Structure Control And Properties Research Of Charged Nanofiltration Membranes With Low Operation Pressure

Nanofiltration (NF) is a pressure-driven membrane process, which was previously considered as "loose" osmosis. It is suitable for applications as water softening, seperation of organic molecules, wastewater treatment and food processing. However, the operation pressure of the NF is relatively high. In order to obtain charged nanofiltration membranes with low operation pressure, a series thin polyelectrolyte films were constructed on the surface of polysulphone (PSf) or polyacrylonitrile (PAN) ultrafiltration membranes by interfacial polymerization, dynamic-self-assembly and UV-graft polymerization of appropriate substances, respectively.A series of NF membranes were prepared by interfacial polymerization of 2, 5-diaminobenzenesulfonic acid (DBSA) with trimesoyl chloride (TMC). The results indicated that the interfacial polymerization rate was reduced by the steric hindrance effect of -SO₃H group on 2, 5-diaminobenzenesulfonic acid. A loose structure surface can be observed on the surface of the substrate membrane. FTIR-ATR and XPS results confirmed the existence of polyamide on the surface of the substrate membrane. The concentration of the reaction monomers has great effects on the performances of the NF membranes. First, the salts rejection increased with the increase of the concentration of DBSA or TMC, however, it decreased by further increasing the concentration of DBSA or TMC, while the change of the solution flux was reverse. Adding NaOH was benefit for polymerization. The rejection of Na₂SO₄ first increased and then decreased with prolonging the reaction time accompanied with a decrease of solution flux at first and then increase. When the concentration of DBSA and TMC were 0.5%, the NF membrane prepared with twice-reaction showed higher salts rejection than the NF membrane prepared with once-reaction. For example, the Na₂SO₄ rejection of the twice-reaction sample was 6% higher than that of the once-reaction sample. The flux of the twice-reaction sample was 59.7 L/m².h at 0.4 Mpa. According to the performances at different operation pressure, there were two types NF membranes in this experiment: (1) The salts rejection of the NF membranes is independent of operation pressure, (2) The salts rejection of the NF membranes dose decrease with increasing the operation pressure. When the concentration of salts increase, the concentration of counter-ion increase too which will screen some surface charge and reduce the dielectric exclusion effect, so the salts rejection decreased with increasing the salts concentration. Taking into account of the performances of these NF membranes, these NF membranes may suitable for dye/salt separation.N-(2-aminoethyl)-piperazine cored amine ended Poly (amido-amine) (PAMAM) dendrimers were synthesized by divergent procedure. A series NF membranes with high salts rejection were prepared with PAMAM and trimesoyl chloride (TMC) via in situ interfacial polymerization. SEM images indicated that the surface morphology of these membranes were different from those of the membranes prepared with 2, 5-diaminobenzenesulfonic acid (DBSA) with TMC, it is comoposed of compact particles. AFM suggested that the substrate membrane became rougher by covering the polyamide layer. The effects of generation number, concentration of PAMAM and reaction time were discussed in this paper. Because the degree of cross-linking increased with the increase of PAMAM generation number, the NF membranes prepared by higher generation number PAMAM showed higher salts rejection and lower solution flux. The performances of the NF membranes changed with the concentration of PAMAM. A higher salts rejection coefficient could be obtained by polymerization with higher concentration of PAMAM, while the salts rejection difference among the different salts reduced. When the concentration of PAMAM(NG2) was 2.79×10^-4mol/L, the rejection of Na₂SO₄,MgCl₂,MgSO₄were 31.0%, 30.2% and 38.9%, respectively. The salts rejection of Na₂SO₄,MgCl₂,MgSO₄ increased to 75.7%, 87.1% and 80.7% , repectively when the concentration of NG2 was 75.7%, 87.1% and 80.7%. Further increasing the concentration of NG2 to 4.46×10^-3mol/L, the rejection of these three salts was very close. The salts rejection of Na₂SO₄,MgCl₂,MgSO₄ were 93.0%, 92.3% and 91.4%. Prolonging the reaction time was in favor of increasing the salts rejection. And the salts rejection difference among Na₂SO₄,MgCl₂,MgSO₄ was reduced when prolonging the reaction time. The salts rejection was increased accompanied with a similar solution flux by increasing the operation pressure. The salts rejection and the solution remained stable when the salts concentration was varied.Compared with interfacial polymerization, layer-by-layer self-assembly is an environmental benign technique for making NF membrane. Driven forces for self-assembly of molecules are electrostatics, hydrogen bonding, Vander Walls forces, hydrophobic interaction and others. Till now, layer-by-layer self assembly is a time-consuming method. In this experiment, a series of NF membranes with high solution flux at low operation pressure were prepared by dynamic self-assembly of poly (4-styrenesulfonic acid-co-maleic acid) sodium salt (PSSMA), poly (styrenesulfonic acid sodium salt) (PSS) and poly (allylamine hydrochloride) (PAH) on the modified poly-acrylonitrile ultra-filtration membrane. The [PAH/PSS]₁PAH/PSSMA (3:1) membrane composed of only 2 bilayers is a kind of negtively charged NF membrane, which exhibited 91.4% rejection of Na₂SO₄ and allowed solution flux of 28.6 L/m²h at 0.2 MPa. The solution flux increased to 106.6 L/m²h without obvious decrease in Na₂SO₄ rejection when the operation pressure increased to 0.8 MPa. The [PAH/PSS]₁PAH/PSSMA/PAH NF membrane with 2.5 bilayers is a kind of positively charged NF membrane, which exhibited a 90.4 % rejection of MgCl₂ and a permeate flux of 23.8 L/m².h at 0.2MPa, a 91.0% rejection of MgCl₂ and a permeate flux of 83.8 L/m².h at 0.8 MPa. The permeate flux was higher than that of the NF membranes in market.The effects as substrate, assembly method, assembly pH, concentration of polyelectrolyte, SS/MA ratio, bilayer number and others on the performences of the NF membranes were discussed in this paper. Compared with static self-assembly method, the NF membranes prepared by dynamic self-assembly method showed better performance. In order to improve the attachment of active substance, PAN ultra-filtraton membranes were modified by NaOH solution. Excessive hydrolysis would affect the mechanical strength and integrity of the substrate membrane. The appropriate modification condition was as follows: NaOH 1.5M, reaction temperature 45℃, reaction time 1.5 h. FTIR-ATR results indicated PAN was partly hydrolyzed by NaOH. The membranes assembled on the modified PAN membrane showed higher salts rejection than on the unmodified one. It reveals that the weakly charged groups in the mutilayer acted as potential charge contributors. Both the surface structure and the charge density of the membrane can be optimized by controlling the assembly pH. The NF membrane prepared with PSSMA (SS/MA=3:1) showed better properties than that of the membrane prepared by PSSMA (SS/MA=1:1). When the assembly bilayer number was larger than 2, the flux decreased with higher assembly number, while the salts rejection nearly remained unchanged. Positively and negatively charged NF membranes were obtained easily by changing the component of the terminal layer. A Positivly charged membrane can obtain when the terminal layer is PAH. The salts rejection order is as follows: MgCl₂> MgSO₄> Na₂SO₄≈NaCl. It is suitable for CuCl₂ removal. The rejection of CuCl₂ remained at about 88% when the concentration of CuCl₂ ranged from 500 mg/L to 2000 mg/L.UV-induced polymerization of 2-methacryloyl oxyethyl trimethyl ammonium chloride (MOTAC) on the polysulfone ultrafiltration membrane was applied to prepare NF membranes. Compared with the works mentioned above, the connection force between the substrate membrane and the active layer is chemical bonds which will be of benefit to increase the stability of membrane. FTIR-ART and XPS results confirmed the existence of the poly (2-methacryloyl oxyethyl trimethyl ammonium chloride) on the surface. The membrane prepared by polymerization of MOTAC with concentration of 1.5 M for 5min demonstrated high MgCl₂ rejection (94.8%) accompanied with high flux (20.3 L/m².h, 0.2 Mpa). Different NF membranes can be prepared by adjusting the monomer concentration, radiation time and radiation distance et al. The degree of grafting and salts rejection increased with increasing the concentration of monomer concentration. However, further increasing the concentration of MOTAC, the degree of grafting and salts rejection did not change a lot. Prolonging the radiation time demonstrated the similar phenomena. The degree of grafting and salts rejection decreased with enlarging the radiation distance accompanied with the increase of solution flux. The salts rejection of the NF membrane with high MgCl₂ nearly maintained stable at different operation pressure, while the flux increased with the increase of operation pressure. These membranes showed typical positively charged characteristic that the salts rejection order of them is MgCl₂> NaCl>MgSO₄(?)Na₂SO₄. It should be mentioned that the MgCl₂ rejection is high accompanied with very low MgSO₄ rejection. So these membranes are suitable for the hardness removal from sulfate-poor water. The primary experiments suggested that these membranes have well antifouling properties. Contributed to the existence of the poly (2-methacryloyl oxyethyl trimethyl ammonium chloride) which endowed the NF membranes with good hydrophilic and well anti-virus properties, the NF membranes demonstrated better anti-fouling properties in the primary experiments. Compared with the original PSf membrane, there were fewer pollutants on the NF membranes surface after the preservation in water for 25 days.