Study On Tailoring The Molecular Structure And Desalination Performance For Pervaporation Membranes With Facilitated Transport Mechanism

With the development of society and economy,freshwater scarcity has become a global problem.Desalination is an effective way to solve the problem by removing salt(NaCl)from seawater.With the development of membrane materials and technology,reverse osmosis(RO)has dominated the desalination application(over 80%),but its process is accompanied by a large number of concentrated salt wastewater,which is difficult to be further treated and the discharge will cause the ecological environment damage.In recent years,the most promising membrane distillation(MD)technology,with its high flux and good water quality,has been widely studied.However,the results show that MD membrane cannot effectively continue the desalination process because of the serious pore wetting and membrane fouling phenomenon caused by pollution or salt crystallization.Another membrane technology,pervaporation(PV),which is close to the working principle of MD,has gradually entered people's field of vision.Different from MD using hydrophobic porous materials,PV uses hydrophilicity dense films.However,the flux efficiency of dense film is usually low,which has become the Achilles heel to the successful application of this potential desalination technology.In order to improve the PV flux,the following three aspects are studied in this paper:1.Facilitated mass transport in pervaporation desalination.Previously studies demonstrated that the trade-off between films hydrostability and water transport efficiency can be overcome through the judicious selection of a crosslinking agent,sulfonated crosslinker,which also functioned as a fixed carrier,enhances chemical stability of polymer films and increases water flux.Through molecular dynamic simulations and a series of complementary experiments,here this work reveals that higher crosslinker loading capacity can increase local water concentration close to the sulfonic acid groups,and enhance both long-distance water-sulfonic acid-interactions and water molecule mobility.These underpinned the significant increase in water flux of crosslinked pervaporation membranes during desalination.Results show that the dependence of water diffusivity on water concentration is reduced with higher crosslinker loading.More importantly,water diffusivity at the "dry membrane region" determined the overall water transport behaviour which is also enhanced by higher sulfonic acid group content.These findings can potentially impact on the design of pervaporation membranes for production of potable water from brackish,sea and brine water.2.Optimization of sulfonated crosslinker for PVA films.Molecular dynamics simulations and complementary experiments were deployed to reveal the thermodynamic compatibilities between the monomeric or polymeric crosslinkers with the host polymer(PVA).Results showed that the monomeric crosslinkers preferred to interact with each other,forming carboxylic dimers with strong hydrogen bonds,carboxylic were difficult to contact PVA hydroxyl groups,and difficult to form esterification crosslinking site,lower films crosslinking density.PVA films crosslinked with monomeric compounds demonstrated poor hydrostability in drip-washing tests as weight loss was 5-6 folds higher than the polymeric compound crosslinked PVA films.The mechanical properties(Young's modulus,fracture strength,and onset fracture strain)of polymeric compounds crosslinked PVA films were far more superior than those of monomeric compounds crosslinked films.The film with the best mechanical properties enabled a thin-film composite membrane the highest fracture pressure during operation i.e.integrating of the selective layer at the highest possible transmembrane pressure.3.Preparation and performance of PV desalination composite membranes.PVA dope solution were dip coated on CP VC ultrafiltration membrane to fabricated PVA/CPVC composite membrane after dry and crosslinking.At 70?,the salt rejection of PVA/CPVC composite reached 99.9 ± 0.1%,while water flux was 57.9 ± 1.8 kg m-2 h-1·By spray coating PVA dope solution on to alumina membrane,ultra-thin PVA coating layer with thickness of 72 nm was made.Benefitting from the low transport resistance afforded by these ultrathin films,at 75?,the water fluxes of thin-film composite membranes reached 148.1±7.7 kg-2 mh-1 with a NaCl rejection rate of over 99.6± 0.4%.A defect-free PVA layer of 0.73?m was spray coated on electrospun PAN nanofibrous mats to produce PVA/PAN nanofiber composite.Larger substrate pores were beneficial for lowing gas penetrate resistance,enhancing the water fluxes of these nanocomposites reached to 211.4±11.3 kg m-2 h-1 with NaCl rejection of 99.8 ± 0.2%.In the temperature range of 35-75? and across salt concentrations of 1.5 to 20 wt.%,PVA/PAN nano fiber thin-film composites outperformed all materials deployed in PV and MD with excellent anti-fouling properties.