step photoinduced reversible addition-fragmentation chain transfer (RAFT) graft polymerization of 2-hydroxylethyl acrylate (HEA) was carried out. Benzyl dithiobenzoate (BDTB) was used as the chain transfer agent (CTA). Firstly, benzophenone was immobilized on the membrane surface under UV irradiation; secondly, polyHEA (PHEA) was grafted on the membrane surface under UV irradiation in the presence of HEA and BDTB; thirdly, the PHEA grafted membranes with and without CTA moieties were respectively immersed in a thermostated water bath at 55 oC for the further grafting polymerization of HEA; in this step, PHEA was continuously grafted on the second-step modified membrane, with the grafted membrane containing CTA moieties served as macro CTA and azodiisobutyronitrile (AIBN) as initiator. Then physical adsorption of zinc oxide nanoparticles (ZnO NPs) on the modified membrane surface was performed. Degradation of methyl orange (MeOr) was carried out in a PMR with the ZnO NPs immobilized on the membrane surface or suspended in the mixture under solar light and UV irradiation.The degree of grafting (DG) of PHEA increased with UV irradiation time and monomer concentration in the second step. In the third step, DG continuously increased with reaction time under thermostated conditions without adding the free radical initiator; for the PHEA grafted membranes with CTA moieties on the grafting chain, DG was relatively higher than that for the PHEA grafted membranes without CTA moieties; also PHEA was grafted on the membrane surface by using the second-step modified membrane as the macro CTA and AIBN as initiator, DG continued to increase with the reaction time.The pure water flux increased with the rise of DG up to 4.48 wt.%, then it decreased gradually, which shared the same trend with the water flux during the filtration of protein dispersion and the flux recovery ratio after water cleaning. But the rejection of protein dispersion followed the reversed trend of the pure water flux due to the combined effect of surface hydrophilicity increasing and pore narrowing.For the degradation of MeOr using ZnO NPs in suspension, the photocatalytic degradation percents (PDPs) of MeOr were over 98% (99.4%, 30 min UV light irradiation; 98.22%, 40 min solar light irradiation). For the PMR with ZnO NPs immobilized on the membrane surface, PDP was 74.3% (6 h UV light irradiation), PDP maintained at a relatively high level after repeated uses for five times (about 55%, 6 h UV irradiation). These results demonstrated that the immobilized ZnO NPs was stable for repeated uses.