In 2013, Zafra et al. evaluated the electrosorption capacity of high surface area electrodes using single-salt solutions consisting of nutrients (Cl −, NO 3 −, and H 2PO 4 −/HPO 4 2−). 40 The authors found a lower phosphate electrosorption compared to nitrate or chloride. It was suggested that this reduced capacity was caused by the sieving effect of the prepared activated carbon (average pore size of 0.855 nm) towards to the smaller ions (Cl − and NO 3 −) compared to the large phosphate species (H 2PO 4 −/HPO 4 2−). This investigation agrees well with the report about the sieving effect of the porous carbon described by former authors ( Fig. 6B). In the same line of nutrient recovery, Ge et al. investigated the competition between physical adsorption and electrosorption of phosphate anions. 69 The authors suggested that electrosorption could only overcome the effect of physical adsorption at very high cell voltages. Therefore, to improve phosphate electrosorption the authors applied a cell voltage as high as 3.0 V, which also cause faradaic reactions. Although the authors suggest that some species formed during the faradaic reactions could also promote a disinfection of the treated water, there is an expressive reduction of the charge efficiency. Nevertheless, this work is important in understanding the lower electrosorption capacity of phosphate at neutral pH compared to other ions.

S. Porada, R. Zhao, A. van der Wal, V. Presser and P. M. Biesheuvel, Prog. Mater. Sci., 2013, 58, 1388–1442 CrossRef CAS. Akin to the work of Yeo et al., Zuo et al. investigated the viability of a resin to selectively remove sulfate from a mixture with chloride. 131 An experiment with the pristine high surface area carbon electrode demonstrated a higher selectivity towards chloride than sulfate ( S i/ j = 2.2), in agreement with the work of Sun et al. 75 The authors were able to reverse the selectivity (SO 4 2−/Cl − of 2.4) by coating the activated carbon electrode with the selective resin. The resin-coated carbon was able to maintain the selectivity of 1.9 towards sulfate even upon increasing the chloride concentration by a factor of 100. In contrast to some of the studies using nitrate-selective resins, 129,130 the authors did not report any issue during the desorption of the electrosorbed sulfate anions. Recently, Hu et al. proposed a new electrode based on layered metal oxide with Pd to remove nitrate using an approach similar to CDI. 113 However, the main difference was the reduction of NO 3 − to N 2 in the cathode of the cell by faradaic reactions. Although the authors did not provide a selectivity value, the electrodes are expected to exhibit high selectivity towards NO 3 − since its concentration in the electrode did not reach saturation.K. Singh, S. Porada, H. D. de Gier, P. M. Biesheuvel and L. C. P. M. de Smet, Desalination, 2019, 455, 115–134 CrossRef CAS. Another recent approach that has provided viable results for selectivity between mono/divalent ions is the use of monovalent ion-selective membranes. Pan et al. investigated the use of such membranes to separate fluoride and nitrite from sulfate. 137 Using an equimolar solution, the authors observed a selectivity ( ρ) of ≈1.4 for fluoride ions over sulfate ions. Furthermore, it was found that the pH of the feed solution was an important parameter to control and improve the ion selectivity. Higher pH values increased the selectivity towards fluoride, while for acidic solutions the selectivity was lost due to an interaction between protons and the surface of the membrane. The effect of the feed concentration was also explored, keeping the concentration ratio between the two anions constant. An increasing fluoride selectivity was observed upon increasing the concentration of both F − and SO 4 2−. When the cell voltage was increased, the selectivity was reduced towards F − demonstrating that high cell voltages cannot attain high selectivity. This result is in line with other works that show lower selectivity at higher cell voltages. 41,77 where η′ is a modified volume fraction of ions in the pore, which is the real volume fraction η, to which is added an empirical term γα′ which relates to the ion size to pore size ratio. The volume fraction η is given by a summation over all ions in the pore of their concentration in the micropores times the molar volume, i.e., the volume (per mole of ions), which can include the water molecules that are tightly bound to the ion (ion plus hydration shell). For larger ions, the γα′ term is larger, and thus for this ion, Φ exc, i will be lower and it will be excluded from the pores relative to the smaller ion. Though this function is derived from a Carnahan–Starling equation of state, which considers mixtures of ions of the same size, 160 we utilize this simplified expression here to describe a size-based selectivity in mixtures of ions of different sizes. D. I. Kim, R. R. Gonzales, P. Dorji, G. Gwak, S. Phuntsho, S. Hong and H. Shon, Desalination, 2020, 484, 114425 CrossRef CAS.

L. Eliad, G. Salitra, A. Soffer and D. Aurbach, J. Phys. Chem. B, 2001, 105, 6880–6887 CrossRef CAS.

d Department of Chemical and Environmental Engineering, Yale University, New Haven, CT 06520-8286, USA L. Gan, Y. Wu, H. Song, S. Zhang, C. Lu, S. Yang, Z. Wang, B. Jiang, C. Wang and A. Li, Sep. Purif. Technol., 2019, 212, 728–736 CrossRef CAS. S. P. Hong, H. Yoon, J. Lee, C. Kim, S. Kim, J. Lee, C. Lee and J. Yoon, J. Colloid Interface Sci., 2020, 564, 1–7 CrossRef CAS. Dr Rafael Linzmeyer Zornitta is a postdoctoral researcher in the Organic Chemistry group at the Wageningen University & Research, The Netherlands. He received his BSc in Chemical Engineering from State University of Maringa (Brazil), MSc and PhD from Federal University of Sao Carlos (Brazil), with internships at the Malaga University (Spain), and Leibniz Institute for New Materials (Germany). His research interests include the development of new electrode materials, ion-selective membranes, and optimization of cell design for water desalination and selective ion recovery using electrochemical technologies.