| Nanofiltration membranes, different from reverse osmosis membrane and ultrafiltration membrane, have high rejection rate for divalent and multivalent ions or organic compounds with relative molecular mass larger than 200. Their investment costs and operation and maintenance costs are relatively low. Therefore, nanofiltration membranes become a focus of research interest and industry application. Their separation characteristics were ultisied in the fields of water treatment, food processing and medicine. They also have broad application potentials.A series of modified chitosan compounds were prepared:(1) 6-(4'-(4-methoxybenzoyloxy)biphenyl-4-yloxy)-6-oxohexanoic acid (M1). (2) 6-((8R,9R,1 OR,13 S,14R,17S)-10,13-dimethyl-17-((S)-6-methylheptan-2-yl)-2,3,4,5,8,9,1 0,11,12,13,14,15,16,17-tetradecahydro-1 H-cyclopenta[a]phenanthren-3-yloxy)-6-oxohex anoic acid (M2). (3) 4-(2-isopropyl-5-methylcyclohexyloxy)-4-oxobutanoic acid (M3). (4) N-(carboxymethyl)-N,N-diethylethanaminium chloride (M4). (5) 2-(4-((4-sulfophenyl)diazenyl)phenoxy)acetic acid (M5). Infrared spectroscopy (IR), differential scanning calorimeter (DSC), and polarized microscopy (POM) and specific rotations meter were used to characterize their properties, results showed that:M1 was liquid crystalline mesogen compound, whose melting point was 187?, clear point was 361?. Schlieren texture was presented in heating process while cross extinction ball texture was presented in cooling process. M1 was a typical of nematic liquid crystal. M2 was cholesteric liquid crystalline mesogen compound, whose melting point was 137 "C, clear point was 147?, had a helical structure. Oil silk was presented in heating process while broken focal conic texture was presented in cooling process which showed it was typical cholesteric liquid crystal. The optical rotation of M2 was-2.4°, L-substance. Chiral M3 had lower melting point (67?), the optical rotation of it was 0.005°, as dextrose substances had a helical structure. M4 was melting at 200.4?, decomposed while the temperature increased. Melting point of anionic primitives M5 was 301?, and it was decomposed with increasing temperature.Series of modified chitosan were prepared with various monomers, in accordance with a certain proportion:(1) P1 series of modified chitosan were obtained by graft polymerized M1 with chitosan in different proportions (0% to 100%). (2) P2 series of modified chitosan were obtained by graft polymerized M2 with chitosan in different proportions (0% to 100%). (3) P3 series of modified chitosan were obtained by graft polymerized M3 with chitosan in different (0% to 100%). (4) P4 series of modified chitosan were obtained by graft polymerized M2 and M4 with chitosan in proportion that 10% M2 and (0%-90%) M4. (5) P5 series of modified chitosan were obtained by graft polymerized M2 and M5 with chitosan in proportion that 10% M2 and (0%-90%) M5. (6) P6 series of modified chitosan were obtained by graft polymerized M3 and M4 with chitosan in proportion that 5% M3 and (0%-95%) M4. (7) P7 series of modified chitosan were obtained by graft polymerized M3 and M5 with chitosan in proportion that 5% M3 and (0%-95%) M5. Infrared spectroscopy showed that the modified primitives successfully were grafted onto chitosan.We used self-made polysulfone ultrafiltration membrane as a base film, and the mixture of modified chitosan and chitosan as cast film material, acetic acid as solvent, a polyvinyl alcohol as a pore-forming agent to prepare seven series of nanofiltration membrane with glutaraldehyde cross-linking agent by the use of a casting method.(1) Pi series of nanofiltration membrane were prepared using the mixture of P1 series and chitosan. Comparing with the nanofiltration membrane using chitosan as casting material, the flux became large. The reason was that the rigid structure of M1 led to nanofiltration membrane pore size increasing. The maximum flux was 68705L.m-2.h-1, while the rejection was very low the maximum was 13.5 with 1000mg/L NaCl solution.(2) P2 series of nanofiltration membrane were prepared using the mixture of P2 series and chitosan. Comparing with the nanofiltration membrane using chitosan as casting material, the rejection reached the maximum of 64.4% and corresponding flux was 2133L.m-2.h-1 when the grafting degree of M2 to chitosan was 10%. The reason was that the helical structure of M2 caused the increase of the tortuosity of the pore. In order to obtain the best performance nanofiltration membrane the experimental conditions was researched, and the best conditions were as the following:glutaraldehyde concentration was 1%, polyvinyl alcohol concentration was 0.06%, acetic acid concentration was 6%, cross-linking time was 14 hours at room temperature, the ratio of P2-4 and the chitosan was 4:5. The test was conducted after a pre-pressure at 0.4 MPa for 0.5 hour. The rejection was 65%, and the corresponding flux was 2062L.m-2.h-1 with the 1000 mg/L of NaCl. Comparing with conventional NF membrane, this membrane operated at low pressure with high rejection, and flux increased significantly, it was three orders of magnitude.(3) P3 series of nanofiltration membrane were prepared using the mixture of P3 series and chitosan. Comparing with the P2 series nanofiltration membrane, the rejection and the flux of P3 series of nanofiltration membrane raised significantly. The reason was that the tortuosity of the pore increased more volume because of the smaller volume of M3. The rejection of nanfiltration membrane reached the maximum of 97.3% and corresponding flux was 360L.m-3.h-1 when the grafting degree was 5%. In order to obtain the best performance nanofiltration membrane the experimental conditions was researched, and the best conditions were as the following:glutaraldehyde concentration was 1.25%, polyvinyl alcohol concentration was 0.06%, acetic acid concentration was 5%, cross-linking time was 18 hours at room temperature, the ratio of P3-3 and the chitosan was 3:6. The test was conducted after a pre-pressure at 0.4 MPa for 0.5 hour. The rejection was 98.2%, and the corresponding flux was 351L.m-2.h-1. Comparing with conventional NF membrane, the rejection and the flux increased significantly.(4) P4 series of positively charged nanofiltration membrane were prepared using the mixture of P4 series and chitosan. The rejection of P4 nanofiltration membrane rose remarkablely because of the introduction of positively charged unit M4. The rejection of nanofiltration membrane reached the maximum of 95.7% with CaCl2 when the grafting degree was 70%, while the rejection with NaCl and Na2SO4 were only 47% and 20%. The rejection with NaCl and Na2SO4 decreased with the increase of grafting degree. The flux increased with the increase of grafting degree. The order was Na2SO4>NaCl>CaCl2, the flux was 2202-3385L.m-2.h-1 The nanofiltration membrane was typical positively charged nanofiltration membrane, and fit for the separation of the cation from the mixed solution.(5) P5 series of negatively charged nanofiltration membrane were prepared using the mixture of P5 series and chitosan. The rejection of P5 nanofiltration membrane rose remarkablely because of the introduction of negatively charged unit M5. The rejection of nanofiltration membrane reached the maximum of 93% with Na2SO4 when the grafting degree was 20%, while the rejection with NaCl and CaCl2 were only 47% and 33.4%. The rejection with NaCl and CaCl2 decreased with the increase of grafting degree. The flux increased with the increase of grafting degree. The order was CaCl2>NaCl>Na2SO4, the flux was 2202-5249L.m-2.h-1. The nanofiltration membrane was typical negatively charged nanofiltration membrane, and fit for the separation of the anion from the mixed solution.(6) P6 series of positively charged nanofiltration membrane were prepared using the mixture of P6 series and chitosan. The rejection of P6 nanofiltration membrane rose remarkablely because of the introduction of positively charged unit M4. The rejection of nanofiltration membrane reached the maximum of 95.8% with CaCl2 when the grafting degree was 95%, while the rejection with NaCl and Na2SO4 were only 39% and 30%. The rejection with NaCl and Na2SO4 decreased with the increase of grafting degree. The flux increased with the increase of grafting degree. The order was Na2SO4>NaCl>CaCl2, the flux was 381?987 L.m2.h-1. The nanofiltration membrane was typical positively charged nanofiltration membrane, and fit for the separation of the cation from the mixed solution.(7) P7 series of negatively charged nanofiltration membrane were prepared using the mixture of P7 series and chitosan. The rejection of P7 nanofiltration membrane rose remarkablely because of the introduction of negatively charged unit M5. The rejection of nanofiltration membrane reached the maximum of 94% with Na2SO4 when the grafting degree was 20%, while the rejection with NaCl and CaCl2 were only 68.6% and 61.3%. The rejection with NaCl and CaCl2 decreased with the increase of grafting degree. The flux increased with the increase of grafting degree. The order was CaCl2>NaCl>Na2SO4, the flux was 381-957.1 L-m-2.h-1. The nanofiltration membrane was typical negatively charged nanofiltration membrane, and fit for the separation of the anion from the mixed solution.The best nanofiltration membrane structures were characterized by SEM and three-dimensional infrared imaging systems. They shows the composite film had two sections, the upper layer was thin but very dense functional layer who played a important role in its retention of membrane performance, while the lower loose polysulfone ultrafiltration membrane support layer. |
PDF Full Text Request