| Water scarcity is one of the most serious global challenges.The efficient strategies to increase water supply are brackish/sea water desalination and wastewater reuse.Reverse osmosis(RO),one of the membrane-based desalination technologies,has displaced the traditional thermal-based desalination as the most developing and popular technique for seawater desalination,with its advantages such as high-quality effluent,low energy consumption,and less footprint.Application of RO in treating high salinity water is constrained by its inherent limitations:an upper salinity level of water that can be treated.Forward osmosis(FO),an emerging osmotic-pressure-driven separation process,is a promising desalination strategy for the treatment of high salinity water,due to its lower energy cost and fouling potential.Thin-film composite(TFC)membranes facilitated the application of RO and FO owing to the high water permeability and salt rejection and became the benchmark to evaluate desalination properties of other novel membranes.However,the widely application of TFC membrane is also hindered by its inherent structure and surface properties,e.g.,ionic exchange behavior and internal concentration polarization(ICP)in FO process and fouling phenomena in RO and FO desalination processes.This study aims to improve the desalination performance of membrane through giving a deep insight into ionic exchange behavior in TFC membranes and developing novel membrane materials to eliminate the ICP and fouling.The influence of ionic species and ionic strength on ion mass transport in TFC FO membranes was systematically studied.Ternary and quaternary electrolyte experiments were conducted to study the effect of ionic species and ionic strength in FO process,respectively.To investigate the role of water transport in cation exchange,a diffusion test was designed with equal osmotic pressure on the two sides of membranes.Our results suggested that the transport flux of monovalent cations with smaller hydrated radius were much larger than those of anions and divalent cations with larger hydrated radius in FO processes.Cations which paired with nitrate could transport through the TFC membrane more readily.Additionally,the cation transport capacity of TFC membranes was mainly determined by the concentration of feed solution rather than that of draw solution.Furthermore,the result of diffusion test indicated that the generation of cation exchange behavior was independent of water transport.Symmetric membrane was fabricated to minimize ICP on account of the occurrence mechanism of ICP in FO process.Polybenzimidazole(PBI)was chosen as membrane materials and sulfonated at various temperature to obtain SPBI.A series of self-standing,and symmetric FO membranes with various thickness were prepared by solution-casting method.Property characterizations such as cross-sectional SEM,tensile test,and water contact angle measurements demonstrated that the fabricated membranes performed symmetric structures with a thickness range of 250-650 nm.Sulfonation was able to improve surface hydrophilicity and mechanical property of the prepared membranes.The near zero structure parameter of membranes with three thicknesses indicated the weak or no ICP effect in FO process for the fabricated SPBI membranes.In addition,the thinnest membrane achieved the highest water permeability and lowest reverse salt flux simultaneously,with an outstanding selectivity of 1882 L mol-1,breaking the permeability-selectivity trade off of membrane materials.We surmised that was associated with a thickness-induced surface segregation effect,which significantly affect the salt permeability via Donnan exclusion,as evidenced by the characterization of surface morphology,surface zeta potential,and the interaction between membrane surface and carboxyl groups modified AFM probe.(PDADMAC/PSS)n polyelectrolyte membranes were prepared through layer-by-layer method to selective removal of scale-forming divalent cations.Properties such as surface morphologies,surface hydrophilicity,and surface charge were determined by the terminated polyelectrolyte.The PDADMAC-terminated membranes showed higher salt rejection and selectivity than the PSS-terminated ones,and the membrane with 5.5 bilayers exhibited 98%rejection of Mg2+with selectivity(Na+/Mg2+)greater than 38.The poly electrolyte membranes exhibited enhanced anti-scaling properties.The rejection order of divalent cations(Ba2+<Sr2+<Ca2+<Mg2+indicated that the rejection mechanism of polyelectrolyte membranes is a combination of both Donnan-and size-exclusion.In feed solutions with high ionic strength,abundant NaCl screened the charge of the polyelectrolytes and led to swelling of the multilayers,resulting in the decrease of membrane selectivity(Na+/X2+)and the increase of membrane water permeability.In real world application,the polyelectrolyte membrane can be used for scaling control through pretreatment of feedwaters with mild-salinity,such as brackish water with<5000 mg L-1 of total dissolved solids.Bioinspired polydopamine chemistry was employed to fabricate graphene oxide(GO)functionalized membranes to mitigate biofouling on membrane surface in desalination process.Raman spectra was used to confirm the successful grafting of GO nanosheets on membrane surfaces.Water contact angle,surface roughness,water flux,and molecular weight cut-off were tested to evaluate the surface properties and water treatment performances of the GO-functionalized membranes.GO nanosheets were exposed on the surface of GO-coated membrane,demonstrated by a directly observation of GO edges on the membrane surface in SEM images and a lower surface charge in zeta potential measurements.Furthermore,the GO-coated membrane exhibited enhanced biofouling resistance as evidenced by lower viable cells in static adsorption experiments,lower water flux decline,and higher flux recovery in three cycle fouling/cleaning dynamic biofouling experiments.Taken together,our findings indicate the significant anti-biofouling properties of GO-modified membranes as a pretreatment strategy in membrane desalination process. |
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