The Preparation Procedure Optimization And Performance Evaluation Of Monolayer Porous Graphene Forward Osmosis Membranes

Thin film composite membranes,which are widely used in forward osmosis(FO)process so far,seldom possess both high permeability and high salt rejection due to the structural and material limitations.The resulted problems including draw solute reverse permeation,water flux decrease and membrane fouling exacerbation limit the further application of FO and become urgent issues.The emerging two-dimensional material monolayer porous graphene(PG)which features a controllable nano/sub-nano scale pore structure may provide a potential way to solve the problems.With an excellent permselectivity,PG has good application prospects as an ultrathin robust filtration membrane.While studies on PG separation performance are mostly theoretical simulations,application researches are scant.Thus,to solve the reverse draw solute permeation problems and investigate the practical PG permselective performance,this thesis focused on the preparation,optimization and performance evaluations of novel composite FO membranes based on PG,by studying the effects of preparation method and operation conditions variation on the structure,properties and FO performance of PG membranes,the optimal preparation method and critical operation parameters were determined.Furthermore,on the basis,oxygen plasma treatment and polylactic acid(PDLLA)doping were used to enhance the hydrophilisity and reinforce mechanical properties of the PG membrane substrate,respectively.By investigating the effects of oxygen plasma treatment duration and PDLLA doping amount on the structure,properties and FO performance of PG composite membranes,the optimal modification conditions were determined.Moreover,based on these results,a hydrophilic thin PG composite membrane was prepared via oxygen plasma treatment,PDLLA doping and membrane thickness reducing modifications,and its FO performance was evaluated.In this dissertation,after study,the PG composite membrane(S-PG)preparation method was determined as below:First,monolayer graphene grown on copper foil by chemical vapor deposition was transferred onto a substrate to prepare graphene membrane(S-G),then S-PG was obtained by treating S-G with oxygen plasma.During preparation,the combination method of graphene and the polymer substrate(heat-press and electrostatic),the separation method of graphene from its copper substrate((NH4)2S2O8 and FeC13 etchant)and oxygen plasma treatment duration(10 s and 20 s,30 s)were the key influencing factors.Results showed that,compared with heat-press method and(NH4)2S2O8 etchant,electrostatic method made graphene adhered tightly onto substrate without any gap and 2.5 wt.%FeC13 etched the copper foil effectively,the prepared S-G membranes via these method and condition had a complete structure,no obvious lattice defects were found.The contact angle,XPS and Raman spectra results showed that oxygen plasma treatment introduced both artificial pores and-OH,-COOH functional groups into graphene structure,making S-PG hydrophilic and permeable.While with oxygen plasma treatment duration increase,the S-PG membrane lattice defects were enlarged and ions rejection were decreased.The FO experiment results showed that the PG composite membrane treated by 10 s oxygen plasma possessed selective pores with a density of 2×1011/cm2 and the best separation performances,its KCl,NaCl and MgC12 rejections were all over 99.99%.The hydrophilic modification results of PG composite membrane by oxygen plasma showed that,with oxygen plasma treatment duration increase,the substrate pore size increased,dense layer disappeared,structure parameter decreased,-COOH and-OH functional groups increased and permeability increased.The PG composite membrane modified by a short time oxygen plasma treatment(5 min)had enhanced FO performances,its water flux increased to 2 times compared with the membrane before modification,while its reverse salt flux remained unchanged.The results of PDLLA doping modification experiments showed that with the increase of PDLLA doping amount,the substrate mechanical strengths were enhanced,porosity increased and permeability increased.While when the PDLLA doping amount was 10 wt.%,the porous structure of the substrate disappeared,leading to its permeability decline.FO experiment results showed that PG composite membrane performed best when PDLLA doping amount was 0.1 wt.%.Based on the comprehensive results above,in this paper,0.2 wt.%PDLLA doping amount was applied to prepare 30 ?m thick substrates,after treated by oxygen plasma treatment for 1 min,a hydrophilic thin PG composite FO membrane(TS*-PG)was prepared using the optimal preparation method and operating conditions,and the FO separation experimental results showed that TS*-PG composite membrane performances were improved significantly.