Impact of Ca(II) on the aqueous speciation, redox behavior, and environmental mobility of Pu(IV) in the presence of EDTA

Nicole A DiBlasi; Agost G Tasi; Xavier Gaona; David Fellhauer; Kathy Dardenne; Jörg Rothe; Donald T Reed; Amy E Hixon; Marcus Altmaier

doi:10.1016/j.scitotenv.2021.146993

Impact of Ca(II) on the aqueous speciation, redox behavior, and environmental mobility of Pu(IV) in the presence of EDTA

Sci Total Environ. 2021 Aug 20:783:146993. doi: 10.1016/j.scitotenv.2021.146993. Epub 2021 Apr 9.

Authors

Nicole A DiBlasi¹, Agost G Tasi², Xavier Gaona³, David Fellhauer², Kathy Dardenne², Jörg Rothe², Donald T Reed⁴, Amy E Hixon⁵, Marcus Altmaier²

Affiliations

¹ Department of Civil & Environmental Engineering & Earth Sciences, University of Notre Dame, 301 Stinson-Remick, Notre Dame, IN 46556, United States of America.
² Karlsruhe Institute of Technology, Institute for Nuclear Waste Disposal, P.O. Box 3640, 76021 Karlsruhe, Germany.
³ Karlsruhe Institute of Technology, Institute for Nuclear Waste Disposal, P.O. Box 3640, 76021 Karlsruhe, Germany. Electronic address: xavier.gaona@kit.edu.
⁴ Los Alamos National Laboratory, 1400 University Dr., Carlsbad, NM 88220, United States of America.
⁵ Department of Civil & Environmental Engineering & Earth Sciences, University of Notre Dame, 301 Stinson-Remick, Notre Dame, IN 46556, United States of America. Electronic address: ahixon@nd.edu.

PMID: 33866175
DOI: 10.1016/j.scitotenv.2021.146993

Abstract

The impact of calcium on the solubility and redox behavior of the Pu(IV)-EDTA system was investigated using a combination of undersaturation solubility studies and advanced spectroscopic techniques. Batch solubility experiments were conducted in 0.1 M NaCl-NaOH-HCl-EDTA-CaCl₂ solutions at constant [EDTA] = 1∙10^-3 M, 1 ≤ pH_m ≤ 11, and 1∙10^-3 M ≤ [CaCl₂] ≤ 2∙10^-2 M. Additional samples targeted brine systems represented by 3.5 M CaCl₂ and WIPP simulated brine. Redox conditions were buffered with hydroquinone (pe + pH ≈ 9.5) with selected samples prepared in the absence of any redox buffer. All experiments were performed at T = 22 °C under Ar atmosphere. In-situ X-ray absorption spectroscopy indicated that PuO₂(ncr,hyd) was the solubility-controlling phase during the lifetime of all experiments and that aqueous plutonium was present in the +IV oxidation state across all experimental conditions except at pH_m ≈ 1, where a small fraction of Pu(III) was also identified. Current thermodynamic models overestimate Pu(IV)-EDTA solubility in the absence of calcium by approximately 1-1.5 log₁₀-units and do not describe the nearly pH-independent, increased solubility observed with increased calcium concentrations. The ternary Pu(IV)-OH-EDTA system without calcium was reevaluated using solubility data obtained in this work and reported in the literature. An updated thermodynamic model including the complexes Pu(OH)(EDTA)^-, Pu(OH)₂(EDTA)^2-, and Pu(OH)₃(EDTA)^3- was derived. Solubility data collected in the presence of calcium follows a pH-independent trend (log m(Pu)_tot vs. pH_m), which can only be explained by assuming the formation of a quaternary complex, tentatively defined as CaPu(OH)₄(EDTA)^2-, in solution. The significant enhancement of plutonium solubility observed in the investigated brine systems supports the formation of a quaternary complex that is not outcompeted by Ca(EDTA)^2-, even in concentrated CaCl₂ solutions. Although the exact stoichiometry of the complex may need to be revisited, this new quaternary complex has a pronounced impact on plutonium predominance diagrams over a broad range of pH, pe, and calcium concentrations that are relevant to nuclear waste disposal.

Keywords: Calcium; Complexation; EDTA; EXAFS; Plutonium; Thermodynamics; XANES.