Preparation And Characterization Of The Novel Charged Composite Nanofiltration Membranes With The High Salt Permeabilities

In modern chemical industry, such as biochemistry, pharmacy, food, dyeing, etc., questions are frequently brought forward regarding the removal of trace electrolytes from water-soluble organics. Therefore, Nanofiltration (NF) membrane technique has been widely applied to these fields. At present, most of NF membranes are charged NF membranes of which the mechanism for the separation is based on the sieving effect of membrane pore and the Donann effect between electric charges and ions.In general, traditional NF membranes allow monovalent ions to be effectively transmitted whereas with divalent ions mostly retained. That is, traditional NF membrane is good at removing monovalent salts from low-molecular-weight organics. However, removal of divalent salts from organics is critical for those fields mentioned above in term of desalination and purification. In this paper, based on our advantages in many years research of charged mosaic membrane, and based on the principle and related experiences on the preparation of charged mosaic membrane by interfacial polymerization (IP) or coating-crosslinking methods, intensive studies were made in an attempt to develop a novel, composite NF membrane which contains both positive and negative charges and with high salt permeabilities. The main research contents and results are as follows:1. Sulfonation of polyethersulphone (SPES) and its application for preparation of a negatively charged support membraneSulfonated polyethersulphone (SPES) was chemically fabricated using polyethersulphone (PES) as raw material, concentrated sulfuric acid as solvent and chlorosulfonic acid as sulfonated agent. The effects on the sulfonation degree (DS) of SPES by various reaction conditions, such as reaction time, reaction temperature, reactant dosage etc., were investigated. Then, by means of L-S phase inversion method, SPES membrane was fabricated used as the support membranes of charged NF membranes. For SPES support membrane, the uniform design was applied to the experiment plan, and the experimental data were processed by SPSS software so as to establish a model equation of water flux. On the base of above, the experiment was optimized further to get the technique conditions of membrane preparation.The results of experiments show that:it is better to set sulfonated reaction time as 2h at 10℃. The optimal preparation conditions of SPES negatively charged support membrane were:SPES 20(wt)%, polyvinyl pyrrolidone (PVP) 5(wt)%, butanone 6(wt)%, phosphoric acid 4(wt)%, N,N-Dimethylacetamide (DMAc) 65(wt)%, and evaporation time 60 s. Under the operating pressure of 0.1 MPa at room temperature, water flux of the support membrane was 95.1 L·m-2·h-1 and the rejection of polyvingl alcohol (PVA) 88000 solution was 96.7%.2. Chloromethylation of polysulphone (CMPSF) and its application for preparation of a positively charged support membraneChloromethylated polysulphone (CMPSF) was synthesized using Polysulfone (PSF) as raw material, dichloroethane as solvent, anhydrous zinc chloride as catalyst and chlorine methyl as chloromethylation agent. The effects on the chlorine content of CMPSF by various reaction conditions, such as reaction time, reaction temperature, reactant dosage etc., were investigated. And then, CMPSF/PSF membrane was prepared by means of blend method. For CMPSF/PSF membrane, the uniform design was also applied to the experiment plan, and the experimental data were processed by SPSS software to get the technique conditions of membrane preparation. In addition, CMPSF/PSF membrane has to be quaternized in trimethylamine (TMA) solution in order to obtain QAPSF/PSF positively charged support membrane.The results of experiments show that:it is better to set the reaction time of chloromethylation is about 4 h. The optimal preparation conditions of CMPSF/PSF blend membrane were:the total concentration of CMPSF and PSF 22 (wt)%, CMPSF/PSF= 1/4, polyethylene glycol (PEG 600) 6 (wt)%, DMAc 72 (wt)% and evaporation time 90 s. Optimal quaternization conditions were:quaternizing 6h at 50℃with 33(wt)% TMA. Under the operating pressure of O.1MPa at room temperature, water flux of positively charged support membrane QAPSF/PSF was 154.3 L·m-2·h-1, and PVA 31000-50000 rejection was 91.5%.3. Preparation of the charged composite NF membranes (NF1 & NF2)(1) Using above SPES negatively charged support membrane, polyethyleneimine (PEI) as coating material and glutaraldehyde & sulfuric acid mixed solution as cross-linking agent, the first charged NF membrane, NF1, was fabricated through dip-coating and cross-linking processes. Effects on membrane performance by preparation conditions and operation conditions, such as PEI concentration, dip-coating time, the type and concentration of cross-linking agent, curing temperature & time etc., were investigated. In addition, contact angle measurement, scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to characterize the microstructure of NF1 membrane. Suggested by the experiment results, good preparation conditions are:1.5 (wt)% PEI dip-coating solution, dip-coated for 25 min, cross-linked for 5 min with cross-linking agent solution mixed by 1.0 (wt)% glutaraldehyde and 1.0 (wt)% sulfuric acid, and curing treatment time was 20 min at 70℃. Under the operating pressure of 0.4 MPa at room temperature, water flux of NF1 membrane was 5.8 L·m-2·h-1 and polyethylene glycol 1000 (PEG 1000) rejection was 87.8%. The rejections to MgSO4, MgCl2, Na2SO4 and NaCl are 40%,35%,29% and 18%, respectively.(2) Using above QAPSF/PSF positively charged support membrane and using interfacial polymerization (IP) technology, the second charged NF membrane, NF2, was prepared by the IP reaction of 2,5-diaminobenzene sulphonic acid (DIA) with trimesoyl chloride (TMC). Effects on membrane performance by preparation conditions and operation conditions, such as DIA concentration and TMC concentration, acid accepter (Na2CO3) concentration, emulsifier (SDS) concentration, IP time, curing temperature & time etc., were investigated. Fourier transformation infrared spectroscopy (FT-IR), contact angle measurement, SEM, AFM and pore distribution measurement were used to characterize the microstructure of NF2 membrane. The optimal membrane preparation conditions were as follows:for the liquor-phase:0.3 (wt)% DIA,0.1 (wt)% Na2CO3,0.1 (wt)% SDS,99.5 (wt)% water; while for the organic-phase:0.3 (wt)% TMC and 99.7 (wt)% n-Hexane. In addition, immersion time was 20 min in liquor-phase; IP time lasted 4 min; curing time were 20 min at 80℃. Under the operating pressure of 0.4 MPa at room temperature, water flux of NF2 membrane was 14.2 L/(m2·h), while PEG 1000 rejection was 97.9%, and the rejections to Na2SO4, MgSO4, NaCl and MgCl2 were 34.8%,10.4%,9.6% and 4.6%, respectively.4. Determination of the structural parameters of NF1 and NF2 membranesUtilizing classical Spiegler-Kedem equation when analyzing permeation experiments of polyethylene glycol solution system, reflection coefficient and solute permeability coefficient of NF1 membrane and NF2 membrane were acquired separately. Further more, the pore radius and the ratio of membrane porosity to membrane thickness of NF1 and NF2 membranes were obtained by the define pore model. The results showed that pore radius of the selection layer of NF1 and NF2 membranes were about 0.83 nm and 0.816 nm, respectively. The ratios of membrane porosity to membrane thickness of NF1 and NF2 membranes were about 2.172×105 m-1 and 6.474×104 m-1, respectively. Calculation values from theoretical model agree with experimental data.In addition, experiments were also attempted using NF1 and NF2 membranes in the desalination process of dye solutions. The results showed that both NF1 and NF2 membranes can effectively remove inorganic salts from dye mixture system with high divalent salt permeability.The outstanding features and innovations of this paper are that:adjust, control and optimize the properties of the charged NF membrane by charging support membrane. An attempt has been made to introduce anions and cations exchanging groups into the membrane which will enhance its permeabilities to inorganic salt, especially to bivalent salt ion. This paper has some novelties specifically in following three aspects:(1) The preparation and optimization of negatively charged SPES support membrane and positively charged QAPSF/PSF support membrane not only provides a good basis for further development of charged NF membrane but also provides two kinds of potential charged ultrafiltration membranes for the'ultrafiltration membrane family'.(2) considering the characteristics of positively charged selective layer of the NF membrane that formed by the coating-crosslinking technology using polyethylenei-mine (PEI), the negatively charged SPES support membrane can, without question, help to fine tune the charging property of NF membrane which will strengthen the electrolyte transport.(3) Compared with negatively charged group, positively charged group is harder to be introduced into the selective layer of the NF membrane by interfacial polymeri-zation (IP) technique. Based on the knowledge that negatively charged membrane selective layer forms in IP reaction of DIA and TMC, positively charged QAPSF/PSF support membrane is adopted in this paper which can improve the anion exch-anging capacity and meanwhile help to optimize the separation performance of charged NF membrane.