Extraction of uranium (VI) with neutral organic phosphorus extractants: A calorimetric and spectroscopic comparison of di(1-methylheptyl) methylphosphonate vs. tri-n-butylphosphate

Abstract

Both tri-n‑butyl phosphate and di(1-methylheptyl) methylphosphonate demonstrate exceptional extraction capabilities for ²³³U from irradiated thorium. While extensive literature exists regarding tri-n‑butyl phosphate-based extraction systems, comparative studies on di(1-methylheptyl) methylphosphonate remain notably limited. This systematic investigation provides a parallel comparison of the chemical behaviors between these two extractants under identical experimental conditions. Results reveal that the di(1-methylheptyl) methylphosphonate system exhibits enhanced hydrophobicity relative to tri-n‑butyl phosphate, correlating with reduced water co-extraction during the uranium recovery process. Raman spectroscopic analysis demonstrated about ∼30 cm^–1 shifts in the uranyl symmetric stretching vibration for di(1-methylheptyl) methylphosphonate complexes compared to ∼15 cm^–1 shifts of tri-n‑butyl phosphate complexes. This significant spectral displacement indicates stronger coordination bonding between phosphoryl oxygen of di(1-methylheptyl) methylphosphonate and uranyl ions. Isothermal titration calorimetry measurements quantified the extraction thermodynamics, yielding enthalpy changes of -12.83 kJ/mol and -11.32 kJ/mol for tri-n‑butyl phosphate and di(1-methylheptyl) methylphosphonate systems, respectively. Di(1-methylheptyl) methylphosphonate exhibits marginally less exothermic enthalpy despite its superior extraction efficiency. The counterintuitive observation suggests distinct thermodynamic compensation mechanisms. This likely involves more favorable entropy contributions in the di(1-methylheptyl) methylphosphonate extraction system and greater energy consumption during desolvation. This comprehensive comparison clarifies fundamental differences in extraction mechanisms between tri-n‑butyl phosphate and di(1-methylheptyl) methylphosphonate extractants, providing critical thermodynamic parameters for optimizing ²³³U recovery processes.