| Two different quaternized chitosans are prepared by two different methods, 2-hydroxy-3-trimethyl ammonium propyl chitosan (HAPC) and graft copolymer of trimethylallyl ammonium chloride onto chitosan (GCTACC), respectively. Eleven kinds of positively charged NF membranes are prepared by method of coating and cross-linking using HAPC and GCTACC as surface active layer; PSF UF membrane and PAN UF membrane with different hydrophilicity as support layer and different organic reagents such as epoxide, diisocyanate, etc as cross-linking reagents. Their structure images are characterized by scanning electron microscope (SEM) and atomatic force microscopy (AFM), and their cross-linking on the surface is analyzed by infrared spectrum (IR). Besides, the permeation characteristic is investigated as well as other ones.HAPC is prepared using HClO4 as catalyst and reaction medium,which makes chitosan and diglycidyl ether trimethyl ammonium react under mild conditionsTwo kinds of HAPC/PAN composite NF membranes are prepared using HAPC as surface active layer; PAN as support layer and epoxide such as epichlorohydrin (ECH) with short chain; glycerol tridiglycidyl ether (GCGCE) with long chain as cross-linking reagents. At room temperature, the water permeability through the membrane from ECH cross-linking is 9.25 L·.h-1·m-2·MPa-1, and its molecular weight cut-off is 560, with a pore diameter of 0.74nm or so, and the root-mean square roughness is 8.50±1.5(103nm), and its pressure osmotic coefficient of 12.0 mv·MPa-1. Addtionally, at a pressure of 1.0MPa, the rejection for 1000 mg·L-1MgCl2, CaCl2, KCl, NaCl and Na2SO4 is 96.6, 96.6, 56.1, 57.5 and 26.6%, respectively, with a flux of 7.00, 7.30, 16.3, 15.9 and 9.86 L·m-2·h-1, respectively. At room temperature, the water permeability through the membrane from GCGCE cross-linking is 16.6 L·.h-1·m-2·MPa-1, and its molecular weight cut-off is 800, with a pore diameter of 0.84nm or so, and the root-mean square roughness is 22.3±0.30 (103nm), and its pressure osmotic coefficient of 4.43 mv·MPa-1. Addtionally, at a pressure of 1.0MPa, the rejection for 1000 mg·L-1 MgCl2, CaCl2, MgSO4, KCl and K2SO4 is 87.9, 72.4, 25.4, 19.1 and 11.3%, respectively, with a flux of 12.2, 22.9, 28.9, 29.6 and 31.8 L·m-2·h-1, respectively.It is found that NF membrane from GCGCE cross-linking shows stronger permeability and bigger cut-off molecular weight (MWCO) than the one from ECH cross-linking, which results from the relatively longer chain in GCGCE.Two kinds of HAPC/PAN composite NF membrane are prepared using HAPC as surface active layer; PAN membrane as support layer and diisocyanate such as hexamethylene diisocyanate (HDI) with clinical structure; toluene diisocynate (TDI) with rigid structure as cross-linking reagents. The performance and characterization about them are listed as follows. At room temperature, the water permeability through the membrane from HDI cross-linking is 17.2L·.h-1·m-2·MPa-1, and its molecular weight cut-off is 520, with a pore diameter of 0.72nm or so, and its pressure osmotic coefficient of 6.31 mv·MPa-1, and the root-mean square roughness is 15.6±0.24(103nm). Addtionally, at a pressure of 1.0MPa, the rejection for 1000 mg·L-1 MgCl2, CaCl2, MgSO4, NaCl and Na2SO4 is 92.9, 93.4, 50.3, 67.3 and 19.6%, respectively, with a flux of 12.3, 32.8, 31.3, 10.4 and 32.3 L·m-2·h-1, respectively. At room temperature, the water permeability through the membrane from mixed diisocyanate cross-linking is 10.6 L·.h-1·m-2·MPa-1, and its molecular weight cut-off is 560, with a pore diameter of 0.74nm or so, and its pressure osmotic coefficient of 7.73 mv·MPa-1, and the root-mean square roughness is 15.6±0.24(103nm). Addtionally, at a pressure of 1.0MPa, the rejection for 1000 mg·L-1 MgCl2, CaCl2, MgSO4, KCl, NaCl and Na2SO4 is 96.6, 95.6, 50.4, 61.5, 61.5 and 24.7%, respectively, with a flux of 12.9, 14.7, 15.3, 14.1, 13.2 and 12.6 L·m-2·h-1, respectively.The composite NF membrane from HDI cross-linking has stronger permeability and allows higher rejection. And the one from HDI cross-ling shows some disadvantages, i.e. this membrane is fragile and easily crackle, and its conditions of preparation are comparatively severe. However, the composite NF membrane from mixed diisocynate containing TDI and HDI has higher rejection, and it allows moderate permeability.Another HAPC/PAN composite NF membrane is prepared using HAPC as surface active layer; PAN as support layer and mixed anhydride of acetic anhydride \and hexane diacid as cross-linking reagent. At room temperature, the water permeability through this membrane is 6.99L·.h-1·m-2·MPa-1, and its molecular weight cut-off is 565, with a pore diameter of 0.74nm or so, and the root-mean square roughness is 7.50±0.31(103nm). Addtionally, at a pressure of 1.0MPa, the rejection for 1000 mg·L-1 MgCl2,NaCl,KCl,MgSO4,K2SO4,Na2SO4 andCaCl2 is 95.8,69.7, 65.7, 38.1, 21.9, 27.5and 97.2%, respectively, with a flux of 4.81, 7.96, 8.88, 7.96,8.19,8.27and 7.65 L·m-2·h-1, respectively. The resultant membrane has higher rejection, but it shows weaker permeability, which results from the stronger hydrophobicity due to cross-linking.Two kinds of HAPC/PAN composite NF membranes are prepared using HAPC as surface active layer; PSF membrane as support layer; both HDI and ECH as cross-linking reagent. At room temperature, the water permeability through the membrane from HDI cross-linking is 13.6 L·.h-1·m-2·MPa-1, and its molecular weight cut-off is 500, with a pore diameter of 0.71nm or so, and its pressure osmotic coefficient of 10.8 mv·MPa-1, and the root-mean square roughness is 9.63±0.18(103nm). Addtionally, at a pressure of 1.0MPa, the rejection for 1000 mg·L-1 MgCl2, MgSO4, Na2SO4, K2SO4, KCl and NaCl is 96.9, 58.5, 36.5, 33.1, 76.0 and 80.9%, respectively, with a flux of 12.9, 11.9, 11.6, 11.6, 13.8 and 13.9 L·m-2·h-1, respectively. At room temperature, the water permeability through the membrane from ECH cross-linking is12.6 L·.h-1·m-2·MPa-1, and its molecular weight cut-off is 720, with a pore diameter of 0.82 nm or so, and its pressure osmotic coefficient of 34.2 mv·MPa-1, and the root-mean square roughness is 15.6±0.24(103nm). Addtionally, at a pressure of 1.0MPa, the rejection for 1000 mg·L-1 MgCl2, MgSO4, Na2SO4, K2SO4, KCl and NaCl is 96.5, 45.0, 31.8, 22.5, 67.1 and 70.8%, respectively, with a flux of 15.6, 14.1, 14.4, 13.9, 15.5 and 14.1L·m-2·h-1, respectively.Compared with the HAPC/PAN composite NF membrane from HDI cross-linking, the HAPC/PSF composite one from HDI cross-linking has higher rejection, but shows weaker permeability. Compared with the HAPC/PAN composite NF membrane from ECH cross-linking, the HAPC/PSF composite one from ECH cross-linking has better performance with stronger permeability and almost unchanging rejection.Graft copolymer of trimethylallyl ammonium chloride onto chitosan (GCTACC) is obtained by grafting trimethylallyl ammonium chloride on chitosan. GCTACC/PAN composite NF membrane is prepared using GCTACC as surface active layer; PAN membrane as support layer and ECH as cross-linking reagent. At room temperature, the water permeability through this membrane is 6.30 L·.h-1·m-2·MPa-1, and its molecular weight cut-off is 490, with a pore diameter of 0.70nm or so, and the root-mean square roughness is 10.2±0.13(103nm). Addtionally, at a pressure of 1.2MPa, the rejection for 1000 mg·L-1 MgCl2, CaCl2, MgSO4, NaCl and Na2SO4 is 97.6, 97.2, 89.7, 65.0 and 40.7%, respectively, with a flux of 6.80, 6.12, 6.12, 5.57 and 5.51 L·h-1·m-2, respectively. This membrane has weaker permeability due to the introduction of hydrophilicity groups from cross–linking, in good agreement with small MWCO.GCTACC/PAN composite NF membranes are prepared using GCTACC as surface active layer; PAN membrane as support layer; both HDI and TDI as cross-linking reagents. At room temperature, the water permeability through the membrane from HDI cross-linking is 6.42 L·.h-1·m-2·MPa-1, and its molecular weight cut-off is 900, with a pore diameter of 0.87nm or so, and its pressure osmotic coefficient of 6.80 mv·MPa-1, and the root-mean square roughness is 8.76±0.23(103nm). Addtionally, at a pressure of 1.2MPa, the rejection for 2000 mg·L-1 MgCl2, CaCl2, MgSO4, KCl, NaCl and Na2SO4 is 95.6, 95.4, 80.8, 53.7, 66.4 and 30.7%, respectively, with a flux of 6.73, 6.73, 6.12, 6.43, 7.35 and 6.73 L·m-2·h-1 L·h-1·m-2, respectively. At room temperature, the water permeability through the membrane from HDI cross-linking is 14.1L·.h-1·m-2·MPa-1, and its molecular weight cut-off is 930, with a pore diameter of 0.89 nm or so, and its pressure osmotic coefficient of 6.80 mv·MPa-1, and the root-mean square roughness is 7.28±0.07(103nm). Addtionally, at a pressure of 1.2MPa, the rejection for 2000 mg·L-1 MgCl2, CaCl2, MgSO4, KCl, NaCl and Na2SO4 is 94.0, 91.7, 61.9, 41.9, 57.0 and 20.1%, respectively, with a flux of 9.48, 10.1, 11.3, 8.57, 8.57 and 8.57 L·h-1·m-2, respectively.GCTACC/PAN composite NF membrane from HDI cross-linking shows weaker permeability and has low molecular weight cut-off, which can results from the reason that GCTACC is polymer with rigid structure, while HDI has clinical structure, thus the compatibility between GCTACC and HDI is better and the cross-linking reaction is carried on easily, resulting in the compact cross-linked network and smaller pore diameter of membrane.Another GCTACC/PAN composite NF membrane is prepared using GCTACC as surface active layer; PAN membrane as support layer and mixed anhydride of acetic anhydride and hexane diacid as cross-linking reagent. At room temperature, the water permeability through this membrane fis 7.13 L·.h-1·m-2·MPa-1, and its molecular weight cut-off is 900, with a pore diameter of 0.87 nm or so, and the root-mean square roughness is 7.70±0.76 (103nm). Addtionally, at a pressure of 1.0MPa, the rejection for 1000 mg·L-1 MgCl2, MgSO4, Na2SO4, K2SO4, KCl and NaCl is 90.0, 56.0, 26.4, 14.0, 41.3 and 37.4%, respectively, with a flux of 6.12,5.90,5.42,5.72,6.12 and 5.67 L·h-1·m-2, respectively. It is found that the resultant membrane shows weaker permeability,as a result of stronger hydrophobicity due to cross-linking.Thus it is not difficult to find that the characteristic and performance of composite membranes are related to the physicochemical characteristic of film-forming material; the type of cross-linking reagent and the compatibility between surface active layer and support layer and roughness, etc.The structure images of composite NF membrane are observed by SEM. The results suggest the cross-section of membrane is consist of adding-layer or a thin active layer on finger-like support layer, and the surface of membrane has a contribution of some gel particle or adding gel-layer. Similar to other nanofiltration membrane, the rejection decreases with an increasing in feed concentration; with an increase of operating pressure, the flux almost increases linearly; and the rejection increases slowly, then approaches a constant value or shows the trend to decrease when operating pressure exceeds a certain value. And the feed cross-flow rate has little influence on membrane performance. Besides, the rejection of composite NF membranes is related to the existent form of ions. The rejection order for different salts is MgCl2 > MgSO4 > NaCl > Na2SO4 or MgCl2 > NaCl > MgSO4 > Na2SO4, and the pressure osmotic coefficients are positive, suggesting the characteristic of positively charged NF membrane.It is reported first that a positively charged NF membrane is adopted to carry on the preparation of carnallite by rejection of Ca2+ and Mg2+brackish water through NF and addition of MgCl2·6H2O. The method has some advantages including high efficiency, conveninent operation, etc. And carnallite is characterized by XRD.Additionally, the positively charged composite membrane prepared is applied to treat wastewater that drained from the production of sodium alginate.The results suggest that after the treatment of nanoflitration , the desalted solution might be reused as technological water , with a rejection of 93.0% for CODcr (21.4mg/L), a rejection of 89.0 % for Ca2+ and a rejection of 88.3% for rigidity.
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