Steric-hindrance Pore Research Articles

A theoretical approach on membrane characterization: the deduction of fine structural details of asymmetric nanofiltration membranes

In this study, the effects of polymer concentration on the performances and fine structural details of asymmetric nanofiltration (NF) membranes were investigated. Based on the well known models/equations on pore flow, solution diffusion and extended Nernst–Planck, the experimental data (electrolyte/ions rejection) has been modeled. Spielger–Kedem equations were used to determine the membranes parameters such as reflection coefficient, solute permeability and steric hindrance effects. Employing steric hindrance pore model (SHP) model and Teorell–Meyer Sievers (TMS) model, important membranes structural details in terms of effective pore radius, effective charge density and ratio of effective membrane thickness to membrane porosity have been measured. From the modeling results, it was found that the polymer concentration can influence the membrane performances by varying of structural details. Through the observation using scanning electron microscopy (SEM), it was shown that the produced membranes exhibited a finger-like structure. According to the results obtained from the modeling, these membranes are in range of the commercially available NF membranes.

Formation and characterization of asymmetric nanofiltration membrane: Effect of shear rate and polymer concentration

In this study, we report the effects of shear rates and polymer concentrations in the formation of asymmetric nanofiltration membrane using a simple dry/wet phase inversion technique. Employing the combination of irreversible thermodynamic model, solution-diffusion model (Spiegler–Kedem equation), steric-hindrance pore (SHP) model and Teorell–Meyers (TMS) model, the transport mechanisms and membrane structural properties were determined and have been characterized for different cases of those formation parameters. The experimental and modeling showed very promising results in terms of membrane performance with interesting structural details. The optimum shear rate (critical shear rate) was found to be at about 203.20 s −1 and the best polymer concentration toward the formation of high performance nanofiltration membrane is in the range of 19.60–23.10%. The modeling results suggested that the pore radius of the membranes produced lies within the range of pore radius of 29 commercial available membranes. This study also proposed that the electrolytes transport through nanofiltration membrane was dominated by a convection factor which accounted approximately 30% more than a diffusion factor. This study also indicated that shear rate and polymer concentration were found to affect the membrane performance and structural properties by providing, to a certain extent, an oriented membrane skin layer which in turn exhibiting an improvement in membrane separation ability.

Study on nanofiltration for purifying fructo-oligosaccharides: II. Extended pore model

Based on the basic pore model and steric hindrance pore model, the extended pore model was founded to predict and optimize the nanofiltration processes for purifying fructo-oligosaccharides. In this model, the factors on which the rejection properties depended were summed up in pressure gradient, steric hindrance and wall friction. Correspondingly, the previous pore models were extended to different combinations for different factors. The relatively suitable model combination was determined through the comparison between the results of calculation and experiment. Then, all transport parameters of FOS were obtained based on the extended pore model. It also showed that the main factors influencing the transport property in nanofiltration membranes might be different considering different sugars.

The characterization of flat composite nanofiltration membranes and their applications in the separation of Cephalexin

We have characterized two flat composite nanofiltration membranes (NADIR ® N30F and NF PES10) by using neutral molecules and electrolytes and investigated their suitability for the separation of Cephalexin. For the first objective, the real rejection was firstly obtained by correcting the observed rejection based on the concentration polarization model. Then both irreversible thermodynamic and steric-hindrance pore (SHP) models were applied in order to estimate membrane structural parameters. It is found that the average pore radius r p are in the range 0.74–1.33 nm and the ratios of membrane porosity to membrane thickness A k/Δ x, are in the range 30,058–35,757 m −1, which are consistent with the experimental results based from the solute transport method. The effective charge density ( ϕX) of membranes, determined from the Teorell–Meyer–Sievers (TMS) model through fitting the NaCl rejection data, varies as a function of electrolyte concentration. These two negatively charged membranes express different rejection performance for single electrolytes in aqueous solutions, which in the order of R(Na 2SO 4) > R(MgSO 4) > R(NaCl) > R(MgCl 2). When investigating the ion rejection performance in the binary salt mixture solutions (Na 2SO 4/NaCl) at different pressures, these two membranes also exhibit a negative rejection to Cl − ion due to its lower valence. Through adjusting the pH of aqueous solutions, the separation of Cephalexin can be effectively manipulated up to 98% and 88% for N30F and NF PES10, respectively. In addition, increasing pressure was propitious to enhance the real rejection of solutes with higher permeate flux. N30F membrane shows higher rejection for Cephalexin due to its smaller pore size and larger charge density than NF PES10 membrane.

The deduction of fine structural details of asymmetric nanofiltration membranes using theoretical models

Asymmetric cellulose acetate nanofiltration (NF) membranes were prepared at different shear rate between 152.0 and 506.67 s −1 to investigate the effect of casting shear rate on the fine structural details of the membrane and membrane performance by using sodium chloride solution. The experimental data is modeled based on the pore flow, solution–diffusion mechanisms and the extended Nernst–Plank equation. The Spiegler–Kedem membrane transport model was used to evaluate the membrane parameters such as reflection coefficient, and solute permeability, P s. The fine structural details of the nanofiltration membrane were evaluated in terms of effective pore radius r p, effective charge density X d, ratio of effective membrane thickness to membrane porosity. The measurement was conducted using steric-hindrance pore (SHP) model. The effective charge density X d, was determined using Teorell–Meyer–Sievers (TMS) model. The modeling results show that, the obtained values were in the range of the commercial available NF membranes.

Observations on solvent flux and solute rejection across solvent resistant nanofiltration membranes

Organic solvent nanofiltration is an emerging technology made possible by the recent development of solvent resistant nanofiltration (SRNF) membranes. These membranes have many potential applications from continuous operation over many months in refinery systems [1,2] to short term operation for a few hours in batch chemical processes [3]. In this paper, solvent flux decline and membrane separation properties are investigated (including their dependence on pressure), using methanol with quaternary alkyl ammonium bromide salts with molecular weights (MW) in the range 322 to 547 Daltons as solutes. The membranes are characterised in terms of an equivalent uniform pore size using three simple pore flow models: Ferry model, Steric Hindrance Pore (SHP) model and Verniory model.

Comparison between rejection characteristics of natural organic matter and inorganic salts in ultra low pressure nanofiltration for drinking water production

Comparisons between rejection characteristics of natural organic matter and inorganic salts by nanofiltration in a very low operational pressure (<0.5 MPa) are demonstrated here. A nanofiltration pilot plant has been operated at the Tamagawa water purification plant, which is located downstream on the Tama River, Japan. The molecular weight distribution and concentration of natural organic matter that account for trihalomethane formation potential, and concentrations of various inorganic salts have been measured in the raw water from the Tama River, microfiltration pretreated effluent and nanofiltration production during 6-months operation of the pilot plant. This paper discusses the water quality and rejection mechanism of natural organic matter and inorganic salts. In addition, the steric-hindrance pore (SHP) model and the Nernst-Planck equation have been used to evaluate membrane pore structures and rejection of ionic solutes, respectively.

A Comparison of Models to Describe the Maximal Retention of Organic Molecules in Nanofiltration

Nanofiltration is used in a growing number of applications for the treatment of drinking water, wastewater, and process water. Trial-and-error is generally used to test the applicability of nanofiltration and to select the membranes. In particular for organic molecules, a model that describes retention as a function of molecular parameters and membrane characteristics has not yet been established. In this paper four models for maximal retention, represented by the reflection coefficient, were compared: the steric hindrance pore model, the model of Zeman and Wales, the log-normal model, and an adapted version of the log-normal model. The calculated results were compared to reflection coefficients determined experimentally for a broad range of relatively small organic molecules. Each of the models yielded acceptable results, although the steric hindrance pore model and the model of Zeman and Wales are based on a somewhat idealized view of membrane structure. The log-normal model calculates reflection coefficients from a distribution of pore sizes. The adapted log-normal model also includes hydrodynamic lag, caused by sterical hindrance in the membrane pores. It was found that this effect is very small. The log-normal model appeared to be most useful to predict reflection coefficients in practical applications.

Analysis of Linear Macromolecule Transport through Aluminum Anodic Oxide Membranes by Pore Model.

An aluminum anodic oxide (AlAO) membrane consisting of highly regulated cylindrical pores of 20-30 nm pore radius was prepared to clarify the applicability of two transport models, the original pore (OP) model and the steric-hindrance pore (SHP) model, to the structural characterization of ultrafiltration (UF) membranes. Filtration and diffusion experiments were performed with the linear macromolecules dextran and polyethyleneglycol. The reflection coefficient (σ) and solute permeability (P) were determined by analyzing rejection properties with the Spiegler-Kedem equation. In addition, the value of P was measured by diffusion experiments. Comparing the observed values with the theoretical predictions makes it clear that membrane structure can be quantitatively evaluated with the OP model by characterizing the size of the macromolecular solute by its radius of gyration (r g ), but not by the Stokes-Einstein radius (r s ).

Rétention aiguë d'urine: présentation clinique et prise en charge dans un service de médecine interne

Three commercial nanofiltration membranes were studied: two negatively charged (NF 40 and NTR 7450) and one positively charged membrane (UTC 20). The membrane pore radii were estimated using filtration experiments with uncharged solutes. For the analysis of the results the Spiegler–Kedem equation and the steric hindrance pore model were used. Salt retention measurements were carried out for NaCl, MgCl2, Na2SO4 and MgSO4 at different concentrations. The experimental results were evaluated with the Donnan exclusion theory. For both NF 40 and UTC 20 the results could not be ascribed to charge effects alone and the retention sequence of the salt solutions was found inversely proportional to the salt diffusion coefficients in water. For the NTR 7450 membrane, which had larger pores, the Donnan exclusion theory could be used and salt retention was mainly determined by charge effects. It was also shown that transport by diffusion could not be neglected.

Influence of ion size and charge in nanofiltration

Three commercial nanofiltration membranes were studied: two negatively charged (NF 40 and NTR 7450) and one positively charged membrane (UTC 20). The membrane pore radii were estimated using filtration experiments with uncharged solutes. For the analysis of the results the Spiegler–Kedem equation and the steric hindrance pore model were used. Salt retention measurements were carried out for NaCl, MgCl2, Na2SO4 and MgSO4 at different concentrations. The experimental results were evaluated with the Donnan exclusion theory. For both NF 40 and UTC 20 the results could not be ascribed to charge effects alone and the retention sequence of the salt solutions was found inversely proportional to the salt diffusion coefficients in water. For the NTR 7450 membrane, which had larger pores, the Donnan exclusion theory could be used and salt retention was mainly determined by charge effects. It was also shown that transport by diffusion could not be neglected.

The electrostatic and steric-hindrance model for the transport of charged solutes through nanofiltration membranes

Nanofiltration (NF) membranes possess the intermediate molecular weight cut-off between reverse osmosis membranes and ultrafiltration membranes, and also have rejection to inorganic salts. So one can assume that NF membranes have charged pore structure. We have developed the electrostatic and steric-hindrance (ES) model from the steric-hindrance pore (SHP) model and the Teorell-Meyer-Sievers (TMS) model (Wang et al., J. Chem. Eng. Japan, 28 (1995) 372) to predict the transport performance of charged solutes through NF membranes based on their charged pore structure. In this article, by doing the permeation experiments of aqueous solutions of neutral solutes and sodium chloride, the structural parameters (the pore radius and the ratio of membrane porosity to membrane thickness) and the charge density of NF membranes (Desal-S, NF-40, NTR7450 and G-20) were estimated on the basis of SHP model and the TMS model, respectively. Then, we selected an aqueous solution of different tracer charged solutes (sodium benzenesulfonate, sodium naphthalenesulfonate and sodium tetraphenyl-borate) and a supporting salt (sodium chloride) to verify the ES model. The prediction based on the ES model was in good agreement with the experimental results.

Transport of organic electrolytes with electrostatic and steric-hindrance effects through nanofiltration membranes.

A new model that considers both the electrostatic and steric-hindrance effects based on the space-charge model and the steric-hindrance pore model has been proposed to evaluate the transport phenomena of an organic electrolyte across a nanofiltration membrane. The dependence of the membrane parameters, reflection coefficient and solute permeability on the electrostatic and steric-hindrance effects has been analyzed in the system of an organic electrolyte and a negatively charged nanofiltration membrane. This model has been verified in the system of a tracer organic electrolyte (sodium benzenesulfonate), a supporting salt (sodium chloride) and commercial nanofiltration membranes (Desal-5, NF-40, NTR7450 and G-20); the theoretical values of the membrane reflection coefficients agreed very well with the experimental results of them.

Evaluation of pore structure and electrical properties of nanofiltration membranes.

Structural parameters and electrical properties of nanofiltration membranes (Desal-5, NF-40, NTR7450 and G-20) were determined with permeation experiments of aqueous solutions of neutral solutes (alcohols and saccharides having different molecular weights) and sodium chloride. The pore radii of these nanofiltration membranes were estimated to range from 0.4 to 0.8 nm based on the steric-hindrance pore model. Neither the constant surface charge density nor the constant surface electrical potential were suitable for interpreting NaCl concentration dependency of reflection coefficients. The fixed charge density of these membranes was evaluated on the basis of the Teorell-Meyer-Sievers model; a simple empirical equation was proposed to represent the dependency of the fixed charge density on NaCl concentration.

Structure and properties of charged ultrafiltration membranes made of sulfonated polysulfone.

Negatively charged ultrafiltration membranes were prepared from Na-form sulfonated polysulfone (SPS), which was made by sulfonation of polysulfone using SO3-triethyl phosphate complex. Two kinds of SPS, having charge densities of 1.60 and 0.776meq/g, were obtained. Casting conditions were optimized by varying polymer concentration, evaporation temperature and casting period. This membrane showed less fouling when ovalbumin was negatively charged at high pH. Tubular membranes were used to get reflection coefficient and solute permeability with PEG #600 to #6, 000. Using these values and pure water permeability, pore radius and Ak/ΔX were calculated on the basis of the steric hindrance pore model. The radius was 18.7Å and 24.0Å for the high and the low charge density membrane, Ml and M2, respectively. Ak/ΔX calculated from water and solute permeabilities coincided well for Ml, but not for M2. This may be due to the hydrophobicity of the latter.