Hafnium(IV) complexation with oxalate at variable temperatures

Abstract

Abstract Appropriate management of fission products in the reprocessing of spent nuclear fuel (SNF) is crucial in developing advanced reprocessing schemes. The addition of aqueous phase complexing agents can prevent the co-extraction of these fission products. A solvent extraction technique was used to study the complexation of Hf(IV) – an analog to fission product Zr(IV) – with oxalate at 15, 25, and 35°C in 1 M HClO4 utilizing a 175+181Hf radiotracer. The mechanism of the solvent extraction system of 10−5 M Hf(IV) in 1 M HClO4 to thenoyltrifluoroacetone (TTA) in toluene demonstrated a 4th-power dependence in both TTA and H+, with Hf(TTA)4 the only extractable species. The equilibrium constant for the extraction of Hf(TTA)4 was determined to be log K ex=7.67±0.07 (25±1°C, 1 M HClO4). The addition of oxalate to the aqueous phase decreased the distribution ratio, indicating aqueous Hf(IV)-oxalate complex formation. Polynomial fits to the distribution data identified the formation of Hf(ox)2+ and Hf(ox)2 (aq) and their stability constants were measured at 15, 25, and 35°C in 1 M HClO4. van’t Hoff analysis was used to calculate Δr G, Δr H, and Δr S for these species. Stability constants were observed to increase at higher temperature, an indication that Hf(IV)-oxalate complexation is endothermic and driven by entropy.