Electrodeionization Research Articles

Numerical simulation of the electrodeionization (EDI) process for producing ultrapure water

A steady-state model was established to simulate EDI process for producing ultrapure water (MixEDI), the dilute compartment of which is filled with mixed cation and anion-exchange resins. By calculating the mathematical model which includes water dissociation mechanism, ionic status of ion-exchange resin, concentration polarization status and the concentration distribution of water dissociation products are obtained. The influence of water dissociation on the current efficiency, removal rate and pH value of EDI effluent is investigated. The existence of water dissociation catalyst at anion-exchange membrane (AM) makes the water dissociation current of the AM much larger than that of CM. The result is that the amount of electro-regenerated cation-exchange resins is much larger than that of anion resins. This is the reason why the removal rate of salt cation much larger than that of salt anion in EDI for producing ultrapure water. Thus, at the target percentage removal, water dissociation at AM surface is excessive and the one at CM surface is insufficient. We assume that there is also some water dissociation catalyst at CM surface. It is found that the improved water dissociation at CM could increase the percentage removal of salt anions and the current efficiency.

Recovery of nickel ions from simulated electroplating rinse water by electrodeionization process

Pure water production and recovery of Ni(2+) from dilute NiSO(4) solution via improved electrodeionization (EDI) process were investigated. It was indicated that, with the feed water containing 50 mg L(-1) Ni(2+), the dilute resistivity of the EDI could achieve 3 MOmega cm which gave a Ni(2+) rejection more than 99.8% while the Ni(2+) was concentrated as high as 1,583 mg L(-1) in the concentrate stream. The current efficiency was as high as 40% and the concentrate factor could reach 31.7. The experiment results demonstrated that, just in a single process, it was feasible to purifying and concentrating dilute nickel wastewater synchronously by the EDI technology.

ESIEX - electrical swing ion exchange: pH control for biomolecule purifications or separations

Due to their amphoteric properties, biomolecules such as amino acids, peptides and proteins are separated as a function of their charges. A classical method to separate these biomolecules is column ion exchange with pH steps. The proper pH is usually obtained in a chemical way with different buffers of variable dilutions, and this is the source of large amount of effluents. The objective of this work is to study the possibility to control the pH in an electrodeionization (EDI) process without using buffer solutions.EDI process is a hybrid technique combining electrodialysis and ion exchange (IEX). In EDI, three types of phenomena are likely to generate protons and hydroxyl ions and to play a role in pH: formation by water oxidation or reduction at electrodes, water splitting and IEX. Two parameters play a role in these processes: the choice of current density and the nature of IEX resins (anion, cation or mixed).To purify a molecule, it is for instance possible to: -keep key molecule in feed compartment and generate the other species migration: the choice of resin type and of current density must allow maintaining the pH at isoelectric point of key molecule -generate selective migration of key molecule through an ion exchange membrane: pH must allow this molecule to have the required charge-fix selectively key molecule on ion exchange resin. The experimental method realized in the present work specifically aims at identifying the coupling of processes for a dipeptide fixation on an ion exchange resin placed in one compartment of electrodialysis with bipolar membrane system.

Purification of Dialysate Water by Electro Deionization System for Hemodialysis

We introduced a new EDI (Electro Deionization) system into a conventional type of RO system for the purification of the dialysate water. The EDI stack consists of a dialyzer where a cation exchange membrane and an anion exchange membrane were alternately arranged, and an ion exchange resin was filled between the membranes. Electric conductivity of the RO water decreased from 3.85 μS/cm to 0.07 μS/cm after the treatment of EDI, and silica and aluminum with a difficult removal by a conventional RO system were removed by the EDI system. Furthermore, endotoxin can be highly removed, and the rejection of the endtoxin was increased with the current. The EDI system was available for the purification of the dialysate water.

Numerical simulation of the electrodeionization (EDI) process accounting for water dissociation

The electrodeionization process (EDI) is usually operated at overlimiting current density, and is thus characterized by water dissociation and concentration polarization. We attempt to study the useful and harmful effects of water dissociation on the EDI process. A numerical steady state model was established to simulate the process of EDI, accounting for the effects of water dissociation. The differences in concentration polarization of membranes were investigated to study the effects of water dissociation on cation and anion membranes. Protons produced by water dissociation caused the resin to transform into the H-form. The H-form resin, which has high conductivity and high transport number, depletes protons in the interstitial solution. This explains the experimentally detected phenomenon that at high current densities, the pH value of the effluency of the dilute compartment (DC) stops decreasing when current increases. We suggest that the useful role of water dissociation in EDI is due to the H-form resin bringing more salt cations of the interstitial solution into the resin phase, thus producing a high conductivity channel for the electro-migration of the salt cations. This mechanism avoids the decrease in salt ion conductivity brought about by concentration polarization. The disadvantageous effect of concentration polarization on the transportation of salt ions in interstitial solution is thus lessened. An intermediate point between the useful and harmful effects of water dissociation was determined by the dependence of current efficiency and removal rate for both cations and anions as a function of current density.

Simulation and Optimal Design of Electrodeionization Process: Separation of Multicomponent Electrolyte Solution

Electrodeionization (EDI) technology combines the operation principle of both ion exchange and electrodialysis in one single unit and overcomes the disadvantages of either unit. This technology has been widely used in the production of ultrapure water due to its better performance and economical operation at low feed concentrations. As a result, there have been several studies that deal with application of this technology for the removal of ions from water. However, except for a few studies, most of the reported works deal with the experimental study of EDI. This study deals with the development of a mathematical model for deionization of electrolyte solutions containing more than one ion and ions that are multivalent in nature. The model validity is verified by comparing the results with a few experimental observations. This is followed by a sensitivity study of the EDI unit to explore the effect of various operating and system parameters on the ion removing ability of the EDI unit. Consequently, a syste...

Removal of low concentrations of hardness ions from aqueous solutions using electrodeionization process

A specially designed electrodeionization (EDI) stack was used for deep softening from simulated aqueous solutions containing low concentrations of hardness ions. Characteristics of the current–voltage ( I– V) and resistance–voltage ( R– V) curves of EDI and electrodialysis (ED) were compared at first, and then removal of Ca 2+ ions was carried out in a continuous experiment under a constant applied voltage. In order to further investigate the abilities of EDI to remove hardness without chemical regeneration, performances of different processes were compared under various experimental conditions of applied voltage, concentrate flow rate, and feed solution composition. It was shown that with feed Ca 2+ concentration of 5 mg dm −3, the dilute resistivity of 3.45 MΩ cm was obtained when under optimized operation conditions. It was also found that applied voltage and concentrate flow rate had remarkable impacts on the migration of Ca 2+ ions. A dimensionless coefficient K t was put forward to distinguish the “enhanced transfer” and “electoregeneration” mechanisms. It was found that scaling formation due to water dissociation on the membranes surface could be avoided under reasonable operation conditions. In addition, no significant influence on final hardness rejection was revealed with multi-species of feed solutions (Ca/Mg). The experimental results have demonstrated the feasibility of deep water softening by improved EDI technology.

Removal of copper ions from electroplating rinse water using electrodeionization

An improved configuration of the membrane stack was adopted in the electrodeionization (EDI) cell to prevent precipitation of bivalent metal hydroxide during the running. The operational parameters that influenced the removal of copper ions from the dilute solution were optimized. The result showed that a moderate decrease in the inlet pH value and a moderate increase in the applied voltage could achieve a better removal effect. The steady process of electroplating wastewater treatment could be achieved with a removal efficiency of more than 99.5% and an enrichment factor of 5∼14. The concentration of copper in purified water was less than 0.23 mg/L. This demonstrated the applicability of recovering heavy metal ions and purified water from electroplating effluent for industrial reuse.

Performance evaluation of electrodeionization process based on ionic equilibrium with plate and frame modules

The paper describes results of experiments carried out in plate and frame electrodeionization (EDI) modules, aiming to elucidate ion mass transfer phenomena between ion-exchange resin and membrane for more effective desalination. The selective separation rate of different ions in process was investigated with conventional theories such as double electric layer and the Donnan theory to check its consistence. Further, as comparing with ordinary electrodialysis (ED) process, no obvious limiting current point was observed in EDI performance, which was usually used as a criterion for process optimization. Behavior of ion migration in the fluids with different configurations under different operating parameters was also examined and the ionic equilibrium analysis of EDI process was found to be beneficial to determine whether it was in the steady state or in the regeneration stage of operation.

Comparison of different ED stack conceptions when applied for drinking water production from brackish waters

Three types of electrodialysis (ED) and electrodeionization (EDI) stacks are studied when treating NaCl solutions in the range of concentrations from 10 − 5 to 3 × 10 − 2 M: (1) with a non-conducting network spacer, (2) with a mixed ion-exchange monolayer bed, (3) with profiled membranes. The Sherwood number characterizing the desalination rate is measured in recirculation regime when the concentration of feed solution is slowly decreasing in time. The data obtained are compared with the results calculated and measured for “empty” desalination channels not containing any filling. It was established that the measured Sherwood numbers at overlimiting currents in stacks with filling are higher than that in “empty” channel under the same potential drop, while the latter, in turn, are higher than the Sherwood numbers calculated for limiting current density. The mass transfer rate in several cases (under 2.5 V par cell pair in diluted, < 10 − 4 M, solutions and high flow rates, > 7 dm 3 h − 1 dm − 1 , a stack with profiled membranes) may be 4 times higher than that in “empty” channel at limiting current density. The stacks with profiled membranes are shown to be effective in the overall range of concentrations. The stacks with non-conducting spacer can be used if the feed solution concentration is not too low, > 2 × 10 − 3 M in our experiments. The stacks with ion-exchange resins are more preferable in the case of treatment of strongly diluted solutions (e.g. at the second stage following an ED with profiled membranes), as well as in the cases where it is necessary to use a compact unit characterized by a low flow velocity, though the power consumption is slightly higher.

Description of mass transfer characteristics of ED and EDI apparatuses by using the similarity theory and compartmentation method

A semi-empirical approach to treat, predict and scale transfer characteristics of electrodialysis (ED) and electrodeionization (EDI) stacks used for the desalination of dilute solutions in overlimiting current modes is described. The approach is based on the recent theoretical ideas about the mechanisms of overlimiting current transfer in membrane systems. In particular, electroconvection and exaltation effects are discussed. For scaling, the similarity theory and compartmentation method are used. As a result, relatively simple equations are obtained, which describe the ED (EDI) characteristics (the mass transfer coefficient, the Sherwood number, the degree of desalination and others) as functions of four parameters: the inlet concentration, the solution flow rate, the potential drop over a cell pair and the length of desalination channel. The comparison of experimental measurements and calculations is examined for several types of ED and EDI stacks: without filling or containing non-conducting spacer, ion-exchange bed or profiled membranes.

Removal of metal ions from electroplating effluent by EDI process and recycle of purified water

In this paper, an improved configuration of the membrane stack was adopted in the electrodeionization (EDI) cell to prevent precipitation of bivalent metal hydroxide during the running. The operational parameters that influenced the electro-regeneration of cation exchange resins loaded with nickel ions were optimized, and the result showed that addition of a small amount of Na 2SO 4 into the electrode solution, moderate increase of applied voltage and initial metal ion concentration could get better regeneration effect. A steady and continuous process of electroplating wastewater treatment could be achieved with current efficiency of 33–38%, removal efficiency of at least 99.8% and enrichment factor of 22–36. Concentration of metal ions in purified water was 0.03–0.09 mg/L for nickel, 0.07–0.10 mg/L for copper, 0.06–0.15 mg/L for zinc, 0.07–0.10 mg/L for cadmium and 0.08–0.17 mg/L for chromium, respectively. This demonstrated the applicability of recovering heavy metals and purified water from electroplating effluent for industrial reuse.

Influence of voltage and flow rate on electrodeionization (EDI) process efficiency

Electrodeionization (EDI) is being applied more and more to produce ultrapure water, especially in the semi-conductor industry. The continuous electrical regeneration of the ion-exchange (IEX) mixed bed is the main advantage of this recent technology. EDI couples two well known effects: electrodialysis and IEX. In spite of its rapid development, there are no established theories, design equations nor clear mechanisms of regeneration and transport. The present research work deals with EDI process efficiency. We have investigated the influence of the applied voltage V and the flow rate Q on cell efficiency by measuring current intensity I, and inlet C in and outlet C out concentrations of the treated solution. The efficiency R is defined by: R (%) = 100 (∆ C/ C in) Q/ Q where C = C in− C out and Q∆ C = J is the mass transfer flux. The main finding was that an original, empirical, and simple equation between the efficiency R and the flow rate Q is established: R = K′∆ VQ − n where n ≈ 0.5 or Log R = Log ( K′∆ V)− n Log ( Q), a linear logarithmic equation between R and Q. The mass transfer flux J = Q∆ C is then directly proportional to Q and inversely to Q : J = K ⁢ Q n ≈ K ⁢ Q 0.5 . This is an important result because it presents strong analogies with the habitual equations of electrochemical hydrodynamics (rotating, porous and packed-bed electrodes). The other interesting results are: (1) the current vs. voltage characteristic curve is constituted of three main linear parts, with an optimal zone (4.5–12 V); and (2) the water dissociation threshold voltage was clearly shown (12 V) and coincides with the beginning of a decrease in efficiency.

The removal of silicon and boron from ultra-pure water by electrodeionization

The method for removal of silicon and boron from ultra-pure water by EDI (electrodeionization) was investigated. By investigating electric current, conductivity, flow of feed water in dilute and concentrate chambers, and pH value, the optimal conditions for removal of silicon and boron are obtained. With 1000 μg/L SiO 2 in feed water, the concentration of silicon in product water can be decreased to 2.66 μg/L, which is currently the best result in our country. With 50 μg/L boron in feed water, less than 1 μg/L boron in product water can be obtained. All these results meet the requirements for ULSI (ultra large scale integrated circuits) (silicon: ≤3 μg/L, boron: ≤1 μg/L).

Development of an electrodeionization process for removal of nitrate from drinking water Part 2: Multi-species testing

A series of bench-scale experiments was performed with a novel electrodeionization (EDI) process to determine its capacity to remove NO 3 − from water in the presence of competing species. Experiments were performed with deionized water that was dosed with selected anionic and cationic species and with two natural waters that contained differing concentrations of dissolved solids. Extended duration experiments were performed to assess the ability of the process to sustain NO 3 − removal over a longer period of time. It was found that the use of ion-selective resins in the EDI unit resulted in a process that was more nitrate selective and energy efficient that when a general anion-exchange resin was employed. Operation in EDI mode was found to be superior to operation without resins (ED mode). The most highly selective and energy-efficient operation was at the lowest applied voltage of 2 V per membrane pair. The impact of competing species on NO 3 − flux was independent of the competing species when expressed on an equivalence basis. At elevated concentrations of multiple competing species the flux of NO 3 − was reduced. The removal of NO 3 − from two natural water streams revealed the significance of the water composition on the performance of the EDI unit with respect to the selectivity of NO 3 − removal and energy consumption. When treating the Manotick well water, the EDI unit required more energy per equivalence of NO 3 − removed and was less selective than when treating the Ottawa potable water that was lower in dissolved solids. Extended duration experiments with the dosed Ottawa water over a period of 48 h did not reveal any decline in fluxes.

Purification of caprolactam by means of an electrodeionization technique

In this study an experimental method for purification of a system containing a 30% caprolactam aqueous solution was attempted at room temperature. The impurities in the system were removed by using the continuous electrodeionization (EDI) device, which is an ion-exchange membrane stack with 40 diluting compartments filled with mixed ion-exchange resin beads. The optimal working parameters for this technology were obtained during a 5-month test. It was shown that the conductivity of the treated caprolactam solution was considerably lower (10 μS/cm) in comparison to the feed caprolactam solution (30 μS/cm). In addition the absorption value had decreased by 20%.

Qualification of an electro-deionization module via experimental design and ion-chromatographic studies

To meet the needs of the laboratory-water market, a modified electro-deionization (EDI) module has been developed to produce Type 2 purified water. An EDI module consists of desalting and concentrating fluidic compartments that are both filled with anion and cation ion-exchange resins; an anode and a cathode electrode are at opposite ends. In the design in this research, the anode electrode is segmented into three parts and individual dc amperages are applied to each segment; the cathode electrode is a single common electrode. Critical to the performance and longevity of this type of EDI module are: (1) the optimization of the applied dc amperages and (2) the ionic mass balance (i.e., the concentrations of specific and total ions of the RO feedwater to the module compared to the concentrations in the water exiting the module via the desalting and concentrating compartments). To determine a suitable current for each electrode pair, a full-factorial experimental design was developed and employed. For the application of this combination of amperages, the critical parameter of specific-ion mass balance was determined using ion-chromatographic measurements.

Qualification of an electro-deionization module via experimental design and ion-chromatographic studies

To meet the needs of the laboratory-water market, a modified electro-deionization (EDI) module has been developed to produce Type 2 purified water. An EDI module consists of desalting and concentrating fluidic compartments that are both filled with anion and cation ion-exchange resins; an anode and a cathode electrode are at opposite ends. In the design in this research, the anode electrode is segmented into three parts and individual dc amperages are applied to each segment; the cathode electrode is a single common electrode. Critical to the performance and longevity of this type of EDI module are: (1) the optimization of the applied dc amperages and (2) the ionic mass balance (i.e., the concentrations of specific and total ions of the RO feedwater to the module compared to the concentrations in the water exiting the module via the desalting and concentrating compartments). To determine a suitable current for each electrode pair, a full-factorial experimental design was developed and employed. For the application of this combination of amperages, the critical parameter of specific-ion mass balance was determined using ion-chromatographic measurements.

Performance of a novel electrodeionization technique during citric acid recovery

This paper concerns with the behavior of an electrodeionization (EDI) system for concentration of citric acid from fermentation broth. Commercially cation-exchange membrane (MC-3470) and anion-exchange membrane (MA-3475) were used as ionic selective barriers of the EDI stack. The diluted compartments of the EDI stack were filled with mixed ion-exchange resins (purolite strong acid cation-exchange, C-100E and strong base type I anion resins, A-400). The experiment used feeds with citric acid concentration in the range of 500–10,000 ppm and feed flow rate in the range of 1–4 l h −1. The V– I characteristics indicated that there were essential differences in current transport and electrical resistance between the EDI and the electrodialysis (ED) processes. Moreover, the overall current efficiency was in the range of 40–96% and has been found to be a function of feed concentration and current density. The performance of the EDI system was also stable during 24 h operation.

Ultrapure Water Production by Continuous Electrodeionization Method: Technology and Economy

Electrodeionization (EDI) and mixed bed deionization (MBDI) processes usually occupy the same position in the technological scheme of ultrapure water production, being used for the additional purification (conditioning) of reverse osmosis (RO) permeate (see, [1, 2]). A comparative analysis of the main differences of EDI and MBDI allows these methods to be more expediently used in water conditioning technology. The MBDI process has been used on a commercial scale for more than three decades, mostly in combination with a two-stage RO process. For this method, the content of active chlorine in the initial water flow is maintained within 0.05 – 0.08 mg liter in order to provide for a 3 – 4 month cellulose acetate regeneration cycle in the RO membranes. The ion exchanger is regenerated by treatment with strong acid and alkali solutions, which yields considerable volumes of highly mineralized waste waters implying the need for thoroughly monitoring the pH level. The EDI technology does not involve chemicals and does not require scrubbers for the flow neutralization after acid and alkali regeneration, reservoirs for the storage of concentrated acids and alkalis, the corresponding pumping equipment and valves, and double pipelines of corrosion-resistant steel, which significantly reduces both capital equipment and operating costs.