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)<i>_n</i>^4-2<i>n</i> complexes (<i>n</i> = 1-4), multiple sets of spectrophotometric data were analyzed to estimate their stepwise equilibrium constants (log <i>K_n</i>°): 10.49 ± 0.18, 7.09 ± 0.18, 5.34 ± 0.04, and 2.33 ± 0.14, for <i>K</i>₁°, <i>K</i>₂°, <i>K</i>₃°, and <i>K</i>₄°, respectively. The corresponding values of reaction enthalpy (Δ_r<i>H</i>_n), entropy (Δ_r<i>S</i>_n), and specific ion-interaction coefficients (ε<i>_n</i>) were further assessed for each U(IV)-Ox complex. Dissolution measurements of synthetic U(Ox)₂·<i>x</i>H₂O(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.