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

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.