Study On Preparation And Performance Of Acid-Resistant TFC NF Membrane Based On Sol-gel And Surface Crosslinking

Membrane separation technology has been widely used in the industrial field due to its advantages such as low energy consumption.In the treatment of acidic waste liquids,membranes are not only required to possess separation performance,but also possess certain chemical stability.However,the currently widely used polyamide membrane and cellulose acetate membrane have poor acid resistance due to the acid-unstable amide bonds and ester groups,respectively.Additionally,membrane materials such as polysulfone,polyvinylidene fluoride and polypropylene with better chemical stability have poor membrane separation performance due to their hydrophobicity.Therefore,it is of important practical value to research and develop nanofiltration membranes with good separation performance and acid resistance.This study proposed to develop acid-stable nanofiltration composite membrane via material and structure innovation.Polypropylene(PP)microfiltration membrane with good chemical stability was selected as the support,a loose separation layer was firstly deposited on PP support by the in-situ sol-gel process of polyvinyl alcohol(PVA)and tetraethyl orthosilicate(TEOS),the skin of the loose layer was then densified via surface cross-linking by GA to produce PP-(PVA/TEOS)-O-C composite nanofiltration membrane with good separation performance and excellent acid resistance.Analyses such as ATR-FTIR,FE-SEM,contact angle measurement,zeta potential measurement were performed to characterize membrane physic-chemical property.Cross-flow permeation tests were conducted to permeation property.Finally,the acid resistance of PP-(PVA/TEOS)-O-C composite nanofiltration membrane was evaluated through static immersion experiment and long-term dynamic separation experiment using strong acid solution and compared with commercial polyamide-based nanofiltration membrane.The conclusions obtained from the experiments are as follows:(1)A loose separation layer could be successfully deposited on the surface of PP microfiltration membrane through in-situ sol-gel of PVA and TEOS by coating-cross-linking method.Sequential surface cross-linking with GA could generate a dense skin on the loose separation layer.ATR-FTIR spectrum of the composite membrane exhibited the characteristic peaks of Si-O-Si and Si-O-C.The incorporation of-OH groups endowed the composite membrane with good hydrophilicity and negative charge under neutral pH.(2)The separation performance of the PP-(PVA/TEOS)-O-C composite membrane was affected by PVA concentration,TEOS concentration,TEOS hydrolysis time,curing temperature and curing time.With increasing concentration of the reactant,membrane flux decreased and the rejection increased.The TEOS hydrolysis time not only affected membrane separation performance,but also affected membrane structure.The amounts of Si-O-C and Si-O-Si groups of separation layer increased with increasing TEOS hydrolysis time.(3)The optimal conditions for fabrication of PP-(PVA/TEOS)-O-C membrane were as follows:PVA content=1.02 w/v%,TEOS content=3.0 v/v%,TEOS hydrolysis time=40 min,heat treatment temperature=60℃,heat treatment time=15 min.The desired membrane was a loose nanofiltration membrane with a MWCO of 3120 Da and an effective membrane pore size of 1.47 nm.Under neutral pH and 0.5 MPa,the desired membrane showed a pure water flux of 24.9 L/m2·h,a rejection of 70.3%to 500 ppm Na2SO4,and a rejection order of RNa2SO4(70.3%)RMgSO4(45.4%)>RNaCl(18.2%)>RMgCl2(12.1%).(4)After soaking the membrane in an aqueous solution 4.9 w/v%H2SO4 for one week,PP-(PVA/TEOS)-O-C membrane retained its separation performance with no generation of crack defect and any changes in chemical structure and hydrophilicity,while the polyamide(PA)based membrane exhibited significant degradation in separation performance with evident cracking,etching,and an increase of surface water contact angle.Moreover,PP-(PVA/TEOS5)-O-C membrane also retained its water flux and solute rejection during a 120-h continuous permeation test with an aqueous solution of 4.9 w/v%H2SO4 and 50 ppm PEG.