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

초록

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 degrees 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 Kn degrees): 10.49 +/- 0.18, 7.09 +/- 0.18, 5.34 +/- 0.04, and 2.33 +/- 0.14, for K 1 degrees, K 2 degrees, K 3 degrees, and K 4 degrees, respectively. The corresponding values of reaction enthalpy (Delta r H n), entropy (Delta r S n), and specific ion-interaction coefficients (epsilon n ) were further assessed for each U(IV)-Ox complex. Dissolution measurements of synthetic U(Ox)2 & centerdot;xH2O(cr) in a free oxalate (similar to 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.

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