Effect of magnesium/calcium ratios in solutions treated by electrodialysis: morphological characterization and identification of anion-exchange membrane fouling

The present study aimed the characterization of the fouling formed on anion-exchange membrane during electrodialysis treatment of model salt solutions at different Mg/Ca ratio (0, 1/20, 1/10, 1/5 and 2/5). The membrane fouling was characterized by membrane parameters (membrane thickness and electrical conductivity) and identified by membrane surface analysis (elemental analysis and X-ray diffraction). The mineral deposit was identified as Ca(OH)2 when no magnesium was present in the model salt. As soon as magnesium was present in the model salt solution for neutral pH((concentrate)) conditions a mix between CaCO3 and Ca(OH)2 was formed. This study is the first one to report the influence of magnesium in solution on the formation of CaCO3 fouling at the interface of anion-exchange membrane during electrodialysis.     

Nature identification and morphology characterization of cation-exchange membrane fouling during conventional electrodialysis

The aim of this work was to study the effect of a concentrate solution pH value and of the composition in calcium, carbonate, and protein of a diluate solution to be treated by conventional electrodialysis on the fouling of cation-exchange membranes (CEM). It appeared that after demineralization of solutions containing CaCl(2) and CaCl(2)+Na(2)CO(3) using a concentrate solution maintained at a pH of 12, mineral fouling appeared on both sides of the CEM. The nature of the deposits was identified as calcium hydroxide and/or carbonate on both surfaces. The mineral fouling presented an aggregation-like crystal following a carnation-like pattern of aggregates of small rhombohedral crystals with CaCl(2) added alone, while CaCl(2)+Na(2)CO(3) yielded a smoother spherical crystal. Protein fouling was detected only on the CEM surface in contact with the diluate after demineralization of a solution containing CaCl(2)+Na(2)CO(3) using a concentrate pH value of 2.     

Effect of calcium and carbonate concentrations on cationic membrane fouling during electrodialysis

Fouling, which is the accumulation of undesired solid materials at the phase interfaces of permselective membranes, is one of the major problems in electrodialysis. The aim of the present work was to investigate the effect on the fouling of cation-exchange membranes of the composition in calcium and carbonate of a model solution to be treated by electrodialysis. No fouling was observed at 400 and 800 mg/L of CaCl(2) in the absence of carbonate, while at only 400 mg/L CaCl(2) with carbonate, a deposit was observed. This difference could be explained by the buffering capacity of the carbonate, which affects the treatment duration with and without sodium carbonate. Since the duration was longer with carbonate, more calcium ions were able to migrate across the CMX-S membrane, which explained the higher deposit on its surface. Furthermore, whether there was carbonate in the solution treated by electrodialysis or not, the deposit on the surface of the cationic membrane was calcium hydroxide. However, this fouling formed during conventional ED was easily cleaned by an acid procedure.     

Effect of calcium and carbonate concentrations on anionic membrane fouling during electrodialysis

A previous study on electrodialysis of calcium and carbonate high concentration solutions demonstrated that calcium migrated through the cation-exchange membrane (CEM) was blocked by the anion-exchange membrane (AEM) where it formed another fouling. The aim of the present work was to complete the identification of the deposit formed on AEM during electrodialysis and to characterize its physical structure at the interface of the membrane. No fouling was found on the anionic membranes treated without calcium chloride in presence of sodium carbonate, while membranes used during ED process of solutions containing calcium chloride and sodium carbonate were slightly fouled. A thin layer of precipitates was observed on the anionic membrane surface. The appearance of precipitates was typical of a crystalline substance. The size and form of crystal increased in proportion to the concentration of calcium chloride in solution. Large and cubic crystals were the best defined on the membrane treated at 1600 mg/L of CaCl2. The precipitate was identified as calcium hydroxide. However, this fouling was not found to affect significantly the electrical conductivity and the thickness of the membranes. Furthermore, the fouling formed was reversible.     

Impact of pulsed electric field on electrodialysis process performance and membrane fouling during consecutive demineralization of a model salt solution containing a high magnesium/calcium ratio

Pulsed electric fields (PEFs), hashed modes of current consisting in the application of a constant current density during a fixed time (Ton) followed by a pause lapse (Toff), were recently demonstrated as an effective alternative for mineral fouling mitigation and process intensification during electrodialysis (ED) treatments. Recent ED studies have continuously reported a considerable mineral fouling formation on ion-exchange membranes, especially during the demineralization of solutions containing a high Mg/Ca ratio and a basified concentrate solution. The aim of this study was to evaluate the process performance under two different PEF conditions on a mineral solution containing a mineral mixture giving a high Mg(2+)/Ca(2+) ratio of 2/5. Two different pause-lapse durations (PEF ratio 1 (Ton/Toff 10s/10s); PEF ratio 0.3 (Ton/Toff 10s/33.3 s)) during consecutive ED treatments and their comparison with dc current were evaluated at a current density of 40 mA/cm(2). Our results showed that PEFs resulted in an intensification of ED process, enhancing the demineralization rates (DRs), reducing the system resistance (SR), and reducing the fouling and energy consumption (EC). PEF ratio 1 was the most optimal condition among the current regimes applied, leading to faster and higher demineralization rates due to a lower fouling and with low energy consumption during all consecutive runs.     

Nature identification and morphology characterization of anion-exchange membrane fouling during conventional electrodialysis

The aim of this work was to study the effect on the fouling of anion-exchange membranes (AEM) of (1) the pH value of the concentrate solution and (2) the composition in calcium, carbonate, and protein of the diluate solution to be treated by conventional electrodialysis. It appeared that after demineralization of solutions containing CaCl(2) using a concentrate solution maintained at a pH value of 7 or 12, mineral fouling appeared on the AEM surface in contact with the concentrate. The mineral deposits presented a cylindrical filament shape for conditions with a concentrate solution pH value of 7, while, for a pH value of 12, the mineral deposit had a crumbly and spongy texture formed by irregular aggregates. The nature of the fouling was identified as a calcium phosphate with or without calcium hydroxide. In addition, gel-like protein fouling was detected on the AEM surface in contact with the diluate after demineralization procedures using a concentrate pH value of 2 or 7, regardless of the mineral composition of the diluate.     

Multistep mineral fouling growth on a cation-exchange membrane ruled by gradual sieving effects of magnesium and carbonate ions and its delay by pulsed modes of electrodialysis

The aim of this study was to reveal the mechanisms ruling a fouling growth on both sides of a CMX-SB cation-exchange membrane (CEM), run after run during three consecutive electrodialysis (ED) treatments. A model solution containing a high magnesium/calcium ratio (2/5) was demineralized under two different pulsed electric field (PEF) on-duty ratios and dc current. The results showed a series of mechanisms ruling a multilayer mineral fouling growth and its delay by PEFs. The nature of the fouling layer, during a first run, depended on the diluate pH-value evolutions and the ion migration rates through the membrane. A subsequent multilayer fouling growth during consecutive treatments was ruled by the already formed mineral layers, where gradual sieving effects inverted the migration rates and led to a multistep crystal growth. Calcium carbonate grew on the diluate side of CEM, starting from its amorphous phase to then crystallize in a coexisting presence of aragonite and calcite. Amorphous magnesium hydroxide appeared on CEM apparently through fouling dehydration ruled by the mineral layers themselves and by overlimiting current regimes. A delayed fouling growth was observed for PEF ratio 0.3. A long pause lapse during pulse modes was demonstrated as an important parameter for fouling mitigation.     

Study on the recovery of ionic liquids from dilute effluent by electrodialysis method and the fouling of cation-exchange membrane

With the wide application of ionic liquids(ILs)in various fields,developing efficient techniques to recover ILs from effluent is an urgent demand for the cost reduction and the environmental protection.In this study,an electrodialysis(ED)method was used to recover 1-butyl-3-methylimidazolium chloride([Bmim]Cl)IL from aqueous solution as model effluent.The influences of initial IL concentration and applied voltage on the current efficiency,removal ratio,desalination ratio,membrane flux and specific energy consumption during the ED process were investigated.It was found that the removal ratio and desalination ratio increases with the increasing of initial IL concentration and applied voltage.The current efficiency decreases with the increasing of initial IL concentration and the current efficiency reached the maximum value of 94.3%at 25 V.Besides,as the applied voltage increases,the membrane flux increases and the specific energy consumption decreases.Moreover,the fouling of cation-exchange membrane was also discovered after the desalination of IL.The deposits on the surface or into the membrane which is probably caused by[Bmim]+was characterized by scanning electron microscopy,elemental analysis and Fourier transform infrared.     

Current-voltage curves for a cation-exchange membrane in methanol-water electrolyte solutions

The current-voltage curves for a cation-exchange membrane separating two equal methanol-water electrolyte solutions were determined under different experimental conditions. From these curves, the values of the limiting current density, I(L), were determined. The influence of the volume percentage of methanol in the solvent on the results was analyzed. The presence of methanol in the solutions was shown not to affect the shape of the current-voltage curves typical of aqueous solutions. However, the system resistance and the values of the limiting current density were both greatly influenced by the content of methanol. Thus, the value of the resistance increased and the limiting current decreased with increasing methanol content of the solution.     

Effect of modification of a sulfonic cation-exchange membrane with cross-linked chitosan on the selectivity of the transport of magnesium and calcium ions relative to sodium ions

A procedure was suggested for modification of MK-40 sulfonic cation-exchange membrane by application of a chitosan layer onto its surface, followed by cross-linking of this layer with epichlorohydrin. The transport of magnesium, calcium, and sodium ions through cation-exchange membranes modified with cross-linked and non-cross-linked chitosan was studied.     

Application of relaxation periods during electrodialysis of a casein solution: Impact on anion-exchange membrane fouling

Electrodialysis (ED) is a membrane process used on a large scale. However, one of the common problems is fouling of ion-exchange membranes stacked in the cell. The use of pulsed power, consisting in applying a constant current density during a fixed time of application (T_on) followed by a pause duration (T_off), was demonstrated recently as an effective fouling mitigation method for electrodialysis. Up until now, no work has investigated the potential of electrodialysis using pulsed electric field on protein fouling. The aim of the present work was to study the influence of pulsed electric field (PEF) with a low frequency square shaped periodic signal (T_on = 10 s–T_off = 10 s, T_on = 10 s–T_off = 40 s) in comparison with dc current during electrodialysis of a casein solution at different current densities (10, 20 and 30 mA/cm²) on membrane fouling. It appeared from these results that PEF, under certain conditions of pulse, would avoid fouling on anion-exchange membranes. For 10 s–40 s pulsed electric field conditions, no fouling was observed with any density, while for 10 s–10 s PEF conditions, fouling appeared only at current density over 10 mA/cm². dc current, whatever the current density conditions, led to a fouling on the diluate side of the AEM. Furthermore, when fouling occurred, magnitude layer thickness and dry weight increased with the applied current density. The nature of the fouling was identified as 97% protein. The protein fouling would be due to the dissociation of water molecules and/or heat increase at the anion-exchange membrane interface. The relaxation time of the pulse would limit both phenomena on the membrane.     

Electrodialysis of calcium and carbonate high concentration solutions and impact on composition in cations of membrane fouling

Fouling, which is the accumulation of undesired solid materials at the phase interfaces of permselective membranes, is one of the major problems in electrodialysis. The objectives of the present work were to investigate the effect of the composition in calcium and carbonate of a model solution to be treated by conventional electrodialysis on their migration kinetics and the composition in cations of the membrane fouling. In the absence of sodium carbonate in the solution, no fouling was visually observed on anion-exchange membranes (AEM) and fouling was observed only at 1600 mg/L CaCl2 on cation-exchange membrane (CEM), while at only 800 mg/L CaCl2 with sodium carbonate, a deposit was observed on both membranes. This difference could be explained by the fact that carbonate has a high buffer capacity, and the time to reach pH 4.0 was then longer than the one without carbonate. Consequently, the migration of the ionic species was carried out over a longer period of time during ED treatment with sodium carbonate addition and in extent the demineralization rates were higher: 43 vs 86%. For treatment with sodium carbonate and 1600 mg/L CaCl2, the higher migration during ED treatment, increased the concentration of calcium, from 14.24 to 93.38 mg/g dry membrane and from 0.74 to 10.27 mg/g dry membrane for CEM and AEM, respectively. Due to the basic pH on the side of the membrane in contact with the NaCl solution, the calcium would precipitate to form calcium hydroxide on CEM while the calcium migrated through the CEM was blocked by the AEM where it formed another fouling.     

A microfluidic membrane chip for in situ fouling characterization

A new method for non-invasive in situ monitoring of a microfiltration process is described. In microfiltration systems, local information on the deposition characteristics can be used to determine the cake behavior during a filtration run. Typically, non-invasive methods of fouling study are restricted to specialized membranes, or require highly complex systems. This study employs the use of synthetic embedded channel membranes, with channels separated by a porous structure (active membrane). The characteristics of the active membrane have been analyzed. Deposition on the membrane surface can be observed and monitored optically across the width of the feed channel. This can be used to observe the liquid hydrodynamics in the channel as well as the local cake properties in time. In dead end filtration, it has been observed that with 6 μm particles, the cake initially deposits towards the end of the membrane. However, as filtration continues, the deposition changes with more local deposition towards the channel entrance, leading to a more homogeneous cake layer.     

An approach to membrane fouling characterization by confocal scanning laser microscopy

Confocal scanning laser microscopy (CSLM) is an optical microscopic technique that, among other advantages, can provide high-resolution images from different depths of a three-dimensional object, therefore rendering invasive techniques unnecessary for sample preparation. CSLM in fluorescence mode is a powerful technique in biological applications and in the microscopy of food materials. The main goal of the present study is to develop the appropriate strategies so that CSLM can be used for membrane fouling characterization during the filtration of protein solutions. Single and binary solutions of BSA–fluorescein and ovalbumin–Texas red conjugates were filtered using 0.8 μm polycarbonate membranes. Samples of the membranes at the end of the filtration runs were analyzed by CSLM. A standardized protocol for sample analysis by CSLM was developed and applied in this study. The most significant results show that CSLM can be used to visualize BSA–fluorescein and ovalbumin–Texas red conjugates on top of and inside the membranes, and that they can be distinguished when they jointly foul the membrane. Finally, if the appropriate sectioning is applied a 3D reconstruction of the membrane and the adsorbed/deposited protein can be obtained which give information on the fouling morphology.     

Impact of electrodialytic parameters on cation migration kinetics and fouling nature of ion-exchange membranes during treatment of solutions with different magnesium/calcium ratios

During electrodialysis (ED) treatment of solutions with different Mg/Ca ratios (R = 0, 1/20, 1/10, 1/5 and 2/5) and in different pH conditions (acid, neutral and basic), foulings on ion-exchange membranes were previously characterized and identified, by the way of X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses. A mineral fouling was observed in neutral and basic conditions (for R = 1/5 and 2/5) on the anion-exchange membrane (AEM) concentrate side and in basic conditions on the cation-exchange membrane (CEM) concentrate side as well as on the diluate side for R = 1/5 and 2/5. The objectives of this present work were to link the morphological characterization and identification of membrane fouling to electrodialytic parameters and cation migration kinetics. It appeared that the CEM permselectivity was severely affected in basic conditions for R ≥ 1/5. The consequence of this alteration was the migration of OH⁻ through the CEM, a pH increase in the diluate compartment and different treatment durations. The calcite observed on AEM concentrate side for Mg/Ca ≥ 1/5 would be due first to the particular operating conditions such as the recirculation of the concentrate solution, and also to the supersaturated conditions reached or not at the AEM interface and favourable pH conditions.     

Effect of concentrate solution pH and mineral composition of a whey protein diluate solution on membrane fouling formation during conventional electrodialysis

The aim of this work was to study the effect of the concentrate solution pH and the composition in calcium, carbonate and protein of the diluate solution to be treated by conventional electrodialysis on the fouling of ion-exchange membranes. Conductivity, system resistance, pH of the diluate and cation migration were monitored to follow the evolution of the demineralization. Total cation migration was similar for all conditions although different forms of fouling were identified after three consecutive 100 min electrodialysis treatments. The nature of fouling and the membrane surface fouled depended on the concentrate pH value, the diluate mineral composition and the intrinsic composition of the whey isolate. Once conditions leading to membrane fouling were identified, an alternative configuration for our electrodialysis stack is proposed to prevent fouling onset.     

Electric mass transport of sodium chloride through cation-exchange membrane MK-40 in dilute sodium chloride solutions: A rotating membrane disk study

The polarization properties of an electromembrane system consisting of an MK-40 membrane and a dilute sodium chloride solution are investigated with an experimental apparatus, which includes a rotating membrane disk with a horizontally positioned membrane. For the electrochemical systems of MK-40/0.01 M NaCl and MK-40/0.001 M NaCl, effective ion transport numbers and partial current-voltage curves are determined for sodium and hydrogen ions, and limiting-current densities and the diffusion-layer thickness are calculated as functions of the rotation rate of the membrane disk. The space-charge distribution in the diffusion layer and in the membrane is calculated for various current densities and rotation rates of the membrane. It is shown that when electric-current densities are greater than the limiting value, ion fluxes of the salt increase as a result of a decrease in the effective thickness of the diffusion layer. This decrease is caused by the development of space charge, electroconvection, water dissociation, and the exaltation effect in the region near the membrane. It has been established that in dilute solutions the limiting current is not purely electrodiffusive in nature.     

The quantitative separation of calcium from magnesium,aluminium and other elements by cation-exchange chromatography in ethanol-hydrochloric acid

Magnesium can be separated from calcium by elution with 3.0 M hydrochloric acid containing 60% ethanol from a column of AG₅₀W-X8 cation-exchange resin. Calcium is retained and can be eluted with 3.0 M hydrochloric acid or 2.0 M nitric acid. The separation factor of (α_Mg^ca=5.6 is considerably higher than that in aqueous hydrochloric acid and comparable to those obtained with organic complexing reagents. Separations are sharp and quantitative; up to 10 mmol of magnesium can be separated from 0.01 mmol of calcium and vice versa on a 60-ml column. Al, Fe(III), Mn, Ni(II), Co(II), Zn, Cd, Cu(II), Pb(II), U(VI), Be, Ga, Ti(IV) in the presence of H₂O₂ and many other elements accompany magnesium and can be separated from calcium quantitatively. Sr, Ba, Zr, Hf, Th, Sc, La and the rare earths are retained together with Ca, but can be separated by other methods.     

An approach to fouling characterization of an ion-exchange membrane using current-voltage relation and electrical impedance spectroscopy

Fouling phenomena of an anion-exchange membrane by bovine serum albumin (BSA) were investigated using current-voltage relation and electrical impedance spectroscopy (EIS) in this study. Electrochemical parameters of the Neosepta CMX cation- and AMX anion-exchange membrane (Tokuyama Corp., Japan) such as limiting current density (LCD), transport number, plateau length, and fraction of the conducting phase were measured. Fraction of the conducting phase of the ion-exchange membranes, calculated from the modified Sand equation, played an important role in determining the electrochemical parameters in the presence of foulants such as BSA. Fraction of the conducting phase of the AMX membrane significantly decreased in the presence of BSA. Two distinguishable slopes were observed in the over-LCD region of the current-voltage (I-V) curve, indicating the change of resistance. To further elucidate the phenomena, the electrical impedance spectroscopic study was carried out using the offset alternating current. It was found that the negatively charged loose fouling layer changed to the dense deposited BSA on the surface of the AMX membrane occurring along with enhanced water dissociation phenomena at the surface of the fouled AMX membrane at a higher current density. This result was confirmed by water dissociation experiments in a six-compartment electrodialysis cell.     

Chitosan solubilization by bipolar membrane electroacidification: Reduction of membrane fouling

Chitosan, a biopolymer obtained from chitin deacetylation, was solubilized by bipolar membrane electroacidification (BMEA). We showed earlier that limitation in solubilization process was mainly due to chitosan precipitation in the acidified compartment. If fouling can be reduced or prevented, BMEA could be an environmentally attractive method for chitosan solubilization. The purpose of the present work was to identify process conditions that could reduce chitosan fouling in BMEA. The factors studied were: the type of salt in the acidified compartment (NaCl or CH₃COONa); the type of electrolyte in the basified compartment (KCl or HCl); and the current density (4 or 20 mA/cm²). Chitosan fouling was successfully reduced by a combination of NaCl salt and HCl electrolyte, while 98% chitosan solubilization yield was achieved by operating at a current density of 4 mA/cm² with NaCl/KCl configuration with no apparent fouling. This work showed that water dissociation at the interface of the anionic membranes was the main factor responsible for chitosan precipitation.