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.     

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.     

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.     

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.     

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.     

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.     

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.     

Impact of cation concentrations on fouling in membrane bioreactors

In this study, the interaction of calcium, magnesium, and sodium as well as impact of monovalent to divalent (M/D) cation ratio and magnesium to calcium (Mg/Ca) ratio in the feed wastewater on membrane fouling in submerged membrane bioreactor (MBR) was investigated. The protein and carbohydrate content of soluble microbial products (SMP) and extracellular polymeric substances (EPS) as well as their relative hydrophobicities was examined. The mixed liquor and its components (soluble and suspended solids) were analyzed for their filtration resistance, as reflected by the modified fouling index (MFI). Based on the findings of this study, the optimum conditions with respect to fouling rate were calcium and sodium concentrations of 36 and 140 mg/L, respectively, M/D of 1:1 and Mg/Ca of 5:1, with all parameters on an equivalent basis. High sodium concentration at high M/D ratio was found to decrease the floc size and increase the fouling rate. At the low M/D ratio of 1:1, introduction of magnesium was beneficial in reducing the fouling rate by increasing the EPS concentration and floc size and decreasing the SMP concentration and relative hydrophobicity in the supernatant. The fouling rate was found to be statistically correlated with the concentrations of Ca, Mg, and Na, with both Ca and Na adversely impacting fouling and Mg alleviating fouling propensity.     

Analysis of fouling potential in the electrodialysis process in the presence of an anionic surfactant foulant

Fouling of ion exchange membranes in an electrodialysis process is highly sensitive to the concentration of a surfactant. To investigate the influence of the fouling on the process performance, an anion exchange membrane was characterized by electrochemical properties as well as physical and chemical properties. The fouling potential was then quantitatively analyzed using the membrane fouling index as a function of the surfactant concentration. It was observed that the fouling mechanism is initiated by the micelle formation. That is, most of SDBS molecules form a fouling layer on the membrane surface at a higher concentration than the critical micelle concentration. Also the SDBS fouling mechanisms caused by the fouling layer were examined by the electrochemical impedance spectroscopy. The equivalent circuits show that the fouling potential of the system was increased by an additional layer, simultaneously increasing the electrical resistance to permeation of ions through the membrane. However, the SDBS fouling on the membrane was a reversible process.     

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.     

Composite phospholipid-calcium carbonate microparticles: influence of anionic phospholipids on the crystallization of calcium carbonate

The synthesis and characterization of calcium carbonate microparticles by reaction of calcium chloride and ammonium bicarbonate in the presence of negatively charged phospholipid mixtures of negative and zwitterionic phospholipids has been reported. Negatively charged phospholipids influence the crystal morphology of calcium carbonate and induce the formation of thermodynamically less stable veterite polymorph as opposed to calcite polymorph. The phospholipids are entrapped in the calcium carbonate microparticles during the crystallization process, with a uniform distribution of phospholipids in the interior of the microparticles. This phenomenon was exploited to encapsulate a model hydrophobic fluorophore, the tris(4,7-diphenyl-1,10-phenanthroline)ruthenium(II) dichloride complex, to simulate encapsulation of hydrophobic drug molecules. Thermogravimetric analysis reveals that, in these microparticles, the calcium carbonate and the phospholipid exhibit strong interactions.     

Effect of membrane morphology on the initial rate of protein fouling during microfiltration

Protein fouling remains a major problem in the use of microfiltration for many bioprocessing applications. Experiments were performed to evaluate the effect of membrane morphology and pore structure on protein fouling using different track-etched, isotropic, and asymmetric microfiltration membranes. Fouling of membranes with straight-through pores occurred by pore blockage caused by deposition of large protein aggregates on the membrane surface. However, the rate of blockage was a function of the membrane porosity due to the possibility of multiple pore blockage by a single protein aggregate on high porosity membranes. Membranes with interconnected pores fouled more slowly since the fluid could flow around the blocked pores through the interconnected pore structure. This behavior was quantified using model membrane systems with well-defined pore morphology constructed from track-etch and isotropic membranes in a layered series combination. These results provide important insights into the effects of membrane pore structure and morphology on protein fouling.     

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.     

Influence of membrane structure on fouling layer morphology during apple juice clarification

The flux behavior of 0.2 μm nylon, polysulfone (PS), polyvinylidene fluoride (PVDF) and polyethersulfone (PES) membranes was examined during dead-end microfiltration of commercial apple juice. On nylon membranes, a 0.1 μm thick surface fouling layer rapidly formed that acted as a secondary membrane. The colloidal particles retained by this surface layer aggregated to form a thick loose gel structure, producing an anisotropic fouling structure. In contrast, the 4 μm thick surface fouling layer of PES was slower to form and had a more open structure with a lower flux resistance per unit thickness. The morphology of the PES surface layer also did not differ dramatically from the loose gel structure that subsequently formed on top of this secondary membrane. The PS surface fouling layer was similar in structure to nylon whereas the PVDF layer more closely resembled that found with PES. The density of the surface fouling layer did not directly correlate to membrane surface hydrophobicity or pure water flux. Atomic force microscopy (AFM) indicated that surface roughness strongly influenced surface fouling layer morphology. The membrane surface appears to act as a template for the fouling process; therefore, smooth membranes (nylon and PS) produce a dense surface fouling layer whereas this same layer on rough membranes (PES and PVDF) is much more open. Consequently, the fluxes of PES and PVDF membranes are less affected by fouling formation.     

Morphological control of calcium carbonate crystallized in reverse micelle system with anionic surfactants SDS and AOT

The influence of a surfactant over water on the polymorphism and crystal size of calcium carbonate produced by reaction crystallization in microemulsion systems was investigated in a mixing tank reactor. The crystallization was induced by the reaction between two aqueous micelle solutions (Na2CO3-CaCl2) stabilized by anionic surfactants, SDS (sodium dodecyl sulfate) or AOT (sodium bis(2-ethylhexyl) sulfosuccinate). With increasing surfactant ratio to water, the water-in-oil microemulsion was stably developed and the morphology of the calcium carbonate crystallized in the micelles sharply transformed from calcite to vaterite. The influence of SDS on the polymorphism and crystal size of calcium carbonate was much clearer than that of AOT. In addition, with AOT, certain step changes in the morphology and crystal size occurred around a surfactant ratio to water (R=[H2O]/[surfactant]) of 15 due to a two-phase separation of the microemulsion.     

Effect of sodium-carboxymethylcellulose on inhibition of scaling by calcium carbonate and sulfate

Sodium-carboxymethylcellulose inhibits the crystallization of calcium carbonate and sulfate on the surface of stainless steel in a supersaturated aqueous solution at 80°C. An increase in the size of calcium carbonate crystals and a change of their structure is observed in the presence of sodium-carboxymethylcellulose. The data obtained indicate that development of new “green” reagents for oil and gas extraction, scale inhibitors, on the basis of sodium-carboxymethylcellulose is promising.     

Pore blocking mechanisms during early stages of membrane fouling by colloids

A method based on a simple linear regression fitting was proposed and used to determine the type, the chronological sequence, and the relative importance of individual fouling mechanisms in experiments on the dead-end filtration of colloidal suspensions with membranes ranging from loose ultrafiltration (UF) to nanofiltration (NF) to non-porous reverse osmosis (RO). For all membranes, flux decline was consistent with one or more pore blocking mechanisms during the earlier stages and with the cake filtration mechanism during the later stages of filtration. For ultrafiltration membranes, pore blocking was identified as the largest contributor to the observed flux decline. The chronological sequence of blocking mechanisms was interpreted to depend on the size distribution and surface density of membrane pores. For salt-rejecting membranes, the flux decline during the earlier stages of filtration was attributed to either intermediate blocking of relatively more permeable areas of the membrane skin, or to the cake filtration in its early transient stages, or a combination of these two mechanisms. The findings emphasize the practical importance of the clear identification of, and differentiation between mechanisms of pore blocking and cake formation as determining the potential for the irreversible fouling of membranes and the efficiency of membrane cleaning.     

Effect of electric fields on the thermal decomposition of calcium carbonate

The thermal decomposition of CaCO₃ powder and single crystal samples to CaO has been shown by X-ray diffraction and ESCA to be assisted by the application of external DC electric fields during the reaction. The effect is particularly marked at the positive electrode. The results are discussed in terms of both ionic migration effects and the field-induced electro-adsorption/electro-desorption of gaseous CO₂ at the surface of a semiconductor. Approximate calculations based on both theories show that ionic migration cannot account for the magnitude of the observed effects, but a reasonably satisfactory explanation is provided by the application of semi-conductor surface theory to the system.