Influence of ethylenediaminetetraacetic acid on the long-term oxidation state distribution of plutonium

Spectrophotometry was used to study the effect of EDTA on plutonium oxidation state distribution as a function of time, pH, and ligand-to-metal ratio (L/M) under anoxic conditions. Novel Pu(V)-EDTA absorption bands were identified at 571, 993, 1105, and 1150 nm with molar absorption coefficients of 15 +/- 1, 6 +/- 1, 10 +/- 1, and 10 +/- 1 cm(-1)M(-1), respectively. Pu(V)-EDTA spectral changes occurred at L/M < 1, indicating only PuVO2(EDTA)(3-) formed with logK = 3.6 +/- 0.3. Time-resolved experiments showed EDTA drastically increased the Pu(V/VI) reduction rate, which we propose is driven by amine lone-pair electron donation and the oxidative decarboxylation of EDTA. Oxidation of Pu(III)-EDTA to Pu(IV)-EDTA occurred on a slower time scale (110-237 days) than previously reported (<15 min) and is hypothesized to be radiolysis driven. Pu(V/VI)-EDTA and Pu(III)-EDTA both approached Pu(IV)-EDTA stabilization over time, yet Pu(V/VI)-EDTA solubility data was >= 1.0 log(10) units higher than predicted by Pu(IV)-EDTA solubility models, indicating that current thermodynamic models are incomplete. Ultimately, the data show EDTA preferentially stabilizes Pu(IV) over time regardless of initial oxidation state, but Pu(V)-EDTA can persist under environmentally-relevant conditions, emphasizing the need to continue investigating redox reactions, speciation, and behavior of these complexes to support the transuranic waste disposal and surface remediation/containment efforts. (C) 2021 Elsevier Ltd. All rights reserved.

Automated summary

The study indicates that EDTA can affect the distribution of plutonium oxidation states, and time, pH, and ligand-to-metal ratio also play a role. EDTA mainly stabilizes the Pu(IV) oxidation state, but Pu(V)-EDTA may persist under environmentally-relevant conditions.