Fabarication And Performance Of Nanofiltration Membrane With Polyvinyl Alcohol Ultra-thin Selective Layer

Nanofiltration has become one of the key processes in water treatment due to its excellent retention efficiency.However,the high operating pressure and trade-off effect between permeation flux and rejection of nanofiltration membranes have greatly affected the water production rate in actual projects.The thicker selective layer of the nanofiltration membrane will also reduce the membrane flux.Therefore,in this study,the hydrophilic material polyvinyl alcohol(PVA)was selected as the main material of the selective layer of the nanofiltration membrane,which was prepared by coating and gradient crosslinking with the crosslinking agent to reduce the membrane size.Thinner thickness,while obtaining high-flux nanofiltration membranes,it ensures better retention efficiency and improves the performance of PVA membranes.First part of this article,the nanofiltration membranes were prepared by coating and gradient crosslinking methods with different crosslinking conditions.The conditions for the coating crosslinking method as follows:0.15%sodium dodecyl sulfate(SDS),which is used as a hydrophilic pretreatment agent for the surface of the base membrane.0.1%PVA(the alcoholysis degree is 99-99.4 mol%),which is used as membrane material.Add0.5ml/L PVA catalyst,while let the mixed solution stays for 5 minutes,and make crosslinked at 80℃for 10 minutes.The pure water flux of the prepared nanofiltration membrane by the coating crosslinking method is 35.63 L/(m~2·h),while the layer thickness is 410 nm.The difference between the conditions of the gradient crosslinking and the coating crosslinking method is that the residence time of the former consists of two parts,including the crosslinking agent stays for 5 minutes,then the PVA solution is allowed to stay on it for 30 seconds.The pure water flux of the prepared nanofiltration membrane by the gradient crosslinking method is 66.25 L/(m~2·h),and the layer thickness is 249 nm.Since the gradient crosslinking method can prepare nanofiltration membranes with thinner layers and better performance,it is preferred for the modification of PVA composite nanofiltration membranes.The second part includes to change the content of three cross-linking agents,including pyromellitic anhydride(PMDA),4,4’-(Hexafluoroisopropylidene)diphthalic anhydride(6FDA)and octyl maleate-POSS,and the cross-linking temperature.It aims to study the changes in nanofiltration membrane performance.When the PMDA content is0.10%and the crosslinking temperature is 80℃,the pure water flux of the modified membrane is 53.48 L/(m~2·h),and the membrane thickness is 223 nm.When the 6FDA content is 0.15%,the pure water flux is 96.26 L/(m~2·h),the membrane thickness is only210 nm.When the POSS content is 0.05%,the pure water flux is 73.34 L/(m~2·h),and the membrane thickness is 204 nm.The thickness of the selective layer of the three modified nanofiltration membranes is much smaller than that of the nanofiltration membrane prepared by the coating crosslinking method,while the former is about half of the latter.Among them,it is better to use the 6FDA-PVA modified nanofiltration membrane because of its outstanding performance and the thinner selected layer overall,which is preferred for the next step of comprehensive performance exploration.Finally,it is obtained that the 6FDA-PVA ultra-thin selective layer nanofiltration membrane has good permeation flux and good retention effect on typical and emerging organic pollutants.The pollution resistance and mechanical properties of the modified membrane have improved,with a better long-term operation stabilit.The experimental results show that the removal rate of bisphenol A,estriol,ethinyl estradiol,estradiol,and estrone can reach 58%,78%,90%,68%and 55%by 6FDA-PVA modified nanofiltration membrane,which is better than most of the current nanofiltration membranes.The anti-fouling performance is about 4%higher than that of the nanofiltration membrane prepared by the coating and crosslinking method.The fracture stress of the nanofiltration membrane prepared by the gradient crosslinking method is 87%higher than the membrane prepared by the coating crosslinking method,while the elongation is more three times than the nanofiltration membrane prepared by the coating crosslinking method.