Preparation Of CEL/CS Blend Nanofiltration Membrane For Dye Desalination

Nanofiltration is a pressure-driven membrane process between reverse osmosis and ultrafiltration.It is often used to remove salt ions and other small molecules in aqueous solutions.Cellulose(CEL)is the most widely distributed,abundant,renewable biomass material in nature.It is applied to prepare nanofiltration membrane successfully due to excellent mechanical stability and membrane-formation property.However,the surface of bare cellulose membrane is negatively charged because of plenty of electron donating group-hydroxyl.And then it has high rejections of Na2SO4 and low molecular weight water-soluble dyes,so it is difficult to achieve an ideal selectivity between dyes and divalent ions.Based on Donnan effect,the positively charged nanofiltration membrane can promote the penetration of the divalent anion salt to a certain extent,and then it can achieve dye desalination.The previous studies are mainly obtained by coating the positive charge separation layer on the supporting layer or grafting the positive charge group on the surface of the membrane.However,the coating method always increases the mass transfer resistance of the membrane and reduces the flux of the membrane to a certain extent.The surface grafting method is often harsh in order to prevent the occurrence of side reactions,so it is difficult to control.Furthermore,the steps of this method are numerous and complicated.Compared to the coating and grafting method,the blending method is simple and easy to operate.Also,it can organically combine the advantages of different materials,so it is widely used in membrane modification researches.Also,as an abundant,renewable biomass material,chitosan(CS)contains a large number of amino groups which show high positive charge.Because the two structures are similar and the compatibility of them is good,it is hopeful to blend the two polymers to prepare the positively charged membrane.Overall,in this study,positively charged CEL/CS blend NF membranes were prepared successfully via phase separation method using cellulose and chitosan as membrane materials,ionic liquid 1-ethyl-3-methyl imidazolium acetate(EMIMAc)as solvent and dimethyl sulfoxide(DMSO)as co-solvent.Based on water flux and dye rejection,the preparation conditions were optimized.Then the effects of the casting solution compositions on the membrane structure and property were investigated through various different characterization methods.The separation performance of membranes in mixed aqueous solutions of different dyes and different inorganic salts were investigated to explore the dye desalination performance of blend membranes.Finally,the effects of pressure,feed temperature,dye concentration and inorganic salt concentration on the separation performance of the membranes were investigated.Specific contents and conclusions were as follows:(1)Preparation of CEL/CS nanofiltration membranes:using water flux and dye rejection as evaluation indicators,the effects of polymer concentration and the ratio of CEL/CS on membrane separation performance were investigated,and the optimal casting solution concentration and polymer ratio were obtained.The results showed that the water flux increased with the increase of the content of CS and DMSO.In the separation of Congo red/salt system,the best EMIMAc:DMSO mass ratio was 30:70.In the separation of Reactive black 5/salt and Reactive blue 19/salt system,the best EMIMAc:DMSO mass ratio was 50:50.and these membranes with optimal dye desalination performance were positive under neutral conditions.(2)The dye desalination performance of the membranes:using Congo red,Reactive black 5 and Reactive blue 19 as dye components,NaCl and Na2SO4 as salt components to simulate the dye/salt mixture solutions,dye desalination experiments were performed using the optimal membranes.The results showed that the rejection of Congo red was higher than 98%,and the rejection of Reactive dyes reached 80-90%,while the retention rates of inorganic salts NaCl and Na2SO4 were below 4%and 1%,respectively.The membrane had good dye desalination performance.(3)Membrane dye desalination mechanism analysis:according to the surface morphology analysis of the scanning electron microscope,the surface structures of the membranes are dense and defect-free.Zeta potential of the membranes show that the surface charge of the membrane under neutral conditions are about 2mV.Therefore,the rejections of the dyes on the membranes mainly depend on the steric effect,and membranes have high rejection for all dyes.For inorganic salts,the molecular sizes are small compared to the dyes,and the retentions of the salts on the membrane mainly depend on Donnan effect.Therefore,the rejections of Na2SO4and NaCl are low.The rejections of dyes and salts of the membrane show great difference,which can achieve dye desalination.In summary,in this paper,under the enlightenment that positively charged nanofiltration membranes increased the permeability of Na2SO4 to achieve dye desalination,non-solvent-induced phase separation method was used to prepare CEL/CS blend nanofiltration membranes.Membranes can separate dyes and salts in wastewater,which provided a new approach to prepare positively charged nanofiltration membranes for dye desalination.