Study On The Structure Regulation And Dye Separation Mechanism Of PES/SPSf/SPES Blend Membrane

The effective separation of saline dye wastewater has always been an urgent problem in the field of industrial wastewater treatment.The preparation of high-performance loose nanofiltration(LNF)membrane is an effective way to solve this critical issue.Polyethersulfone(PES),sulfonated polyethersulfone(SPES)and sulfonated polysulfone(SPSf)are blended to prepare PES/SPES/SPSf LNF using non-solvent induced phase separation(NIPS).The salt/dye selective separation performance of the membrane is explored.Especailly,their separation mechanism including the size effect and the synergistic effect between steric hindrance-and charge interference is investigated.PES,SPES(sulfonation degree-DS=25%)and SPSf(DS=25%)as raw materials(31 wt%polymeric concentration),N,N-dimethylacetamide(DMAc)as solvent,adipic acid(AA)as a small molecular pore-forming and water as coagulanting bath(at)were employed to fabricate PES/SPES/SPSf LNF membrane.The effect of SPES concentration in casting solution on the morphology and performance was investigaged.Results showed that the PES/SPSf/SPES blend system is a completely compatible system.The resulting PES/SPSf/SPES LNF membranes displayed a typical asymmetric structure with a dense skin layer and a sponge like sub layer.The viscosity of the casting solutions raised with the incease of AA due to hydrogen bonding between the polymer chains and AA,which caused a delay phase inversion during the membrane formation.Consequently,the resultant LNF membrane presented a fully sponge-like structured substrate with thicker dense skin layers.Speficically,when SPES centration was 10wt%of polymers(PES/SPSf/SPES)(mass ratio of PES and SPES to SPSf=86/14)in the casting solution,the LNF membrane(M10)with the molecular weight cut off(MWCO)of 5900 Da and pore size of 1.38 nm was obtained.The pure water flux(PWF)of the resultant LNF membrane(M10)was 120.9 L m^-2 h^-1.Additionally,the zeta potential of the membrane(M10)was up to-40.1 m V(p H=7).The rejection of M10 was over 99%for Congo Red(CR)and lower than 25%for Na₂SO₄.Simutaneously,after being contaminated with Bovine serum albumin(BSA)solution and Congo Red(CR)solution,the flux recovery rate was up to 91.1%and 90.2%.Further,PEG as the neutral model molecule and 9 kind of dyes such as methyl red,methyl orange,and acid blue 25 as the charged model molecules were used to explore,the interaction between the size and charge characteristics of the model molecules and the separation performance of the membrane M10.Meanwhile,the quantitative method about the contribution rate of size effect and the steric hindrance/charge synergistic effect was developed.Results showed that the molecular weight and Stokes radius of the dye molecules are exponentially related to the membrane rejection rate(R²=0.99),which is in accordance with the Exp Assoc function distribution law.When the molecular weight of the model molecules is less than 800 Da,the synergistic effect between the steric hindrance and charge repulsion of the membrane played the main role in the rejection performance.At the same time,as the molecular weight of the dyes increased from 269.3 Da to 696.7 Da,the membrane rejection rate increased sharply from 64.7%to 97.9%.Alternatively,when the molecular weight of the model molecule was greater than 800 Da,the size effect contribution rate is greater than 50%,and the dye rejection rate was approximately100%.In addition,the separation performance LNF membrane for of Disperse Red 74and Blue 79,Reactive Blue 4 and Red 11 and actual dye wastewater was also explored.The results show that,the rejection rate of LNF membrane(M10)was>99%for the above-mentioned sing dyes.In a salt/dye mixed system,the membrane rejection rate remained>99%for disperse dyes,whereas the rejection rate was at the range of 66.4-70.1%for reactive dyes.Finally,the integrated technology of Na₂S₂O₈advanced oxidation and LNF membrane filtration was developed to treat the actual dye wastewater.The solid suspension(SS)and chromaticity removal rate was up to100%,and the chemical oxygen demand(COD)removal rate was 95.4%.The main parameters of the obtained water from the intergrated process meet the key technical index of National First-level Disposal Standard.