Preparation And Characterization Of Silica In-situ Crosslinking Solvent-resistant Composite Nanofiltration Membrane

Solvent resistant nanofiltration?SRNF?is a promising technology for the separation and purification of organic solvents,including solvent recovery from lube oil filtrates,deacidification of vegetable oil and recycling of homogeneous catalysts in the pharmaceutical industry.The core of SRNF is the the separation stability of nanofiltration membrane used.Compared with organic nanofiltration membranes,inorganic nanofiltration membranes possess excellent solvent resistance.Therefore,it is of great importance to prepare solvent resistant nanofiltration membranes with good separation performance,heat resistance and mechanical strength by forming organic-inorganic cross-linking networks.In this study,hydroxyethyl cellulose?HEC?and tetraethyl orthosilicate?TEOS?hydrolyzate were selected as the functional coating material and cross-linking agent,respectively,to prepare HEC/SiO₂ nanofiltration membrane by the method of dip-coating and in-situ condensation and cross-linking on porous polypropylene support.Membrane properties including morphology,surface chemical composition,hydrophilicity and charge were characterized by Fourier transform infrared spectroscopy?FTIR,ATR-FTIR?,scanning electron microscopy?SEM?,atomic force microscopy?AFM?,zeta potential measurement,ultraviolet-visible spectrophotometer?UV-vis?and surface contact angle measurement.Molecular weight cut-off,inorganic salt and dye rejection performance of HEC/SiO₂ nanofiltration membranes were evaluated through cross-flow separation measurement.The solvent resistance and separation stability of the membranes were evaluated by both static immersion and dynamic running tests.The results obtained from the experiments are as follows:?1?Ultra-thin separation layer could be formed on the surface of PP through the in-situ condensation and cross-linking of HEC and hydrolyzed TEOS.There was good adhesion between the active layer and PP film,and the surface of the nanofiltration membrane was smooth and compact.And the surface of the membrane exhibited negative charge under the pH higher than 4.The permeation property of the HEC/SiO₂ nanofiltration membrane could be tuned through varying HEC content,TEOS content and annealing temperature.The membrane exhibited a trade-off between its pure water flux and Na₂SO₄ rejection.The the ratio of Si-O-C to Si-O-Si of the separation layer of the HEC/SiO₂ membrane could be adjusted through the control of TEOS hydrolysis time.The molecular weight cut off and the Stokes radius of HEC/SiO₂ nanofiltration membrane could effectively adjust by changing the TEOS content.?2?HEC/SiO₂ nanofiltration membrane prepared under the optimized conditions possesses a molecular weight cut-off of 1900 Da,corresponding to an average pore radius of 1.11 nm.Under the pressure of 0.5 MPa,the pure water flux of the membrane was 45.7 L/?m²·h?,the rejections to 500 ppm electrolytes followed the order of R_Na2SO4?69.7%?>R_MgSO4?67.5%?>R_NaCl?13.3%?>R_MgCl2?10.3%?>R_CaCl2?7.7%?;the removals to 50 mg/L dye aqueous solutions followed the order of RMethylblue?99.9%?=RCongored?99.9%?>RSunsetyellow?99.2%?>RVictoriablue _B?98.9%?>R_{Neutral red}?70.1%?.The inorganic salts and dyes were rejected by the membrane via both size exclusion effect and Donnan electrostatic repulsion.The HEC/SiO₂ nanofiltration membrane maintained its separation performance after being tested with 1.0mol/L HCl or 0.5mol/L NaOH aqueous solution for 48h,showing good resistance to acid and base.?4?HEC/SiO₂ nanofiltration membrane possessed good stability to organic by forming organic-inorganic networks.The HEC/SiO₂ nanofiltration membrane was slightly swelling in static immersion test with organic solvents such as carbon tetrachloride,ethyl acetate,absolute ethanol,N,N-dimethylformamide,dimethyl sulfoxide,acetonitrile and tetrahydrofuran,showing slightly decline of pure water flux and increase of rejection to Na₂SO₄.The swelling effect of tetrahydrofuran on the membrane was most obvious.Continuous filtration of DMF and 50mg/L polystyrene n-hexane solution showed that HEC/SiO₂ nanofiltration membranes were stable in long-term filtration of organic solution.During the 12-h filtration,the DMF permeability and polystyrene?Mn=2200g/mol?/n-hexane rejection maitanied at 9.8L/?m²·h·bar?and 94.8%,respectively.In addition,affected by the physical properties of the solvents and swelling degree of membranes,the solvent flux of HEC/SiO₂ nanofiltration membrane followed the order of pure water>absolute ethanol>isopropyl alcohol>n-butanol.When tested with dye ethanol solutions,the removals of methyl blue and Congo red of HEC/SiO₂ nanofiltration membrane were better than Victoria Blue B.