Anthony Szymczyk

Analysis of the salt retention of a titania membrane using the “DSPM” model: effect of pH, salt concentration and nature

Abstract Retention measurements with single salt solutions of KCl, LiCl, K2SO4, MgCl2 and MgSO4 were carried out as a function of the permeate flux for a commercial titania membrane close to the nanofiltration (NF) range. The effect of both pH and salt concentration was studied. The membrane shows amphoteric behavior with an isoelectric point (iep) at pH 6.2 (in presence of an indifferent electrolyte: KCl or LiCl). The obtained results agree qualitatively with the Donnan exclusion principle, characteristic of electrically charged membranes: a higher co-ion valence leads to a higher retention, a higher counter-ion valence leads to lower retention and retention decreases with increasing concentration. The analysis of the retention data by the Donnan steric partitioning pore model (DSPM) allowed to evaluate the effective volume charge of the membrane. It was shown that the membrane volume charge depends not only on pH, but also salt and its concentration. At low pH values (when the membrane is positively charged), the membrane charge is higher for magnesium salts than for potassium salts and lower for sulfate salts than for chloride salts. Also, in the high pH range (when the membrane is negatively charged), the membrane charge is higher in absolute value for sulfate salts than for chloride salts and lower (in absolute value) for magnesium salts than for potassium salts. Moreover, it was shown that the membrane charge does not increase with concentration for sulfate salts unlike chloride salts, when the membrane is positively charged. Also, the membrane charge does not increase (in absolute value) with concentration for magnesium salts unlike potassium salts, when the membrane is negatively charged. These results have been attributed to specific adsorption of magnesium and sulfate ions on the membrane material.

Comparison of two electrokinetic methods – electroosmosis and streaming potential – to determine the zeta-potential of plane ceramic membranes

Abstract Electroosmotic flow rate and streaming potential measurements are used to characterise electrokinetic properties of plane ceramic membranes. The study is carried out at different pH, ionic strengths and electrolytes. Effects of pH and ionic strength are studied for both techniques which lead to very close values of isoelectric points. The specific adsorption of Ca 2+ cations is observed with the two methods. For identical pH and ionic strength electroosmosis gives greater zeta-potential values than those determined from streaming potential measurements. The gap between the two methods increases as the pH move of the isoelectric point and ionic strength increases. These results suggest that the location of the shearing plane depends on the electrokinetic method used.

Evaluation of three methods for the characterisation of the membrane–solution interface: streaming potential, membrane potential and electrolyte conductivity inside pores

Filtration and separation performances of microfiltration, ultrafiltration and nanofiltration membranes can be greatly affected by the charge (or electrical potential) on their surface. These surface properties can be characterised in terms of potential in the Outer Helmholtz Plane (Ψd). A theoretical analysis of electrical and electrokinetic phenomena (electrolyte conductivity inside pores λpore, membrane potential Em and streaming potential SP) occurring in charged capillaries was developed in the framework of the linear thermodynamics of irreversible processes with the aim of studying the variation of SP, Em and λpore as a function of Ψd for various pore sizes and electrolyte concentrations. From these results, the accuracy on the determination of Ψd from experimental measurements of λpore, Em and SP could be estimated and a ‘method limitation’ diagram was constructed using the constraints of pore size and surface charge.

A simple and accurate determination of the point of zero charge of ceramic membranes

Abstract A simple and accurate determination of the point of zero charge of a ceramic membrane is reported. It is based on pH variation measurements on adding an amphoteric oxide in a solution of a given pH. Up to now this method was effective for studying powder dispersions. In this work it is extended to ceramic membranes. In fact, we present and test a new experimental set-up that allows the performance of pH measurements on a solution continuously circulating through the membrane. It is verified that the point of zero charge, determined in presence of an indifferent electrolyte, is the same as the isoelectric point (or point of zero charge) determined with standard electrokinetic methods. The shift in the point of zero charge towards a higher and a lower pH, in presence of Na2SO4 and CaCl2 solutions, respectively, confirms the specific adsorption of sulphate and calcium ions and validates the method used. We have compared this new method with the known salt addition method, carried out on the crushed membrane. This last one leads to a different value of the pzc, thus showing the importance in performing measurements directly on membranes themselves. Another interesting aspect in the method presented here is that it allows to assess directly to the absolute value of the surface charge density of the membrane. Results obtained are in good agreement with the data reported in the literature on mineral oxides.

Characterisation of surface properties of ceramic membranes by streaming and membrane potentials

Abstract The charge of ceramic UF membranes is studied in NaCl and CaCl2 media from streaming potential and membrane potential measurements. The amphoteric behaviour of these materials is observed with both methods. The apparent transport numbers of cations in the membrane are determined from cell potential measurements. Streaming potential measurements and the study of the transport properties lead to similar isoelectric points of the membrane. It also appears that the presence of Ca2+ cations leads to a more positive net charge of the membrane at pH lower than the isoelectric point and a less negative charge for the higher pH.

Characterisation of the electrokinetic properties of plane inorganic membranes using streaming potential measurements

Abstract We present and test a device designed to measure the streaming potential of plane inorganic membrane during filtration. Two kinds of microporous membranes (a membrane made of a mixture of alumina-titania and this same type of membrane covered with an additional titania layer) are studied with different pH, ionic strength and electrolyte nature. The modification of the surface acid-basic equilibriums is analysed from the streaming potential measurements. The pores size of the studied membranes is large enough to avoid overlapping of the double layers. Streaming potential measurements are used to determine the zeta potential of the membranes from the Helmholtz-Smoluchowski relationship, corrected for the lowest ionic strengths studied. The shifting of the isoelectric point of the membranes studied with CaCl2 and Na2SO4 solutions shows specific adsorption of calcium and sulfate ions onto the surface. The additional titania layer on the alumina-titania support does not seem to modify the electrokinetic properties of the membrane. The interactions of the alumina-titania membrane with the H+ and OH− ions are analysed by studying the variations of pH between permeate and retentate compartments. These variations allow determining the isoelectric point of the membrane with a reasonable precision.

Evaluation of the "DSPM" model on a titania membrane: measurements of charged and uncharged solute retention, electrokinetic charge, pore size, and water permeability.

The DSPM (Donnan steric partitioning pore model) was evaluated in the case of a titania membrane with nanofiltration properties by measuring the electrokinetic charge, pore size, and water permeability of the membrane, along with charged and uncharged solute retention. The zeta potential values (zeta) were determined from measurements of the electrophoretic mobility (EM) of titania powder forming the filtering layer of the membrane. Zeta potential values were converted into membrane volume charge (X) by assuming two limiting cases: a constant surface charge (sigma(s)(cst)) and a constant surface potential (psi(s)(cst)). The mean pore radius and thickness/porosity ratio of the membrane were determined by permporometry and from water permeability measurements, respectively. Retention measurements were carried out as a function of the permeate volume flux for both neutral solutes (polyethylene glycol PEG of different size) and salts (KCl, MgSO4, K2SO4, and MgCl2) at various pH values. Ionic retentions showed minimum values near the IEP of the membrane. Retention data were analyzed using the DSPM. Very good agreement was found between the pore radius calculated by the model and that determined by permporometry. X values calculated from fitting retention data using the DSPM were also in satisfactorily agreement with X values calculated from EM measurements assuming a constant surface potential for a large pH range. Furthermore, the DSPM leads to X values (X(DSPM)) between those calculated from EM (X(EM)) using the two limiting bounds. In other words, X(DSPM) was higher than X(EM) assuming psi(s)(cst) at pH values far from the isoelectric point (IEP) and lower than X(EM) assuming sigma(s)(cst). These results show that the DSPM is in qualitative agreement with the charge regulation theory (increase of the pore surface potential and decrease of the pore surface charge density with decreasing the pore size). On the other hand, the thickness/porosity ratio of the membrane calculated from solute retention data differed significantly from that determined from water permeability measurements. Moreover, a single value of Deltax/Ak could not be determined from PEG and salt retention data. This means that the Deltax/Ak parameter loses its physical meaning and includes physical phenomena which are not taken into account by the DSPM. Nevertheless, the model satisfactorily predicted the limiting retention, as this is not influenced by the Deltax/Ak parameter.

Membrane potential in charged porous membranes

Abstract For charged porous membranes, the separation efficiency to charged particles and ions is affected by the electrical properties of the membrane surface. Such properties are most commonly quantified in terms of zeta-potential. In this paper, it is shown that the zeta-potential can be calculated numerically from the membrane potential. The membrane potential expression for charged capillary membranes in contact with electrolyte solutions at different concentrations is established by applying the theory of non-equilibrium thermodynamic to the membrane process and considering the space-charge model. This model uses the Nernst–Planck and Navier–Stokes equations for transport through pores, and the non-linear Poisson–Boltzmann equation, which is numerically solved, for the electrostatic condition of the fluid inside pores. The integral expressions of the phenomenological coefficients coupling the differential flow (solute relative to solvent) and the electrical current with the osmotic pressure and the electrical potential gradients are established and calculated numerically. The mobilities of anions and cations are individually specified. The variations of the membrane potential (or the apparent transport number of ions in the membrane pores) are studied as a function of different parameters: zeta-potential, pore radius, mean concentration in the membrane, ratio of external concentrations and type of ions.

An application of the space charge model to the electrolyte conductivity inside a charged microporous membrane

Abstract In this study, we test the validity of the space charge model in the case of a ceramic microporous membrane. To this end, experimental measurements of the electrical resistance in pores are performed with the membrane filled with KCl solutions of various concentrations. The electrolyte conductivity within the membrane pores is deduced from these experiments. In situations where the contribution of the surface conduction is important (i.e. at low salt concentration or/and high zeta potential), the conductivity of the electrolyte inside pores substantially exceeds the conductivity of the external solution. Experimental results are compared with the theoretical predictions based on the Nernst–Planck and Navier–Stokes equations for flow in pores and the non-linear Poisson–Boltzmann equation for the electrostatic potential profile. For numerical calculations, the membrane is assumed to be a set of parallel cylindrical pores having an identical mean radius. The zeta potential is determined numerically from streaming potential measurements and used in the model to compute the electrolyte conductivity within the membrane pores. The space charge model provides rather good predictions for all the concentrations under consideration.

Influence of steric, electric, and dielectric effects on membrane potential.

The membrane potential arising through nanofiltration membranes separating two aqueous solutions of the same electrolyte at identical hydrostatic pressures but different concentrations is investigated within the scope of the steric, electric, and dielectric exclusion model. The influence of the ion size and the so-called dielectric exclusion on the membrane potential arising through both neutral and electrically charged membranes is investigated. Dielectric phenomena have no influence on the membrane potential through neutral membranes, unlike ion size effects which increase the membrane potential value. For charged membranes, both steric and dielectric effects increase the membrane potential at a given concentration but the diffusion potential (that is the high-concentration limit of the membrane potential) is affected only by steric effects. It is therefore proposed that membrane potential measurements carried out at high salt concentrations could be used to determine the mean pore size of nanofiltration membranes. In practical cases, the membrane volume charge density and the dielectric constant inside pores depend on the physicochemical properties of both the membrane and the surrounding solutions (pH, concentration, and chemical nature of ions). It is shown that the Donnan and dielectric exclusions affect the membrane potential of charged membranes similarly; namely, a higher salt concentration is needed to screen the membrane fixed charge. The membrane volume charge density and the pore dielectric constant cannot then be determined unambiguously by means of membrane potential experiments, and additional independent measurements are in need. It is suggested to carry out rejection rate measurements (together with membrane potential measurements).