High Na⁺ levels are detrimental for most crops. Selective membranes provide the possibility for the selective removal of Na⁺ while preserving beneficial ion species. The challenge is to separate two ion species of the same charge. This study evaluates the implementation of an electrodialysis (ED) system equipped with a supported liquid membrane (SLM) and a commercially available monovalent cation-selective membrane (CIMS) in the treatment of greenhouse drainage water. The SLM shows a (minimum) K⁺ over Na⁺, Ca²⁺ and Mg²⁺ permselectivity of 9, 15 and 30, respectively. Whereas the CIMS holds a high K⁺ over Ca²⁺ and Mg²⁺ permselectivity of 10 and 16, respectively, the K⁺ over Na⁺ permselectivity is just 1.3. With the experimentally obtained membrane characteristics at hand, the treatment of drainage water was simulated by a two-steps process with the two membrane types operating in series. Using real-life operational parameters, analysis revealed the optimal configuration and the ability to recover 96% of the K⁺ and approximately 80% of the water, Ca²⁺ and Mg²⁺. Summarized, this study not only shows the efficient separation of two ion species of the same valance but also the implementation of this technology in a real-life application.

We investigated in detail the permeation selectivity in the electro-dialysis of Na⁺, K⁺, Mg²⁺ and Ca²⁺ in both binary and quaternary mixtures using a supported liquid membrane (SLM). The SLM consisted of the organic liquid 2-nitrophenyl octyl ether (NPOE) containing a lipophilic anion, i.e. tetrakis[3,5-bis(trifluoromethyl)phenyl]borate, as the cation-exchanging site, which was used to fill the pores of the supporting membrane Accurel^R. We first determined the electro-phoretic mobilities of the migrating cations in single salt solutions, yielding: Na⁺ > K⁺ > Mg²⁺ > Ca²⁺. This order reflects the different size of the migrating cations. The monovalent cations Na⁺ and K⁺ migrate in the dehydrated state and the divalent cations Ca²⁺ and Mg²⁺ migrate in a (partly) hydrated state, a conclusion was supported by Karl Fisher titrations. Both binary and quaternary salt experiments showed a permeation selectivity in the following order: K⁺ > Na⁺ > Ca²⁺ > Mg²⁺. Since this order does not correlate with the order of electro-phoretic mobilities, we have determined the ion-exchange selectivity constant (K_ex) and found: K⁺ > Ca²⁺ > Mg²⁺ ≈ Na⁺. We conclude that the overall permeation selectivity is determined by the combination of ion-exchange selectivity and electro-phoretic mobility of the cations present in the membrane.

This study demonstrates the effective separation of alkali metal cations using a Supported Liquid Membrane (SLM) containing lipophilic, negatively charged borate moieties, operating under electro dialysis conditions. The selectivity of the membrane is essentially based on differences in dehydration energy and mobility between ion species. The system favors the ion species with the largest crystal radius, despite its lower mobility. In mixtures of K⁺ and Na⁺, the SLM separates K⁺ from Na⁺ with a separation efficiency ranging from ~20% to 90%, depending on the feed solution composition. With solutions containing either K⁺ or Na⁺ and Li⁺, the K⁺/Na⁺ over Li⁺ separation efficiency is nearly 100%. Addition of 15-crown-5 derivative does not improve SLM behavior, but slows down the K⁺ current by approximately 30% whereas the Na⁺ current remains unaffected. As supported by simulations, the free K⁺ and Na⁺ ratio in the membrane (and with that the current ratio) is entirely defined by partitioning and the feed concentration ratio, regardless the presence of 15-crown-5. As a result, the current ratio of two ion species can be described exclusively in terms of their feed concentrations and crystal radii because the latter parameter defines both partitioning and mobility.