Enhanced cesium removal by K2Ni[Fe(CN)6]·H2O: structural optimization, adsorption behavior, and competitive ion selectivity compared to Ni2Fe(CN)6·3H2O

Abstract

In this study, two hexacyanoferrate-based materials, Ni₂Fe(CN)₆·3H₂O (NiHCF) and K₂Ni[Fe(CN)₆]·H₂O (KNiHCF), were synthesized and comprehensively characterized using EDS, FTIR, XRD, TEM/HR-TEM, and BET analyses. The results demonstrated that both materials possess high crystallinity, uniform cubic morphology, stable [Fe(CN)₆]⁴⁻ frameworks, and mesoporous structures. Adsorption experiments showed that KNiHCF exhibited significantly enhanced cesium ion (Cs⁺) removal capacity, with a higher maximum adsorption capacity (182.54 mg g⁻¹), faster adsorption kinetics (equilibrium reached within 18 minutes), and remarkable selectivity in the presence of competing ions (K⁺, NH₄⁺, Na⁺, and Ca²⁺). The adsorption process conformed to the Sips isotherm model and the pseudo-second-order kinetics model. Thermodynamic analysis indicated that the adsorption was spontaneous and exothermic for both materials. While KNiHCF demonstrated stronger enthalpic interactions, NiHCF showed higher affinity for Cs⁺ under ambient conditions. Ion-exchange was identified as the predominant adsorption mechanism in KNiHCF, facilitated by the structural compatibility between Cs⁺ ions and the host lattice. These findings suggest that precise control over synthesis conditions to obtain K₂Ni[Fe(CN)₆]·H₂O is essential for optimizing cesium removal efficiency in practical water treatment applications.