Fabrication of a dual-layer (CA/PVDF) hollow fiber membrane forRO concentrate treatment

In this study, a dual-layer hollow fiber nanofiltration (NF) membrane was fabricated with an inner layer of poly(vinylidene fluoride) (PVDF) via thermally induced phase separation (TIPS), and an outer layer of cellulose diacetate (CA) via non-solvent induced phase separation (NIPS). The intermediate-treatment using dimethyl formamide (DMSO) between the two layers increased the porosity of inside surface of outer layer. The nanopore of the outside surface of outer layer was molecularly designed by controlling the dope solution composition, i.e., the polymer concentration, the co-solvent additive (acetone) ratio and TiO2 nanoparticle loading. By increasing the acetone ratio and TiO2 nanoparticle loading, the pore size became narrower and the pore size exhibited dominate effects on both filtration and separation performances. Moreover, biofouling and bacterial growth on the membrane were reduced by increasing the TiO2 nanoparticle loading. The dual-layer (CA/PVDF) hollow fiber membranes had high strengths larger than 8 MPa, Na2SO4 rejections of 90–95% and pure water permeabilities of 1–4 L m−2 h−1 bar−1. The feasibility of treating RO concentrates by the resultant membrane was proven by the effective removal of total organic compounds (N90%) and low rejection of total dissolved salts (b60%). © 2015 Elsevier B.V. All rights reserved.

A novel dual-layer (CA/PVDF) hollow fiber membrane was fabricated via the two step (NIPS/TIPS) process. The dope solution composition of the outer layer, including the polymer concentration, the co-solvent ratio and nanoparticle loading were found to influence the morphology and pore size of the resultant membranes. The following conclusions can be drawn from this study: (1) The dual-layer hollow fiber membrane has outstanding mechanical strength compared to the CA flat sheet membrane, while the pore size of the CA outer layer in the hollow fiber membrane is much narrower than in the flat sheet membrane. (2) By increasing the ACE ratio as the co-solvent additive, the pore size and the outer layer thickness decrease simultaneously. As a result, the dextran and electrolyte rejections improve and the PWP of the membranes is maintained. (3) By increasing the TiO2 nanoparticle loading in the outer layer, the pore size was narrowed down and the hydrophilicity was improved. Moreover, the incorporated TiO2 nanoparticle improved the anti-biofouling and antibacterial performance of the dual-layer hollow fiber membrane, characterized by BSA and E. coli as model protein and microorganism foulant, respectively. In addition to the superior anti-fouling performance and high strength (N8 MPa), the newly developed NF membrane has a MWCO of 580–810 Da, Na2 SO 4 rejection of 90–95% and PWP of 1.5– 4 L m − 2 h− 1 bar− 1 . The feasibility of treating RO concentrates by the dual-layer (CA/PVDF) hollow fiber membrane was proved by the effective removal of total organic compounds (N 90%) and low rejection of total dissolved salts (b40%), and the purified effluents (total organic compounds b 50 ppm) can be safely discharged into the surface and underground waters.