Structural regulation of manganese ferrocyanide compounds for palladium separation: improved crystallinity and enhanced selectivity

Abstract

Ferrocyanide compounds are considered as promising materials for the efficient separation of palladium from high level liquid waste (HLLW), which is essential for the full utilization of palladium resources. This study proposes a structural regulation strategy to synthesize potassium manganese ferrocyanide (KMnFC) with enhanced crystallinity and selectivity for Pd(II) ions. A series of synthesis conditions were optimized to improve adsorption properties, with sodium citrate employed as a representative chelating agent to achieve high selectivity for Pd(II). Exhibiting optimized grain distribution, lattice spacing, and negligible content of defects, the adsorption ratio of the regulated KMnFC for main competitive ion Cs(I) decreased significantly from 99.2% to 42.5%. By optimizing the amount of sodium citrate, a balance between the adsorption capacity and selectivity was achieved. The regulated KMnFC demonstrated an enhanced maximum adsorption capacity for Pd(II), while maintaining fast kinetics. The novel adsorbent exhibited excellent reusability by using 0.2 mol per L thiourea, with desorption ratio over 99% in the first cycle and adsorption ratio of 96% after three cycles. Density functional theory (DFT) calculations and multiple characterizations were utilized to investigate the Pd(II) incorporation mechanisms. Through the pathways created by the expelled K(I) ions, Pd(II) ions are adsorbed and captured by the surface-centered sites in the KMnFC lattice, with a theoretically spontaneous occupancy of 75%. These findings demonstrate that the KMnFC adsorbent exhibits significant potential for the highly efficient and selective separation of Pd(II) with excellent reusability, while providing a novel strategy to enhance the adsorption performance of ferrocyanides by structural design and regulation.