Polyethersulfone/sulfonated Polysulfone Loose Nanofiltration Membrane Structure Regulation And Dye/salt Selective Separation Mechanism

The development of high performance loose nanofiltration（NF）membrane for the dye and salt selective separation is still a challenge for the scientifica research and industry.The aim of this study is to fabricate polythersulfone（PES）/sulfonated polysulfone（SPSf）blend loose NF membrane by non-solvent induced phase separation（NIPS）method using adipic acid as additive.The separation mechanism including size effect and charge effect was explored.PES and SPSf as raw materials,N,N-dimethylacetamide（DMAc）as a solvent,adipic acid as an additive,and H₂O as coagulation bath,were employed to prepare a PES/SPSf blend loose NF membrane by NIPS method.The influence of adipic acid cocentration（0-13 wt%）on the structure and properties of PES/SPSf blend loose NF membrane was investigated.Results showed that the morphology of blend membrane M11 with the mass ratio of PES/SPSf=84/16 changed from an asymmetry with dense surface layer and firger-like support layer into sponge-like structure as an increase of adipic acid concentration to 11 wt%in the casting solution.Simultaneously,the resultant membrane with the molecular weight cut-off of 7250 Da and an average pore size of 1.81 nm displayed a high permeability and separation performance.The pure water flux was up to 144.4 L h^-1m^-2 under the operating pressure of 0.2 MPa.The rejection for the mixture of Congo red and Na₂SO₄ was 100%and 25%,respectively,indicating a high selective separation for dye and salt.PEGs（Mw 400-10000Da）as neutral models,Methyl Orange（MO）,Acid Blue25（AB25）,Indigo Carmine（IC）,Acid Fuschin（AF）,Congo Red（CR）,Direct Red 23（DR23）and Evans Blue（EB）as charge model dye moleculars were employed to analyze the relationshop between blend membrane（M11）rejection and and the molecular structure parameters of PEG and dyes such as molecular weight,stokes radius and hydration radius.Results showed that the correlation coefficients（R²）between PEG molecular weight and stokes radius and membrane rejection were 0.86and 0.84 respectively.The R² between dye molecular weight and stokes radius and membrane rejection were 0.75 and 0.62 respectively.Besides,the rejection of blend membrane M11 for PEG400（MW=400 Da）and AB25（MW=416 Da）were 19%and92%,respectively.It implied that the rejection of PEG and dyes by the blend membrane is attributed by sieving effect and the synergy of seiving-charge effect,respectively.The intereaction between p H and ionic strength of dye and salt mixture and the surface Zeta potential of blend membrane（M11）was investigated.Results showed that surface Zeta potential of membrane increased with the increase of p H or the decline of ionic strength.Further,the rejection of MO,AB25,IC and AF（Mw<696Da）by the blend membrane increased from 51%、79%、90%and 89%to 79%、92%、99%and 100%,respectively,with the increase of p H value from 3 to 10,whereas the rejection of CR,DR23 and EB（Mw≥696 Da）kept stable.Besides,the rejection of MO,IC and AF（Mw<696 Da）by the blend membrane decrease from 69%,93%,96%to 17%,43%and 85%,respectively,with an increase in the ionic strength of feed solutions from 0 mol L^-1to 0.84 mol L^-1,suggesting the the synergy of seiving-charge effect,whereas the rejection of CR,DR23 and EB（Mw≥696 Da）remained unchanged,indicating the sieving effect.