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 2H3BO3·3CsCl (BC) was observed in CsCl(aq) but not in RbCl(aq). The phase equilibria of the H3BO3 + RbCl + CsCl + H2O 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 H3BO3, 2H3BO3·3(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 βCs/Rb between the solid Bss phase and the corresponding solution was 2.0, significantly lower than that of (Cs,Rb)Cl (βCs/Rb = 9.5) in the ternary system without H3BO3. This indicates a stronger coordination between CsCl and H3BO3 relative to that of RbCl in the ternary system without H3BO3. The above understanding provides new strategies designing novel B–OH compounds to enhance Cs/Rb separation efficiency in aqueous systems.