基于电荷模型的荷电膜传递现象的研究进展

根据电荷模型（空间电荷和固定电荷模型）,将荷电膜传递现象分为膜的分离性能、电性质及传递参数3类.其中膜的分离性能用截留率和通量来表征;膜的电性质包括膜的电化学性质和膜的动电性质,膜的电化学性质用膜电位来表征,膜的动电性质用流动电位、Zeta电位和电滞效应等参数来表征;膜的传递参数则用反射系数、溶质透过系数、纯水透过系数、电导率、离子的迁移数及电渗系数6个系数来表征.概述了空间电荷和固定电荷模型描述电解质溶液在荷电膜中的传递现象的研究进展,介绍了两种模型在荷电膜中应用时的各自优势,展望了电荷模型在荷电膜体系中应用的发展方向.     

Characterization of transport across cellulose acetate membranes in the presence of strong solute–membrane interactions

Certain organic solutes, including phenol, undergo anomalous enrichment when hyperfiltered through cellulose acetate membranes: the solute concentration is higher in the permeate than in the feed solution. A number of existing theoretical approaches describing hyperfiltration phenomena are presented and their merits and limitations upon application to the transport of phenol discussed. A new two-parameter transport relationship is derived based on an extension of the solution–diffusion model. The enrichment, or negative solute rejection by the membrane, is predicted to occur whenever the pressure-induced solute permeation velocity exceeds that of water. By acknowledging and incorporating the effect of pressure on the chemical potential of the solute, the present extended solution–diffusion model relationship successfully describes hyperfiltration data of phenol in homogeneous and asymmetric cellulose acetate membranes provided the contribution of convective flow to the overall solute transport is insignificant. In addition to the transport parameters of the extended solution–diffusion model, the transport parameters of the phenomenological, Kedem–Spiegler, and combined viscous flow–frictional relationship are evaluated from hyperfiltration data obtained with 0.05 and 0.1 wt % phenol feed solutions and homogeneous cellulose acetate membranes of different acetyl content.     

Solute transport in non‐aqueous nanofiltration: effect of membrane material

Nanofiltration experiments in methanol and ethanol were carried out for six reference components with different molecular weights (MW 228–880) and polarities (logP 0–12). The contribution of diffusion to solute transport, calculations based on results from cell diffusion experiments, was found to be only 1–7%; solute transport occurs mainly by convection. Furthermore, it was found that solute transport is influenced by solute–solvent–membrane interactions. Solvent–solute interactions (solvation) cause a different effective solute diameter in each solvent: it is smaller in ethanol than in methanol, resulting in lower rejections in ethanol than in methanol. Solute rejection increases with increasing molecular size (for components with similar polarity). Solute–membrane interactions were expressed in polarity terms and charge effects. A decrease of the rejection with decreasing solute polarity (for components with similar MW) was observed. Since non‐polar components (high logP) are exposed to smaller repulsion forces from the polymeric membrane material, the resistance against solute permeation is lower for these components. The solvent–membrane interactions were found to result in solvation of the pore wall; the degree of membrane solvation is different for each solvent. It is determined by the affinity between the solvent and the membrane, and by the molecular size of the solvent. In ethanol, hydrophilic membranes show a larger drop in solute rejection than hydrophobic membranes. The differences in solvent–membrane affinity (measured by contact angle) are much smaller for the first membranes, and therefore pore wall solvation decreases with increasing solvent size. Hydrophobic membranes have a much larger affinity for ethanol than for methanol, leading to stronger interactions, but undergo competitive forces due to the larger solvent size. Therefore, the difference in degree of solvation and effective pore diameter is less pronounced. Based on these three observed or postulated interactions, rejections of all six reference solutes in methanol and ethanol could be explained. Copyright © 2005 Society of Chemical Industry     

Nanofiltration Separation of Aqueous Polyethyleneglycol – Salt Mixtures

To investigate nanofiltration (NF) separation for recycling polyethylenglycol (PEG) from an ion partition process using an aqueous two-phase system, fractionation performance of five different NF membranes (NF270, SR3, SR100, SR2, and BW30) with solutions of NaNO₃, KClO₄, and PEG 4000 in water comprising various mixtures were studied. PEG rejections and salt passage were analyzed and explained based on size exclusion as well as electrostatic interactions. The highest permeate flux at high rejection of PEG as well as the lowest salt rejections were obtained with SR2 and NF270 membranes. Similar salt rejections were observed for mixed solute solutions and complex mixtures, all following this trend: SR3 > NF270 > SR2. The PEG rejections were well above 95%. This study also revealed that high salt passage of above 90% could be achieved with the same NF membrane only by unstirred conditions through concentration polarization mechanism; however, at the expense of low flux, especially with high PEG concentrations.     

Polymer alloy membrane. I. Cellulose acetate–poly(bromophenylene oxide phosphonate) dense and asymmetric membranes

Cellulose acetate (CA) and poly(bromophenylene oxide, dimethylphosphonate) (PPOBrP), which are compatible polymers, have been cast from solution to give both dense and asymmetric alloy membranes. Membranes containing PPOBrP with different degrees of phosphonylation have been prepared. The water absorption of these membranes increases with the number of phosphonate ester groups, but is kept in the range of 12–16 wt-% water for most of the alloy compositions, which contained 20–80 wt-% PPOBrP. The morphologies of asymmetric membranes obtained from various casting formulations were studied by scanning electron microscopy. Two different structures were identified: (1) the well-known dense skin resting on an open-celled foam, and (2) skin resting on a porous layer which displays a two-phase morphology. In the latter, dense spheres (0.1–1 μm) appear to grow out of a continuous polymer network. The membranes have been tested for hydraulic permeability and separation of water from salt solutions by reverse osmosis. In general, the asymmetric alloy membranes that had been annealed at 90–95°C display salt rejections >90% and water permeation rates of 10–30 gfd. Since the phenyl ring of the PPOBrP component was brominated prior to membrane fabrication, the membranes exhibit exceptional tolerance to chlorinated water (20–80 ppm), as demonstrated in short-time durability tests. The irreversible collapse of these membranes occurs at applied hydraulic pressures far above 1200 psi. A cross linking between the two polymer components in the membranes and some suggestions for further improvement of these membranes are also reported.     

Studies on syntheses and permeabilities of special polymer membranes: 33. Permeation behaviour of aqueous alcohol solution through cellulose membranes

The permeabilities of various aqueous alcohol solutions through cellulose membranes were investigated by changing the preparation conditions of membranes, the feed concentration, and the feed solute, etc. The permeation rates for aqueous solutions of alcohols, glycols, glycerol, trihydroxyl benzene were greater than for pure water. This permeation phenomenon could be explained by some permeation models considering water cluster, activation of water molecules (second bound water-like) attached weakly to the bound water in cellulose membrane, and the surface of cellulose membrane, and plasticization of cellulose molecules.     

Preparation of ultrathin reverse osmosis membranes and the attainment of theoretical salt rejection

Cellulose acetate reverse osmosis membranes, 600–2800 A. thick, have been prepared on glass surfaces by dipping a clean glass plate into a dilute solution of cellulose acetate. After drying, the membranes are floated of onto a water surface and placed on molecular filter supports. Theoretical salt rejections, as calculated from the solution-diffusion model of membrane transport for cellulose acetate, were obtained with imperfection-free membranes.     

Theoretical study on Janus graphene oxide membrane for water transport

Graphene oxide (GO) membranes have received considerable attention owing to their outstanding water-permeation properties; however, the effect of the membrane’s microstructures (such as the distribution of oxidized and pristine regions) on the transport mechanism remains unclear. In this study, we performed molecular simulations to explore the permeation of a water–ethanol mixture using a new type of Janus GO membranes with different orientations of oxidized and pristine surfaces. The results indicate that the oxidized upper surface endows the GO membrane with considerable water-capture capability and the in-built oxidized interlayer promotes the effective vertical diffusion of water molecules. Consequently, using the optimized Janus GO membrane, infinite water selectivity and outstanding water flux (~40.9 kg⋅m⁻²⋅h⁻¹) were achieved. This study contributes to explaining the role of oxidized regions in water permeation via GO membranes and suggests that Janus GO membranes could be used as potential candidates for water–ethanol separation.     

THERMODYNAMIC MODELING OF SOLUTE TRANSPORT THROUGH REVERSE OSMOSIS MEMBRANE

A new thermodynamic model is developed for water and solute transports through reverse osmosis membranes. The model is featured with rigorous derivations in theoretical development and clearly defined parameters for membrane transport properties. The new model can correctly describe not only the dependence of salt rejection on pressure and salt concentration, but also the non-linearity between water flux and pressure. Comparisons of model simulations with the reported reverse osmosis experiments demonstrate that the parameters in the new model are concentration-independent. This study shows that water and salt transports through reverse osmosis membranes can be satisfactorily described with irreversible thermodynamics.     

Preparation and modification of nano-porous polyimide (PI) membranes by UV photo-grafting process: Ultrafiltration and nanofiltration performance

Polyimide (PI) membranes were prepared via non-solvent induced phase separation. The prepared PI membranes were modified by ultraviolet light (UV) and graft polymerization of hydrophilic acrylic and amino monomers in the absence and presence of benzophenone (BP) onto the membrane surface to introduce more hydrophilic and lower fouling membranes. Acrylic acid (AA) and 2-hydroxyethylmethacrylate (HEMA) as acrylic monomers, 1,3-phenylenediamine (mPDA) as amino monomer and BP as photo-initiator were used. The unmodified and modified PI membranes were characterized by degree of grafting (DG) and contact angle measurements. They were also characterized by their ultrafiltration performance with pure water and non-skim milk and nanofiltration performance with 500 ppm NaCl and MgSO₄ single solutions. The DG was increased with increasing monomer concentration, especially at presence of BP. The contact angle measurements indicated that hydrophilicity of PI membrane was improved after UV photografting of hydrophilic monomers onto the membrane surface in all cases. The ultrafiltration results showed that the pure water fluxes and milk water permeation of PI membrane declined after monomer photo-grafting while the protein rejection was extremely increased. The decrease in permeability was remarkable in the presence of BP. The mean pore size of base and modified PI membranes ranged from 8.3 to 0.55 nm when calculated from the solute transport data. Moreover, the irreversible flux loss and flux recovery of PI membrane were modified by UV photo-grafting of hydrophilic monomers. All modified membranes showed considerable NaCl and MgSO₄ rejections. In addition, the membrane modified with mPDA at presence of BP showed highest NaCl and MgSO₄ rejections.     

Polyamide-based membranes consisting of nanocomposite interlayers for high performance nanofiltration

It is still a task to synthesize polyamide-based membranes with selective layers down to 10–20 nm for high performance desalination. Herein, cellulose nanocrystals (CNC) were used as one-dimensional (1D) nanorods and graphene oxides (GO) as two-dimensional (2D) nanosheets, respectively, to construct nanocomposite interlayers for synthesizing polyamide layers thinner than 15 nm. The 2D nanosheets are homogeneously mixed with the 1D nanorods and effectively reduce the surface roughness of the nanocomposite interlayers with decreasing the mass ratio of CNC/GO. Polyamide layers with a thickness of 10–15 nm have been synthesized at an ultralow monomer concentration of 0.025 wt% on these CNC/GO nanocomposite interlayers. The polyamide-based membranes exhibit extremely high water permeation (45.9 L/m²·h·bar) without losing their salt rejection ability. The nanofiltration performances of these polyamide-based membranes are higher than most of the reported nanofiltration membranes in recent years.     

GO/Al2O3复合纳滤膜的制备及其稳定性能研究

以平均孔径为20 nm的Al₂O₃管式超滤膜为载体,经多巴胺改性后,利用压力驱动沉积法成功制备出能在水溶液中长期稳定的GO/Al₂O₃复合纳滤膜,并通过改变负载量实现了对GO层厚的调控。结果表明,随错流时间的延长,不同GO负载量下GO/Al₂O₃复合纳滤膜的纯水渗透系数均呈现先降低后稳定的趋势。且随着GO负载量的增加,稳态纯水渗透系数逐渐降低;当GO负载量增加到90 mg/m²后,GO/Al₂O₃复合纳滤膜对一二价盐的渗透系数与截留率均无显著变化。同时,由于盐测试过程中残余的盐离子在GO片层间产生了交联作用,从而导致随着在纯水中存放时间的延长,不同GO负载量的GO/Al₂O₃复合纳滤膜对一二价盐的截留率均呈上升趋势。GO负载量为140 mg/m²的GO/Al₂O₃复合纳滤膜在水中浸泡680 h后对1 mmol/L Na₂SO₄的截留率可达到91.0%。GO/Al₂O₃复合纳滤膜对四种一二价盐的截留率满足：R(Na₂SO₄) > R(MgSO₄) > R(NaCl) > R(MgCl₂)。     

Artificial particulate fouling of hyperfiltration membranes — II analysis and protection from fouling

An artificial fouling method was used to study the effect of suspended particulates on the performance of standard asymmetric cellulose acetate membranes in flat cells. Using a modified filtration theory developed here, membrane and cake resistances to permeation were individually monitored as a function of applied pressure, membrane curing temperature, and feed type. The membranes behaved according to the solution-diffusion model.A new previously unreported method of membrane protection — the protective cover method — for submicron colloidal solutions, resulted in as high as 78% increased fluxes and 43% decreased salt rejections, over the unprotected membrane. This development is applicable where low salt rejection and high fluxes are desired such as in industrial and municipal wastewater renovation.     

Reverse osmosis performance of organosilica membranes and comparison with the pervaporation and gas permeation properties

Hybrid organosilica membranes were successfully prepared using bis(triethoxysilyl)ethane (BTESE) and applied to reverse osmosis (RO) desalination. The organosilica membrane calcined at 300^°C almost completely rejected salts and neutral solutes with low‐molecular‐weight. Increasing the operating pressure led to an increase in water flux and salt rejection, while the flux and rejection decreased as salt concentration increased. The water permeation mechanism differed from the viscous flow mechanism. Observed activation energies for permeation were larger for membranes with a smaller pore size, and were considerably larger than the activation energy for water viscosity. The organosilica membranes exhibited exceptional hydrothermal stability in temperature cycles up to 90^°C. The applicability of the generalized solution‐diffusion (SD) model to RO and pervaporation (PV) desalination processes were examined, and the quantitative differences in water permeance were accurately predicted by the application of generalized transport equations. © 2012 American Institute of Chemical Engineers AIChE J, 59: 1298–1307, 2013     

Enhanced chlorine-resistant and low biofouling reverse osmosis polyimide-graphene oxide thin film nanocomposite membranes for water desalination

The effect of graphene oxide (GO) loading (0.03, 0.06, 0.09, 0.12, and 0.30 wt%) in the aqueous phase on the performance of reverse osmosis (RO) polyimide (PI) thin film composite (TFC) membrane was investigated. TFC and thin film nanocomposite (TFN) membranes were produced through interfacial polymerization and the imide linkage was confirmed by attenuated total reflection Fourier transform infrared spectroscopy. The spongy-like structure with vertical fingers of RO PI-GO TFN membranes was explored by top-surface and cross-sectional field emission scanning electron microscope (FE-SEM). The roughness of the membranes was determined. All PI-GO TFN membranes exhibited enhanced desalination performance in comparison with PI membranes. Samples with 0.06 wt% GO performed the best with a water flux of 31.80 L/m²/h, salt rejection of 98.8%, and very good antibiofouling properties. This hydrophilic membrane displayed significantly enhanced chlorine-resistance with water flux of 36.3 L/m²/h and salt rejection of 98.5%. This work provides a promising start for designing rapid water permeation PI-GO TFN membranes in water desalination.     

Exclusive and fast water channels in zwitterionic graphene oxide membrane for efficient water–ethanol separation

Graphene oxide (GO) membranes have shown great prospects as the next-generation membranes to tackle many challenging separation issues. However, the employment of GO membranes remains difficult for the precise separation of molecules with strong coupling effect and small size discrepancy such as water–ethanol. Herein, a new strategy of constructing exclusive and fast water channels in GO membrane was proposed to achieve high-performance water–ethanol separation via the synergy between zwitterion-functionalized GO and hydrophilic polyelectrolyte. The as-formed ordered and stable channels possess high-density ionic hydrophilic groups, which benefit from inhibiting the strong coupling between water and ethanol, facilitating the fast permeation of water molecules while suppressing ethanol molecules. As a result, the ultrathin GO-based membrane acquires exceptionally high separation performance with a flux of 3.23 kg/m² h and water–ethanol separation factor of 2,248 when separating water–ethanol (10 wt%/90 wt%) mixture at 343 K. This work paves a feasible way to construct 2D channels for the high-efficiency separation of strong-coupling mixtures.     

Enhanced forward osmosis from chemically modified polybenzimidazole (PBI) nanofiltration hollow fiber membranes with a thin wall

To develop high-flux and high-rejection forward osmosis (FO) membranes for water reuses and seawater desalination, we have fabricated polybenzimidazole (PBI) nanofiltration (NF) hollow fiber membranes with a thin wall and a desired pore size via non-solvent induced phase inversion and chemically cross-linking modification. The cross-linking by p-xylylene dichloride can finely tune the mean pore size and enhance the salt selectivity. High water permeation flux and improved salt selectivity for water reuses were achieved by using the 2-h modified PBI NF membrane which has a narrow pore size distribution. Cross-linking at a longer time produces even a lower salt permeation flux potentially suitable for desalination but at the expense of permeation flux due to tightened pore sizes. It is found that draw solution concentration and membrane orientations are main factors determining the water permeation flux. In addition, effects of membrane morphology and operation conditions on water and salt transport through membrane have been investigated.     

Movements of univalent ions in cellulose acetate membranes

Membrane potentials across the asymmetric membranes of cellulose acetate with various salt rejection properties have been measured for univalent ions. The behavior of ions in the membranes is discussed from the viewpoint of relative ionic mobilities calculated from the membrane potentials. The relationship between ionic mobilities and ionic radii in the membranes having salt rejections lower than 80% is almost the same as that in aqueous solutions. This implies that the ions in these membranes behave as if they exist in bulk water. However, the ionic mobilities in the membranes having salt rejections higher than 86% differ significantly depending on the ionic radii. It seems probable that the bound water influences the ionic mobilities in these membranes.     

Studies on syntheses and permeabilities of special polymer membranes. 27. Concentration of poly(styrene sulphonic acid) in various aqueous solutions using poly(vinylidene fluoride) membranes

The permeation characteristics of poly(vinylidene fluoride) membranes in the separation and concentration of poly(styrene sulphonic acid), from various aqueous solutions were investigated under various conditions. The rejection of the polymer from its aqueous solution was high, because electrostatic repulsions between the charges along polymer chains cause chain extension. When a salt, such as sodium chloride, and sulphuric acid were added to the aqueous solution and the pH was changed, the configuration of the poly(styrene sulphonic acid) molecules changed significantly with the added amounts of salt. The permeation characteristics were influenced markedly by the conformational changes of polymer molecules and the viscosities of permeating liquids. The rejections were dependent on the conformational changes: the permeation rates were mainly governed by the viscosities. Poly(vinylidene fluoride) membranes had much superior resistance to acid, i.e. even when immersed in concentrated sulphuric acid for 7 days, the permeation characteristics did not change at all. The membranes were also effective for the concentration of poly(styrene sulphonic acid) and the removal of sulphuric acid from aqueous mixtures since the concentration of these solutes were optimum.