WORCESTER BOSCH SET OF ELECTRODES 87186643010

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X. Zhang, K. Zuo, X. Zhang, C. Zhang and P. Liang, Environ. Sci.: Water Res. Technol., 2020, 6, 243–257 RSC. The removal of an unconventional ion, uranium( VI), using phosphate-functionalized graphene hydrogel electrodes was studied by Liao et al. 66 The electrodes were tested in equimolar solutions (0.3 mM) containing uranium( VI) and a series of interfering metals ions (Cs +, Co 2+, Ni 2+, Sr 2+, and Eu 3+). The authors reported that the electrodes preferred uranium( VI) over all the other metals that were tested. Furthermore, they observed that the uranium( VI) is more selective against monovalent metal ions compared to that of divalent or trivalent ions. This phenomenon was attributed to the stronger electrostatic interaction between trivalent ions and the electrode surface, thus adsorbing more trivalent ions resulting in reduced selectivity of uranium( VI). Apart from ion valence, the selectivity of the electrode is also attributed to the formation of strong acid–base complexes with the phosphate groups attached to the electrode ( Fig. 6E). 2.2 Anion selectivity The mechanisms used to achieve cation selectivity may be extrapolated, and used to achieve anion selectivity in CDI. One of the pioneering studies in CDI anion selectivity is the one of Farmer et al., reported in 1996, which showed a difference in electrosorption capacity of different anions. 67 In their work, the authors employed 192 pairs of carbon aerogel electrodes to investigate the desalination of NaCl and NaNO 3 in single-salt experiments. Although this work was not intended to investigate ion selectivity, the difference in electrosorption observed by the authors was the first clue that CDI could be a valuable technology for selective anion adsorption.

where subscript j indicates the phase, either the electrolyte outside the micropore, ∞, or the micropore region (the subscript j is dropped). Note that all potential terms are without dimension, and can be multiplied by a factor RT to obtain a potential in J mol −1. The parameter μ ref, i is the reference chemical potential of ion i, the second term relates to ion entropy, z i ϕ j is the electrostatic term, while μ exc, i, j represents a contribution due to excess or volumetric interactions, and μ aff, i, j relates to chemical interactions, the interaction of the ion with the environment, not described by volume or charge. The simplest relevant situation is when all ions are ideal point charges, and there are no affinity effects. Then ions are subject to entropic effects, given by ln c i, j, and the electrostatic field, given by z i ϕ j. Potential ϕ j refers to the electric potential of phase j, and ϕ − ϕ ∞ is the dimensionless Donnan potential, ϕ D. This potential can be multiplied by V T = RT/ F to obtain a voltage with unit volt. At phase equilibrium, the chemical potential of ion i is balanced between the micropore and bulk electrolyte, yielding R. Y. Wang, B. Shyam, K. H. Stone, J. N. Weker, M. Pasta, H. W. Lee, M. F. Toney and Y. Cui, Adv. Energy Mater., 2015, 5, 1–10 Search PubMed. Recently, Zhang et al. used activated carbon in flow CDI to selectively remove Cu 2+ from a solution which also contained Na +. 65 A higher affinity towards Cu 2+ was obtained in the system. This was attributed to the preferential adsorption of Cu 2+ on the carbon particles and was also reduced to Cu. The preference of carbon towards divalent over monovalent cations, as shown in Fig. 6A was also reported here. The Na + removed from the feed remained in the electrolyte of the flow electrode. A recent study by Hawks et al. showed a high selectivity for nitrate over chloride and sulfate by using ultra-microporous (pore diameter < 1 nm) carbon electrodes. 41 The idea is similar to the one already explored by Eliad et al. (2001), in which selectivity is achieved due to sieving effect of very small carbon pores. The authors explored the effect of the solvation shell of the ion in aqueous media on their selective adsorption ( Fig. 6D). While chloride and sulfate ions are nearly homogeneously surrounded by water molecules, the solvation shell of a nitrate ion is mostly located at the edge of the ion and is not strongly bound to the molecule. As such, the authors suggested that the position of the solvation shell and the hydration energy instead of the average hydrated radius should be a more accurate parameter to be used in the investigation of ions selectivity based on ion size. The selectivity of nitrate over sulfate was also investigated. In this case, only a small amount of sulfate was electrosorbed inside the miniscule pores of the carbon electrode, which is explained by the higher solvation energy of sulfate compared to nitrate or chloride. In the electrosorption experiments, different cell potentials were applied to achieve the maximum selectivity ( ρ, Table 1) of NO 3 −/Cl − ≈ 6 and NO 3 −/SO 4 2− ≈ 18 at 0.6 V. At a cell voltage of 1.0 V, the NO 3 −/Cl − and NO 3 −/SO 4 2− selectivities were found to be ≈3 and ≈9, respectively. The observed reduction in selectivity with increasing cell voltage is explained by the solvation energy. At higher cell voltages, more energy is available to rearrange the solvation shell, and the ions be stored in the electrode. Consequently, the removal efficiencies of chloride and sulfate increase, reducing nitrate selectivity. In contrast, lower cell voltages limit the ion removal capacity due to co-ion repulsion, reducing the charge efficiency of the electrodes. Therefore, there is an optimum voltage that should be considered to maximize both energy efficiency and nitrate selectivity.S. Porada, A. Shrivastava, P. Bukowska, P. M. Biesheuvel and K. C. Smith, Electrochim. Acta, 2017, 255, 369–378 CrossRef CAS. S. Kim, J. Lee, J. S. Kang, K. Jo, S. Kim, Y. E. Sung and J. Yoon, Chemosphere, 2015, 125, 50–56 CrossRef CAS. For the term, γα′, γ is a constant, namely γ = 0.0725, while α′ = d i/ h p. Here, d i is the (hydrated) ion size and h p is the ratio of pore volume over pore wall area. For a slit-shaped pore, h p is equal to the pore width divided by 2, and for a cylindrical pore it is equal to pore size ( i.e., pore diameter) divided by 4. Thus h p is a characteristic pore size, but because we typically do not know these values exactly, neither the ion size in the pore, nor the factor h p, α′ is typically an empirical factor.

J. Kim, A. Jain, K. Zuo, R. Verduzco, S. Walker, M. Elimelech, Z. Zhang, X. Zhang and Q. Li, Water Res., 2019, 160, 445–453 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. Selective removal of Pb 2+ over Ca 2+ and Mg 2+ was studied by Dong et al. by using activated carbon electrodes in an asymmetric CDI setup. This setup only contained an AEM (hence asymmetric), as the Pb 2+ desorption was reported inefficient when a CEM was used as well, thus hindering its selectivity. 64 The asymmetric system was selective towards Pb 2+ over Ca 2+ and Mg 2+. The selectivity mechanism was hypothesized to be a swapping process where Ca 2+ and Mg 2+ are initially adsorbed due to their higher mobilities, but later replaced by Pb 2+ owing to its higher affinity towards the native functional groups ( e.g., carboxyl groups) present on the electrode. P. Srimuk, J. Lee, S. Fleischmann, M. Aslan, C. Kim and V. Presser, ChemSusChem, 2018, 11, 2091–2100 CrossRef CAS.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. X. Gao, A. Omosebi, N. Holubowitch, A. Liu, K. Ruh, J. Landon and K. Liu, Desalination, 2016, 399, 16–20 CrossRef CAS. Within the last decade, in addition to water desalination, capacitive deionization (CDI) has been used for resource recovery and selective separation of target ions in multicomponent solutions. In this review, we summarize the mechanisms of selective ion removal utilizing different electrode materials, carbon and non-carbon together with or without membranes, from a mixture of salt solutions, by a detailed review of the literature from the beginning until the state-of-the-art. In this venture, we review the advances made in the preparation, theoretical understanding, and the role of electrodes and membranes. We also describe how ion selectivity has been defined and used in literature. Finally, we present a theory of selective ion removal for intercalation materials that, for the first time, considers mixtures of different cations, evidencing the time-dependent selectivity of these electrodes.