Fabrication And Characterization Of TFN Membranes With Carbon Nanotubes

Composite membranes with carbon nanotubes were prepared by interfacial polymerization with forming a thin-film polyamide layer on the PS porous substrates. The concentration of pore-forming agent in the casting solutions was studied. The factors that affected the growth of the thin-film layer were also investigated. Carbon nanotubes were added into the dopamine modified layer and porous polysulfone substrate, as well as the effect of concentration and diameter of carbon nanotubes on the permeation performances and structures of composite membranes was studied. Both forward osmosis process and nanofiltration process were carried out to test the permeation performances of the prepared membranes. Furthermore, the anti-fouling abilities of composite membranes were studied in both forward osmosis and nanofiltration process. The conclusions are as follows:The water flux of composite membranes increased with the concentration of pore-foaming agent in the casting solution, and the swelling of the support layer by isopropyl alcohol can also improve water flux of membranes. However, the improvement of the water flux was less than 1.0 L/(m2h). The optimum preparation conditions for the membranes were:8 g of pore-foaming agent,2 g/L of dopamine solution for modifying of support layer,24 h of modifying time,30 s of interfacial polymerization reaction time,0.5% of interfacial polymerization aqueous phase monomer and 0.1% of oil phase monomer. The membranes with modified support layers obtained a water flux of 4.05 L/(m2h), and lower reverse flux which Js/Jv is about 0.13 g/L.It is found that the water flux of the composite membranes first increased with the increase of carbon nanotube concentration in the modified layer and then decreased with the sequentially adding of carbon nanotubes. When the adding concentration was 0.05%, the composite membranes obtained the highest water flux in forward osmosis process. Specifically, the water flux of composite membranes was 4.92 L/(m2h) compared with the pure polyamide memrbrane 2.30 L/(m2h) for glucose/NaCl solution system, with a reject at 90% of NaCl. For MgCl2/pure water system, the composite membrane obtained a water flux of 7.24 L/(m2h). and the Js/Jv was about 0.42 g/L. Regarding to the effect of carbon nanotube diameter on membrane permeation performances, it is found that composite membranes with multi-walled carbon nanotubes obtained a higher water flux and a lower reverse salt flux than membranes with single-walled carbon nanotubes. When the carbon nanotubes were added into the support layer and when the adding concentration in the substrate was 0.15%, the composite membranes obtained the highest flux of 7.15 L/(m2h) for in forward osmosis process. When carbon nanotubes were added into both support substrate and modified layer, the composite membranes obtained the highest water flux of 8.25 L/(m2h) and a Js/Jv about 0.03 g/L.Compared with pure polyamide membranes, composite membranes with carbon nanotubes in both of their support layers and modified layers obtained a higher water flux and a similiar rejection. Under 4 bar operating pressure, the composite membranes with 0.15% and 0.05% of carbon nanotubes in the support layers and modified layers obtained a water flux of 29.33 L/(m2h) and a rejection of 40% for 200 PPM of MgCl2, while the pure water flux of pure polyamide membranes was 20.89 L/(m2h) and the rejection was 41%. After three times of pollution-cleaning-recover circulation, the composite membranes with adding amount of 0.05% in the modified layers obtained a water flux recovery rate of 80.1% in forward osmosis process and a water flux recovery rate of 51.5% in nanofiltration process, while the water flux recovery rates of the pure polyamide membrane were 73.4% and 35.9% respectively.