Removal of organics from aqueous solutions by commercial RO and NF membranes of characterized porosities

Removal of organic pollutants of petrochemical and agrochemical origin by some commercial reverse osmosis (RO) and nanofiltration (NF) membranes of characterized porosity was investigated. The rejection of organics was shown to depend on both the membrane properties like pore size, membrane material, membrane charge and solute characteristics such as molecule size, charge and polarity. The rejection of the small nonionized organic molecules by the tight pore membranes is influenced by both the sieving parameters (solute and pore size) and by the physicochemical interactions. The rejection of the same pollutants by the wider pore membranes is dominantly influenced by the physicochemical interactions. The rejection of pesticides is prevalently governed by the sieving mechanism based on the size of the solute molecule and the membrane pore size. However, the physicochemical effects cannot be totally neglected, and they can contribute to the rejection of some pesticides by certain membranes.     

Particle Filtration for Determination of Pore Size Characteristics of Microporous Membranes: Applicability to Plasma Separation Membranes

Abstract

Pore characteristics of microporous membranes were studied by filtration with aqueous solutions containing spherical particles of uniform diameter. The rejection values for four types of plasma separation membranes of microporous structure show good linearity to particle size with high correlation on log-normal probability coordinates. The mean pore size of 94 to 866 Å and standard deviation of 1.51 to 2.13 were obtained for these membranes. Such membranes have mean pore sizes of about one order of magnitude larger than that for synthetic dialysis and hemofiltration membranes in addition to having wider pore distributions. The mean pore sizes obtained by this study relate closely to sieving properties of macromolecules from blood.     

Modelling of membrane nanofiltration—pore size distribution effects

The effects of log-normal pore size distributions on the rejection of uncharged solutes and NaCl at hypothetical nanofiltration membranes have been assessed theoretically. The importance of pore radius-dependent properties such as solvent viscosity and dielectric constant is increased by the introduction of a pore size distribution in calculations. However, the effect of porewise variation in viscosity is less apparent when considered at a defined applied pressure rather than at a defined flux, showing a further advantage of basing theoretical analysis of nanofiltration in terms of applied pressure.Truncated pore size distributions gave better agreement than full distributions with experimental rejection data for a Desal-DK nanofiltration membrane. Such truncation is in agreement with the findings of atomic force microscopy (AFM). Analysis of uncharged solute rejection data alone could not give useful information about membrane pore size distribution. Neither could such a distribution be obtained quantitatively directly from AFM images. However, use of the shape of the distribution obtained by AFM in conjunction with experimental rejection data for an uncharged solute allows calculation of corrected distributions. Importantly, incorporation of such a corrected pore size distribution in calculations of NaCl rejection gave better agreement with experimental data, compared to calculations assuming uniform pores, at high pressure, the region of industrial interest.     

Pore Size Distributions of Polysulfonic UF Membranes and Protein Adsorption

Abstract

To compare actual and effective porous structure in operative conditions of ultrafiltration membranes, flux, retention, and the amount of adsorbed protein have been measured for 0.1% w/w aqueous solutions of several proteins [lysozyme, pepsin, bovine serum albumin (BSA), lipase, and γ-globulin] with molecular weights from 14.6 to 150 kD tangentially filtered through two asymmetric polysulfone membranes, E-100 and E-500. From retention and flux experiments, the dependency of mass transfer coefficients on molecular volume has been analyzed. Results imply that protein molecules behave as being slightly uncoiled, especially when filtered through the smallest pore size membrane. By using a simple sieving model, retention data allow pore size distributions to be obtained. The data are modified by taking into account adsorption and volume hindrance effects in operational conditions.     

Polyamide‐imide nanofiltration hollow fiber membranes with elongation‐induced nano‐pore evolution

The molecular design of nanoporous membranes with desired morphology and selectivity has attracted significant interest over the past few decades. A major problem in their applications is the trade‐off between sieving property and permeability. Here, we report the discovery of elongation‐induced nano‐pore evolution during the external stretching of a novel polyamide‐imide nanofiltration hollow fiber membrane in a dry‐jet wet‐spinning process that simultaneously leads to a decreased pore size but increased pure water permeability. The molecular weight cutoff, pore size, and pore size distribution were finely tuned using this approach. AFM and polarized FTIR verified the nano‐pore morphological evolution and an enhanced molecular orientation in the surface skin layer. The resultant nanofiltration membranes exhibit highly effective fractionation of the monovalent and divalent ions of NaCl/Na₂SO₄ binary salt solutions. More than 99.5% glutathione can be rejected by the nanofiltration membranes at neutral pH, offering the feasibility of recovering this tripeptide. © 2009 American Institute of Chemical Engineers AIChE J, 2010     

Using ultrafiltration to analyze the molar mass distribution of kraft lignin at ph 13

We used deliberately calibrated ultrafiltration membranes to analyze the molar mass distribution (MMD) of kraft lignin in alkaline solution. The membranes were first tested with probe macromolecules to obtain sieving curves at the same conditions as the lignin analysis. Using a restricted transport model, we calculated the average effective membrane pore sizes. The results were different from the nominal cut-off values. The MMD of a lignin sample at pH 13 was analyzed by separating the lignin solution into five fractions; the MMD using the calibrated cut-off values was confirmed by gel permeation chromatography.     

Studies on hydrophilic polysulfone ultrafiltration membranes

Ultrafiltration membranes were produced from carboxylated polysulfone polymers having different degrees of substitution (DS). Solute separation was found to increase with increasing DS. The techniques of single solute permeation and permoporometry were used to determine the pore size of these membranes. Permoporometry measurements indicated that pore size decreased with increasing DS. Solute separation was predicted by inserting the distribution of pore sizes obtained from permoporometry into a restricted transport model. Similar results were obtained from a nonlinear least squares fit of the restricted transport model to the experimental data. Both techniques produced similar sieving curves. It was concluded that pore size decreases were not due to the increasing repulsion of solute molecules by hydrophilic polysulfone but to changes in polymer solution properties as the DS increased.     

膜孔形态结构图象分析研究——Ⅱ.膜孔结构定量表征

本文通过对多种微孔滤膜和超滤膜进行电镜照像及计算机图象分析,定义和得出了下列定量表征膜孔形态结构的参数:几何孔径分布、有效孔径分布、孔形不圆度、孔隙率以及孔形的分形(Fractal)维数。这些参数有利于成膜条件—膜孔结构—膜分离性能三者之间关系的研究,有利于膜成孔机理的研究,从而促进研制性能更好的膜。     

Effects of Membrane-Making Conditions and Shrinkage Treatment on Morphology and Performance of Cellulose Acetate Butyrate Membranes

Abstract

The functions of additives in cellulose acetate butyrate (CAB) membrane casting solution, effect of thermal shrinkage treatment on porous CAB membranes, and the changes of CAB membrane surface morphology during the solvent evaporation step have been investigated. Additives (glycerol and lactic acid) in CAB membrane casting solution function only as pore number promoting agents when used at low concentration and function both as pore number and pore size promoting agents when used at higher concentrations. Triethyl phosphate in CAB membrane casting solution functions both as a pore number promoting agent and as a secondary solvent for CAB. Three distinct phases can be observed in the solvent evaporation step in making CAB membranes. With the increase in solvent evaporation time, the number of pores in the first pore size distribution increases in the initial small pore-forming phase and decreases in the large pore-forming phase, and the number of pores in the second pore size distribution always increases with solvent evaporation time. These changes in pore numbers, pore sizes, and pore number ratio in two pore size distributions as well as the membrane skin layer thickness together govern the ultimate membrane performance and result in a maximum solute separation which, in the case of CAB/ acetone membranes, falls at 60 seconds of solvent evaporation time. Significant improvement of the performance of a porous CAB membrane can be achieved by thermal shrinkage treatment. Equally high CAB membrane performance can also be achieved by using a lower concentration of additives in the membrane casting solution.     

Performance Evaluation of Alkaline Treated Poly(vinylidene fluoride) Membranes

A poly(vinylidene fluoride) (PVDF) hollow fiber membrane surface was modified by alkaline treatment in this study. This subject was selected with the aim to confirm the mechanisms of alkaline degradation of PVDF membranes, characterize the variations of membrane surface morphology (e.g., average pore size, pore size distribution, porosity, etc.), and estimate the membrane fouling potential by a bench-scale test with synthetic surface water. The conditions of the alkaline treatment covered various concentrations, temperature, and processing time. The results of this study indicate that the hydrophilic PVDF membranes can be obtained after appropriate treatment without loss of integrity of the membrane surface. All factors, including the concentration of NaOH, temperature, and processing time affect membrane properties. The surface images and air flow rate of unmodified and modified membranes showed difference in their average pore size and pore size distribution. In general, the increase of the processing time decreases the average pore size at constant concentration and temperature; the increase of the NaOH concentration in solution and temperature fastens the degradation process. Membrane pure water flux decreased after alkaline treatment. This can be attributed to the decrease of pore size. However, the membrane anti-fouling potential increased after alkaline treatment due to the enhancement of hydrophilic property of membrane surface.     

Surface Fractal Dimension of Perovskite-Doped Alumina Membrane: Influence of Calcining Temperature

The surface fractal dimension, D , and pore size distribution of perovskite-doped alumina membrane prepared via the sol–gel method were determined from their nitrogen adsorption isotherms. The D value was calculated using the Frenkel–Halsey–Hill method. The D value increased with increasing temperature due to membrane shrinkage. The pore size distribution pattern of perovskite-doped alumina membrane showed a narrow pore size when the temperature was increased from 400° to 800°C. The isotherm type was characterized as Type IV. Smoothing and sintering effects contributed to the decreasing trend of surface fractal dimension at a high temperature. The surface fractal dimension increased when the perovskite ratio in alumina membrane was increased.     

Pore‐size evaluation and gas transport behaviors of microporous membranes: An experimental and theoretical study

A modified gas‐translation (GT) model based on a GT mechanism was successfully applied to the pore‐size evaluation and gas transport behavior analysis of microporous membranes with different pore‐size distributions. Based on the gas permeation results of three microporous membranes derived from different alkoxides, the effects of activation energy and the selection of a standard gas on the pore‐size evaluation were discussed in a comparative study. The presence of nano‐sized defects had an important influence on the gas permeation performance of microporous membranes, depending largely on the original pore size of the membrane in question. Moreover, the gas‐separation effect of the pore‐size distribution in a silica membrane was theoretically studied and revealed a significant increase in gas permeance for relatively large gas species but not for small ones. © 2015 American Institute of Chemical Engineers AIChE J, 61: 2268–2279, 2015     

Effect of polymer precursors on carbon molecular sieve structure and separation performance properties

Polymer precursor and processing method have a significant effect on the separation performance of carbon molecular sieve (CMS) membranes. The authors previously developed a polymer processing method involving oxygen exposure during pyrolysis using synthesized polyimide, 6FDA/BPDA-DAM. The objectives of this work were (i) to demonstrate the generality of the oxygen doping method with a commercially available polymer Matrimid®, (ii) to investigate resultant CMS membrane structures, and (iii) to engineer the CMS performance observed with Matrimid® precursor by tuning the pyrolysis temperature. The investigation of the pore structures is challenging due to their amorphous structures. Various researchers investigated using traditional characterization methods, such as XRD and adsorption, yet molecular sieving structure in ultramicropore region is still not known. Here, the authors investigated using gas molecules as a probe. By interpolating the characterization results, hypothetical ultramicropore size distributions for each CMS membranes are suggested. The results are used to explain dramatically different separation performance trends observed between 6FDA/BPDA-DAM and Matrimid® CMS membranes and to adapt the doping method on Matrimid® CMS membrane for better performance.     

Capillary Pressure Estimation Using Statistical Pore Size Functions

Capillary pressure curves, which have been employed for a long period of time by researchers interested in pore size distribution, are commonly obtained from experimental measurements. The dynamic capillary pressure that influences the flow is affected by many factors including the pore size characteristics and pore scale dynamics. Hence, it is important to investigate the variation of the estimated pore size distribution with capillary number. In this study, a glass type micromodel is considered as the porous media sample. A parametric probability density function is proposed to express the pore size distribution of the porous model, which is also measured using an image analysis technique. The capillary pressure saturation mathematical model is developed by integrating the pore size distribution function. Model parameters with a physical significance are estimated by fitting the model to the measured capillary pressure data at different capillary numbers. The results of capillary pressure obtained are well matched to the measured values. The results show that the trends of the extracted pore size distribution curves have similar trends, but they are not exactly the same. Therefore, the dynamic capillary pressure data alone are not sufficient for estimation of the pore size distribution. As a related development, the prediction of the capillary pressure curves based on measured pore size distributions is also presented. The proposed probability distribution function has the flexibility of representing a wide variety of pore size distributions.     

基于分形理论的油页岩有效扩散系数研究

以直径为1.6—2 mm柳树河油页岩为原料,采用扫描电镜法与氮吸附法得出油页岩吸附-脱附等温线,孔径分布曲线等;分析了油页岩内部孔隙结构,计算出孔道弯曲分形维数与孔隙面积分形维数,并研究油页岩内部的有效扩散系数。结果表明:油页岩吸附-脱附曲线属于Ⅲ型等温线,是一种典型的二端都开放的管状毛细孔型结构的多孔物质;其含有较发达孔隙,孔径分布为0.4—40 nm;油页岩孔道弯曲分形维数越小,孔道越平坦,孔隙面积分形维数为2.464;油页岩有效扩散系数与其内部结构和操作温度有关,温度越高,粒径越小,有效扩散系数越大。     

RO and NF membrane fouling and cleaning and pore size distribution variations

The variations of porosity parameters of some reverse osmosis (RO) and nanofiltration (NF) polyamide thin-film composite membranes were determined in order to explain the changes of membranes' performances caused by membrane fouling and chemical cleaning of the fouled membranes. The pore size distribution curves and the effective number of pores in the membrane surface indicated plugging of the tight network pores in the membrane surface and even their disappearance during fouling. The enlargement of the wider aggregate pores was responsible for the noticed reduction in salt rejection. The initial pore structure of the fouled RO membrane was restored by immediate chemical cleaning. A delay of chemical cleaning of the fouled membranes led to irreversible changes in the porous structure of both the RO and NF membranes, which were caused by a microbial activity.     

Fabrication and characterization of integrally skinned‐oriented highly selective charged asymmetric low pressure poly(ether sulfone) membranes for nanofiltration

BACKGROUND: The effects of shear rate induced molecular orientation in polymeric based asymmetric membranes on performance, structural details, key properties, morphologies and pore size distribution were studied. Asymmetric membranes fabricated at five different potential shear rates ranging from 93.33–466.67 s^?1 were evaluated based on nanofiltration test. The use of Spiegler–Kedem, steric‐hindrance pore and Teorell–Meyers–Sievers models enables an assessment to be made of the relationship between shear rates and membranes properties. RESULTS: Experimental and modeling results show that there is a significant correlation between shear and membrane characteristics, whereby as shear rate increases, the separation performances of nanofiltration membranes increase until an optimum (critical) level of shear is achieved. Beyond the optimum shear, the deterioration in membrane performance suggests that there exists an optimum shear rate which produces optimal structural details, key properties, morphologies and pore size distributions. CONCLUSIONS: Results showed that a skinned‐oriented highly selective charged asymmetric low pressure nanofiltration (ALP‐NF) membrane was successfully developed. The optimum shear rate (critical shear) was found to be 233.33 s^?1. At the optimum shear rate, the fabricated ALP‐NF membranes produced the finest properties, morphology and narrowest pore distributions. The positive improvement in performance properties of ALP‐NF membrane provides the potential for producing a highly selective NF membrane for different applications in the future. Copyright © 2011 Society of Chemical Industry     

REVERSE OSMOSIS SEPARATION OF GLUCOSE-ETHANOL-WATER SYSTEM BY CELLULOSE ACETATE MEMBRANES

The separation of glucose-ethanol mixed solutes from aqueous solutions was attempted by cellulose acetate membranes of different average pore sizes and pore size distributions at the operating pressure of 6895 kPag (=1000 psig) and at the feed glucose concentration ranging from 2000 to 75000 ppm while maintaining ethanol concentrations at 1/5 to 1/2 of the glucose concentration. Using chosen reference solutes the pore size distribution on the membrane surface was characterized by two normal distributions. It was found that ethanol is attracted to the cellulose acetate membrane material more strongly at the higher glucose concentration in the feed solution, and consequently the separation of ethanol solute is brought down at higher glucose concentrations. These results are attributed to lowering of solubility of ethanol in the solvent when a greater amount of glucose is in the solvent.     

Water transport control in carbon nanotube arrays

Based on a recent scaling law of the water mobility under nanoconfined conditions, we envision novel strategies for precise modulation of water diffusion within membranes made of carbon nanotube arrays (CNAs). In a first approach, the water diffusion coefficient D may be tuned by finely controlling the size distribution of the pore size. In the second approach, D can be varied at will by means of externally induced electrostatic fields. Starting from the latter strategy, switchable molecular sieves are proposed, where membranes are properly designed with sieving and permeation features that can be dynamically activated/deactivated. Areas where a precise control of water transport properties is beneficial range from energy and environmental engineering up to nanomedicine.