Adsorption selectivity of nickel hexacyanoferrate foam electrodes and influencing factors: extraction of a 98 % potassium fraction solution from pseudo-seawater

Metal hexacyanoferrates are promising adsorbents for desalination and concentration of seawater and wastewater, because of a high capacity for selective cation intercalation into their three-dimensional lattice through redox reactions. The ratio between fractional quantities of cations removed from a solution is a metric commonly used to evaluate adsorption selectivity. However, this metric also depends directly on cation concentrations in the adsorption solution, thus on the electrode potential through the reaction yield. Here, we analyzed the adsorption selectivity of nickel hexacyanoferrate (NiHCF) foam electrodes, characterized by high porosity and excellent ion diffusivity, using only the selectivity coefficient for ion exchange, i.e., independently of the electrode potential. We conducted a potassium extraction experiment with three consecutive stages, each comprising first an adsorption then a desorption process. From pseudo-seawater (K⁺ = 10 mmol/L, Na⁺ = 495 mmol/L) as first adsorption solution, we obtained a final desorption solution with a high potassium fraction (98 %; K⁺ = 123 mmol/L, Na⁺ = 3 mmol/L). Temporal concentration variations illustrated the close agreement between measurements and values calculated using only the selectivity coefficient for ion exchange, demonstrating that the adsorption selectivity of NiHCF foam electrodes was primarily influenced by ion exchange reactions, and did not depend on the electrode potential. We also demonstrated the usefulness of our foam electrodes for industrial application through a cyclability assessment and a detailed K⁺ adsorption selectivity evaluation in synthetic seawater, a more realistic seawater analogue containing not only Na⁺ and K⁺, but also divalent cations (Mg²⁺, Ca²⁺).