Patrick Fievet

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.

Electrolyte transport through amphoteric nanofiltration membranes

Abstract The electrolyte transport through amphoteric nanofiltration membranes is investigated using a model based on the application of the extended Nernst–Planck equation coupled with the electroneutrality condition in the membrane, and the assumption of a Donnan equilibrium at both membrane/solution interfaces. The model gives the possibility to distinguish between the different transport mechanisms, namely, convection, diffusion and electromigration. The influence of ion valence, ion diffusion coefficient and permeate volume flux on the different transport mechanisms and electrolyte retention is presented. All calculations are carried out for a membrane with fixed structural parameters (effective pore radius of 2 nm and effective thickness/porosity ratio of 7 μm ).

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.

PAH contaminated soil remediation by reusing an aqueous solution of cyclodextrins.

We studied the possibility to re-use an aqueous solution of methyl-beta-cyclodextrin (bpmCD) in order to decontaminate a soil polluted by phenanthrene and pyrene. The loss of bpmCD in the soil was insignificant. In order to eliminate polycylic aromatic hydrocarbons (PAHs) from the contaminated aqueous solution, on one hand we tested their photodegradation using TiO(2) suspensions. But it was inefficient, because of the stabilisation of PAHs within the cavity of bpmCD. On the other hand, we removed PAHs by liquid-liquid extraction with colza oil. This allowed the regeneration of cyclodextrins, by concentrating the pollutants in the organic phase with a small loss of carrier. Contaminated soils were almost completely de-polluted after 2d of re-circulation, using a 10mM solution of bpmCD. To reduce the amount of bpmCD loss in the oil phase, we set the fraction of colza oil low, by using a micro-emulsion or by impregnating an organic membrane with the oil. We found this last possibility more interesting.

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.