Dual Role of Resin Carboxylates in Prussian Blue Formation and Divalent-Ion Sensitivity during Cesium Adsorption

The Prussian blue (PB) lattice is intrinsically selective for Cs+, yet the influence of carboxyl-rich support interfaces on this selectivity under competing ion conditions remains unclear. Here, we constructed IRC-50-PB from the weak-acid cation-exchange resin IRC-50 to examine the dual role of carboxylates in PB formation and ion competition. We combined structural and spectroscopic characterization, comparative titration, competitive adsorption, independent zeta potential measurements, and local density functional theory models. Structural characterization confirmed crystalline PB formation. Spectroscopy revealed changes in the carboxylate environment across the material states, while titration showed a corresponding variation in operational titratable acidity at pH 8.5. In systems containing nominally 1.5 mM Cs+, capacity retention in Ca2+/Mg2+ media was 8.8–24.0% at nominal ionic strengths of 10 and 30 mM for the added electrolytes, compared with 66.0–88.3% in Na+/K+ media. Zeta potentials measured independently in corresponding Cs+-free salt media showed the same divalent–monovalent grouping. Δζ varied with material states, and pure PB showed the smallest mean Δζ, consistent with a contribution from the carboxyl-rich interface. Local models indicated that hydration-shell ligand exchange involving carboxylates was electronically more favorable for Ca2+ and Mg2+ than for the examined monovalent cations. The models also supported favorable local Fe(III)–carboxylate coordination and compatibility between Fe–CN bridging and relaxation of some Fe–carboxylate contacts. These results suggest that the PB lattice provides intrinsic Cs+ selectivity, whereas the carboxyl-rich interface may modulate its expression in the composite.