Study of the mass transfer phenomena involved in an electrophoretic membrane contactor

Electrophoretic separators, in which a porous membrane is used as a contactor, offer the possibility to scale up electrophoresis as well as to extend the field of application of electrodialysis to fractionate polyamino acids, peptides or small proteins for instance. This paper deals with the study of the mass transfer mechanisms involved in such electroseparation processes. On one hand, a theoretical approach is carried out. The different contributions to the mass transfer are considered in order to establish a relationship providing the solute concentration as function of the main parameters of the system, i.e. the operating conditions and the membrane, buffer and solute characteristics. In this expression, a partition coefficient is used to represent the interactions taking place at the membrane–solution interface. Then, an experimental study is performed with different representative solutes using a prototype apparatus in order to determine the dependence of the solvent and solute transfer with respect to the operating and physicochemical parameters of the system. The experimental results show the existence of a limiting electro-osmotic flux, the origin of which is explained. Then the partition coefficient is determined for any set of conditions by fitting the variations of the solute concentration calculated by the model with experimental ones. The dependence of the partition coefficient with respect to the solute and buffer characteristics, together with that of the transmission coefficient obtained during filtration experiments, shows that the main limitation with respect to the mass transfer is due to electrostatic interactions taking place at the membrane–solution interface

Influence of electrostatic interactions in electrophoretic membrane contactors

In electrophoretic separators, a porous membrane is used to put into contact two flowing liquids between which an electrically driven mass transfer takes place. As far as charged solutes are concerned, the mass transfer can be affected by electrostatic interactions taking place at the membrane solution interface. The influence of these interactions on the solvent and solute transfer is investigated by associating a theoretical and an experimental work, carried out with buffered solutions of different solutes, chosen with respect to their size or electrical charge. Experimental variations of the electroosmotic flux as well as those of the solute concentrations are used to get the values of the characteristic parameters involved in the model. Results obtained with binary solutions are then compared to those obtained with single-solute solutions so as to point out the mass transfer limitation

The electrophoretic membrane contactor: A mass-transfer-based methodology applied to the separation of whey proteins

In the electrophoretic membrane contactor (EMC), a porous membrane is used to establish a contact across two flowing liquids between which an electrically driven mass transfer takes places. In this work, a methodology is proposed to select the best operating conditions to separate biomolecules in an EMC. Single-solution experiments were coupled with a theoretical approach to predict the influence of the process parameters (pH, membrane MWCO) on the separation factor.This methodology was applied to the separation of wheyproteins, α-lactalbumin and β-lactoglobulin, which are known to be difficult to separate. Experiments were first carried out with single synthetic protein solutions at different pH values (4.8, 6 and 8) using cellulose acetate membranes of either 30 or 100 kDa molecular weight cut-off. The experimental work was associated with a theoretical approach to study the mass transfer mechanisms. The parameters used in the model were calculated from the experimental variations of the solute and solvent transfer. The dependence of these parameters on the operating conditions gives the extent of electrostatic repulsion and provides information on the steric effect with respect to separation performance.The model was then used to calculate the separation factor for various operating conditions in order to determine the best ones (pH and membrane) for fractionation. Using the results, fractions enriched in α-lactalbumin and in β-lactoglobulin were obtained at pH 4.8 with the 100 kDa membrane

Influence of the ionic composition on the diffusion mass transfer of saccharides through a cation-exchange membrane

Recent studies have pointed out that the presence of salts can change significantly the membrane process performances because of the resulting modification of the neutral solutes transfer through the membrane. The influence of the ionic composition on the transfer of neutral solutes through membranes could be explained by a modification of the membrane properties, due to electrostatic effects, by a modification of the solute radius, likely due to its dehydration induced by the electrolyte, or more probably by a combination of both. This study deals with the investigation of the mechanisms governing the mass transfer of neutral species through an ion-exchange membrane used in electrodialysis, CMX, with a focus on the role of ions. The mass transfer of various saccharides (xylose,glucose and sucrose) as well as the solvent transfer in different electrolytic solutions (NaCl,NH 4Cl, CaCl 2 and MgCl 2) was studied in a diffusion regime. A specific procedure has been used to dissociate the solvent or solute fluxes variations due to the modifications of the solute properties and of the membrane material induced by the electrolyte. The results showed that the transfer modification is mainly due to the influence of the electrolyte on the membrane properties, which is fixed by the membrane soaking. A quantitative correlation has been established between the solvent and solute transfer and the hydration number of the membrane counterion. In presence of electrolyte, the saccharide mass transfer increased but the impact of the presence of electrolyte is much less than the one of membrane soaking. However, in this case, a relationship has been also established between the increase of the saccharide mass transfer and the cation hydration state

Nanofiltration as a purification step in production process of organic acids: Selectivity improvement by addition of an inorganic salt

The aim of this study is to investigate to what extend the addition of an electrolyte (NaCl or Na2SO4) can improve the selectivity of the sodium lactate/glucose separation by nanofiltration. Experimental results were used to get the variation of the observed retentions versus the permeation flux and to evaluate the separation efficiency from the separation factor. In presence of NaCl, both glucose and lactate retentions slightly decrease and remain very close except at low permeation fluxes where the addition of NaCl has more effect on lactate retention than on glucose one. On the contrary, whilst the addition of Na2SO4 has no influence on glucose retention, a strong effect was pointed out on the lactate one, especially for high electrolyte concentrations for which negative retentions were obtained at low permeation fluxes. Then, the separation was much more improved by the addition of Na2SO4 compared to NaCl. A maximum separation factor of 1.9 was obtained with Na2SO4 at 0.25 M added to the glucose (0.1 M)/sodium lactate (0.1 M) solution whereas the separation was impossible without the addition of salt

Ion hydration number and electroosmosis during electrodialysis of mixed salt solution

Water transfer is an important aspect to be considered in electrodialysis since it fixes the performances of the process. It is due to electro-osmosis, i.e. the water carried by the migrating species and is thus related to their hydration. Few results were reported about the hydration number of solutes transferring through ion-exchange membranes. In this work, a methodology is proposed to calculate the hydration numbers of ions transferring through ion exchange membranes during electrodialysis. It is based on the experimental measurements of ion and water transfer under different conditions, like salt compositions and current. Salt hydration is first obtained, and then the hydration numbers of 4 transferring ions (Na+, Mg2+, Cl−, SO42−) are calculated simultaneously. It is shown that these hydration numbers are constant, independent from the salt composition and current. The hydration number for monovalent ions is found to be lower than that of divalent ones, which is in agreement with the values of the hydration free energy. Further comparison with the reported values concerning the hydration of the same ions in solution shows that for monovalent ions the hydration numbers are close to those reported for the 1st hydration shell while much higher values are obtained for divalent ions

Study of biomolecules separation in an electrophoretic membrane contactor

In electrophoretic membrane contactors, a porous membrane is used to put into contact two flowing liquids between which an electrically driven mass transfer takes place. These processes offer the possibility to scale up electrophoresis as well as to extend the field of application of electrodialysis. This paper deals with the study of the mass transfer mechanisms involved in such electroseparation processes for the separation of binary mixtures. This study is carried out by associating an experimental work and a theoretical approach. The parameters involved in the model are calculated according from the experimental variations of the solvent and solute transfer. From the dependence of these parameters with the operating conditions, the importance of electrostatic interactions with respect to the separation performances is pointed out. Finally, the process performances are studied considering the separation of alpha-lactalbumin and bovine haemoglobin as a case study

A phenomenological model to evaluate the performances of electrodialysis for the desalination of saline water containing organic solutes

Electrodialysis is promising to treat saline solutions containing organic solutes. However, there is still a need for a model to describe the desalination performances according to the process parameters. In this work a phenomenological model is proposed based on the analysis of the different contributions to the mass transfer of salt, water, and organic compounds. Once the characteristic parameters are determined experimentally, it is possible to predict the salt and organic concentrations in both compartments according to the operating conditions. The methodology is illustrated for the evaluation of electrodialysis performances considering a case study, and the effect of the current and solution compositions are discussed. To remove a fixed salt quantity, a lower current gives a higher organic solute transfer due to a higher time dependent solute diffusion, the additional transfer, fixed by the quantity of charge, remaining constant. Regarding the influence of the inorganic salts, higher transfer of organics, like glucose, acetic acid or acetate, are observed with sulfate compared to chloride, while a contrary behavior is obtained with phenol. This model also permits to discuss the influence of possible pretreatments to change the pH or the salt composition in order to improve the electrodialysis performances

Relationship between volumetric properties and mass transfer through NF membrane for saccharide/electrolyte systems

Recent studies have shown that unexpected performances can be obtained with NF membranes, when applied to the treatment of solutions containing significant amount of electrolyte. This study deals with the investigation of the mechanisms governing the mass transfer of neutral species through NF membranes with a focus on the role of ions. More precisely, it consists to determine the apparent molar volume of saccharides, which characterizes the hydration state of solutes in presence of electrolyte, and to assess the relationship between these parameters and those characterizing the mass transfer. The transfer of saccharides of increasing molecular weights (xylose, glucose and sucrose) through a NF membrane is studied in a diffusion regime. Different electrolytes (NaCl, Na₂SO₄, CaCl₂, MgCl₂) are chosen with respect to their hydration level. A specific procedure was developed to dissociate the flux variation due to the modifications of the solute properties and of the membrane material induced by the electrolyte. The results show that the mass transfer modification is mainly due to the influence of the electrolyte on the solute properties. Then, the mass transfer parameters are put in parallel with the apparent molar volume of saccharides measured in solutions of different ionic compositions. For a given electrolyte, Na₂SO₄, a good quantitative relationship is obtained regardless of saccharide nature, concerning the influence of the electrolyte concentration. This result confirms that the saccharide transfer increase can be due to its dehydration in presence of electrolyte. However, from these results, it is not possible to establish a clear relationship regardless of the electrolyte nature

Transfer of neutral organic solutes during desalination by electrodialysis : influence of the salt composition

Electrodialysis is a promising process to treat saline water containing organic solute. The desalination performances are fixed by the transfer of salts and organic solutes. On the contrary to the transfer of salts,few results were reported regarding that of organic compounds. This is the objective of this work to investigate the transfer of neutral organic solutes (acetic acid, phenol, glucose) through ion-exchange membranes, focusing on the influence of the salt (NaCl, MgCl2, Na2SO4). Results show that the water transfer due to electroosmosis depends on the salt composition following the ion hydration. Two contributions, diffusion and convection, are pointed out for the transfer of organic solutes. Both are important and mainly fixed by steric effect. Concerning the influence of the ion hydration, reverse trends are found for both contributions. Decreasing diffusion flux is observed for increasing membrane counterion hydration showing the influence of the membrane structural properties. Increasing convection flux is observed for increasing ion hydration showing the influence of the solute hydration