Pu( iii ) and Cm( iii ) in the presence of EDTA: aqueous speciation, redox behavior, and the impact of Ca( ii )

Journal Article · Mon Mar 21 00:00:00 EDT 2022 · RSC Advances

DOI:https://doi.org/10.1039/D1RA09010K· OSTI ID:1856623

DiBlasi, Nicole A.  ^[1]; Tasi, Agost G.  ^[2]; Trumm, Michael  ^[2]; Schnurr, Andreas ^[2]; Gaona, Xavier  ^[2]; Fellhauer, David ^[2]; Dardenne, Kathy  ^[2]; Rothe, Jörg ^[2]; Reed, Donald T. ^[3]; Hixon, Amy E.  ^[4]; Altmaier, Marcus ^[2]

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1. Department of Civil & Environmental Engineering & Earth Sciences, University of Notre Dame, 301 Stinson-Remick, Notre Dame, IN 46556, USA, Karlsruhe Institute of Technology, Institute for Nuclear Waste Disposal, P.O. Box 3640, Karlsruhe, 76021, Germany
2. Karlsruhe Institute of Technology, Institute for Nuclear Waste Disposal, P.O. Box 3640, Karlsruhe, 76021, Germany
3. Los Alamos National Laboratory, 1400 University Dr., Carlsbad, NM 88220, USA
4. Department of Civil & Environmental Engineering & Earth Sciences, University of Notre Dame, 301 Stinson-Remick, Notre Dame, IN 46556, USA

The impact of calcium on the solubility, redox behavior, and speciation of the An(III)–EDTA (An = Pu or Cm) system under reducing, anoxic conditions was investigated through batch solubility experiments, X-ray absorption spectroscopy (XAS), density functional theory (DFT), and time-resolved laser fluorescence spectroscopy (TRLFS). Batch solubility experiments were conducted from undersaturation using Pu(OH)₃(am) as the solid phase in contact with 0.1 M NaCl–NaOH–HCl–EDTA–CaCl₂ solutions at [EDTA] = 1 mM, pH_m = 7.5–9.5, and [CaCl₂] ≤20 mM. Additional samples targeted brine systems represented by 3.5 M CaCl₂ and WIPP simulated brine. Solubility data in the absence of calcium were well-described by Pu(III)–EDTA thermodynamic models, thus supporting the stabilization of Pu(III)–EDTA complexes in solution. Cm(III)–EDTA TRLFS data suggested the stepwise hydrolysis of An(III)-EDTA complexes with increasing pH, and current Pu(III)-EDTA solubility models were reassessed to evaluate the possibility of including Pu(III)–OH–EDTA complexes and to calculate preliminary formation constants. Solubility data in the presence of calcium exhibited nearly constant log m(Pu)_tot, as limited by total ligand concentration, with increasing [CaCl₂]_tot, which supports the formation of calcium-stabilized Pu(III)–EDTA complexes in solution. XAS spectra without calcium showed partial oxidation of Pu(III) to Pu(IV) in the aqueous phase, while calcium-containing experiments exhibited only Pu(III), suggesting that Ca–Pu(III)–EDTA complexes may stabilize Pu(III) over short timeframes (t ≤45 days). DFT calculations on the Ca–Pu(III)–EDTA system and TRLFS studies on the analogous Ca–Cm(III)–EDTA system show that calcium likely stabilizes An(III)–EDTA complexes but can also potentially stabilize An(III)–OH–EDTA species in solution. This hints towards the possible existence of four major complex types within Ca–An(III)–EDTA systems: An(III)–EDTA, An(III)–OH–EDTA, Ca–An(III)–EDTA, and Ca–An(III)–OH–EDTA. While the exact stoichiometry and degree of ligand protonation within these complexes remain undefined, their formation must be accounted for to properly assess the fate and transport of plutonium under conditions relevant to nuclear waste disposal.

• Research Organization: Oak Ridge Inst. for Science and Education (ORISE), Oak Ridge, TN (United States)
• Sponsoring Organization: USDOE Office of Science (SC)
• Grant/Contract Number: SC0014664
• OSTI ID: 1856623
• Alternate ID(s): OSTI ID: 1904990
• Journal Information: RSC Advances, Journal Name: RSC Advances Vol. 12 Journal Issue: 15; ISSN 2046-2069
• Publisher: Royal Society of Chemistry (RSC)
• Country of Publication: United Kingdom
• Language: English

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