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.     

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.     

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.     

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.     

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.     

Influences of colloidal stability and electrokinetic property on electrodialysis performance in the presence of silica sol

Negatively charged silica sol is known to lead to fouling of anion exchange membranes during electrodialysis (ED) as a result of its deposition on the membrane surface. It is known that the fouling potential is related to the physical and electrochemical properties of the silica particles as well as those of the anion exchange membranes. In this study, the properties of the silica sol were characterized in terms of its particle size, turbidity, and zeta potential in order to predict their effects on the electrodialysis performance. In the stability of colloidal particles, the critical coagulation concentrations of silica sol were determined as functions of ionic strength, cation species, and solution pH. In the electrodialysis of NaCl solution containing silica sol with various concentrations of CaCl(2), the colloidal behavior related to deposition and transport was examined during and after electrodialysis. The electrodialysis experiments clearly showed that the deposition and transport of silica sol during electrodialysis were related to the colloidal stability of dispersion.     

Effect of magnesium/calcium ratio in solutions subjected to electrodialysis: characterization of cation-exchange membrane fouling

Electrodialysis is based on the migration of charged species through perm-selective membranes under an electric field. Fouling, which is the accumulation of undesired solid materials at the interfaces of these membranes, is one of the major problems of this process. The aim of the present work was to investigate the nature and the morphology of fouling observed at different Mg/Ca ratios (R=0, 1/20, 1/10, 1/5, 2/5) on cation-exchange membranes (CEM) during conventional electrodialysis treatments. It appeared that for R=0, the fouling observed on the surface in contact with the basified concentrate was formed of only Ca(OH)2. As soon as magnesium was introduced into the solution treated, CaCO3 was observed. Furthermore, the X-ray diffraction results also identified the CaCO3 observed as calcite. To our knowledge, this is the first time that the presence of magnesium has been demonstrated to induce a CaCO3 fouling on CEM during electrodialysis.     

Preparation and electrochemical characterization of cation- and anion-exchange/polyaniline composite membranes

Composite membranes were prepared by chemical polymerization of a thin layer of polyaniline (PANI) in the presence of a high oxidant concentration on a single face of a sulfonated cation-exchange membrane (CEM) and quaternary aminated anion-exchange membrane (AEM). IR and SEM studies for both types of membranes confirmed PANI loading on the ion-exchange membranes. PANI composite ion-exchange membranes were characterized as a function of the polymerization time by ion-exchange capacity, coating density, and membrane conductance measurements. Membrane potential measurements were performed in various electrolyte solutions in order to observe the selectivity of these membranes for different types of counterions. Membrane potential data in conjunction with membrane conductance data was interpreted on the basis of frictional considerations between membrane matrix and solute. Electrodialysis experiments, using PANI composite ion-exchange membranes with 4 h polymerization time, were performed in single and mixed electrolyte solutions for observing electromigration of solute across PANI composite ion-exchange membranes. Relative dialytic rates of Na(2)SO(4), CaCl(2), and CuCl(2) were estimated with reference to NaCl on the basis of electrodialysis experiments and it was concluded that it is possible to separate different electrolytes using PANI composite ion-exchange membranes.     

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.     

Removal of strontium from an aqueous solution using co-precipitation followed by microfiltration (CPMF)

This study characterized the removal of strontium from an aqueous solution via co-precipitation followed by microfiltration (CPMF) on a lab-scale. Sodium carbonate was used as the precipitating agent. Ferric chloride was added to a settler at a dosage of 20 mg/L to improve strontium removal, and it was added to a membrane separator at a dosage of 10 mg/L to reduce membrane fouling. The concentration of strontium in the raw water used in this study was about 5 mg/L. In two intermittent tests, where dosages of sodium carbonate were 2000 mg/L and 1000 mg/L, the mean decontamination factors (DFs) were 237 and 158 and the mean concentration factors (CFs) were 288 and 462, respectively. Although the mean DF value was lower when the sodium carbonate dosage was 1000 mg/L instead of 2000 mg/L, the rate at which the specific flux (SF) of the membrane declined decreased as the amount of the effluent treated increased. The problem of strontium release at the beginning of the operation was controlled by a continuous test in which the dosage of sodium carbonate still was 1000 mg/L and the other parameters were the same as in the other tests. The results showed that the mean DF and CF were 157 and 480, respectively.     

Conversion of sodium lactate to lactic acid with water-splitting electrodialysis

The conversion of sodium lactate to lactic acid with water-splitting electrodialysis was investigated. One way of reducing the power consumption is to add a conductive layer to the acid compartment. Doing this reduced the power consumption by almost 50% in a two-compartment cell, whereas the electric current efficiency was not affected at all. Three different solutions were treated in the electrodialysis unit: a model solution with 70 g/L of sodium lactate and a fermentation broth that had been prefiltered two different ways. The fermentation broth was either filtered in an open ultrafiltration membrane (cut-off of 100,000 Dalton) in order to remove the microorganisms or first filtered in the open ultrafiltration membrane and then in an ultrafiltration membrane with a cut-off of 2000 Dalton to remove most of the proteins. The concentration of sodium lactate in the fermentation broth was 70 g/L, as well. Organic molecules present in the broth (peptides and similar organic material) fouled the membranes and, therefore, increased power consumption. Power consumption increased more when permeate from the more open ultrafiltration membrane was treated in the electrodialysis unit than when permeate from the membrane with the lower cut-off was treated, since there was a higher amount of foulants in the former permeate. However, the electrodialysis membranes could be cleaned efficiently with a 0.1 M sodium hydroxide solution.     

Gluconic acid production in bioreactor with immobilized glucose oxidase plus catalase on polymer membrane adjacent to anion-exchange membrane

Gluconic acid was obtained in the permeate side of the bioreactor with glucose oxidase (GOD) immobilized onto anion-exchange membrane (AEM) of low-density polyethylene grafted with 4-vinylpiridine. The electric resistance of the anion-exchange membranes was increased after the enzyme immobilization on the membrane. The gluconic acid productions were relatively low with the GOD immobilized by any method on the AEM. To increase the enzyme reaction efficiency, GOD was immobilized on membrane of AN copolymer (PAN) adjacent to an anion-exchange membrane in bioreactor. Uses of anion-exchange membrane led to selective removal of the gluconic acid from the glucose solution and reduce the gluconic acid inhibition. The amount of gluconic acid obtained in the permeate side of the bioreactor with the GOD immobilized on the PAN membrane adjacent to the AEM under electrodialysis was about 30 times higher than that obtained with enzyme directly bound to the AEM. The optimal substrate concentration in the feed side was found to be about 1 g/l. Further experiments were carried out with the co-immobilized GOD plus Catalase (CAT) on the PAN membrane adjacent to the AEM to improve the efficiency of the immobilize system. The yield of this process was at least 95%. The storage stability of the co-immobilized GOD and CAT was studied (lost 20% of initial activity for 90 d). The results obtained clearly showed the higher potential of the dual membrane bioreactor with GOD plus CAT bound to ultrafiltration polymer membrane adjacent to the AEM. Storage stability of GOD activity in GOD plus CAT immobilized on PAN//AEM membranes and on AEM.     

Electrodialysis: From an Idea to Realization

The historical validity demands some clarifications in the early history of electrodialysis. Early investigations with non-selective membranes are not to be recognized as electrodialysis-related although some of them have used the term electrodialysis. Only in 1950 the first synthetic ion-exchange membranes were produced (W. Juda and W.A. McRay). Preceding them membranes manufactured by T.R.E. Kressman were in fact a modified natural product—impregnated parchment membranes. Major electrodialysis" shortcomings such as membrane fouling and scaling were mostly overcome with electrodialysis reversal (EDR) commercial introduction by Ionics in 1967. Synthesis of novel sodium-selective membrane did not spark a burst in ED manufacturing. However higher demand for sodium-selective membranes can be expected in conjunction with nitrate-selective membranes for potable water denitrification—the essential step in water treatment in Europe. New break-through in ED progress was associated with development of apparatuses with ion-exchange resin beads filling desalting compartment. The analysis of energy consumption in ED process shows that the possibility of its reduction by traditional ways of membranes and spacers improvements is practically exhausted. However there is practically undeveloped option to lower the polarization resistance with help of electrodynamic instability (volumetric-charge development). Another possible improvement is creating membranes with special profile. As EDI targets higher levels of water purity the removal of weakly ionized ions becomes important issue.     

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.     

Electrodialytic transport properties of cation exchange membranes in the presence of cyclodextrins

After commercial cation exchange membranes (NEOSEPTA) had been immersed in an aqueous cyclodextrin solutions, electrodialysis of a 1:1 mixed solutions of alkaline earth metal ions and sodium chloride was carried out in the presence of cyclodextrins in the desalting side solution. Adsorption of the cyclodextrins in the membrane matrix was confirmed by IR spectrum, X-ray photoelectron spectroscopy (XPS) and the weight increase of each corresponding component of the membranes. As a result of adsorption of the cyclodextrins in the membranes, transport numbers of alkaline earth metal ions relative to sodium ions decreased compared with those of the membranes without cyclodextrins. Water content in the membrane increased and a ratio of calcium ions (alkaline earth metal ions) to sodium ions in the membrane phase decreased after adsorption of the cyclodextrins. This is due to existence of the compounds having hydrophilic outer surface, cyclodextrins, in the membrane matrix. The decrease in the transport number of calcium ions relative to sodium ions was due to both decrease in a ratio of mobility of calcium ions to that of sodium ions and decrease in an ionic ratio of calcium ions to sodium ions in the membrane phase.     

Ultrasonic time-domain reflectometry as a non-destructive instrumental visualization technique to monitor inorganic fouling and cleaning on reverse osmosis membranes

Fouling is readily acknowledged as one of the most critical problems limiting the wider application of membranes in liquid separation processes. A better understanding of fouling layer formation and its monitoring is needed in order to improve on existing cleaning techniques. The overall objective of this research was to develop a non-destructive, real-time, in situ visualization technique or device for fouling layer monitoring. Ultrasonic time-domain reflectometry (UTDR) was employed as a visualization technique to provide real-time characterization of the fouling layer. The fouling experiment was carried out with 2 g/l calcium carbonate solution. Results confirmed that there is a correspondence between the flux decline behavior and the UTDR response from membranes in reverse osmosis (RO) modules. The ultrasonic technique could effectively detect fouling layer initiation and growth on the membrane in real-time at different axial velocities. In addition to the measurement of fouling, the ultrasonic technique was also successfully employed for monitoring membrane cleaning. The UTDR technique, due to its extremely powerful capabilities and its use in monitoring devices, can be of great significance in the membrane industry.     

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.     

Secondary potential in electrodialysis membranes and the effect on permselectivity

The most important factor in the electrodialysis (ED) process is the permselectivity of the ion exchange membranes, which permit not only the separation of cations and anions in a solution, but also the separation of ions with the same sign but different valences. In this work, the mechanism of the permselectivity has been studied through the measurement of the potentials at different planes of the membrane. The experimental results have shown that there was a secondary potential inside ion exchange membranes in an electrodialysis process. At the membrane side touched with dilute solution, this secondary potential enhanced the external electrical field, and thus speeded up the passage of the corresponding ions in the dilute solution through the membranes; at the membrane side touched with concentrated solution, the secondary potential was contrary to the external electrical field and thus counteracted it, which could be very helpful by preventing the ions in the concentrated solution from entering the membranes. Obviously, the existence of the secondary potential might play an important role in the permselectivity of ion exchange membranes in ED processes.