Preparation Of Carbon Nanotube-based Membranes And Investigation Of Their Separation Performance Under Electrochemical Assistance

Membrane separation has been widely used in water treatment due to its high efficiency, environmentally friendly, easy operation and controllability. However, tranditional membrane separation not only suffers from the trade-off relationship between selectivity and permeability, but also is limited by membrane fouling. Furthermore, they can only remove pollutants with size larger than or comparable to pore size by pore sieving, while smaller pollutants would passthrough membranes. To solve these problems, carbon nanotube-based membranes (CNTM) were prepared in this work that ensured an electroconductive platform for integration of membrane separation and electrochemistry. In the experiments, influences of electrochemical assistance on separation performance and fouling resistance were systematically investigated and the results were showed as followed:(1) A graphene-like carbon-carbon nanobute(Gr-CNT)/ceramic membrane was prepared by coating carbon nanotube on ceramic plates from pressing/sintering coupling with vacuum filtration precesses. Effects of electrochemical assistance on separation performance were investigated by applying bias on prepared membrane. By applying a bias of+1.5 V vs. Ag/AgCl on prepared membrane, removal efficiency of silica spheres with size comparable to membrane pore was 1.13 times as high as that for membrane separation alone, and permeate flux was 1.50 times as high as that for the latter. Meanwhile, removal amount of latex particles with size smaller than membrane pore was 4 orders higher than membrane separation alone. The diffusion-adsorption rate of negatively charged latex particles at+1.5 V vs. Ag/AgCl was 7.45 times as high as that for membrane alone. Furthermore, phenol can be completely removed during electrochemically assisted separation at+1.5 V vs. Ag/AgCl, while it can pass through the prepared membrane for membrane separation alone. For natural organic matter (NOM) treatment, electrochemically assisted membrane separation not only presented total organic carbon (TOC) removal and permeate flux of 3.03 and 1.56 times as high as membrane separation alone, respectively. These results mean that electrochemical assitance can improve removal efficiency and permeate flux for prepared membrane via electrostatic repulsion, electrosorption, electrochemical oxidation and electrowetting/electrocapillary.(2) Gr-CNT/ceramic hollow fiber membrane was prepared by dry-weting spinning and vacuum filtration precesses. The effect of electrochemical assistance on membrane separation performance was investigated under different bias polarity. The results revealed that permeate fluxes in separating bacteria suspension or NOM solution under at+1.0 V vs. Ag/AgCl were 2.86 or 1.09 times as high as those for membrane separation alone, respectively. On the other hand, a bias of-1.0 V vs. Ag/AgCl ensured the prepared membrane presented fluxes of 6.67 or 1.44 times as high as those for membrnae alone for separating bacteria suspension or NOM solution, respectively. When the membrane was electropolarized by alternating square-wave bias between+1.0 V vs. Ag/AgCl and-1.0 V vs. Ag/AgCl with a pulse width of 60 s, it exhibited permeate fluxes of 8.10 or 1.53 times as high as those of membrane separating alone, respectively, by synergistic functions under both anodic and cathodic polarization.(3) A pilot-scale membrane separation device (10～15 t-d-1) based on carbon nanotube-ceramic tublar membrane was designed and applied in groundwater treatment (feed water from a reservoir around Dalian). Compared to membrane separation alone, water output was increased by 18% when the prepared membranes worked as cathode (-1.0 V). Meanwhile, the removal efficiencies of TOC and turbidity were increased by 14% and 8.2%, respectively. Furthermore, the energy consumption at voltage of-1.0 V can be decreased by 15% compared to membrane separation alone.(4) A self-standing carbon nanotube hollow fiber membrane with superhydrophobicity was prepared by wet-spinning and fluorination treatment. The results revealed that water contact angle of prepared membrane was 168° that ensured the membrane with wetting resistance of 1.32～1.61 folds as high as polyvinylidene fluoride and polypropylene membranes. During direct contact membrane distillation, the prepared membrane presented a flux of 1.38～1.73 times as high as those of the two control polymeric membranes, while the condu< tivity of its permeate water was only 1/18～1/2 of those from the latters. When the prepared membrane worked as cathode at-0.5 V voltage, it presented a stable flux of 30 L·m-2·h-1without flux loss in 36 h operation for feed containing NOM, which is 1.18 times as high as membrane distillation alone.In summary, electrochemical assistance can improve both selectivity and permeability of CNTM during water treatment. Moreover, membrane fouling can be significantly mitigated during electrochemically assisted membrane separation. This work opens a new avenue for water purification and desalination by membrane separation under electrochemical assistance on CNTM.