Solid–Liquid Phase Equilibrium in the Quaternary System H 3 BO 3 + CsCl + RbCl + H 2 O at 298.15 K and Identification of a Solid Solution Phase 2H 3 BO 3 ·3(Cs,Rb)Cl

Previously, the formation of a stable adduct 2H 3 BO 3 ·3CsCl (BC) was observed in CsCl(aq) but not in RbCl(aq). The phase equilibria of the H 3 BO 3 + RbCl + CsCl + H 2 O system were studied to assess the potential for selective separation of Cs from Rb. Solubility isotherms were determined experimentally. Appropriate liquid composition ranges were preselected using modeling with ISLEC’s equilibrium software. Schreinemakers’ wet solid method, XRD, Raman and IR spectra, Roozeboom’s plots, and SEM-EDS were applied to identify the four phase regions H 3 BO 3, 2H 3 BO 3 ·3(Cs,Rb)Cl (Bss), (Rb,Cs)Cl, and (Cs,Rb)Cl. Notably, in the presence of RbCl, the BC phase incorporates Rb +, forming a substitutional solid solution, Bss, which inhibits selective separation of CsCl. Roozeboom’s plot analysis revealed that RbCl exhibits higher solubility in the Bss phase compared to the (Cs,Rb)Cl solid solution. The separation factor β Cs/Rb between the solid Bss phase and the corresponding solution was 2.0, significantly lower than that of (Cs,Rb)Cl (β Cs/Rb = 9.5) in the ternary system without H 3 BO 3 . This indicates a stronger coordination between CsCl and H 3 BO 3 relative to that of RbCl in the ternary system without H 3 BO 3 . The above understanding provides new strategies designing novel B–OH compounds to enhance Cs/Rb separation efficiency in aqueous systems.