Enhancing the deep removal of trace thorium from rare earths using polycinamic acid resin

Abstract

Precise recognition and deep removal of trace thorium from rare earths remains a significant challenge. Herein, we prepared a polycinnamic acid (PCA) resin by emulsion polymerization to achieve efficient separation of thorium from rare earths. The PCA resin leverages its specific pore size, steric hindrance and relatively large electronegativity of thorium to precisely recognize trace thorium ions in rare earth solutions. For instance, a separation factor of 4158 was achieved for Th/Lu at a Lu/Th molar ratio of 350. The resin also exhibited an exceptional adsorption capacity for Th(IV), reaching 405.2 mg/g at 328 K, surpassing most of the previously reported adsorbents. The adsorption process was determined by a combination of intra-particle diffusion and monolayer chemisorption, driven by entropy in a spontaneous endothermic reaction. Desorption of thorium from the loaded resin ions was effectively accomplished using H₂SO₄ solution at pH < 2. Additionally, the PCA resin exhibited excellent cycling and regeneration performance, maintaining stability under various acidic conditions and immersion durations. In practical applications, the PCA resin reduced the thorium concentration from 5.5 mg/L to 0.005 mg/L in simulated radioactive wastewater and from 0.56 mg/L to 0.007 mg/L in high-purity LuCl₃ solution, with no measurable loss of rare earths. Characterization through FTIR, Raman and XPS analysis revealed that two oxygen atoms of the –COOH group are bidentate chelating coordination with thorium ions. Furthermore, density functional theory (DFT) calculations provided insights into the selective binding mechanism at the –COOH site, highlighting its role in the effective separation of thorium from rare earths.