| Electrodialysis(ED)exhibits wide application potential in the desalination of industrial effluent because of its advantages such as high brine concentration,high desalted water recovery and adjustable desalination rate.In spite of such advantages,membrane fouling,especially the organic fouling of anion exchange membrane(AEM),has become the bottleneck problem which limits the large-scale application of ED in industrial effluent.In this work,the effects of membrane properties,organic molecular structure and organics concentration on fouling of AEM were investigated,and the organic fouling behavior on the AEM and the electrochemical properties changes of AEM were studied,then the organic fouling mechanism of AEM was elucidated.Based on the analysis of organic fouling mechanism,surface modification of AEM was explored in order to improve its antifouling performance.The effects of modifying agent,modified method and technical condition on properties of AEM were studied and the surface modification process was optimized,then the new modified AEM with better antifouling performance was prepared.The main research contents and results are as follows.(1)The effects of organic fouling on properties of AEM and ion transmembrane migration were studied with sodium dodecyl sulfate(SDS)as model foulant.In the low concentration,SDS molecule was distributed evenly in the feed solution,then SDS was adsorbed on the membrane surface or blocked in the membrane channel,resulting in the change of ion transmembrane migration.With 100 mg/L SDS or above in the feed solution,SDS molecule was accumulated to form micelle,then the dense SDS fouling layer was formed and adsorbed on the membrane surface,increasing membrane electrical resistance sharply and almost restricting ion transmembrane migration completely.After membrane cleaning,most of SDS fouling layer could be removed while SDS blocked in the membrane channel couldn’t be removed completely.Results indicated that SDS caused irreversible fouling on the AEM and the dense fouling layer on the AEM was the main factor for deteriorating membrane and restricting ion transmembrane migration.(2)The effects of membrane properties and organic molecular structure on organic fouling of AEM were studied.SDS caused different fouling on the different AEMs.The desalination rate of homogeneous AEM(AMX),semi-homogeneous AEM(AMA)and heterogeneous AEM(HAEM)decreased by 42.3%,15.3%and 9.2%,respectively,because of SDS in the feed solution.It indicated that the denser membrane structure caused severer membrane fouling.The fouling effects of different organics on the AEM were analyzed.Sodium methane sulfonate(MS)almost caused no fouling effect on the AEM,sodium benzene sulfonate(BS)and sodium 6-hydroxynaphthalene-2-sulfonate(NSS)caused slight fouling effect on the AEM,however,SDS and sodium dodecyl benzene sulfonate(SDBS)formed the dense fouling layer on the membrane,causing severe fouling effect on the AEM.(3)The AEM was modified by low temperature plasma and the effects of plasma gas on physicochemical properties and antifouling performance of modified AEM were studied.New polar group were formed on the modified AEM,such as imide group was formed on the N2-treated AEM and ester/carboxyl,carbonic acid groups were formed on the O2-treated AEM.O2 plasma treatment improved surface negative charge density of AEM more but surface hydrophilicity of AEM less than N2 plasma treatment.The antifouling performance of O2-treated AEM was better than that of N2-treated AEM.Results indicated that the increase of membrane surface negative charge density was more beneficial to the improvement of antifouling performance of AEM than the increase of membrane surface hydrophilicity.(4)The AEM was modified by electrodeposition and the effects of modifying agent on physicochemical properties and antifouling performance of modified AEM were investigated.Except poly(vinyl alcohol)(PVA),all the polyelectrolytes such as poly(sodium 4-styrene sulfonate)(PSS),poly(vinylsulfonic acid,sodium salt)(PVS)and poly(sodium acrylate)(PAAS)increased surface hydrophilicity and surface charge negative density of the modified AEM,contributing to the improvement of antifouling performance.The antifouling performance of PSS-modified AEM and PVS-modified AEM was better than that of PAAS-modified AEM.Results indicated that increasing membrane surface negative charge density by surface modification played a key role in improving antifouling performance and the polyelectrolytes with sulfonic group exhibited significant influence on the improvement of antifouling performance of AEM,meaning that the stronger polar group of polyelectrolytes was more beneficial to the improvement of antifouling performance.(5)The electrodeposition and layer-by-layer assembly was combined to modify the AEM and the effect of the deposition of(PSS)/poly(diallyldimethylammonium chloride)(PDADMAC)multilayer polyelectrolytes on the antifouling performance of modified AEM was studied.With the increase of PSS/PDADMAC bilayer number on the AEM,surface hydrophilicity and surface negative charge density of modified AEM also increased,which was more beneficial to the improvement of antifouling performance.As 4.5 PSS/PDADMAC bilayers or above was deposited on the AEM,the formation of SDS fouling layer could be prevented,therefore,SDS adsorbed dispersedly on the modified AEM increased membrane resistance slightly and didn’t restrict the ion transmembrane migration according to the analysis of electrochemical impedance spectroscopy and fitting of equivalent circuit,and the antifouling performance of modified AEM was improved significantly. |
