Fig. 2 A graphical timeline depicting the evolution of ion selectivity in CDI and MCDI. The works employing membranes are denoted in italics. P. Srimuk, J. Lee, Ö. Budak, J. Choi, M. Chen, G. Feng, C. Prehal and V. Presser, Langmuir, 2018, 34, 13132–13143 CrossRef CAS. T. Rijnaarts, D. M. Reurink, F. Radmanesh, W. M. de Vos and K. Nijmeijer, J. Membr. Sci., 2019, 570–571, 513–521 CrossRef CAS. J. E. Dykstra, K. J. Keesman, P. M. Biesheuvel and A. van der Wal, Water Res., 2017, 119, 178–186 CrossRef CAS.

In CDI experiments using a CMX membrane, selectivity towards divalent over monovalent cations was reported. 119,120 Although the CMX membrane was not designed to differentiate between different cations, its negatively charged outermost layer attracts divalent more than the monovalent cations. 121 Hassanvand et al. stated that the implementation of CMX in CDI leads to sharper desorption peaks of divalent cations since larger amounts of di-over monovalent cations are temporarily stored within the CMX membrane. 53 On the other hand, the CIMS membrane resulted in preferential transport of monovalent over divalent cations. 122 Similarly, Choi et al. used a CIMS membrane and obtained monovalent cation selectivity ( R) of 1.8 for sodium over calcium ions. 121 By selectively removing Na +, a Ca 2+-rich solution was obtained. In addition, the selectivity attained its maximum value at higher cell voltages, pH, and lower TDS (total dissolved solids) concentration. S. Ren, M. Li, J. Sun, Y. Bian, K. Zuo, X. Zhang, P. Liang and X. Huang, Front. Environ. Sci. Eng., 2017, 11, 17 CrossRef. To further improve the performance of graphene electrodes, several groups prepared three-dimensional graphene structures by using sponge 61 or polysterene 39 templates, increasing the accessible surface area. In the former, the specific surface area reached 305 m 2 g −1 leading to greater ion adsorption capacity of 4.95 mg g −1 for a 0.5 M NaCl solution. The total electrosorption capacity of graphene-based electrodes was pushed beyond that of activated carbon and carbon aerogels by increasing the frequency of defects in the graphene sheets, which effectively increases the density of micropores and dramatically increases the ion adsorption capacity (see Fig. 8a). 62,63 J. W. Blair and G. W. Murphy, Saline Water Conversion, Washington, DC, 1960, pp. 206–223 Search PubMed.

Conflicts of Interest

On a system level, dependence of cell selectivity on operation parameters such as applied current, cell voltage, ion concentration, and pH among others will have to be systematically studied to find optimum conditions that enhance selectivity for the CDI cell. Intercalation electrodes such as TiS 2 show switchable preference depending on the potential of the electrode, as shown by Srimuk et al. 43 Such insights will be useful in realizing the full potential of existing (and the search for new) electrode materials. M. A. Lilga, R. J. Orth, J. P. H. Sukamto, S. M. Haight and D. T. Schwartz, Sep. Purif. Technol., 1997, 11, 147–158 CrossRef CAS. C. He, J. Ma, C. Zhang, J. Song and T. D. Waite, Environ. Sci. Technol., 2018, 52, 9350–9360 CrossRef CAS.

In summary, the use of various electrode material, operational conditions, and surface modifications for selective ion separation was reviewed in this section. Thus, it is evident that electrodes can act as selective elements in CDI processes. In the following section, we will review the use of membranes for selective ion separation in CDI. 3. Membranes for ion selectivity In the previous section, ion selectivity in terms of electrodes was discussed. The use of membranes also plays a vital role in CDI. This section is dedicated for exploring the studies which rely on membranes for achieving ion selectivity. 3.1 Cation selectivity Several different studies have demonstrated the advantages of using IEMs to prevent co-ion repulsion, reduce anode oxidation, and to boost the salt removal by employing gradient of solutions in multi-chamber cells. 7,114 An IEM can also be used as a barrier for specific ions, and therefore, improve the ion selectivity. The works of Eliad et al, Gabelich et al., and later of Huang et al., provided evidence on electrosorption behavior of different anions on porous carbon electrodes. They demonstrated that CDI could be used to selectively remove different species of ions from aqueous solutions. However, at this early stage of ion selectivity with CDI, some questions regarding the parameters involved and the accurate mechanisms behind the selectivity, still remained unanswered. 68 which is set up and solved at each coordinate twice, first for i = Na + with j = K + and second for the reverse situation. In this equation, parameter V T is the thermal voltage given by V T = RT/ F which at room temperature is around 25.6 mV. All other parameter values are given in ESI (Section 7) of Porada et al. 78 Fig. 5 Typical desalination curves of potentiostatic, galvanostatic, and G/P modes in single-pass systems for CDI intended to support a qualitative comparison. To facilitate this comparison further, the potentiostatic mode in batch systems is also presented. G. R. Iglesias, S. Ahualli, M. M. Fernández, M. L. Jiménez and A. V. Delgado, Environ. Sci.: Water Res. Technol., 2019, 5, 873–883 CAS.Apart from more commonly targeted alkali and alkaline-earth metals, selective removal of heavy metals has also been of interest in CDI. In 2010, Li et al. utilized electrodes made of graphene nanoflakes to remove Fe 3+ and compared the electrosorption capacity with Mg 2+, Ca 2+, and Na + in single-salt experiments. 61 The Fe 3+ were preferred over the others, which was attributed to its higher valence ( Fig. 6A). Between Ca 2+ and Mg 2+, Ca 2+ were preferred due to their smaller hydrated radii ( Fig. 6B), as described before, whereas Na + exhibited the lowest electrosorption among all. In another study, Huang et al. employed activated carbon electrodes to remove Cu 2+ from aqueous solutions. 62 They also compared the Cu 2+ electrosorption in the presence of NaCl, natural organic matter (NOM), and dissolved reactive silica in binary salt solutions, and reported that Cu 2+ removal decreases with an increasing amount of the competitive species. However, no significant decrease in Cu 2+ electrosorption was observed in the presence of dissolved reactive silica.

Activated carbon. Activated carbon, defined by its high surface area to volume ratio, was used in the first CDI system 16 developed in the 1960's; in recent years this material has been modified to achieve even higher surface areas and hierarchical pore geometries with fast charge transfer and ion diffusion kinetics. In general, activated carbon, comprised of aggregates of microporous particles, is fabricated through pyrolysis of a carbon precursor, such as wood, then is activated ( i.e. micropores are created) via chemical etching or gasification of the product. 45 Although the typical performance of activated carbon electrodes does not match those of 1D and 2D materials (see Fig. 8a for a comparison), the low cost of activated carbon makes it an appealing electrode material for commercial applications. 47,48S. Sahin, J. E. Dykstra, H. Zuilhof, R. L. Zornitta and L. C. P. M. de Smet, ACS Appl. Mater. Interfaces, 2020, 12, 34746–34754 CrossRef CAS. Electrochemical deionization processes have found many applications in selective electrosorption/electrodeposition of ions, such as remediation of toxic ions from contaminated freshwater and resource mining from seawater. Tailored electrode coatings have been especially instrumental in advancing the field of selective electrosorption; researchers have used electroactive polymers, chelating polymers and redox-active polymers (conjugated and pendant-bearing) to coat electrodes for high capacity, highly selective separations. 32,33 Some prominent examples of the latter include uranium extraction from seawater 34 and chromium/arsenic oxyanion removal 35 from wastewater. Such electrode modifications coupled with modulated electric field techniques have resulted in further enhanced selectivity. The mechanisms of this process, while capacitive/pseudocapacitive in nature ( Fig. 1a), are covered in more detail in section 5 of this review. Mr Jayaruwan Gunathilake Gamaethiralalage is currently a PhD candidate in the Department of Organic Chemistry at Wageningen University & Research, The Netherlands. He received his BSc in chemistry from Kutztown University of Pennsylvania in the United States of America, joint MSc degrees in analytical chemistry from University of Tartu in Estonia, and Åbo Akademi University in Finland. His research interests include development of new material for ion separation and sensing, wastewater treatment, and electrodriven systems for circular water economy. S. Samatya, N. Kabay, Ü. Yüksel, M. Arda and M. Yüksel, React. Funct. Polym., 2006, 66, 1206–1214 CrossRef CAS. Y. Gao, L. Pan, H. B. Li, Y. Zhang, Z. Zhang, Y. Chen and Z. Sun, Thin Solid Films, 2009, 517, 1616–1619 CrossRef CAS.

L. Han, K. G. Karthikeyan, M. A. Anderson and K. B. Gregory, J. Colloid Interface Sci., 2014, 430, 93–99 CrossRef CAS.E. Avraham, B. Yaniv, A. Soffer and D. Aurbach, J. Phys. Chem. C, 2008, 112, 7385–7389 CrossRef CAS. where c i is the concentration of ion i in the micropores. The chemical potential of ion i is given by 49,77 R. L. Zornitta, K. M. Barcelos, F. G. E. Nogueira and L. A. M. Ruotolo, Carbon, 2020, 156, 346–358 CrossRef CAS. S. Porada, A. Shrivastava, P. Bukowska, P. M. Biesheuvel and K. C. Smith, Electrochim. Acta, 2017, 255, 369–378 CrossRef CAS.