Effective enrichment of Cs+ and Rb+ from high-salty solutions using calcium hexacyanoferrate and followed separation by phase-transformation

Abstract

Enrichment and separation of high-value Cs⁺ and Rb⁺ from K⁺ and Na⁺ are challenging process due to the homologous similarity of the alkali metal ions. Metal hexacyanoferrates demonstrate excellent selectivity for Cs⁺ and Rb⁺, but the products are typically ultrafine powders or colloidal precipitates that are difficult to settle or filter. In this study, a novel metal hexacyanoferrate, calcium hexacyanoferrate (M-CaFC), is employed with micron-sized ultra-large particles to recover Cs⁺ and Rb⁺ from high-salty solutions with K⁺ and Na⁺ over 200 g/L. M-CaFC exhibits a regular cubic structure of 3 μm, which is thousands of times larger in size than that formed using transition metal elements, and completely solves the difficulty of solid-liquid separation. The process exhibits fast reaction velocity (1 min for Cs⁺ and 3 min for Rb⁺) and high recovery rates (R_Cs = 98.66 %; R_Rb = 84.69 %) in the pH range of 2–13. M-CaFC consists of three distinct phases, Cs-CaFC, Rb-CaFC and K-CaFC, with the molecular formula of Cs_1.15Rb_0.57K_0.17Ca_1.05Fe(CN)₆. Then the solid M-CaFC was transformed to K⁺, Rb⁺ and Fe(CN)₆⁴⁻ by the aid of CO₃²⁻, while Cs-CaFC maintains its original solid state, the maximum separation factor for Cs⁺ and Rb⁺ is up to 169.04. DFT calculations confirm that Cs-CaFC exhibits superior structural stability compared to K-CaFC and Rb-CaFC. This technique represents a simple and efficient method for the enrichment and separation of Cs⁺ and Rb⁺ from high-salty solutions.