Membrane Capacitive Deionization Cell Research Articles

Nitrogen recovery from low-strength wastewater by combined membrane capacitive deionization (MCDI) and ion exchange (IE) process

Nitrogen exists in municipal wastewater in the form of ammonia/ammonium which is a recoverable resource. Recovering nitrogen to improve nitrogen-use efficiency should be carefully considered to promote global environmental sustainability. However, ammonia/ammonium is always not well recovered but removed as one of the main pathways for nitrogen release into the environment, which is due to ineffective ammonia capture. In this study, a new method of ion-exchange resin ammonium recovery combined with membrane capacitive deionization (MCDI+IE process) pretreatment is proposed. Selective ion removal by MCDI is demonstrated and ammonium ratio to the total prior ion (K+, Ca2+ and Mg2+) in outflow of MCDI cell is respectively increased from ∼15% to over 30% and 45% after 3 and 5 MCDI cycles. The MCDI+IE process could deal with 2.48 and 3.89 times of inflow, respectively, compared with the single IE process after 3 and 5 MCDI charge–discharge cycles and recover more than 65% of ammonium in a resin regeneration cycle after 3 MCDI cycles. It ascribes to MCDI desalination eliminating the negative effects of divalent cations, such as Ca2+ and Mg2+, on the IE absorb–desorb process.

Capacitive Deionization Using Composite Carbon Electrodes

Brackish water resources may be an attractive option for human consumption, agriculture, and industry if efficient water purification can be implemented. In the past few decades, research and development of various desalination technologies have been carried out, among which distillation, reverse osmosis, and electrodialysis are the most commonly known and widespread.1 Capacitive deionization (CDI) is an alternative, emerging, and energy-efficient technology for water desalination, which employs an electrochemical flow cell configured with polarized porous carbon electrodes to remove ionized salts in a stream with low molar concentration. Briefly, by regulating an external voltage to a CDI cell, ionized salts are electrostatically captured (or released) in the pores of the carbon electrodes, resulting in the stream being deionized (or the electrodes being regenerated).2-4 Recent studies have found that the salt adsorption capacity (SAC) could be substantially improved by using surface modified carbon electrodes resulting from nitric acid and ethylenediamine treatments.5 Combined with the modified Donnan model including a term of chemical surface charge, this improved SAC was accounted for by enhancement of the chemical charges immobilized in the carbon micropores, validating both enhanced CDI (e-CDI) and extended-voltage CDI (eV-CDI) effects in the CDI literature (Fig. 1).6In summary, it is considered that, for the carbon electrodes used in a CDI cell, an increase in the chemical surface charges makes the pores more readily available for salt adsorption under proper applied voltages. In addition to the surface modified carbon electrodes, immobilized chemical charges can be found in ion-exchange materials. For instance, a well-known cation-exchange polymer, Nafion, contains the negative chemical charges, -SO3 -, while an anion-exchange polymer typically holds positive chemical charges, e.g., NR4 + and NR3 +. As a consequence, together with the knowledge gained above, ion-exchange polymers coating were used in our current studies to explore new composite carbon electrodes for CDI cycling tests. As shown in an initial test (Fig. 2), the addition of ion-exchange polymers results in the SAC not only being increased but also being stabilized with operational time when NaCl solution was used. In this presentation, the preparation and characterizations of composite carbon electrodes will be detailed including comparisons to conventional CDI and membrane capacitive deionization cells. Furthermore, these composite electrodes will be configured into a CDI cell to investigate both e-CDI and eV-CDI effects in various salt solutions such as CaCl2, Na2SO4, and NH4NO3. In addition, the relevant charge efficiency and cycling longevity will be reported and discussed. Figure 1. Demonstration of both enhanced CDI (e-CDI) and extended voltage CDI (eV-CDI) effects using the modified Donnan model with the addition of chemical surface charge. The parameters used in the model can be found in ref. (5 and 6). Figure 2. Improved salt adsorption capacity and operational stability using cation- and anion-exchange polymers added to the carbon cathode and anode, respectively, in a CDI cell. The CDI cell was operated using 1 V charging and at 0 V discharging in ~31 L of ~7 mM deaerated NaCl solution.

Improving Electrosorption Performance in Membrane Assisted Capacitive Deionization Cells Using Asymmetric Electrodes Configuration

Membrane capacitive deionization (MCDI) is an emerging salt treatment technology that performs beyond the capabilities of traditional capacitive deionization cells due its ability to mitigate unwanted ion expulsion events. We present a dynamic model for observing the concentration response of deionization cells configured in a recirculating batch setup. The model consists of two volumes representing the deionization cell and the reservoir where salt storage and mixing respectively occur. The model was used to fit a typical MCDI cell made up of two similar electrodes each with potential of zero charge (EPZC) values of ~-0.2 V vs saturated calomel electrode (SCE), and an asymmetric MCDI cell with respective anode and cathode EPZCs at ~-0.2 and ~+0.2 V vs. SCE. The model predicts increases of up to 50% and 10% in capacity and efficiency, respectively, for the asymmetric MCDI cell.

Cost evaluation of large-scale membrane capacitive deionization for biomass hydrolysate desalination

The bio-based industry is striving to replace refined sugars by much cheaper secondary feedstocks for the production of bio-fuels and chemicals. However, due to their higher complexity, a number of technological challenges need to be overcome. One example are the high concentrations of sodium and potassium present in the biomass hydrolysates that inhibit fermentation and hence need to be reduced. Previous research demonstrated the technical feasibility of membrane capacitive deionization (MCDI) for biomass hydrolysate desalination as a chemical/waste free alternative compared to the commonly used ion-exchange process (IEX). In this paper, the economic viability of MCDI was investigated for a production capacity of 500ton sugarday−1 and a target Na removal from 3 to 0.1gkg−1 hydrolysate. Although capital costs were higher for MCDI than for IEX due to the expensive MCDI cells and power supplies, operating costs were lower because less water and chemicals are used and less wastewater is generated. Cost calculations for different initial feed concentrations indicated that IEX was only preferential over MCDI when the feed Na+ concentration was below 0.4gkg−1 hydrolysate. Then the higher chemical, water and wastewater treatment costs for IEX no longer outweighed the higher cost of MCDI cells compared to IEX resins.

TiO2 nanorod-intercalated reduced graphene oxide as high performance electrode material for membrane capacitive deionization

Membrane capacitive deionization (MCDI) attracted unprecedented attention due to energy saving during the deionization process. Based on its excellent characteristics, graphene can be considered an optimum CDI electrode material. However, pristine graphene is still far from the anticipated results. In this study, to improve the electrochemical characteristics, TiO2 nanorod intercalation between graphene nanosheets was successfully performed by hydrothermal treatment. The morphology, crystal structure and elemental analysis were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS), respectively. The electrochemical properties were evaluated by a cyclic voltammetry (CV) test. Moreover, the desalination activity was checked in MCDI unit. Furthermore, the influence of TiO2 loading was studied. It was found that the synthesized reduced graphene oxide (rGO)/TiO2 nanorod composite having 20wt.% TiO2 revealed a remarkable specific capacitance of ~443F/g which is nine fold more than that of the pristine rGO at 10mV/s. In the MCDI cell, the introduced nanocomposite showed high reversibility, excellent cycling stability, full regeneration and distinguished electrosorptive capacity (9.1mg/g) under an applied potential of 0.8V and initial salt concentration of ~300mg/L. Overall, the proposed rGO/TiO2 nanorod composite electrode can be considered a promising material for CDI applications.

Determination of the electrode potential causing Faradaic reactions in membrane capacitive deionization

In this study, the potential at which electrode reactions start and the effects of these reactions on membrane capacitive deionization (MCDI) were investigated. An adsorption experiment was conducted until the cell potential reached 1.2V, applying constant current to an MCDI unit cell. While varying the current density (5.0–20.0A/m2), the changes in cell potential, electrical conductivity and effluent pH were measured. The results of the experiment showed that the ion adsorption decreased significantly as the current density increased. Furthermore, the pH of the effluent rapidly decreased beyond a certain time in the adsorption process, which is judged to be the result of electrode reactions. When the electrode reactions started, the cell voltages were in the range of 0.62–0.90V, which varied greatly depending on the current density. Using a simple model for the MCDI cell, the electrode potential was calculated from the cell potential and current density. Regardless of the current density applied, the electrode reaction was started at the electrode potential of 0.25V. It is desirable to use the electrode potential instead of the cell potential to analyze the electrode reactions in MCDI operation.

Selective removal of nitrate ions by controlling the applied current in membrane capacitive deionization (MCDI)

The use of membrane capacitive deionization (MCDI) to selectively remove nitrate ions from a mixed solution of chloride and nitrate ions was investigated in this study. Adsorption equilibrium experiments were conducted to determine the selectivity of the AMX membrane toward the nitrate ion. Desalination experiments were also performed on a mixed solution (5mM NaCl+2mM NaNO3) using a MCDI cell consisting of carbon electrodes, a Neosepta AMX membrane, and a CMX membrane. The AMX membrane had a selectivity coefficient (kNO3−/Cl−) of 4.37, indicating high selectivity of the AMX membrane toward the nitrate ion. The desalination experiments were performed by applying a constant current to the MCDI cell until the cell potential reached 1.0V. The total amount of adsorbed ions was not affected by changes in the applied current. However, the mole fractions of the nitrate ions relative to all the adsorbed ions were inversely proportional to the applied current: 31.7, 33.7, and 41.0% at 30, 20, and 10mA applied current, respectively. This result is attributed to the increase in the mole fraction of nitrate ions adsorbed by the AMX membrane at lower applied currents.

Enhanced desalination efficiency in capacitive deionization with an ion-selective membrane

In order to investigate ionic adsorption and desorption mechanisms on carbon electrode surfaces, capacitive deionization experiments were carried out with two types of capacitive deionization (CDI) cell configurations. A CDI cell equipped with carbon electrodes only and a membrane capacitive deionization (MCDI) cell having a cation-exchange membrane on the cathode surface were constructed, and desalination experiments were carried out at various operating conditions. The conductivity transients of effluents and currents passed through the cells were accurately measured by a potentiostat in order to analyze the transport mechanism at the carbon electrode surfaces. The salt removal efficiencies of the MCDI cell were enhanced by 32.8–55.9% compared to the CDI cell, depending on the operating conditions. Furthermore, the current efficiencies were improved for the MCDI cell: 83.9–91.3% versus 35.5–43.1% for the CDI cell. It was verified that ions were selectively transported between the electric double layer at the electrode surface and the bulk solution in the MCDI cell configuration when a potential was applied. On the contrary, in the case of the CDI cell, both anions and cations were transported during the sorption and desorption processes, which led to decreased salt removal and current efficiencies.

Improvement of desalination efficiency in capacitive deionization using a carbon electrode coated with an ion-exchange polymer

A composite carbon electrode coated with a cation-exchange polymer, crosslinked poly(vinyl alcohol) with sulfosuccinic acid, was fabricated to enhance the desalination performance of a capacitive deionization (CDI) system. The electrochemical properties of the prepared electrode were characterized by impedance spectroscopy, and desalination experiments were carried out at various operating conditions using a CDI cell with carbon electrodes only, and a membrane-capacitive-deionization (MCDI) cell including a coated-carbon electrode, to evaluate the effect of the coated-carbon electrode on desalination performance. The electrical resistance of the coated electrode was increased by a small amount over the uncoated electrode, but the capacitance was improved by the coating. In the CDI cell, the salt-removal efficiencies were in the range of 50–67%, while the efficiencies increased to 75–85% for the MCDI cell. Depending on the operating conditions, the salt-removal and current efficiencies of the MCDI cell were enhanced by 27–56% and 69–95%, respectively, compared to the CDI cell. The enhanced efficiency for the MCDI cell was attributed to the selective transport of cations between the electrode surface and bulk solution due to the cation-exchange coating layer.

Membrane capacitive deionization

Membrane capacitive deionization (MCDI) is an ion-removal process based on applying an electrical potential difference across an aqueous solution which flows in between oppositely placed porous electrodes, in front of which ion-exchange membranes are positioned. Due to the applied potential, ions are adsorbed in the electrodes and a product stream with a reduced salt concentration is obtained. Including the membranes in the process has two advantages: first, they block co-ions from leaving the electrodes, thereby increasing the salt removal efficiency of the process, and second, when during ion release a reversed voltage is used, counterions can be more fully flushed from the electrode region, thereby increasing the driving force for ion removal in the next cycle. Here we present pilot-plant experimental data for salt removal in MCDI as function of inlet ionic strength and flow rate. In the subsequent stage of ion release the flow rate is temporarily reduced to zero and the voltage sign reversed. This “stop-flow” operation mode results in a small and concentrated product stream. We present a theoretical process model for MCDI which describes the time-dependent electric current and effluent ion concentration, both during the deionization stage and during the subsequent stage of ion release. The process model describes the MCDI cell as a number of stirred volumes placed in-series, and includes the transport resistance of the ion-exchange membrane and of the stagnant diffusion layer in front of the membrane. Ion storage in the electrodes is described according to the equilibrium Gouy–Chapman–Stern model for the electrostatic double layer.