Enhanced partitioning and transport of phenolic micropollutants within polyamide composite membranes

Aromatic phenols represent an important class of endocrine-disrupting and toxic pollutants, many of which (e.g., bisphenol A and substituted phenols) are known to be insufficiently removed by reverse osmosis (RO) and nanofiltration polyamide membranes that are widely used for water purification. In this study, the mechanism of phenol transport across the polyamide layer of RO membranes is studied using model phenolic compounds hydroquinone (HQ) and its oxidized counterpart benzoquinone (BQ). The study employs filtration experiments and two electrochemical techniques, impedance spectroscopy (EIS) and chronoamperometry (CA), to evaluate the permeability of an RO membrane SWC1 to these solutes in the concentration range 0.1-10 mM. In addition, combination of the permeability data with EIS results allows separately estimating the average diffusivity and partitioning of BQ and HQ. All methods produced permeability of the order 10(-7) to 10(-6) m s(-1) that decreased with solute concentration, even though the permeability obtained from filtration was consistently lower. The decrease of permeability with concentration could be related to the nonlinear convex partitioning isotherm, in agreement with earlier measurements by FTIR. The diffusivity of HQ and BQ was estimated to be of the order 10(-15) m(2) s(-1) and partitioning coefficient of the order 10. The high affinity of phenols toward polyamide and their high uptake may change membrane characteristics at high concentration of the solute. EIS results and hydraulic permeability indeed showed that permeability to ions and water significantly decreases with increasing concentration of organic solute.     

编织管增强型聚乳酸中空纤维膜结构及其性能

为制备兼具高强度、高分离精度的聚乳酸(PLA)中空纤维膜,采用同心圆复合纺丝技术,分别制备了PLA同质编织管与聚对苯二甲酸乙二醇酯(PET)异质编织管增强型PLA中空纤维膜。考察聚乙二醇(PEG)相对分子质量对PLA增强膜结构和性能的影响,并通过物理反冲洗、超声水浴振荡实验研究了同质、异质编织管表面分离层与增强体界面结合性能。结果表明:随PEG相对分子质量的增大,纤维膜的表面孔径逐渐减小,渗透通量先增大后减小,牛血清蛋白截留率先增大后保持稳定,添加PEG相对分子质量为10 000的增强膜的渗透通量最大且分离效果最佳;增强膜与同质编织管的界面结合性能明显优于异质的。     

Charge effects in the fractionation of natural organics using ultrafiltration

Comparison of two commonly used techniques for molecular weight determination of natural organics, ultrafiltration (UF) fractionation and high-performance size exclusion chromatography (SEC), shows that neither technique gives absolute measures of molecular weight. Investigations of International Humic Substances Society standard humic and fulvic acids as well as natural organic matter concentrated from surface freshwaters show that charge effects and solution conditions are important in both SEC and UF fractionation with various components of the natural organics being affected differently. Membranes with a smaller molecular weight cutoff (MWCO) produce permeates with a lower UV/DOC ratio, suggesting that the more aromatic components of natural organics are removed by the lower molecular weight cutoff membranes. Variation in ionic strength has little effect on the rejection of humic acid fractions but does significantly influence the rejection of low molecular weight acids. pH and organic concentration do not affect DOC rejection significantly over the pH range of 4.5-10 and the DOC concentration range of 15-60 mgL(-1). These results indicate that UF should not be applied for quantitative "size" analysis unless performed under well-defined conditions. If performed under conditions appropriate to water treatment, UF fractionation can give information of direct applicability to treatment such as the MWCO required to achieve significant organics removal.     

Tailoring the structure of thin film nanocomposite membranes to achieve seawater RO membrane performance

Herein we report on the formation and characterization of pure polyamide thin film composite (TFC) and zeolite-polyamide thin film nanocomposite (TFN) reverse osmosis (RO) membranes. Four different physical-chemical post-treatment combinations were applied after the interfacial polymerization reaction to change the molecular structure of polyamide and zeolite-polyamide thin films. Both TFC and TFN hand-cast membranes were more permeable, hydrophilic, and rough than a commercial seawater RO membrane. Salt rejection by TFN membranes was consistently below that of hand-cast TFC membranes; however, two TFN membranes exhibited 32 g/L NaCl rejections above 99.4%, which was better than the commercial membrane under the test conditions employed. The nearly defect-free TFN films that produced such high rejections were achieved only with wet curing, regardless of other post-treatments. Polyamide films formed in the presence of zeolite nanoparticles were less cross-linked than similarly cast pure polyamide films. At the very low nanoparticle loadings evaluated, differences between pure polyamide and zeolite-polyamide membrane water and salt permeability correlated weakly with extent of cross-linking of the polyamide film, which suggests that defects and molecular-sieving largely govern transport through zeolite-polyamide thin film nanocomposite membranes.     

基于绿色溶剂的聚酰胺纳米纤维膜制备及其空气过滤性能

为避免在聚酰胺纳米纤维过滤材料制备和使用过程中甲酸等溶剂对人体和环境的潜在危害,采用乙醇(溶剂)和水(非溶剂)通过静电纺丝技术制备了绿色溶剂型聚酰胺纳米纤维膜,分析了纺丝液中乙醇与水的质量比对溶液性质和纤维成形的影响,研究了纳米纤维膜本体结构与空气过滤性能之间的关系。结果表明：在聚酰胺/乙醇溶液体系中加入适量的水能减小纤维直径,但过量的水又会使纤维直径增大,当溶剂中乙醇与水质量比为9：1时,聚酰胺纤维最细,平均直径为332 nm;该聚酰胺纳米纤维膜具有小孔径(0.7μm左右)、高孔隙率(84%)的孔结构,对最易穿透粒径颗粒物PM_0.3具有较好的过滤性能,过滤效率为99.02%,阻力压降为158 Pa,品质因子为0.029 3 Pa^-1。     

Membrane Packaging System to Permit Safe Hydration of Freeze-Dried Contents with Impure Water

We tested the feasibility of using polymer membranes for a self-hydrating packaging system to reconstitute freeze-dried foods using nonpurified water. Several commercial membranes were screened according to (1) hydration rate, (2) water permeability, (3) passage of microorganisms, (4) salt rejection, and (5) strength. The most promising membranes were used for developing and testing prototype packaging systems. A feasible self-contained rehydration system was a nylon-6 polyamide membrane that rehydrated the freeze-dried food within 30 min while passage of microorganisms was prevented. The temperature was 37°C, and the food contained 0.5g/5g of a low molecular weight solute (salt). Such self-contained membrane rehydration systems must be designed to accommodate expected environmental conditions such as temperature, relative humidity, and nature of the product.     

Develop a Novel Method for Removing Fusel Alcohols from Rice Spirits Using Nanofiltration

ABSTRACT: The removal effect on excessive fusel alcohols from rice spirits were investigated using nanofiltration (NF) and ultrafiltration (UF). Compared to UF (GE and GH membranes), NF (DK and DL membranes) showed 10 times greater effect for fusel alcohols rejection due to molecular weight cut-off. On operating pressures, 488.95 kPa was suitable with a rejection rate attaining 44.2% for DK membrane. Only slight changes in physicochemical indices including ethanol concentration, flavor, total acidity, pH value, and soluble solid content were observed for rice-spirits after NF treatment. Moreover, rice spirits treated with the DK membrane achieved a higher score in sensory evaluation. We anticipated a practical application of the nonheat processes in rice spirits production.     

Effect of flux (transmembrane pressure) and membrane properties on fouling and rejection of reverse osmosis and nanofiltration membranes treating perfluorooctane sulfonate containing wastewater

Perfluorooctane sulfonate (PFOS) is an emergent contaminant of substantial environmental concerns. In this study, reverse osmosis (RO) and nanofiltration (NF) membranes were used to remove this toxic and persistent compound from PFOS-containing wastewater. Five RO membranes and three NF membranes were tested at a feed concentration of 10 ppm PFOS over 4 days, and the PFOS rejection and permeate flux performances were systematically investigated. PFOS rejection was well correlated to sodium chloride rejection. The rejection efficiencies for the RO membranes were > 99%, and those for the NF membranes ranged from 90-99%. Improvement in PFOS rejection, together with mild flux reduction (< 16%), was observed at longer filtration time. Such shifts in rejection and flux performance were probably due to the increased PFOS accumulation at longer duration, as shown by X-ray photoelectron spectroscopy and liquid chromatograph and tandem mass spectrometry results. A fraction of PFOS molecules might be entrapped in the polyamide layer of the composite membranes, which hindered the further passage of both water and other PFOS molecules. In a similar fashion, PFOS rejection and fouling were enhanced for greater initial flux and/or applied pressure, where PFOS accumulation was promoted probably due to increased hydrodynamic permeate drag. Flux reduction was also shown to correlate to membrane roughness, with the rougher membranes tend to experience more flux reduction than the smoother ones.     

The role of vehicle interactions on permeation of an active through model membranes and human skin

Previous work from this group has focused on the molecular mechanism of alcohol interaction with model membranes, by conducting thermodynamic and kinetic analyses of alcohol uptake, membrane partitioning and transport studies of a model compound (i.e. methyl paraben) in silicone membranes. In this article, similar membrane transport and partitioning studies were conducted in silicone membranes to further extend the proposed model of alcohol interactions with silicone membranes to include other vehicles more commonly used in dermal formulations, that is, isopropyl myristate (IPM), dimethyl isosorbide (DMI), polyethylene glycol (PEG) 200, PEG 400 and Transcutol P^® (TC). More importantly, membrane partitioning studies were conducted using human SC to evaluate the application of the proposed model of solvent‐enhanced permeation in simple model membranes for the more complex biological tissue. The findings support a model of vehicle interactions with model membranes and skin where high solvent uptake promotes drug partitioning (i.e. K) by enabling the solute to exist within the solvent fraction/solvent‐rich areas inside the membrane or skin in a concentration equivalent to that in the bulk solvent/vehicle. High solvent sorption may also ultimately impact on the membrane diffusional characteristics, and thus the diffusion coefficient of the solute across the membrane. The implications for skin transport are that increased partitioning of a drug into the SC may be achieved by (i) selecting vehicles that are highly taken up by the skin and also (ii) by having a relatively high concentration (i.e. molar fraction) of the drug in the vehicle. It follows that, in cases where significant co‐transport of the solvent into and across the skin may occur, its depletion from the formulation and ultimately from the skin may lead to drug crystallization, thus affecting dermal absorption.     

Hyperbranched polyethyleneimine induced cross-linking of polyamide-imide nanofiltration hollow fiber membranes for effective removal of ciprofloxacin

This study aims to develop a positively charged nanofiltration (NF) hollow fiber membrane for effective removal of ciprofloxacin from water. A novel NF membrane was fabricated by hyperbranched polyethyleneimine (PEI) induced cross-linking on a polyamide-imide hollow fiber support. The spongy-like, fully porous membrane support provides minimal transport resistance and sufficient mechanical strengths for water permeation under high pressures. It is found that the PEI modification significantly influences NF performance through the mechanisms of size exclusion, charge repulsion, and solute-membrane affinity. Specifically, after PEI induced cross-linking, the membrane pore size is significantly reduced. The membrane surface becomes more hydrophilic and positively charged. As a result of these synergic effects, the rejection of ciprofloxacin is substantially enhanced. Furthermore, experimental results show that the molecular weight of PEI has tremendous effect on NF performance of the as-modified membrane. The NF membrane modified by a high molecular weight PEI_60K exhibits the highest rejection, the lowest fouling tendency, and keeps a constant flux over the whole pH range. This study may have great potential for developing high-performance antifouling NF hollow fiber membranes for various industrial applications.     

Performance characterization of nanofiltration membranes based on rigid star amphiphiles

The objective of this study was to develop new nanofiltration (NF) membranes capable of providing significantly greater water permeability and higher rejection of water contaminants compared to state-of-the-art NF membranes. The active layer of the new NF membranes is prepared with rigid star amphiphiles (RSAs) synthesized as part of this study. Performance characterization for a first generation of RSA membranes in a bench-scale apparatus reveals that most of the new membranes provide water permeability of 1.3-3.1 times that of two commercial NF membranes with polyamide active layers while providing comparable rejection of the organic contaminant surrogate Rhodamine WT. However, the rejection of arsenious acid (H3AsO3) by most new NF membranes was found to be lower than that by the two commercial NF membranes tested. Future research efforts of this study will focus on exploring if H3AsO3 rejection could be significantly increased, without negatively affecting water permeability and organic contaminant rejection, by addition of various chemical groups to RSA hydrophobic cores and hydrophilic branches, and by RSA cross-linking.     

非织造布增强聚氯乙烯多孔膜的制备及其微结构调控

为了改善聚氯乙烯（PVC）膜通量低、力学性能差等不足,提出一种PVC多孔膜的制备方法。以PVC为成膜聚合物,γ-丁内酯为溶剂,聚乙二醇、纳米二氧化硅和环氧大豆油为添加剂,以非织造布为增强体,采用溶液相转化法制备PVC多孔膜。通过形貌观察、纯水通量、截留率、孔径及其分布、孔隙率以及力学性能测试,考察了PVC固含量对多孔膜结构与性能的影响。结果表明,所得PVC多孔膜横截面为均质海绵状孔结构,上表面均匀分布着大量微孔结构,随PVC固含量的增加,膜横截面及上表面孔径减小,纯水通量降低,膜表面水接触角增大,亲水性降低,膜纯水通量可达1000L/(m2?h);膜对碳素墨水具有良好的截留性能,截留率可达92.8%;膜的断裂强度均大于20 MPa,且随PVC固含量升高而增大。     

Enhancing the performance of nanofiltration membranes by modifying the active layer with aramide dendrimers

The fully aromatic polyamide active layer of a commercial nanofiltration membrane was modified with three generations (G1, G2, and G3) of aramide dendrimers, all with oligoethylene glycol chains on their peripheries. Permeation experiments revealed that the rejection of Rhodamine WT, used as a surrogate for organic contaminants, improved 1-2 orders of magnitude for membranes modified with G2 and G3 dendrimers at loadings of 0.7-3.5 μg/cm(2) (dendrimer layer thicknesses of ～1-6 nm) compared to the performance of unmodified membranes. In contrast, the corresponding water permeability of dendrimer-modified membranes decreased by only ～30%. Although an enhancement in the rejection of H(3)AsO(3), NaCl, and BaCl(2) was also observed for dendritic membranes, the effect was less pronounced than that for rhodamine WT. Characterization of membranes modified with 3.5 μg/cm(2) dendrimers G2 and G3 by Rutherford backscattering spectrometry with the aid of heavy ion probes (Ag(+) and Ba(2+)) revealed that accessibility of the larger Ba(2+) probe to carboxylate groups on the active layer decreased for the membranes modified with dendrimers.     

RBS characterization of arsenic(III) partitioning from aqueous phase into the active layers of thin-film composite NF/RO membranes

The main objective of this study was to apply Rutherford backscattering spectrometry (RBS) for characterizing the partitioning of arsenic(III) from aqueous phase into the active layer of NF/RO membranes. NF/RO membranes with active layer materials including polyamide (PA), PA-polyvinyl alcohol derivative (PVA), and sulfonated-polyethersulfone (SPES) were investigated. The partition coefficient was found to be constant in the investigated As-(III) concentration range of 0.005-0.02 M at each pH investigated. The partitioning of As(III) when predominantly present as H3AsO3 (pH 3.5-8.0) was not affected by pH. In contrast, the partition coefficient of As(III) at pH 10.5, when it was predominantly present as H2AsO3-, was found to be approximately 33-49% lower than that of H3AsO3. The partition coefficients of H3AsO3 and H2AsO3- for membranes containing PA in their active layers were within the respective ranges of 6.2-8.1 and 3.6-5.4, while the corresponding values (4.8 and 3.0, respectively) for the membrane with SPES active layer were approximately 30% lower than the average values for the PA membranes.     

Ion implantation: effect on flux and rejection properties of NF membranes

Nanofiltration (NF) membranes typically carry a net electric charge, enabling electrostatic interactions to play a pivotal role in the rejection of species such as metals, nitrates, and other charged contaminants. In this study, two types of polymeric NF membranes, polyamide and cellulose acetate, were modified by ion implantation to increase the effective surface charge of the membranes. The modified membranes contain implanted ions in the membrane matrix, inducing a discrete, permanent charge in the active membrane layer. The presence of a permanent charge in the membrane matrix allows for increased electrostatic repulsive forces throughout the entire pH range. Streaming potential measurements were conducted as a function of pH for the modified and unmodified membranes to determine the effect of ion implantation on the zeta potential of the membranes. Rejection experiments were performed in order to quantify the effect of increased electrostatic repulsion on ion rejection, and flux measurements quantified the effect of the modification on permeability. Results indicate that electrostatic interactions near the membrane surface can affect rejection; however, the extent of the effect of increased membrane charge depends on physical-chemical characteristics of the membrane. Increased negative zeta potential of the modified membranes resulted in slightly higher rejection of salts with divalent co-ions from the membrane, with less increase observed with salts of monovalent co-ions. Modified membranes were less permeable than the unmodified membranes. Results of this research hold implications in membrane synthesis and modification studies as well as choice of membranes for water treatment applications.     

Positron annihilation spectroscopic evidence to demonstrate the flux-enhancement mechanism in morphology-controlled thin-film-composite (TFC) membrane

In this study, positron annihilation lifetime spectroscopy (PALS) is applied to explain the flux-enhancement mechanism in thin-film-composite (TFC) membranes prepared by using dimethyl sulfoxide (DMSO) as an additive in the interfacial polymerization. The TFC membranes show a large increase in water flux, up to 5-fold, compared to nonadditive membrane. Atomic force microscopy (AFM) shows that surface roughness and surface area increase when DMSO in the aqueous phase solution phase works to increase miscibility of the aqueous and the organic phase by reducing the solubility difference of two immiscible solutions. X-ray photoelectron spectroscopy (XPS) reveals the variation of the chemical compositions to the extent that there is a considerable increase in the cross-linked amide linkages of the flux-enhanced TFC membranes. The effects of these structural changes on the molecular-size free volume properties are evaluated by PALS studies. The PALS results are the first to experimentally show that the thin films of cross-linked aromatic polyamide RO membranes are composed of two types of pores having radii of about 2.1-2.4 A from tau3, network pore, and 3.5-4.5 A from tau4, aggregate pore. The increase in the size and number of network pores by means of DMSO addition during interfacial polymerization enhances the water flux notably. The size of aggregate pores also increases and may contribute to enhance water flux, although their number inevitably decreases as the number of network pores becomes increased. Details on the correlations between RO performances and o-Ps lifetime parameters are clearly described based on the pore-flow model of reverse osmosis developed by Sourirajan et al.     

Rejection efficiency of water quality parameters by reverse osmosis and nanofiltration membranes

The objective of this study was to evaluate the effectiveness of reserve osmosis (RO) and nanofiltration (NF) membranes, under various solution chemistries, on water quality. The effects of organic carbon, divalent and monovalent cations, bacteria, and permeate drag on the rejection efficiencies of three different membranes were investigated through a series of laboratory bench-scale experiments. Quantitative models were successfully developed to predict the rejection of turbidity, divalent and monovalent cations, ultraviolet absorbance at 253.7 nm (UV254), and dissolved organic carbon (DOC) by membrane filtration. It was found that mechanical sieving (measured as molecular weight cutoff, MWCO) and electrostatic interactions were the most significant parameters since they were found to be important in nearly all models developed. For negatively charged membranes, under high ionic strength solution environments that repress electrostatic interaction between charged compounds and membranes, passage of compounds was mainly a function of size exclusion (i.e. MWCO). Further, of the feedwater parameters tested, bacteria concentration was observed to be the most significant influence on UV254, divalent cation and monovalent cation rejections. The developed models revealed that interactions between feedwater composition and membrane properties impacted the rejection efficiency of membranes as significantly as water composition and membrane properties individually.     

Nanofiltration of glucose solution containing salts: Effects of membrane characteristics,organic component and salts on retention

The understanding on nanofiltration especially the rejection of uncharged solutes, charged solutes and mixture of solutes is crucial for its application in food industry. This is because process streams in food industry usually contain organic components without charge and salts. In this work, DK and CK membranes were utilized for the separation of multi-component feeds containing glucose, NaCl and multivalent salts (MgCl₂ or Na₂SO₄). Glucose rejection is slightly affected by salt concentration which may due to pore swelling of CK membranes at high concentration of NaCl. Meanwhile, rejection of NaCl is reduced by the increment of glucose and NaCl concentration which causes concentration polarization. The addition of multivalent salts even induces negative rejection of NaCl in DK membranes. However, both membranes show high rejection of multivalent salts with or without the presence of glucose. The rejection behavior follows the pattern expected from the structural and electrical properties of the membranes.     

纤维素液晶溶液凝固过程的研究

在捏合机中制备纤维素在磷酸/多聚磷酸复合溶剂中的液晶溶液。利用显微镜观察边界层移动,测定凝固速度,研究凝固剂性质、凝固浴温度、溶液中纤维素含量对凝固速度的影响。实验结果表明:凝固剂的分子体积和结构对凝固速度有重要的影响,对于乙醇、正丙醇、异丙醇这3种一元醇其凝固速度随分子体积的增大而减小;温度与凝固速度之间存在Arrhenius式的关系,凝固速度随凝固浴温度的升高而增大,凝固活化能的排序为乙醇<正丙醇<异丙醇;凝固速度随溶液中纤维素含量的增大而减小。在此基础上分析凝固变量对凝固过程的影响。     

膜分离和聚酰胺吸附对金花茶多酚的纯化

结合中空纤维膜分离与聚酰胺吸附对金花茶中的茶多酚进行纯化研究,考察金花茶提取液分别经过分子量为300,100,10,3 k的中空纤维膜分离后,各透过液和截流液中茶多酚的含量情况.当提取液透过分子量为10 k的中空纤维膜时,茶多酚含量提高到39.36%;透过液再经聚酰胺吸附纯化,茶多酚的纯度可提高到79.42%.