| Forward osmosis(FO) membrane process has attracted significant attention because of its like advantages namely low operating cost and low fouling property. But there are still a number of challenges for its practical application. Recent studies has found that the addition of nanomaterials may significantly enhance the membrane properties. Carbon nanotubes(CNTs) have gained more attention as one of these additives due to its high biocompatibility and large specific surface area. But as a result of their low hydrophilicity, the surface modifications of CNTs are suggested. In this research, functionalized CNTs are added into the aqueous solution to improving the performance of traditional forward osmosis membrane.And characterization of the structure and properties of membrane were done.Firstly, the thesis examined the influential factors during the process of producing traditional cellulose acetate(CA) FO membrane and obtained the best parameters of membrane preparation. The impact of concentration polarization was analyzed by comparing the change of water flux and reverse the salt flux in different operating mode. Scanning electron microscopy(SEM) and contact angle analyzer were used to characterize the structure of forward osmosis membrane surface and cross section and hydrophilicity. The optimum conditions for preparation of CA forward osmosis membrane were shown as follows: the content of cellulose acetate was 12%, the pore-form agent was chosen K15 series of polyvinylpyrrolidon which content was 4%, lactic acid content was 6%, polyester screen mesh was 120, the optimal heat temperature was 70 ? and the optimal coagulation bath temperature was 27 ?. At this point the water flux of the membrane was 46 LMH, reverse salt flux was 6 g MH, contact Angle was 65.9 °. The hydrophilic and permeability of the CA forward osmosis membrane was much higher than commercial membrane. And both of the membranes were affected by the concentration polarization during the process of application.Scanning electron microscopy(SEM) results showed that the membrane thickness was significantly greater than that of commercial membrane, and the structures has more pore structures.On this basis, the thesis studied the preparation of membrane using carboxyl carbon nanotubes(MWCNT-COOH) as additive. Adding carboxyl carbon nanotubes into castingsolution to improve membrane performance. By investigating the influence of carboxyl carbon nanotube content, heat treatment temperature, coagulation bath temperature and polyester screen mesh on membrane performance so that to obtained the best parameters of membrane preparation. The penetrating quality test, hydrophilic and membrane structure of FO membrane before and after adding carboxyl carbon nanotubes were studied. The optimal preparation conditions of membrane are as follows: the MWCNT-COOH content is 3.5% of total cellulose acetate, the optimal heat temperature is 70 ?, the optimal coagulation bath temperature is 27 ?, with 120 mesh polyester screen mesh as supporting materials. At this point the water flux was 56.0 LMH, reverse salt flux was 6.0 g MH, and contact angle was57.2 °. By comparing the performance of the membrane before and after adding MWCN-COOH, it can be seen that MWCNT-COOH can dramatically improve the membrane hydrophilicity. And the water flux increased sharply, but the impact of reverse salt flux was not obvious. Moreover, the addition of the MWCNT-COOH aggravated the influence of concentration polarizationThe experiment also made cellulose acetate / hydroxylation carbon nanotubes(CA /MWCNT-OH) FO membrane. Adding hydroxylation carbon nanotubes into casting solution to improve membrane performance. The best parameters of membrane preparation were obtained by investigating the influence of preparation conditions. Results indicated that when MWCNT-OH content was 3.0% of total cellulose acetate content, the optimal heat temperature was 70 ?, the optimal coagulation bath temperature was 30 ?, with 120 mesh polyester screen as supporting materials, the performance of the membrane was optimal. At this point the water flux was 57.4 LMH, reverse salt flux was 4.5 g MH, contact angle was49.3. It is means that the hydrophilicity had a significantly increase compared to traditional CA membrane. It can be seen from the cross section structure that many small holes were formed and the membrane structure become looser. Comparing the osmosis of membrane before and after adding MWCNT-OH, it can be seen that when the support layer face to the draw solution, the addition of MWCNT-OH can significantly improve the water flux, while significantly decrease the value of the reverse salt flux. But when the support level face to the feed solution, the addition of MWCNT-OH fails to improve the reverse salt flux.Finally, the thesis studied the application of membrane made in the former experiment inthe process of simulation of brackish water desalination and compared it with the membrane produced by American HTI company. The experiment was implemented with 5000 mg/L simulate brackish water fluid as feed solution and 2 M glucose solution as draw solution. The water flux and salt rejection of different FO membrane were tested during an hour testing time.The study of water flux showed that several kinds of membrane prepared in this paper were more unstable and required for longer time to reach stability compared with commercial membrane. Observation of changes of salt rejection can find the same situation. In terms of water flux values, the membrane prepared in this paper is superior to commercial membrane.And the addition of the functionalized carbon nanotubes can significantly improve the penetrating quality of membrane. But the salt rejection was not optimistic. As you can see through an hour test, several kinds of permeable membrane prepared in this paper have no obvious advantage as for salt rejection. On the contrary, compared with commercial HTI membrane, they still have spacious room for development. |
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