Visible-NIR absorption spectroscopy study of the formation of ternary plutonyl(vi) carbonate complexes

Abstract

We present the first experimental evidence for the ternary complexation of calcium and magnesium ions with plutonyl(VI)tricarbonate species in carbonate-containing aqueous solutions using visible-NIR spectrophotometric titration. Prior to studying the ternary plutonyl(VI) carbonate complexation, visible-NIR absorption spectral information of PuO₂(CO₃)₂²⁻ and PuO₂(CO₃)₃⁴⁻ was successfully obtained. PuO₂(CO₃)₂²⁻ has a prominent peak at 853 nm and its molar absorptivity was determined to be ε_{853, PuO2(CO3)2²⁻} = 49.0 ± 4.2 M⁻¹·cm⁻¹. The spectrophotometric titration results by adding calcium or magnesium to the plutonyl(VI) carbonate system consisting of PuO₂(CO₃)₂²⁻ and PuO₂(CO₃)₃⁴⁻ indicate the formation of CaPuO₂(CO₃)₃²⁻ and MgPuO₂(CO₃)₃²⁻ complexes and provide the formation constants at 0.1 M H/NaClO₄ for MPuO₂(CO₃)₃²⁻ from PuO₂(CO₃)₃⁴⁻, log K = 4.33 ± 0.50 and 2.58 ± 0.18 for M = Ca²⁺ and Mg²⁺, respectively. In addition, the formation constants of CaPuO₂(CO₃)₃²⁻ and MgPuO₂(CO₃)₃²⁻ from PuO₂(CO₃)₃⁴⁻ at infinite dilution (log K°) were proposed to be 6.05 ± 0.50 and 4.29 ± 0.18, respectively, based on the correction of ionic strength using the Davies equation. The absorption spectrum of the ternary plutonyl(VI) complexes of CaPuO₂(CO₃)₃²⁻ is similar to that of PuO₂(CO₃)₃⁴⁻ with the exception of a characteristic absorption peak at 808 nm (ε_{808, CaPuO2(CO3)3²⁻} = 42.9 ± 1.6 M⁻¹·cm⁻¹). According to the calculated aqueous plutonyl(VI) speciation including the ternary plutonyl(VI) complexes, CaPuO₂(CO₃)₃²⁻ is considered the dominant Pu(VI) species under environmental conditions, and plutonyl(VI) may be more mobile than expected in previous assessments.