| Nanofiltration membrane separation technology has been widely used in the fields of drinking water purification,sewage treatment,industrial fluids concentration and separation for its advantages of low operating pressure,high separation efficiency and easy to scale up.At present,the polyamide nanofiltration composite membrane prepared by interfacial polymerization has become the dominant commercial nanofiltration membrane products.However,due to the microstructure and physicochemical properties of the polyamide separation layer,the type of polyamide-based composite nanofiltration membrane still has the major problems of relatively poor perm-selectivity and membrane fouling in practical applications,especially in processing printing and dyeing wastewaters with complex compositions.Therefore,development of nanofiltration composite membrane with high perm-selectivity and excellent anti-fouling property is of great scientific significance and practical value.In this study,a novel strategy of constructing asymmetric separation layer consisting of loose body and dense skin layer was proposed to fabricate nanofiltration composite membrane for effective improvement of perm-selectivity.Furthermore,functionalization was performed with the asymmetric separation layer simultaneously for further improvement of perm-selectivity and anti-dye-deposition properties.High-permeable nanofiltration composite membrane with loose polyamide(PA)selectively layer was firstly fabricated through interfacial polymerization via the regulations of monomer compositions and preparation conditions.Asymmetrization and functionalization of loose PA separation layer was then complemented through approaches of surface admidation,secondly interfacial reaction and surface grafting.The microstructure and physic-chemical properties of the obtained membranes with asymmetric separation layer were characterized by Fourier transform infrared spectroscopy(ATR-FTIR),X-ray photoelectron spectroscopy(XPS),scanning electronic microscopy(SEM),atomic force microscopy(AFM),zeta potentiometer and contact angle analyzer.Cross-flow filtration experiments were performed to investigate the influences of asymmetrization and functionalization on membrane's perm-selectivity and anti-dye-deposition properties and the mechanism of improvement.The following conclusions were obtained:(1)Asymmetric separation layer could be constructed through surface admidation of loose polyamide separation layer.ATR-FTIR and XPS analyses confirmed that the cross-linking density of the skin of loose polyamide layer could be enhanced via surface amidation reaction between free carboxylic and amino groups of PA layer with the assistance of 1-ethyl-(3-3-dimethylaminopropyl)carbodiimide hydrochloride(EDC)/N-hydroxysuccinimide(NHS).Zeta potential and contact angle measurements revealed that the electro-negativity and hydrophilicity of membrane surface did not change obviously after surface admidation.Permeation tests revealed that the construction of asymmetric separation layer composing loose body and dense top skin could effectively reduce membrane permeation resistance to water.For the membranes having the same molecular weight cut-off(MWCO)of about 330 g/mol,the pure water permeation coefficient of membrane with symmetric PA separation layer was 10.2l/m2 h bar,while that of the membrane with asymmetric separation layer was 12.2l/m2 h bar,showing an enhancement of 19.6%.The construction of asymemetric separation layer via surface admidation could effectively improve perm-selectivity of polyamide-based composite membrane,but had no effect on the improvement of anti-dye-deposition for no change of physicochemical property of separation layer.(2)Hydrophilic asymmetric separation layer could be constructed via secondly surface interfacial reaction.ATR-FTIR,XPS and contact angle measurements confirmed that surface densification and hydrophilization of loose PA layer could be simultaneously completed through the esterification reaction of tannic acid(TA)and residual acyl chloride groups of nascent polyamide layer.The incorporation of TA via surface interfacial reaction led to significant decreases in water contact angle from 36.5°to 29.7° and surface negative charge from-68.8mV to-47.1 mV under neutral pH.Permeation tests revealed the successful construction of asymmetric separation layer.For the membranes having the same molecular weight cut-off(MWCO)of about 330 g/mol,the pure water permeation coefficient of PA-based membrane was 10.0l/m2 h bar,while that of the PA/TA-based membrane was 12.2l/m2 h bar,showing an enhancement of 30.3%.To aqueous Congo red(CR),Victory blue B(VBB)and methylene blue(MB)solutions,the steady-state fouling resistances from the deposition of dye molecules of PA-based membrane were 1.3×1013,2.1×1013 and 1.6×1013 m-1,respectively,while those of PA/TA-based membrane were 1.0×1013,1.5×1013 and 1.3×1013 m-1,respectively,showing decreasing rates of 23.1%?28.6%and 18.8%,respectively.The results indicate that the construction of hydrophilic asymmetric separation layer could effectively improve membrane's perm-selectivity and anti-dye-deposition property,however,the flux decline ratio of PA/TA-based membrane to cationic dye aqueous solution was still higher than that to anionic dye and water flux of membrane fouled by cationic dye was hard to recover through physical cleaning due to its surface negative charge.(3)Zwitterionic asymmetric separation layer could be constructed via surface grafting.ATR-FTIR and XPS analyses confirmed that surface densification and zwitterionic functionalization of loose PA layer could be complemented through EDC/NHS-assisted grafting of polyethyleneimine(PEI)followed by ring-open reaction using 1,4-BS.Water contact angle decreased significantly from from37.9°to 18.4°and surface charge changed from-60.5 mV to 26.3 mV under neutral pH.For the membranes having the same molecular weight cut-off(MWCO)of about 330 g/mol,the pure water permeation coefficient of PA-based membrane was 9.96 l/m2 h bar,while that of the PA/TA/BS-based membrane was high up to 14.7 l/m2 h bar,showing an increment rate of 30.3%.The water permeability of PA/PEI/BS-based membrane was even 13.1%higher than that of commercial nanofiltration membrane NF 270 with a similar MWCO.To aqueous CR,VBB and MB solutions,the steady-state permeation coefficients were 12.7,12.6 and 13.4 1/m2 h bar,respectively.Water flux decline ratios decreased from 24.3,35.6 and 27.5%of PA-based membrane to 13.3,14.3 and 9.1%of PA/PEI/BS-based membrane,fouling resistances from dye deposition decreased from 1.3×1013,2.3×1013 and 1.5×1013 m-1 of PA-based membrane to 0.4×1013,0.5×1013 and 0.3×1013 m-1 of PA/PEI/BS-based membrane,and the resistances from irreversible deposition of dye molecules declined from 0.5×1013,1.1×1013 and 0.9×1013 m-1 of PA-based membrane to 0.1×1013,0.1×1013 and 0.1×1013 m-1 of PA/PEI/BS-based membrane,respectively.The results indicate that the construction of zwitterionic asymmetric separation layer could effectively improve membrane's water permeability and anti-depositing properties to both cationic and anionic dye aqueous solutions,and thus assure high and stable efficiency in treating printing and dyeing waste waters. |
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