Flow-Electrode Capacitive Deionization for Double Displacement

Research Article pubs.acs.org/journal/ascecg

Flow-Electrode Capacitive Deionization for Double Displacement Reactions Christian J. Linnartz,† Alexandra Rommerskirchen,†,‡ Matthias Wessling,†,‡ and Youri Gendel*,§ †

Aachener Verfahrenstechnik-Chemical Process Engineering, RWTH Aachen University, Turmstraße 46, 52064 Aachen, Germany DWI Leibniz Institute for Interactive Materials, Forckenbeckstraße 50, 52056 Aachen, Germany § Faculty of Civil and Environmental Engineering, Technion, Haifa 32000, Israel ‡

ABSTRACT: We report the design and analysis of a salt metathesis process using Flow-Electrode Capacitive Deionization (FCDI) for the generation of a concentrated valuable magnesium sulfate solution from dilute MgCl2 and Na2SO4 (or K2SO4) solutions. First, a batch mode decomposition and recombination of the MgCl2, Na2SO4, and K2SO4 solutions was studied with varying initial concentrations. Current efficiencies of ∼100% were observed for each cycle. In a so-called decomposition step, two different salt solutions are decomposed into electrically charged slurries having the counterions adsorbed. Swapping the slurries with the stored counterions during the recombination step results into new salt solutions upon discharge including the desired product. Both, purity of products and overall conversion of ions into the products, depend on operational parameters, while maximal achieved MgSO4 purity was as high as 93% with a concentration factor of 6.3 and a discharge current efficiency of ∼85%. Finally, a semicontinuous FCDI metathesis system was investigated. Performing the recombination step at appropriate process conditions also allows the concentration of the resulting product solutions by a factor as high as 81.5 with MgSO4 purity of ∼80% and current efficiencies of 96%. Future improvements in process configurations and membrane ion selectivity will render the process even more selective. KEYWORDS: Capacitive deionization, Recycling, Salt metathesis, Water reuse, Deionization

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INTRODUCTION Growing freshwater demand and a depletion of natural freshwater resources result in water scarcity in many areas of the planet.1,2 The global water demand is expected to increase by ∼55% from approximately 3600 km3 in 2000 to 5500 km3 in 2050.1 Desalination using thermal distillation (e.g., multieffect distillation (MED) and multistage flash (MSF) distillation) and membrane processes (e.g., reverse osmosis (RO), nanofiltration (NF), and electrodialysis (ED)) are the primary techniques today which are applied for the fresh water production from brackish and seawater.2,3 Water reuse via biological and/or physicochemical processes is yet another great source of water.4 Capacitive deionization (CDI) is normally an electrochemical technique for water desalination.5,6 CDI is based on electrosorption of ions in the electric double layer of highly porous electrodes.5,6 A basic CDI reactor comprises two solid carbon electrodes (e.g., activated carbon, carbon black, carbon felt, carbon cloth, carbon nanotubes, and carbon aerogels) separated by a spacer which avoids short circuiting of the electrodes and forms the channel for the flow of the solution to be desalinated. By application of an electrical potential between the anode and cathode, ions are adsorbed on the electrode’s surfaces. The maximum applied potential is normally < 1.23 V to minimize unwanted Faradaic reactions. Once the maximum capacitive adsorption of the electrodes is achieved, the cell is regenerated via short circuiting of the electrodes or by application of reversed potential. Membrane capacitive deion© 2017 American Chemical Society

ization (MCDI) reactors contain next to the two porous electrodes additionally cation- and anion-exchange membranes placed onto the surfaces of the cathode and anode, respectively.5−7 These additional membranes result in a better deionization efficiency as compared to membrane free CDI. Jeon et al.8 were the first suggesting the application of flowelectrodes to improve the performance of MCDI and termed this process flow-electrode capacitive deionization (FCDI). Rather than solid porous carbon electrodes, two aqueous solutions of either positively or negatively charged carbon particles (carbon slurry) flow between the ion conductive membranes and the corresponding current collectors.8 Ions from the feedwater channel pass the membranes and are adsorbed on the suspended carbon particles (e.g., activated carbon). The system described was operated in a batch mode process. Gendel et al.9 reported for the first time a continuous process for the desalination of water using two FCDI modules: one module was used for water desalination and another one for the regeneration of the slurry electrodes. Rommerskirchen et al.10 reported on a continuous desalination using a single module FCDI system. Instead of the ion-exchange membranes, Hatzell et al.11 used a porous separator in the FCDI cell. Energy Received: December 19, 2016 Revised: February 8, 2017 Published: March 7, 2017 3906

DOI: 10.1021/acssuschemeng.6b03086 ACS Sustainable Chem. Eng. 2017, 5, 3906−3912

Research Article

ACS Sustainable Chemistry & Engineering production from water salinity gradient was demonstrated by Hatzell et al.12 and Porada et al.13 A fluidized bed FCDI reactor was developed by Doornbusch et al.14 Other studies on FCDI can be found in relevant literature.15−17 Yet, the applications of FCDI are limited to desalination applications. This contribution aims to broaden the application window to metathesis reactions. The primary goal of the present study was the development of a FCDI−based configuration for the recovery of valuable chemicals from dilute water streams using a double displacement reaction (metathesis). Metathesis reactions that involve the interchange of ions between two salts (eq 1) are widely applied in the chemical field for acid−base neutralization (eq 2), potassium nitrate production (eq 3), and other processes.18 AB + CD → AD + CB

(1)

HCl(aq) + NaOH(aq) → NaCl(aq) + H 2O

(2)

XNO3(aq) + KCl(aq) → KNO3(s) + XCl(aq) (X = NH4 +or H+)

(3)

Electrochemical salt metathesis was previously reported in several publications using the electrodialysis technique. Thampy et al.19 reported the production of potassium carbonate from potassium sulfate and sodium carbonate. Ochoa et al.20 studied the production of iminodiacetic acid (IDA) from the sodium salt of iminodiacetic acid (Na2IDA) and sulfuric acid. Magnesium sulfate production from magnesium chloride and sodium sulfate was reported by Alheritiere et al.21 However, the main drawback of electrodialysis driven salt metathesis is the parasitic crossover of ions through the ion exchange membranes. Pisarska22 showed that during the metathesis of magnesium sulfate and potassium chloride magnesium ions contaminated the potassium sulfate solution. Magnesium chloride as product was contaminated with sulfate and potassium ions. The main hypothesis of the present study is that FCDI can be applied to the salt metathesis process aimed at the recovery of high purity valuable products with minimal parasitic crossover of ions as well as large concentration factors. The principle of the proposed electrochemical process is shown in Figure 1. The system comprises four identical FCDI modules. Two feed streams (MgCl2 and Na2SO4 in Figure 1) are pumped into the first pair of FCDI modules and the ions are charged separately on the four flow-electrodes. Next, the flow electrodes are led into the next pair of FCDI cells where the electrodes are discharged and the ions migrate into the two product streams: MgSO4 and NaCl in Figure 1. It is important to notice that each of the eight streams (two products, two feeds, and four flow-electrodes) can be operated in a batch, continuous, or a single pass mode, depending on the specific requirements and conditions of the process.

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Figure 1. Proposed process for the salt metathesis using a four-module FCDI system. MgCl2 and K2SO4 are decomposed in the left two FCDI cells. The electrode flows are swapped into the upper and lower flow cells on the right for the recombination into MgSO4 and KCl. for the recirculation of the treated water. Titanium fleece inserts (121 cm2) were installed in each slurry electrode compartment to increase the effective surface area of the cell. Two double-channel DC supplies (DPS 5315, ELV Elektronik AG) were used to control the FCDI cell potentials. The currents were measured with a standard optical digital ampere-meter with an accuracy of 100 μA. The liquids were recirculated through the FCDI modules by peristaltic pumps (Digital Drive 600 rpm, two-channel Easy-Load II pump head, 16′ neoprene tubing, Masterflex L/S). The electrical conductivity (EC) of the aqueous solutions was measured with Metrohm 914 pH/Conductometers reading the data from Metrohm 6.0919.140 conductivity probes and a Thermo Fischer Orion Star A 215 connected to a ORION 013005MD conductivity cell. The flow-electrodes comprised a 5 wt % solution of Norit SX Ultra Activated Charcoal (SigmaAldrich) in HPLC grade deionized water (Sigma-Aldrich). Analytical grade potassium sulfate, sodium sulfate, and magnesium chloride hexahydrate salts (Merck and Bio-Lab, Jerusalem) were used to prepare the electrolyte solutions. The concentrations of anions and cations in water samples of every experiment were determined using a Metrohm 881 Compact IC Pro-Anion chromatograph and a Thermo Scientific iCAP 6300 ICP spectrometer, respectively. The rate of water crossover between electrolyte and the flow electrode compartments was determined by weighing or volume measurement. Three types of experiments were conducted in the present study: (1) desalination and regeneration as performed in regular FCDI as reference experiments; (2) two-step batch metathesis comprising the two subsequent steps of (a) desalination/decomposition and (b) discharge/recombination combined with concentration; and (3) semicontinuous salt metathesis combining batch desalination with batch recombination/concentration using a continuous flow electrode. Desalination and Regeneration Experiments. These were done to study the performance of regular FCDI during the desalination of Na2SO4, K2SO4, and MCl2 solutions followed by the slurry regeneration of the flow-electrodes. These experiments are classical FCDI experiments, yet they generate new and important results as the different salts have not been reported before in literature. The system was operated in batch mode at a constant voltage of 1.20 V. Two flow-electrodes (100 mL each) and a saline water (100 mL) with different initial salt concentrations (3.5, 14, and 35 mM) were recirculated through the module at constant flow rates of 60 and 30

EXPERIMENTAL SECTION

The construction of the FCDI modules applied in the present study was identical to the cells used by Gendel et al.9 and Rommerskirchen et al.10 Each cell was made of PVC end-plates; epoxy-impregnated graphite electrodes with the engraved flow channels required for the recirculation of the flow-electrodes; cation and anion exchange membranes (Fumasep FKS-PET-130/ED-100 and Fumasep FASPET-130/ED-100, Fumatech GmbH) with an effective surface area of 121 cm2; and a 0.5 mm thick PVC spacer (Fumatech GmbH, ED-100) 3907


ACS Sustainable Chemistry & Engineering

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mL/min, respectively. Samples of electrolyte solutions were taken periodically from the water storage vessels for chemical analysis. The pH of the electrolyte solutions was measured periodically during several separate FCDI experiments. Once a complete desalination or the maximum salt adsorption capacity (SAC) of the slurries was reached, the electrical circuit was opened. After recirculating of the liquids for another couple of minutes, the regeneration was done by applying a reversed potential (−1.20 V) to the FCDI electrodes. The same flow-electrodes were used for all experiments conducted with the same salt type. After each experiment, the water compartment of the system was washed with plenty of deionized water while applying a discharge potential to remove adsorbed ions from the electrodes. Two-Step Salt Metathesis Experiments. During the first step of the process shown in Figure 2, K2SO4 (or Na2SO4) and MgCl2

Research Article

RESULTS AND DISCUSSIONS

Desalination and Regeneration Experiments. Figure 3 shows the results of several batch mode desalination− regeneration experiments performed with Na2SO4, K2SO4, and MgCl2 solutions starting with different initial salt concentrations. Decomposition and recombination of all three studied solutions was achieved successfully. Charge and discharge current efficiencies higher than 90% were obtained for Na2SO4, K2SO4, and MgCl2 solutions. Apparent SAC values for Na2SO4, K2SO4, and MgCl2 obtained in this study are 18.5, 27.1, and 22.3 mg/gcarbon, respectively. These values equal SAC values of 7.6, 9.1, and 13.7 mg NaCl/gcarbon for Na2SO4, K2SO4, and MgCl2, respectively. The maximum transferred amount of water was always lower than 2% of the initial volume of electrolyte solution (100 mL). During two FCDI experiments with 3.5 mM Na2SO4 and 3.5 mM MgCl2 solutions, the pH of salt water changed from initial almost neutral pH to a minimum of pH 5.2−5.5 during the charge and up to a maximum of about 9.0 during the discharge. This pH behavior is apparently due to the Faradaic side reactions that occur in CDI.23 Two-Step Salt Metathesis Experiments. Table 1 lists major operational parameters and results of the two-step FCDI metathesis experiments. The volumes of feed and product solutions defined the desired concentration factors of 1, 5, and 10 that were examined during these experiments. The amount of activated carbon, electrolyte volume, and its concentrations were adjusted to achieve complete desalination during the decomposition step of the process. Figure 4 shows the change of the conductivity of the input and output electrolyte solutions measured during the metathesis of 3.5 mM MgCl2 and 3.5 mM Na2SO4 solutions (exp. #1 from Table 1). Figure 4 shows that both solutions were completely desalinated (EC < 2 μS/cm) within the first step (decomposition) of the process. The reason for the different time evolution might be due to the different kinetics of the salt ions. In future research, we will discuss the influence of molar conductivity or hydration radius of the ions and the membrane properties in more detail to be able to predict the kinetics of decomposition. Next, the recombination step was initiated by swapping the loaded electrodes, and the product water solutions became enriched with KCl and MgSO4 products observed as an increase in conductivity. The final electrical conductivity values of feed and product solutions are different due to the different ionic compositions. Current efficiencies of almost 100% were achieved during the decomposition steps (CEcharge [%] in Table 1). The current efficiency of the recombination steps was less than 85% for most of the experiments. The purity of MgSO4 was in the range of 82.0− 97.9%. Parasitic crossover of sodium or potassium ions into the sulfate slurry combined with the sodium ions initially present in activated carbon was the main reason for the magnesium sulfate impurity. By calculating the species balance for each ion in the system, it becomes clear that not all ions are recombined into the product salts. Some ions remain on the flow-electrodes’ particles, which also causes the lower concentration factors that depend on this balance. Semicontinuous Salt Metathesis Experiments. The experimental system shown in Figure 1 represents the operation mode of semicontinuous metathesis. It is semi-

Figure 2. Two step metathesis of K2SO4 and MgCl2 using FCDI. solutions were deionized using a pair of FCDI modules in batch mode with a potential of 1.20 V. Once the complete deionization (EC < 2 μS/cm) of both feed solutions was achieved, the flow-electrodes were redirected into the recombination modules (also operated in batch mode) for the discharge with a reversed potential of −1.20 V. During both steps, the electrical currents and EC values were recorded continuously. Samples of electrolyte solutions were taken periodically from the system and analyzed for pH, Na+, K+, Mg2+, Cl−, and SO42−. Semicontinuous Salt Metathesis Experiments. These were performed in the system shown in Figure 1. The pairs of flowelectrodes were continuously recirculated (60 mL/min) through the corresponding decomposition and recombination modules and the storage vessels. Two inlet solutions (K2SO4 and MCl2) were recirculated (30 mL/min) in batch mode through the two decomposition modules. Products were accumulated in the product solutions, initially made with an appropriate volume of deionized water. Choosing this volume smaller than the volume deionized allows one to establish a concentration factor larger than 1. These solutions were also recirculated in batch mode through the recombination FCDI modules. Electrical currents and EC values were continuously recorded during all experiments. Samples of electrolyte solutions were periodically taken from the system and analyzed for pH, K+, Mg2+, Cl−, and SO42−. After each experiment, the system was washed in the same way as it is described for the two-step experiments. 3908


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Figure 3. Results of batch-mode FCDI decomposition−regeneration experiments.

Table 1. Operational Parameters and Results of Two-Step FCDI Metathesis Experiments exp. number 1 2 3 4 5

electrolyte

aimed conc. factor

C0 [mol/L]

V0 [mL]

CEcharge [%]

product

Vend [mL]

Cend [mmol/L]

CEdischarge [%]

product purity [%]

achieved conc. factor

anion mol. balance [%]

Na2SO4 MgCl2 Na2SO4 MgCl2 Na2SO4 MgCl2 K2SO4 MgCl2 K2SO4 MgCl2

1 1 5 5 10 10 1 1 10 10

3.5 3.5 1.4 1.4 0.7 0.7 3.5 3.5 0.7 0.7

080 080 450 450 950 950 080 080 950 950

99.9 87.9 96.0 97.3 98.3 101.2 104.7 97.7 90.3 97.2

MgSO4 NaCl MgSO4 NaCl MgSO4 NaCl MgSO4 KCl MgSO4 KCl

80 80 90 90 95 95 80 80 95 95

2.89 9.83 4.83 9.53 5.36 12.57 1.72 0.88 4.60 8.15

84.0 98.8 79.7 70.8 93.1 93.5 70.7 25.0 84.9 83.2

82.0 97.5 89.4 98.0 97.9 99.0 85.5 99.0 95.1 99.9

0.8 1.3 3.2 3.3 7.0 8.0 0.5 0.1 6.3 5.9

82.5 134.0 64.7 66.5 69.6 80.3 48.5 12.6 63.4 59.0

continuous because it never reaches steady state. Feed and product solutions are recycled through the FCDI modules in batch mode, while the flow-electrodes are continuously recirculated between the decomposition and recombination FCDI modules. Table 2 summarizes all operational parameters and the corresponding results of semicontinuous FCDI metathesis experiments conducted with K2SO4 and MgCl2 feed solutions. Figure 5 represents the change of EC in feed and product solutions versus time observed during experiments #6 and #7 (see Table 2).

The conductivity increase of the product MgSO4 solutions starts at about the same time as the conductivity decrease of the reagent solution. Yet, at the end of the experiment, the conductivities of the feed solutions reached values of deionized water remarkably faster compared to the product solution conductivities reaching a maximum value. This effect can be explained by the overall system setup. Ions are first charged onto the carbon particles and are discharged subsequently in the recombination module. Because the flow-electrode has the capability of storing ions, the charging and discharging step may not be directly connected. The transfer rate of ions charged onto the slurry electrodes and the transfer rate from slurry 3909


Research Article


Figure 4. Time evolution conductivity of MgCl2 and Na2SO4 metathesis in a two-step FCDI process.

particles which are discharged and recombined are not stringently equal. Consequently, the charging step can be faster compared to the discharging step, which is also dependent on the availability of counterions provided from the second decomposition module, causing the delay in magnesium sulfate formation. A closer look at the slope of the magnesium chloride conductivity curves (Figure 5) reveals that at the beginning of the process the decomposition of the feed salts is faster than the recombination, accompanied by the storage (accumulation) of ions in the flow-electrodes. Once the amount of ions stored in the slurry reached a certain maximum value (governed by the SAC), the decomposition rate slows down because it became dependent on the rate of recombination. In a final third phase, when the reagent solutions are almost deionized, the amount of ions stored in the flow-electrodes decreased again. In this part of the process, the rate of discharge governs the recombination rate. This time-dependent imbalance and storage can also be observed by measuring the electrical currents in each module. As the voltage on the electrodes is kept constant, the evolution of current corresponds to the amount of decomposition or recombination. Figure 6 shows the course of the currents of all modules of experiment #6 (see Table 2). The patterns of this figure support the above discussion. It appears in Figure 6 that electrical currents obtained in decomposition and recombination modules are very high and low, respectively, at the very beginning of the studied semicontinuous FCDI metathesis processes. Next, the values of all four currents became quite close and decreased afterward until the end of the experiment. Initial currents are high due to

Figure 5. Time evolution of electrical conductivity versus time observed during the two-step FCDI metathesis of 3.5 mM MgCl2 and 3.5 mM Na2SO4 solutions. Initial volumes of feed and product solutions −80 mL. Volume of flow electrodes (5% wt activated carbon) − 100 mL.

the availability of lots of free ions in the feed solutions and the strong driving force of the uncharged flow-electrodes. As no ions were stored on the particles at the beginning of the process, the currents of the product modules increase from zero as more and more ions are charged into the electrodes. At the point where nearly the same amount of ions is charged and discharged on and off the electrodes, all four currents

Table 2. Operational Parameters and Results of the Semi-continuous FCDI Salt Metathesis Experiments Conducted with K2SO4 and MgCl2 Feed Solutions with Varied Initial Concentrations and Concertation Factors exp. number 6 7 8 9

electrolyte

aimed conc. factor

C0 [mol/ L]

V0 [mL]

CEcharge [%]

product

Vend [mL]

volume c

Cend [mmol/L]

CEdischarge [%]

product purity [%]

achieved conc. factor

anion mol. balance [%]

K2SO4 MgCl2 K2SO4 MgCl2 K2SO4 MgCl2 K2SO4 MgCl2

001 001 020 020 050 050 100 100

3.5 3.5 3.5 3.5 1.0 1.0 1.0 1.0

0080 0080 1580 1580 3950 3950 7900 7900

100.3 104.6 076.7 106.1 079.0 132.5 083.5 097.4

MgSO4 KCl MgSO4 KCl MgSO4 KCl MgSO4 KCl

80 80 84 83 80 82 85 97

0.0 0.0 6.3 5.1 1.3 3.8 7.6 22.8

002.44 006.57 054.76 109.16 033.20 078.07 081.19 140.23

75.6 84.4 71.3 78.8 70.7 78.0 79.6 80.6

81.5 99.9 99.1 99.0 98.1 97.4 95.9 95.2

00.3 00.9 15.7 14.4 33.4 38.8 81.5 70.0

69.0 94.5 83.5 75.8 67.6 80.5 87.7 85.9

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steps of the chemical industry, reaction and concentration, is the key aspect that underlines the innovation of the shown process.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +972 4 8293627. Fax: +972 4 8228898. E-mail: ygendel@ tx.technion.ac.il, Ravitz fellow. ORCID

Youri Gendel: 0000-0002-3029-5028 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was funded by the Israeli Ministry of Science, Technology and Space (MOST) and Bundesministerium für Bildung und Forschung (BMBF) under the German-Israeli Water Technology Cooperation Program (research grant number - GR2572). Y.G. appreciates the financial support of the Ravitz Foundation. C.L. appreciates the financial support of the BMBF-MOST Young Scientists Exchange Program.

Figure 6. Variation of electrical currents measured in decomposition and recombination modules during semicontinuous FCDI metathesis of MgCl 2 and K 2 SO 4 solutions. Exp# 8 (Table 2); initial concentrations for MgCl2, K2SO4: 1 mM, designed concentration factor: 50.

approximately level each other. Thus, the behaviors of charging and discharging currents show effects which are in accordance with the information gained from the conductivity curves that appear in Figure 5. Because the SAC of the slurries is not limited in the semicontinuous mode, concentration factors up to 100 were aimed. These aimed concentration values were not reached completely. For instance, aimed and apparent concentration factors of experiment #9 (Table 2) were 100 and 70, respectively. Even if the achieved concentration factors are closer to the aimed ones, there is still a lack of about 20%. The same follows from the related anion substance balance. Since the samples reveal that anions and cations are in an electrically neutral proportion, the species balance is not closed because of ions left charged in the slurry particles. Since the amount of such leftover ions is lower for the semicontinuous mode compared to that in the two-step mode, we expect that by using a fully continuous steady state process much higher concentrating factors and product purities can be achieved. Nevertheless, the product purities achieved in semicontinuous FCDI metathesis are very high. A minor impurity in the magnesium sulfate compartment is potassium sulfate, which is a result of a parasitic crossover of potassium ions. We attribute this relatively high purity to the lower feed concentration applied in semicontinuous experiments, compared to two-step studies. Moreover, sodium ions that are initially present in the activated carbon cause the major amount of the impurities. Without that sodium, the product purities would be >98%. Finally, the current efficiencies of the decomposition step are again higher and closer to 100%, whereas the recombination step reaches only values of up to 84%. For the first time, we report a salt metathesis reaction using the FCDI technology. High desalination rates with simultaneous high current efficiencies underline that the FCDI technique is competitive with other electrochemical deionization techniques, but offer the possibility to transport ions separately without a counterion. The ability of conducting the salt metathesis reaction while concentrating the product was never shown before and is another benefit of the FCDI technique. Hence, the presented process is capable of using weak reagent solutions and turning them into concentrated valuable product streams. The combination of these two major

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REFERENCES

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Flow-Electrode Capacitive Deionization for Double Displacement

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