This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes.
Letter pubs.acs.org/journal/estlcu
Faradaic Reactions in Water Desalination by Batch-Mode Capacitive Deionization Di He, Chi Eng Wong, Wangwang Tang, Peter Kovalsky, and T. David Waite* School of Civil and Environmental Engineering, University of New South Wales, Sydney, NSW 2052, Australia S Supporting Information *
ABSTRACT: Non-Faradaic (ion electrosorption) and Faradaic (oxidation−reduction) effects in a batch-mode capacitive deionization (CDI) system were investigated, with results showing that both effects were enhanced with an increase in charging voltage (0.5−1.5 V). Significant concentrations of hydrogen peroxide (H2O2) were observed with the generation of H2O2 initiated by cathodic reduction of O2 with subsequent consumption occurring as a result of cathodic reduction of H2O2. A kinetic model of the Faradaic processes was developed and found to satisfactorily describe the variation in the steady-state concentration of H2O2 generated over a range of CDI operating conditions. Significant pH fluctuations were observed at higher charging voltages. While the occurrence of Faradaic reactions may well contribute to pH fluctuations and deterioration of electrode stability and performance, the presence of H2O2 could provide the means of inducing disinfection or trace contaminant degradation provided H2O2 could be effectively activated to more powerful oxidants (by, for example, ultraviolet irradiation).
■
INTRODUCTION An important role of water desalination treatment plants is to satisfy the increasing civilian and industrial demands for clean water through harvesting water resources from widely available seawater and brackish water. Traditional desalination techniques, such as reverse osmosis (RO), have now achieved energy consumption rates of 1−3 kWh/m3 of freshwater produced but, in many instances, are susceptible to membrane fouling and/or scaling.1,2 While by no means entirely free of fouling and/or scaling problems, capacitive deionization (CDI) is an emerging water desalination technology that has the potential to achieve even lower energy consumption rates (0.5−1 kWh/m3 of freshwater produced) for industrial process stream and subsurface brackish water desalination, particularly if energy recovery can be achieved.2,3 Recent developments in ionselective membrane/carbon electrode combinations,4 new designs of porous carbon electrodes,5−8 and energy recovery techniques3,9,10 could potentially render CDI competitive with traditional desalination techniques in the near future.11 While non-Faradaic processes, such as ion transport and capacitive storage, are at the heart of the CDI process, Faradaic reactions (i.e., redox reactions both on the surface of and within the carbon electrodes) may lead to the formation of chemical byproducts and/or pH fluctuations of the produced water12,13 and the deterioration of the long-term stability and performance of the electrode, which could be attributed to the occurrence of reduction reactions at the cathode automatically driving the anode potential to values at which subsequent oxidation of the anode occurred.14,15 As such, it is essential that these electrochemical processes be clearly understood if electrode stability, energy efficiency, ion removal, and treated © 2016 American Chemical Society
water quality are to be optimized. Importantly, it may be possible to make positive use of Faradaic effects with, for example, the production of reactive species such as hydrogen peroxide (H2O2) and/or free chlorine (HOCl) that may be used to induce water disinfection and/or degradation of organic contaminants. In the capacitive electrochemical process, water, dissolved oxygen (DO), and Cl− are the most likely electron acceptors and donors, with the possibility that HOCl (pKa = 7.55) may be produced by anodic oxidation of Cl− and, for the CDI system at least, that H2O2 (pKa = 11.64) may be generated by cathodic reduction of O2 (eqs 1−4).16,17 H+
cathode: O2 + e− ⎯→ ⎯
1 H 2O2 E 0 = 0.69 V/SHE 2
(1)
H+ 1 H 2O2 + e− ⎯→ ⎯ H 2O E 0 = 1.78 V/SHE 2
(2)
1 Cl 2 + e− E 0 = 1.36 V/SHE 2
(3)
hydrolysis: Cl 2 + H 2O → HOCl + Cl− + H+
(4)
cathode:
anode: Cl− →
The presence of electrochemically generated HOCl or H2O2 combined with UV irradiation or O3 would be expected to lead to 2.0−5.0 log inactivation of microorganisms,18−21 with the Received: Revised: Accepted: Published: 222
April April April April
3, 2016 25, 2016 25, 2016 25, 2016 DOI: 10.1021/acs.estlett.6b00124 Environ. Sci. Technol. Lett. 2016, 3, 222−226
Letter
Environmental Science & Technology Letters
value between 0.5 and 1.5 V. During discharging, simple shortcircuit was applied, i.e., 0 V. Experimental Methods. The schematic diagram of the batch-mode CDI test procedure is shown in Figure S3. The whole system consisted of a CDI cell, a recycling tank, a peristaltic pump (DAB Pumps S.P.A.), a digital electrical conductivity (EC) meter (F-54, Horiba), a pH meter (F-51, Horiba), and a DO probe (EZ-DO T031). The feed solution containing air-saturated electrolytes with an initial DO concentration of ∼250 μM was pumped from the recycling tank, passed through the CDI system where ion electrosorption took place, and then transported back to the recycling tank. The EC, as a representative measure of ion concentration, pH, and DO were monitored during charging. Further details of the batch-mode CDI procedure used are provided in section S2 of the Supporting Information. H2O2 concentrations were determined using the Amplex Red (AR) method in which AR is oxidized by H2O2, in the presence of horseradish peroxidase (HRP), leading to the formation of a highly fluorescent product that emits light at 587 nm upon excitation at 563 nm, allowing the quantification of H2O2 in the solution.23 Concentrations of HOCl generated via the anodic oxidation of Cl− were measured using the N,N-diethyl-pphenylenediamine (DPD) method, where DPD reacts with HOCl and produces the radical cation DPD•+, which exhibits an absorption peak at 551 nm.22 Kinetic modeling of the chemical reactions accounting for the production and consumption of H2O2 in solution was undertaken using rate expressions corresponding to the reactions considered to be important in the system. The concentrations of reactants and products (given specific initial conditions and rate constants) at any given time were determined by numerically solving the differential equations corresponding to these rate expressions using the kinetic modeling software Kintek Explorer.24
potential to combine salt removal and disinfection in a single process. On the other hand, Faradaic (eqs 1 and 2) and hydrolysis reactions (eq 4) in the CDI system may lead to pH fluctuations in the treated water stream. Fluctuations in pH may also arise from a variety of additional side Faradaic reactions, including anodic water electrolysis (eq 5) and oxidation of the carbon electrode itself (eq 6),12 and non-Faradaic reactions (e.g., H+/OH− transport and surface complexation in the carbon micropores): anode:
1 1 H 2O → O2 + H+ + e− E 0 = 1.23 V/SHE 2 4 (5)
1 1 1 anode: C + H 2O → CO2 + H+ + e− E 0 = 0.7−0.9 V/SHE 4 2 4
(6)
In the studies reported here, we describe the extent and rate of Faradaic reactions in a batch-mode CDI system operating at various charging voltages (0.5−1.5 V) using sodium fluoride (NaF) and sodium chloride (NaCl) as electrolyte solutions. Because of the extremely high redox potential for the anodic oxidation of F− (eq 7),16 cathodic reduction reactions in NaF (eqs 1 and 2) will constitute the major Faradaic reactions responsible for the generation of oxidants, with comparison of oxidant generation behavior in NaCl allowing clarification of the importance of chloride-mediated redox processes in NaCl. For example, it is expected that the reaction between HOCl (if produced) and H2O2 (eq 8)22 could reduce the oxidant yield in NaCl. anode: F− →
1 F2 + e− E 0 = 2.87 V/SHE 2
(7)
reactions: HOCl + H 2O2 → O2 + H+ + Cl− + H 2O
■
(8)
As an aid to clarifying key reaction pathways, a mechanismbased kinetic model is developed to describe the variation of the steady-state concentration of H2O2 in the batch-mode CDI system over a range of charging voltages. In addition, ion (Na+, F−, and Cl−) removal efficiency and pH fluctuation are also examined as a function of charging voltage.
RESULTS AND DISCUSSION
Ion Electrosorption and O2 Decay in NaF. Results of ion electrosorption (Na+ and F−) and decay of DO during charging at different charging voltages (0.5−1.5 V) and ion desorption during discharging at 0 V are shown in Figure 1 and Figure S4. It can be observed from Figure 1 and Figure S4 that both ion
■
MATERIALS AND METHODS Fabrication of a CDI Cell. A photograph of the CDI cell used in this study and a schematic of its structure are shown in Figures S1 and S2 of the Supporting Information, respectively. The CDI cell consisted of a pair of parallel porous carbon electrodes separated by a 200 μm thick nonconductive nylon cloth to prevent electrical short circuit and to act as a spacer channel. The electrode sheets, 10 cm × 10 cm in area and 100 μm in thickness, were composed of powdered activated carbon and a polymer binder (i.e., polytetrafluoroethylene) with this mixture bound to the graphite sheets using carbon conductive adhesive. The total micropore (0.5 V), additional reactions, including the anodic electrolysis of water (eq 5) and oxidation of the carbon electrodes (eq 6), occur with the associated production of protons. Also, with the decay of O2 during charging (Figure 1), the protons generated from anodic oxidation processes (eqs 5 and 6) can outcompete the consumption of protons by cathodic reduction (eq 1), resulting in a subsequent rapid decrease in pH following an initial increase in pH at higher charging voltages (1.2 and 1.5 V). It should be noted here that not only Faradaic reactions (eqs 1, 2, 16
224
DOI: 10.1021/acs.estlett.6b00124 Environ. Sci. Technol. Lett. 2016, 3, 222−226
Letter
Environmental Science & Technology Letters
Table 1. Proposed Conceptual Kinetic Model for CDI Faradaic Reactions Occurring during Charging at Different Charging Voltages (in the presence of NaF electrolytes)a rate constant (s−1) no.
reaction + − 2H
1
O2 + 2e ⎯⎯⎯→ H 2O2
2
H 2O2 + 2e− ⎯⎯⎯→ 2H 2O
3
H 2O2 + C a ⇌ > H 2O2
2H+
0.5 V
0.9 V
1.2 V
1.5 V
5.4 × 10−5
5.0 × 10−4
1.6 × 10−3
2.7 × 10−3
6.0 × 10−4
1.8 × 10−3
2.0 × 10−3
5.0 × 10−3
K = 1.4 × 103 M−1; k+ = 0.7 M−1 s−1; k− = 5.0 × 10−4 s−1
a
C represents the surface sites of porous carbon electrodes; the total number of adsorption sites for H2O2 on carbon electrodes is 1.2 mM (more details are given in section S6 of the Supporting Information).
5, and 6) but also non-Faradaic reactions (e.g., H+/OH− transport and surface complexation in and/or on the carbon micropores) could contribute to pH fluctuations.25 Chemical Kinetic Modeling for H2O2 Generation in NaF. To examine whether the conceptual framework presented above could lead to a quantitative description of the time dependence of H2O2 concentration on charging and discharging of the CDI cell, a chemical kinetic model (Table 1) was developed with core components of this model, including (i) cathodic reduction of O2 (eq 1), (ii) cathodic reduction of H2O2 (eq 2), and (iii) surface association of H2O2 with the porous carbon electrodes. With the equilibrium constant for the adsorption of H2O2 to the electrodes fixed at 1.4 × 103 M−1 (as discussed above), values of 1.2 M−1 s−1 and 8.6 × 10−4 s−1 were deduced for the rate constants of adsorption and desorption of H2O2, respectively, based on the best fit to the experimental data for adsorption of H2O2 (no discharging) (Figure S8). Via the incorporation of the calculated rate constants for cathodic reduction of O2 (eq 1) and adsorption and desorption of H2O2 to the carbon electrodes into the model, the rate constants for the cathodic reduction of H2O2 (eq 2) during charging at various charging voltages were determined on the basis of the best fit to the variation of H2O2 following a different initial dosage of H2O2 (Figure S10). Overall, the relatively simple kinetic model presented in Table 1 is able to successfully describe the experimentally determined profiles of H2O2 arising from the proposed Faradaic reactions over a range of charging voltages (Figure 2 and Figure S10). As demonstrated in Table 1, the ratio of rate constants for eqs 1 and 2 determined the steady-state concentration of H2O2. An increase in this ratio with a charging voltage increasing from 0.5 to 1.2 V caused an increase in the steady-state concentration of H2O2, while a decrease in this ratio with a further increase in charging voltage from 1.2 to 1.5 V leads to a decrease in the steady-state concentration of H2O2 even though the generation of H2O2 occurred at a relatively faster rate at 1.5 V. It should be noted that kinetic modeling of pH fluctuations is not provided here considering that uncertainties exist in not only Faradaic reactions (eqs 5 and 6) but also non-Faradaic reactions, such as H+/OH− diffusion and surface complexation, all of which could contribute to pH fluctuations. Oxidant Production in NaCl. Undetectable concentrations of HOCl and very similar profiles of H2O2 in NaCl and NaF were observed during charging at various charging voltages (0.5−1.5 V) (cf. Figure 2 and Figure S11), demonstrating that anodic oxidation of Cl− was unlikely to have occurred under the CDI operating conditions applied here. In addition, no significant difference in H2O2 production was observed with an increase in NaCl concentration from 5 to 100 mM (Figure S12), further confirming the minor role of anodic oxidation of
Cl− in the CDI system used here. As such, it would seem reasonable to conclude that H2O2 was the dominant oxidant produced in both NaF and NaCl solutions. Environmental Implications. Our preliminary study has addressed two key issues associated with CDI Faradaic reactions: (i) pH fluctuations and (ii) H2O2 generation. pH fluctuations are particularly important to the efficacy of CDI water treatment as they not only affect the quality of the treated water stream but also might lead to carbonate scaling (and associated reduction in ion retention capacity), the severity of this issue clearly being increased at intermediate operating potentials. During charging at relatively high cell potentials (1.2 and 1.5 V), a decrease in pH was also observed after a few minutes, implying that oxidation of the porous carbon electrodes (eq 6) may have occurred. As such, CDI performance is likely to be influenced not only by factors controlling non-Faradaic ion sorption processes (such as electrode area, total pore volume, size, and connectivity) but also by Faradaic processes that may influence electrode stability and longevity [due particularly to the occurrence of anode oxidation (eq 6)] as well as overall system performance. While Faradaic processes may lead to problems with CDI performance, these processes may also be used beneficially. For example, in situ-produced H2O2, if managed appropriately, may be used, in combination with UV irradiation or O3, for effective disinfection and/or degradation of organic contaminants in the treated water stream. Previous studies have reported that small dosages of H2O2 (20−150 μM) in combination with UV or O3 disinfection can lead to improvement in bacterial inactivation by 0.5−2.0 log units.18−21 While the relatively high cost and low efficiency still render either UV/H2O2 or O3/H2O2 uncompetitive with Cl2 with respect to disinfection, growing health and environmental concerns regarding the use of chlorine for the disinfection of drinking water (particularly the formation of harmful chlorinated organic byproducts) have been widely raised. For this reason, it is important to find alternative technologies for water disinfection.
■
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.estlett.6b00124. Twelve figures showing the CDI cell and its structure, a schematic diagram of the batch-mode CDI setup, ion electrosorption, rate constant calculation for O2 decay, variation of voltage during discharging, a Langmuir isotherm of H2O2 adsorption, calculation of E0W, the effect of the initial dosage of H2O2 on cathodic production of H2O2, cathodic production of H2O2 in 225
DOI: 10.1021/acs.estlett.6b00124 Environ. Sci. Technol. Lett. 2016, 3, 222−226
Letter
Environmental Science & Technology Letters
■
(15) Cohen, I.; Avraham, E.; Bouhadana, Y.; Soffer, A.; Aurbach, D. Long term stability of capacitive de-ionization processes for water desalination: The challenge of positive electrodes corrosion. Electrochim. Acta 2013, 106, 91−100. (16) Morel, F. M. M.; Hering, J. G. Principles and Applications of Aquatic Chemistry; John Wiley & Sons: New York, 1993. (17) Qiu, S.; He, D.; Ma, J. X.; Liu, T. X.; Waite, T. D. Kinetic modeling of the electro-Fenton process: Quantification of reactive oxygen species generation. Electrochim. Acta 2015, 176, 51−58. (18) Cho, M.; Gandhi, V.; Hwang, T. M.; Lee, S.; Kim, J. H. Investigating synergism during sequential inactivation of MS-2 phage and Bacillus subtilis spores with UV/H2O2 followed by free chlorine. Water Res. 2011, 45, 1063−1070. (19) Rubio, D.; Nebot, E.; Casanueva, J. F.; Pulgarin, C. Comparative effect of simulated solar light, UV, UV/H2O2 and photo-Fenton treatment (UV Vis/H2O 2/Fe 2+ ,Fe 3+) in the Escherichia colt inactivation in artificial seawater. Water Res. 2013, 47, 6367−6379. (20) Cho, M.; Yoon, J. Enhanced bactericidal effect of O3/H2O2 followed by Cl2. Ozone: Sci. Eng. 2006, 28, 335−340. (21) Lanao, M.; Ormad, M. P.; Ibarz, C.; Miguel, N.; Ovelleiro, J. L. Bactericidal effectiveness of O3, O3/H2O2 and O3/TiO2 on clostridium perfringens. Ozone: Sci. Eng. 2008, 30, 431−438. (22) Garg, S.; Ma, T.; Miller, C. J.; Waite, T. D. Mechanistic insights into free chlorine and reactive oxygen species production on irradiation of semiconducting silver chloride particles. J. Phys. Chem. C 2014, 118, 26659−26670. (23) He, D.; Jones, A. M.; Garg, S.; Pham, A. N.; Waite, T. D. Silver nanoparticle-reactive oxygen species interactions: Application of a charging-discharging model. J. Phys. Chem. C 2011, 115, 5461−5468. (24) Johnson, K. A.; Simpson, Z. B.; Blom, T. Global Kinetic Explorer: A new computer program for dynamic simulation and fitting of kinetic data. Anal. Biochem. 2009, 387, 20−29. (25) Biesheuvel, P. M. Activated carbon is an electron-conducting amphoteric ion adsorbent. arXiv:1509.06354, 2015.
NaCl, and the effect of NaCl concentration on H2O2 production (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: +61 2 9385 5060. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS We gratefully acknowledge funding support from the Australian Research Council and partners Beijing Origin Water, Western Australian Water Corporation, Northern Territory Power & Water and Water Research Australia through Linkage Grant LP130101107.
■
REFERENCES
(1) Greenlee, L. F.; Lawler, D. F.; Freeman, B. D.; Marrot, B.; Moulin, P. Reverse osmosis desalination: Water sources, technology, and today’s challenges. Water Res. 2009, 43, 2317−2348. (2) Zhao, R.; Porada, S.; Biesheuvel, P. M.; Van der Wal, A. Energy consumption in membrane capacitive deionization for different water recoveries and flow rates, and comparison with reverse osmosis. Desalination 2013, 330, 35−41. (3) Dlugolecki, P.; van der Wal, A. Energy recovery in membrane capacitive deionization. Environ. Sci. Technol. 2013, 47, 4904−4910. (4) Porada, S.; Zhao, R.; van der Wal, A.; Presser, V.; Biesheuvel, P. M. Review on the science and technology of water desalination by capacitive deionization. Prog. Mater. Sci. 2013, 58, 1388−1442. (5) Wen, X. R.; Zhang, D. S.; Yan, T. T.; Zhang, J. P.; Shi, L. Y. Three-dimensional graphene-based hierarchically porous carbon composites prepared by a dual-template strategy for capacitive deionization. J. Mater. Chem. A 2013, 1, 12334−12344. (6) Wang, H.; Shi, L. Y.; Yan, T. T.; Zhang, J. P.; Zhong, Q. D.; Zhang, D. S. Design of graphene-coated hollow mesoporous carbon spheres as high performance electrodes for capacitive deionization. J. Mater. Chem. A 2014, 2, 4739−4750. (7) El-Deen, A. G.; Barakat, N. A. M.; Khalil, K. A.; Kim, H. Y. Hollow carbon nanofibers as an effective electrode for brackish water desalination using the capacitive deionization process. New J. Chem. 2014, 38, 198−205. (8) Lei, H.; Yan, T. T.; Wang, H.; Shi, L. Y.; Zhang, J. P.; Zhang, D. S. Graphene-like carbon nanosheets prepared by a Fe-catalyzed glucose-blowing method for capacitive deionization. J. Mater. Chem. A 2015, 3, 5934−5941. (9) Pernia, A. M.; Norniella, J. G.; Martin-Ramos, J. A.; Diaz, J.; Martinez, J. A. Up-down converter for energy recovery in a CDI desalination System. IEEE T. Power Electr. 2012, 27, 3257−3265. (10) Pernia, A. M.; Alvarez-Gonzalez, F. J.; Prieto, M. A. J.; Villegas, P. J.; Nuno, F. New control strategy of an up-down converter for energy recovery in a CDI desalination system. IEEE T. Power Electr. 2014, 29, 3573−3581. (11) Suss, M. E.; Porada, S.; Sun, X.; Biesheuvel, P. M.; Yoon, J.; Presser, V. Water desalination via capacitive deionization: what is it and what can we expect from it? Energy Environ. Sci. 2015, 8, 2296− 2319. (12) Lee, J. H.; Bae, W. S.; Choi, J. H. Electrode reactions and adsorption/desorption performance related to the applied potential in a capacitive deionization process. Desalination 2010, 258, 159−163. (13) Choi, J. H. Determination of the electrode potential causing Faradaic reactions in membrane capacitive deionization. Desalination 2014, 347, 224−229. (14) Bouhadana, Y.; Avraham, E.; Noked, M.; Ben-Tzion, M.; Soffer, A.; Aurbach, D. Capacitive deionization of NaCl solutions at nonsteady-state conditions: Inversion functionality of the carbon electrodes. J. Phys. Chem. C 2011, 115, 16567−16573. 226
DOI: 10.1021/acs.estlett.6b00124 Environ. Sci. Technol. Lett. 2016, 3, 222−226
