Supercritical CO ₂ Solubility in TBP–HNO ₃ Complexes: Thermodynamic Insights and Implications for Rare Earth Element Extraction Efficiency

To effectively design and optimize rare earth element (REE) extraction processes, supercritical carbon dioxide (SC–CO2) solubility behavior in mixed solvent systems is crucial. This study systematically examined the thermodynamic solubility of CO2 in tributyl phosphate (TBP)–nitric acid (HNO3) complexes at three molar ratios (2:1, 1:1, and 1:2), temperatures between 40 and 60 °C, and a maximum pressure of 12 MPa, respectively. This paper used a static high-pressure equilibrium cell coupled with mole balance calculations to quantify solubility. In addition, Fourier Transform Infrared (FTIR) spectroscopy was employed to clarify the molecular interactions triumphing within the solvent system. The findings of this study show a robust positive correlation between pressure and carbon dioxide solubility with a steep increase being observed above the critical pressure of CO2 (about 7.38 MPa). Contrarily, temperature and CO2 solubility showed an inverse relation; for the 1:2 TBP:HNO3 complex, the maximum CO2 absorption was consistently recorded at 40 °C across the investigated pressure range. Consequential spectral shifts in the phosphorylation (P═O) and nitrate (N–O) regions were revealed and detected by Fourier Transform Infrared (FTIR) spectroscopy, substantiated by hydrogen bonding interactions and a reorganization of the solvent structure upon CO2 dissolution. Generally, the observed CO2 solubility behavior gives important mechanistic insight into the solvent–CO2 interactions essential for efficient ligand delivery and complexation in SC–CO2-mediated REE separation processes. Additionally, preliminary extraction experimental runs were conducted to validate the solubility studies. Results from ICP-MS report confirmed that high solubility favors effective REE extraction, especially Cerium and Neodymium, and a complex ratio of 2:1 resulted in a higher yield of extraction. These research findings reveal that solubility is a crucial mechanism for good extraction performance and have provided valuable thermodynamic understanding that can inform the future development of more environmentally benign and solvent-efficient extraction technologies.