Spectroscopic and Thermodynamic Investigation of U(IV)–Oxalate Complexation Equilibria: Effects of Ionic Strength and Temperature

Aqueous equilibria involving multiple tetravalent uranium (U(IV)) complexes with oxalate (Ox) were investigated by using various spectroscopic techniques over a wide range of pH values (1-7), ionic strengths (0.02-2 M), and temperatures (0-35 °C), as well as quantum-chemical calculations based on density functional theory (DFT). Ultraviolet-visible (UV-vis) absorption spectroscopy coupled with a liquid waveguide capillary cell revealed distinct red shifts in the U(IV) absorption spectra upon binding with multiple Ox ligands. Based on an equilibrium model involving U(Ox)_n^4-2n complexes (n = 1-4), multiple sets of spectrophotometric data were analyzed to estimate their stepwise equilibrium constants (log K_n°): 10.49 ± 0.18, 7.09 ± 0.18, 5.34 ± 0.04, and 2.33 ± 0.14, for K₁°, K₂°, K₃°, and K₄°, respectively. The corresponding values of reaction enthalpy (Δ_rH_n), entropy (Δ_rS_n), and specific ion-interaction coefficients (ε_n) were further assessed for each U(IV)-Ox complex. Dissolution measurements of synthetic U(Ox)₂·xH₂O(cr) in a free oxalate (∼pH 5) solution using attenuated total reflectance FTIR spectroscopy along with DFT calculations support the proposed complexation-equilibrium model, showing that the stable binding geometry of all four U(IV)-Ox complexes adopts a side-on bidentate or 5-membered-ring coordination structure; in particular, the formation of a ternary complex, U(OH)(Ox)⁺, was not energetically favorable.