Complexation of Plutonium(IV) with trans-1,2-Diaminocyclohexane-N,N,N′,N′-tetraacetic Acid (CDTA) in Acidic Solution

Abstract

Understanding the interaction of Pu(IV) with complexing agents present in the nuclear fuel cycle is important for predicting the performance of used nuclear fuel separations. The complexation of Pu(IV) with trans-1,2-diaminocyclohexane-N,N,N′,N′-tetraacetic acid (CDTA) was studied in acidic solutions of 0.10–0.50 mol·L−1 HClO4 with 1.00 mol·L−1 (Na,H)ClO4 total ionic strength by a liquid–liquid extraction method using tracer quantities of 238Pu. The acid dissociation constants of CDTA and the autoprotolysis constant of water were determined via potentiometric titrations in 0.10–2.00 mol·L−1 NaClO4 and 25.0 ± 0.1 °C. The variation of the dissociation constants with ionic strength was modeled with the Specific ion Interaction Theory (SIT) and the associated SIT parameters were obtained. The thermodynamic dissociation constants at zero ionic strength for water and CDTA were determined from this analysis as $${\text{p}}K_{\text{w}}^{\text{o}}$$ = 14.00 ± 0.03, $${\text{p}}K_{\text{a2}}^{\text{o}}$$ = 1.52 ± 0.04, $${\text{p}}K_{\text{a3}}^{\text{o}}$$ = 2.78 ± 0.05, $${\text{p}}K_{\text{a4}}^{\text{o}}$$ = 4.17 ± 0.04, $${\text{p}}K_{\text{a5}}^{\text{o}}$$ = 6.75 ± 0.02, and $${\text{p}}K_{\text{a6}}^{\text{o}}$$ = 10.64 ± 0.04 at 25.0 ± 0.1 °C. The results of the liquid–liquid extraction experiments indicated the formation of a 1:0:1 complex, PuCDTA0, and the presence of additional protonated species, Pu(HCDTA)+ and Pu(H2CDTA)2+, at these acidities. The corresponding stability constants in 1.00 mol·L−1 (Na,H)ClO4 and 23 ± 1 °C were determined to be log10 β101 = 24.2 ± 0.3, log10 β111 = 25.4 ± 0.2, and log10 β121 = 25.8 ± 0.1.