A comparison of multicomponent electrosorption in capacitive deionization and membrane capacitive deionization
Graphical abstract
Introduction
The shortage of fresh water has become a severe problem in our time owing to population and economic growth, as well as the impacts of climate change. This has made desalination of sea and brackish water stand out as an increasingly necessary answer to resolve the water crisis. Among desalination technologies, capacitive deionization (CDI) has attracted attention as an energy-efficient and promising electrochemical desalination technology, especially for low salinity brackish water (Bouhadana et al., 2010, Subramani and Jacangelo, 2015). In the most common approach to CDI, the influent stream passes between two high-capacitance electrodes made of porous carbon materials to which an electrical voltage or current is applied. As a result, anions and cations are temporarily stored on the porous surface of the oppositely charged electrode and a deionized stream with lower ion concentration flows out of the cell. Ion electrosorption is based on the formation of electrical double layers (EDLs) inside the micropores (<2 nm) of the electrodes (Porada et al., 2013). After a period of operation, the electrodes become saturated and require regeneration. In this step, the cell voltage or current is reduced to zero and adsorbed ions are released into a wastewater stream. To summarize, a CDI cycle consists of two steps, ion adsorption and ion desorption. While CDI is only economic for relatively dilute solutions, it has low energy consumption as it removes ions from the electrolyte rather than separating water from the salty stream, such as in reverse osmosis and distillation (Asquith et al., 2015, Liu et al., 2015).
To improve performance, ion-exchange membranes (IEMs) can be placed in front of the electrodes. This approach, which is one of the most recent developments in CDI, is called Membrane Capacitive Deionization (MCDI) (Biesheuvel and van der Wal, 2010). In this case, cation and anion exchange membranes placed in front of the negatively and positively charged electrodes, respectively, will only allow counter-ions to move from the bulk solution toward the electrode. By blocking almost all of co-ions, the desalination process is more efficient as there is less co-ion repulsion. Furthermore, the use of IEMs enables us to reverse the polarity of the cell during desorption, which leads to a more complete expulsion of counter-ions from the micro and macropores of the carbon (Zhao et al., 2012a). In addition to the favorable features of CDI including low energy consumption, easy regeneration and maintenance (Wang et al., 2015), MCDI operation is more stable which makes this technique an attractive water treatment technology for industrial applications (Kim et al., 2010). Biesheuvel et al., 2011, Zhao et al., 2012a, Zhao et al. (2012a) and Dykstra et al. (2016a) have presented comprehensive ion transport models for desalination using MCDI.
Ion charge and size plays an important role, given that the CDI process is based on temporary adsorption of ions inside the EDLs of the carbon micropores. However, few studies on CDI performance have focused on salts other than NaCl. Pioneers of this work were Gabelich et al. (2002) who investigated the sorption capacity of carbon aerogel electrodes for various monovalent and divalent ions. They reported that monovalents are preferentially removed over divalent ions due to smaller hydrated radii. Zhao et al. (2012b) similarly observed preferential adsorption of Na+ over Ca2+; however, they reported Na+ replacement with Ca2+ later during adsorption. In contrast, Xu et al. (2008), Mossad et al. (2013) and Mossad and Zou, 2012) recognized ionic charge as the factor controlling the electrosorption preference in a competitive environment. It is worth mentioning that the last three research groups studied the CDI performance with an electrolyte consisting of non-equal concentration of ions. In other words, as the ion concentration is one of the variables influencing the removal rate of that specific ion, it is questionable to attribute the preferential electrosorption sequence reported by them to ionic charge alone. A few studies have investigated the effect of ion properties on electrosorption while keeping the concentration equal for different salts. Seo et al. (2010) reported selective ion removal for a mixture of cations including sodium, potassium, magnesium and calcium at different flow rates. They attributed the adsorption sequence to the pore size and structure of the carbon material. In another work, Huyskens et al. (2013) measured the ion removal for various monovalent and divalent salts; however, their result was not in agreement with that of Seo et al. (2010). Later, Han et al. (2014), in a comprehensive data collection on removal of various monovalent ions in CDI using different activated carbon cloths, showed that sorption capacity and competitive ion removal can be tuned by varying the accessible surface area of carbon and its micro to meso-porosity ratio. Discrepancies between these reports indicate that more research is needed to fully understand the competitive electrosorption of different ions, especially in the presence of divalent ions. In the area of EDL modelling, Suss (2017) extended the existing models by accounting for ion volume exclusion interactions to demonstrate selective ion removal based on ion size.
The focus in MCDI has predominantly been on the removal of different salts using novel electrodes or IEMs (Kim et al., 2016, Kim and Choi, 2012). However, very few research groups have compared the removal rate of different ions in MCDI using commercially available IEMs. In 2012, Kim et al. (2013) manipulated the removal of chloride and nitrate in single and mixed solutions by varying the current density in MCDI. In a recent publication, Tang et al. (2017b) studied the removal of sulfate in MCDI and observed more sulfate removal in a mixture of sulfate and chloride with equal molar concentrations. As diffusion of the ions through the IEMs occurs prior to ion adsorption inside the micropores, these are crucial in controlling the diffusion. To date, little effort has been made to compare the competitive removal of different cations and anions in CDI to that in MCDI at milliequivalent concentrations.
Another phenomena that is mostly overlooked in this area is the pH fluctuation over one adsorption/desorption cycle. Only recently have He et al. (2016) and Gao et al. (2017) addressed this issue over a range of CDI operating conditions. Tang et al. (2017a) probed into details of pH fluctuation in batch mode operation of CDI and MCDI by monitoring the concentration of H2O2 and dissolved oxygen, and measuring the electrode potentials. Yet, we believe this phenomenon requires more research especially for a wide range of monovalent and divalent salts.
In this work, we aim to investigate the role of ion affinity to both the carbon electrode and the ion exchange membrane. To cover a wide range of ionic properties, experiments are conducted with NaCl, KCl, CaCl2, NaNO3, and Na2SO4 for single and mixed electrolyte solutions in both CDI and MCDI cells. To better understand the competitive electrosorption process, experiments were conducted at milliequivalent concentrations.
Section snippets
Materials
In this work, we utilized the analytical grade of all chemicals. Activated carbon (AC Norit SA 4, Cabot Norit Activated Carbon, USA), polyvinylidene fluoride (PVDF, Mw ∼530,000, Sigma-Aldrich), NN dimethylformamide (DMF, 99.8%, Merck Millipore) and graphite sheet (DSN 530, Suzhou Dasen Electronics Material Co., China) were utilized for electrode fabrication. Sodium chloride (NaCl, 99.7%), potassium chloride (KCl, 99%), calcium chloride (CaCl2, 99%), sodium nitrate (NaNO3, 99%) and sodium
Feed solutions containing a single salt
To probe the comparative removal of different cations, 10 mM solutions of NaCl, KCl or a 5 mM solution of CaCl2 was fed to the (M)CDI cell in a single pass mode. Fig. 1(a) shows the concentration variation over one cycle in CDI. It was previously reported for CDI that, at the same initial concentration, the hydrated radius governs the sorption capacity (Gabelich et al., 2002). Similarly, in this experiment, KCl is removed marginally faster from the electrolyte in comparison with NaCl in terms
Conclusion
In this work, activated carbon electrodes were prepared and then were utilized in a CDI and (M)CDI setup. Single salt electrolyte solutions of NaCl, KCl, CaCl2, NaNO3, Na2SO4 were tested as were solutions containing mixtures of three cations and three anions. The results show that the charge efficiency of NO3− and SO42− is lower in CDI than for other anions which can be attributed to strong co-ion adsorption and to slow rates of diffusion. Salt adsorption and charge efficiency boosted
Acknowledgements
Armineh Hassanvand acknowledges The University of Melbourne for the IPRS (International Postgraduate Research Scholarship) and APA (Australian Postgraduate Awards) scholarships, which are funded by the Australian Government. George Chen and Sandra Kentish acknowledge research funding from the Australian Research Council Industrial Transformation Research Program (ITRP) scheme (Project Number IH120100005). The ARC Dairy Innovation Hub is a collaboration between The University of Melbourne, The
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