J. Pagetti

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.

Surface electrochemical properties of mixed oxide ceramic membranes : Zeta-potential and surface charge density

Abstract The surface electrochemical properties of alumina based ceramic microfiltration membranes were studied by measuring electroosmotic rates and surface charge densities obtained from potentiometric titrations. The rate of electroosmosis, which determines the zeta-potential, was measured on the membrane itself, whereas the surface charge was titrated on a suspension obtained by crushing of the membrane. The zeta-potential was measured in the presence of salts including NaCl, CaCl 2 and Na 2 SO 4 , for a wide range of pH values (4–9) at ionic strengths of 0.01 and 0.001 M. The pH value of the isoelectric point (iep) show a specific adsorption of SO 4 2− and Ca 2+ ions onto the membrane surface. The iep in NaCl solutions occurs at pH 4.7 ± 0.1. The low iep is due to the large amount of silicium oxide in the membrane. The surface charge density is relatively high with respect to the low values of zeta-potentials. The point of zero charge pH(pzc) determined from surface charge and pH profiles occurs at pH 8.2 ± 0.1 in NaCl solution. The pH(pzc) value was also determined by two ‘addition’ methods. Similar pH(pzc) values were obtained. The difference between the pH(pzc) and pH(iep) may be correlated to a loss of acidity that is due to using crushed-membrane powder to perform potentiometric measurements.

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.

Effects of proteins on electrokinetic properties of inorganic membranes during ultra- and micro-filtration

Abstract The streaming potential is measured across inorganic ultra-filtration and micro-filtration membranes to characterize them. Concurrently, electrophoretic mobility measurements are carried out to study the membrane material. The influence of whey proteins on the streaming potential and electrophoretic mobility is also studied. Results show that the membrane interface after adsorption has a surface property behavior similar to that of the proteins from a charge point of view. Moreover, the streaming potential considers two aspects of filtration which must take into account the steric and electric effects. In an electrolyte solution without proteins, the streaming potential depends only on charge repartition, determined by pH value. With whey proteins, the streaming potential depends on both charge repartition and permeate flux. If there is no inner fouling, the charge repartition is not affected and the streaming potential depends exclusively on flux; when there is an inner fouling by protein adsorption, the pore surface is modified and the streaming potential depends on both permeate flux and on electric charge of the oxide-solution interface.

Streaming potential and protein transmission ultrafiltration of single proteins and proteins in mixture: β-lactoglobulin and lysozyme

Abstract A study of single proteins, β-lactoglobulin and lysozyme of different sizes and electrical characteristic as a function of pH, ionic strength and nature of salt (NaCl or CaCl 2 ) allows to evaluate the filtration performances. Streaming potential measurements confirm that proteins contribute to the net charge of the system and that the protein tranfer through mineral membranes is governed by ionic and steric exclusion phenomena. This work shows the correlation between protein transmission and streaming potential values which takes into account steric and ionic exclusion. The ionic repulsion decreases the transmission. This one depends on membrane net charge characterised by streaming potential, which depends on the solution composition. This model, which does not take into account interactions between protein and membrane fouling leads to an overestimation of calculated transmission values when proteins are in a mixture. However, it allows a correct estimation of transmission variations versus the studied variables: pH and ionic strength.

Use of electrophoretic mobility and streaming potential measurements to characterize electrokinetic properties of ultrafiltration and microfiltration membranes

Abstract The electrophoretic mobility of mineral oxide particles and the streaming potential of UF and MF inorganic membranes both depend on the electrical charge distribution at the oxide–solution interface. They are related to the zeta potential. The aim of this work is to evaluate experimentally how measuring the electrophoretic mobility and streaming potential complement each other for understanding the physical-chemical phenomena linked to filtration. We investigated the effect of pH and ionic strength on electrophoretic mobility and streaming potential values. The isoelectric point (i.e.p.) determined by microelectrophoretic measurements on zirconia particles obtained by membrane scraping is different from the i.e.p. obtained from pure zirconia powder. The i.e.p. obtained from scraped particles is the i.e.p. obtained from streaming potential measurements. Zeta potential values calculated from measurements using both techniques are compared to ascertain the accuracy of the conditions of validity of the Helmoltz–Smoluchowski relation for the streaming potential and zeta potential. Well-known inadequacies of this relation stem from double layer overlapping, the conductivity in the pores, etc. A correction factor, between zero and one is calculated. It is related to the zeta potential obtained from electrophoretic mobility measurements.

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.