Tangential Streaming Potential Research Articles

Tangential streaming potential, transmembrane flux, and chemical cleaning of ultrafiltration membranes

Transmembrane flux measurements are the only practical tools used to evaluate the degree of organic fouling and the efficiency of chemical cleaning of ultrafiltration membranes in situ. Tangential pH-streaming potential profiles may become a comprehensive in situ method to analyse cleaning efficiency versus potential membrane damage. A parallel implementation of the two methods was used to assist in tuning an efficient cleaning protocol for 300 kDa polyethersulfone membranes. The membranes were fouled with a mixture of organics and cleaned with nitric acid, acetic acid, caustic soda or liquid bleach, each at concentrations of 1, 5, or 10 mg/L. A modified Kolmogorov-Smirnov test for divergence in datasets clearly indicated cleaning with 5 mg/L NaOH or NaOCl. These findings were confirmed by atomic force microscopy surface contouring and infrared spectra recording.Tangential pH-streaming potential profiling is easy in terms of operation and maintenance, inexpensive, and may be conducted in situ. Implementation of two independent tests is instrumental in the validation of the cleaning agent efficiency, optimisation of the cleaning dose and pH, and assessment of membrane fouling potential by complex organic mixtures. A combination of transmembrane flux and tangential streaming potential tests may reduce the cost of chemical cleaning and suspend membrane ageing.

Prediction of single salt rejection by NF membranes: An experimental methodology to assess physical parameters from membrane and streaming potentials

Modeling of mass transport through nanofiltration membranes has seen an increasing expansion over the last decades in view of proposing predictive tools. Many models were devoted to the prediction of separation by NF and the model proposed here is classically based on the extended Nernst–Planck equation and equilibrium partitioning at the interfaces. The key of such models lies in the estimation of the adjustable parameters. Hence, the feasibility of fully predictive models requires an accurate methodology to identify easily and quickly the input parameters. Among them, the two parameters governing electric and dielectric exclusions have aroused much scientific interest because of their difficult assessment. In this paper, a methodology based on membrane potential (MP) and tangential streaming potential (TSP) measurements is proposed to estimate their values. MP is used to identify the dielectric constant of ternary mixtures which are extrapolated to determine those of salt solutions. In this study, the membrane charge is supposed to vary along the pore length because of the concentration variation. The TSP method is thus implemented to correlate membrane charge and salt concentration via adsorption isotherms, which are introduced in the model to calculate the charge along the pore length. Finally, the ability of the Pore Transport Model (PTM) to predict salt rejections is investigated for two membranes and a satisfactory prediction is obtained. The methodology appears to be convenient for a preliminary estimation of separation performances.

Novel in situ normal streaming potential device for characterizing electrostatic properties of confluent cells.

The characteristics of transport across confluent cell monolayers may often be attributed to its electrostatic properties. While tangential streaming potential is often used to quantify these electrostatic properties, this method is not effective for transport normal to the apical cell surface where the charge properties along the basolateral sides may be important (i.e., confluent cells with leaky tight junctions). In addition, even when cells have a uniform charge distribution, the shear stress generated by the conventional tangential flow device may dislodge cells from their confluent state. Here we introduce a novel streaming potential measurement device to characterize the normal electrostatic properties of confluent cells. The streaming potential device encompasses a 24 mm cell-seeded Transwell(®) with two AgCl electrodes on either side of the cell-seeded Transwell. Phosphate buffered saline is pressurized transversal to the Transwell and the resultant pressure gradient induces a potential difference. Confluent monolayers of HEK and EA926 cells are used as examples. The corresponding zeta potential of the cell-membrane configuration is calculated using the Helmholtz-Smoluchowski equation and the zeta potential of the confluent cell layer is deconvolved from the overall measurements. For these test models, the zeta potential is consistent with that determined using a commercial dispersed-cell device. This novel streaming potential device provides a simple, easy, and cost-effective methodology to determine the normal zeta potential of confluent cells cultured on Transwell systems while keeping the cells intact. Furthermore, its versatility allows periodic measurements of properties of the same cell culture during transient studies.

Electrical characterization of NF membranes. A modified model with charge variation along the pores

Nanofiltration rejection measurements have been performed for KCl solutions to characterize a non-commercial NF membrane. Membrane Potential (MP) and Tangential Streaming Potential (TSP) measurements have also been carried out for a wide range of concentrations. The charge of the membrane and the pore dielectric constant has been obtained as functions of the salt concentration from these three ensembles of experiments, showing a fair agreement. For the data treatment, two modifications of the Steric Electric and Dielectric Exclusion (SEDE) model have been proposed to allow the electrical characterization of the NF membrane: (1) the volumetric charge density is considered to change along the pores by means of the inclusion of a charge isotherm (which correlates the membrane charge with the ion concentration) in the transport equations, and (2) the porous support is considered by the inclusion of the adequate transport equations similarly to how the active layer is considered.

Investigating nanofiltration of multi-ionic solutions using the steric, electric and dielectric exclusion model

The aim of this work was to test the ability of the steric, electric and dielectric exclusion (SEDE) model to describe the separation of multi-ionic solutions (3 and 4 ions) by nanofiltration. To this end, mixed solutions of Pb ( NO 3 ) 2 and Co ( NO 3 ) 2 · 6 H 2 O at various pH were filtered through a nanofiltration polyamide membrane. The structural features of the membrane active layer were inferred from glucose rejection rates and water permeability. The charge state of the membrane active layer was evaluated for the various solutions from coupled tangential streaming potential (TSP) and electric conductance measurements. Calculations were carried out using different approaches. Modelling results show that the experimental rejection rates are not well described incorporating the two types of dielectric exclusion mechanisms, i.e. Born dielectric effect and image forces contribution, in the steric/electric model. However, the description is found to be satisfactory considering (i) steric, electric and Born dielectric exclusion mechanisms, with two fitted parameters: the effective volume charge density, X, and the dielectric constant of the solution inside the pores, ε p , or (ii) steric, electric exclusion mechanisms and image forces, with two fitted parameters, X and the dielectric constant of the active layer material, ε m . ε p and ε m were assessed in the range of 56.3–74.6 and 7–10, respectively. The values of 7–10 for ε m are different from the commonly used value for dry polyamide (i.e. ε m = 3 ). It might be due to the wetting of the polymer. In both approaches, the fitted values of X were found to be smaller than X values inferred from TSP measurements, which is in agreement with the charge regulation phenomenon.

Tangential streaming potential as a tool in modeling of ion transport through nanoporous membranes

Tangential streaming potential (TSP) measurements have been carried out so as to assess the electrokinetic properties of the active layer of organic nanofiltration (NF) membranes. Due to the porous structure of NF membranes, cares must be taken to convert the experimental data into zeta potential. Indeed, an assumption that is implicitly made in Smoluchowski's theory (or in related approaches accounting for the surface conduction phenomenon) is that both streaming and conduction currents involved in the streaming potential process flow through an identical path. Such an assumption does not hold with porous membranes since the conduction current is expected to flow wherever the electric conductivity differs from zero. Consequently, a non-negligible share of the conduction current is likely to flow through the membrane body filled with the electrolyte solution. This phenomenon has been taken into account by carrying out a series of TSP measurements at various channel heights. Experiments have been conducted with various electrolyte solutions. The inferred zeta potentials have been further converted into membrane volume charge densities which have been used to predict the membrane performances in terms of rejection rates. The conventional NF theory, i.e. based on a steric/Donnan exclusion mechanism, has been found to be unable to describe the experimental rejection rates. Using the volume charge density of the membrane as an adjustable parameter, it has been shown that the conventional theory even predicts the opposite sign for the membrane charge. On the other hand, the experimental rejection rates have been well described by including dielectric effects in the exclusion mechanism. In this case, a noticeable lowering of the effective dielectric constant of the electrolyte solution inside pores has been predicted (with respect to the bulk value).

Characterisation of microporous tubular membranes by tangential streaming potential

Streaming potential measurements can beperformed in two different ways: by flowthrough the membrane pores (transmembranestreaming potential) or by flow across the topsurface of the membrane (tangential streamingpotential, TSP). The se cond procedure is usuallyused for the characterisation of asymmetric orcomposite membranes because it has the advan-tage of avoiding undesirable effects such as thecontribution of both supporting layer(s) to themeasured signal and the membrane potentialinduced by the concentration difference acrossthe active layer. Up to now the tangential tech-nique was applied to flat membranes but notto mono- or multi-channels tubular membranesprobably due to the largeness of channels insidewhich the solution flows. Indeed, in these condi-tions, the flow becomes rapidly turbulent andthe classical Hagen-Poiseuille equation cannotbe used anymore in the derivation of the stream-ing current expression. Although the flow is notwholly laminar, it is show n that the electrokinet-ics theory can be used to convert TSP data intoz-potentials provided that the electrical doublelayer lies within a laminar sublayer near thechannel walls. In this study, we present and test an electro-kinetic setup for the determination of z-potentialof tubular membranes from tangential streamingpotential measurements under conditions of tur-bulent flow. The study was performed with athree-channels tubular membrane close to theNF range over a range of pH. z-potentials deter-mined by combining TSP with electrical con-ductance measurements were compared withthose calculated from the classical Helmholtz–Smoluchowski (H–S) equation.

Tangential streaming potential as a tool in the characterisation of microporous membranes

Tangential streaming potential as a tool in the characterisation of microporous membranes

Effects of solution pH and UV irradiation on the streaming potential of the titanium dioxide membranes

This study focused on the effect of electrolyte solution pH, fluid pressure and UV irradiation on the tangential streaming potential (TSP) and filtration streaming potential (FSP) of Degussa P-25 titanium dioxide membrane. Both TSP and FSP measuring systems for TiO 2 membrane were developed. Experimental results depicted TSP and FSP measurements were sensitive to solution pH, temperature, fluid pressure and UV irradiation. Zeta potentials of TiO 2 membranes calculated with TSP data were different from those of TiO 2 suspensions measured by electrophoretic light-scattering technique, but the isoelectric point (pH iep) was measured to be almost the same value of 6.8. The pH drifts (difference of initial and final pH) were observed during the TSP measurements of TiO 2 membranes. A modified model was developed to calculate the change in Fermi energy of the electrolyte/TiO 2 membranes system presented at different solution pH levels. A hysteresis phenomenon was found that two zeta potentials were observed at the same pH through different pH adjustment paths. Slight decreases of TSP (<15 mV) were observed in acidic solutions under UV irradiation; however, more considerable decreases of FSP (20–60 mV) were examined in both acidic and alkaline solutions. The FSP increased with increasing filtration pressures and exhibited an exponential decay with increasing light intensity.

A New Tangential Streaming Potential Setup for the Electrokinetic Characterization of Tubular Membranes

ABSTRACTA new electrokinetic setup was developed for assessing the active layer ζ-potential of tubular membranes based on tangential streaming potential and electrical resistance measurements. Although the flow was not wholly laminar (because of the large hydraulic diameter of channels), the electrokinetics theory could be used to convert the streaming potential data into ζ-potentials because the electrical double layer lay within a laminar sublayer near the channel walls. Electrical resistance data allowed for the account of the conduction phenomenon through the membrane porous body. The new device was tested over a range of pH with a tubular ceramic membrane composed of three channels with a titania active layer. The isoelectric point was found to be in good agreement with that determined from salt retention data. The ζ-potential value determined at pH = 3.5 using the present device was compared with that obtained on a flat membrane made of the same material using the traditional microslit electrokinetic setup. A good agreement between the two measurements was observed. It was shown that neglecting the electric conduction phenomenon through the membrane porous body leads to a low underestimation of the ζ-potential (less than ~20%). This is related to the large size of channels. The contribution of the membrane porous body was found to be independent of the pH of solution. This suggests that the support layer of the membrane would make a decisive contribution to the electric conductivity of membrane porous body.

Influence of the Membrane Pore Conductance on Tangential Streaming Potential

Tangential streaming potential technique is an attractive way to characterize the electrokinetic properties of various kinds of materials such as films or flat membranes with a dense or a porous structure. However, the interpretation of data in terms of zeta potential is usually carried out by doing the implicit assumption of nonconducting substrates. In the present paper, we investigate the electrokinetic properties of a commercial ultrafiltration membrane, the porous structure of which affects the streaming potential because streaming and conduction currents involved in the streaming potential process do not flow through identical paths (the streaming current flows only through the slit channel formed by the two membrane samples facing each other whereas a non-negligible part of the conduction current is likely to flow through the membrane pores filled with electrolyte solution). The correct zeta potential value is determined from an extrapolation method for which a set of measurements with various channel heights is required. A very good agreement is obtained with zeta potential values deduced from consecutive streaming potential and total conductance measurements (referred in the text as the direct method). The ratio of the correct zeta potential to the apparent one (given by Helmholtz-Smoluchowski equation) is dependent on the pH which suggests a non-negligible contribution of surface conductance within the membrane pores.

Effect of pH on electrokinetic and electrochemical parameters of both sub-layers of composite polyamide/polysulfone membranes

Electrokinetic and electrochemical characterisations of a composite polyamide/polysulfone membrane (PAC-1) were carried out by measuring the streaming potential along the membrane surface (polyamide active layer) and the membrane potential at different pHs (3<pH<9). These results allow the determination of some characteristic parameters such as the active layer charge density and the ionic transport number. Porous sublayer characterisation was obtained from measurements made with an ultrafiltration polysulfone membrane (PPS) similar to that used as support of the composite one. Measurements of tangential streaming potential at different pHs allow the determination of some PAC-1 and PPS surface material parameters (polyamide and polysulfone, respectively), such as the isoelectric point (ie.p.), or the pK a and the number of acidic sites on the membrane surface (N a), if a dissociation site model is assumed. The following values are been obtained: polyamide surface, N a = (1.4±0.2)×10 16 m −2, pK a = (5.7±0.3) and ie.p. = (4.3+0.2); polysulfone surface, N a = (1.4±0.1)×10 16 m −2, pK a = (4.2±0.1) and ie.p. = (3.1±0.1). From both types of measurements, a change in the electrical character of the polyamide active layer was obtained, from a negative behaviour at acidic pH (7.6) to slightly positive when pH<4, while the porous sublayer behaves as a very weak electronegative material for the whole range of pH studied.

Effect of Hydration of Polyamide Membranes on the Surface Electrokinetic Parameters: Surface Characterization by X-Ray Photoelectronic Spectroscopy and Atomic Force Microscopy

The surface and the solid/liquid interface of two polyamide membranes, one experimental (B0) and one commercial (NF45), have been characterized by X-ray photoelectronic spectroscopy (XPS), atomic force microscopy (AFM), and ζ potential, respectively. The surface roughness, determined by AFM data analysis, is different for the two membranes, and results show that the commercial NF45 membrane presents a much lower roughness than the experimental B0 membrane. XPS data indicate that the surface of membrane NF45 is similar to that of pure polyamide, while membrane B0 contains a considerable amount of impurities. The homogeneity in depth of both membranes was also studied by determining the composition profile at different analysis angles. Streaming potential along the membrane surface or tangential streaming potential (TSP) measurements with NaCl solutions at different concentrations were carried out with both membranes to determine the ζ potential and the electrokinetic surface charge density, and a correlation between membrane surface and interface parameters is made. Some differences in atomic concentrations of membrane surface elements and X-ray photoelectronic spectra of the samples used in TSP measurements and after a drying process at 90°C for 24 h can be observed when they are compared with those for fresh membranes. Electrokinetic parameters for membrane NF45 (TSP, ζ potential, and surface electrokinetic charge density) obtained from three different series of measurements strongly decrease as a result of membrane use, but for membrane B0 they are practically independent of the number of measurements. This difference in the electrokinetic behavior of the two membranes has been related to the hydration process of the surface for each sample studied by XPS and AFM.

A Comparison of Electrochemical and Electrokinetic Parameters Determined for Cellophane Membranes in Contact with NaCl and NaNO3 Solutions

Electrochemical and electrokinetic characterizations of cellophane membrane samples have been carried out by measuring membrane potential, salt diffusion, and tangential streaming potential, which allow the determination of different characteristic membrane parameters. Experiments were made with the membrane samples in contact with NaCl and NaNO3 solutions at different concentrations and under different external conditions (concentration gradients), in order to obtain differences in transport and membrane characteristic parameters, depending on the electrolyte considered. Salt permeability across the membrane, which was obtained from diffusion measurements, is about twice as high for NaCl solutions as for NaNO3 solutions, which is attributed to the different sizes of the electrolytes. Membrane potential measurements keeping the concentration ratio constant (C1/C2=2) were used to determine both the effective fixed charge concentration in the membrane, Xf, and the average value of transport numbers, 〈ti〉; taking into account these values, concentration dependence of membrane potential under a different external condition (C1=cte=0.01 M, 5×10−3≤C(M)≤5×10−2) was predicted. Results show that cellophane membrane behaves as a weak cation-exchange membrane and its permselectivity to cations is practically independent of the electrolyte considered. From electrokinetic results, assuming a Langmuir-type adsorption of anions on the cellophane surface, the number of accessible sites per surface unit was obtained, which is higher for Cl− than for NO3−, in agreement with the small radii of chlorine ions; however, no significant differences in the specific adsorption free energy were found (ΔGNacl=−22.0×103 J/mol) and (ΔGNaNO3=−23.2×103 J/mol).

Electrokinetic and electrochemical characterizations of porous membranes

Surface and bulk membrane have been electrically and chemically characterized by different kinds of measurements. Tangential streaming potential, impedance spectroscopy, membrane potential and salt diffusion were measured with a porous polysulfone membrane for ultrafiltration when it was in contact with NaCl solutions at different concentrations (5×10 −5< c<5×10 −2 M). From these experiments some membrane and transport characteristic parameters such as the ζ potential, the surface charge density, ionic transport numbers, concentration of fixed charge in the bulk membrane and salt and ionic permeabilities across the porous membrane were determined. From electrokinetic results, assuming for the membrane a Freundlich heterogeneous adsorption of Cl − ions, the following parameters were determined: membrane proper charge density ( σ o=−1.2×10 −3 C m −2), density of accessible sites ( N=6×10 17 m −2) and mean adsorption free energy (Δ G ads=−3.9 kJ mol −1); membrane potential results also indicate a weak cation exchange character for this ultrafiltration membrane: fixed charge concentration X f=−(5.0±0.2)×10 −3 M, cation transport number t +=(0.41±0.4). A reduction in salt permeability due to concentration–polarization was determined by measuring salt diffusion at different stirring rates. Chemical composition of the membrane surface was determined by X-ray photoelectron spectroscopy (XPS); the homogeneity of the membrane surface in the range between 2 and 9 nm was obtained from XPS data by using five different take-off angles (chemical depth profile).

Characterization of active and porous sublayers of a composite reverse osmosis membrane by impedance spectroscopy, streaming and membrane potentials, salt diffusion and X-ray photoelectron spectroscopy measurements

Electrokinetic, structural and surface chemical characterizations of both active and porous sublayers of a composite (polyamide/polysulfone) reverse osmosis membrane were carried out by different techniques: (i) electrochemical impedance spectroscopy (EIS) allows both electrical and geometrical characterizations using equivalent circuits as models; (ii) surface chemical composition of the active layer was determined by X-ray photoelectron spectroscopy (XPS); (iii) salt diffusion, tangential streaming potential and membrane potential measurements allow the determination of different transport and electrokinetic parameters such as salt and ionic permeabilities, zeta potential and surface charge density. Electrical and electrokinetic parameters were determined with the membrane in contact with NaCl solutions at different concentrations, although the influence of pH and the use of the membrane on zeta potential values was also studied. Changes in the electrokinetic parameters as a result of the membrane use were obtained and, according to XPS results, they can be attributed to the hydratation of the polyamide active layer. For comparison, electrochemical measurements with a polysulfone ultrafiltration membrane similar to the porous sublayer of the composite RO membrane were also carried out. Good agreement was obtained for the electrical resistance and ionic permeabilities determined for both the ultrafiltration membrane and the porous sublayer of the RO membrane, which confirms the two-layer model assumed for composite membranes.

Electrical and surface chemical characterizations of the active layer of composite polyamide/polysulphone nanofiltration commercial membranes

Membrane separation techniques are now commonly used in many industrial processes. Because the membrane surface chemistry, morphology and the effective surface charge can influence the membrane separation properties to a large extent, it is important to get information about these parameters. In this work, the active layer (polyamide) of two commercial composite membranes is characterized. Morphological, electrokinetic and surface chemical characterizations of both active layers were carried out by: impedance spectroscopy measurements, using equivalent circuits as models, which allow their electrical and geometrical characterization; tangential streaming potential and membrane potential measurements, to obtain the charge arrangement in both bulk and in the surface; and surface chemical composition, obtained by x-ray photoelectron spectroscopy. Electrokinetic parameters (electrical resistance, transport numbers and zeta potential) were determined with the membranes in contact with NaCl solutions at different salt concentrations. Differences in transport parameters obtained with both nanofiltration membranes may be related to differences in the membrane morphology and chemical composition of their active layers. Copyright © 2000 John Wiley & Sons, Ltd.

Electrochemical characterization of charged membranes and membrane systems

The electrochemical behaviour of polyelectrolyte complex (PELC) membranes, prepared by simultaneous interfacial reaction between aqueous solutions of Na-cellulose sulphate (CS) on the one side and different polycations as well as cationic macroions on the other side, was investigated. Tangential streaming potential and concentration potential measurements were carried out in order to obtain a complete information on the nature and density of their effective surface charges. In the case of PELC membranes obtained from CS and polycations, their weak ion exchange properties are confirmed. On the other hand, in the case of PELC membranes obtained from CS and cationic surfactants, the results of tangential streaming potential measurements lead to a reinterpretation of earlier observations based on concentration potential measurements: Their properties are governed by specific sorption of anions from surrounding solutions. For the case of membrane systems obtained by laminating both types of PELC membranes one over the other, the characterization of gradients with respect to fixed charge density and/or morphology is demonstrated.