Interaction of Np( v ) with borate in alkaline, dilute-to-concentrated, NaCl and MgCl ₂ solutions

Journal Article · Wed Feb 05 00:00:00 EST 2020 · Dalton Transactions

DOI:https://doi.org/10.1039/C9DT04430B· OSTI ID:1582687

Hinz, K. ^[1]; Fellhauer, D. ^[1]; Gaona, X.  ^[1]; Vespa, M. ^[1]; Dardenne, K.  ^[1]; Schild, D.  ^[1]; Yokosawa, T. ^[1]; Silver, M. A. ^[2]; Reed, D. T. ^[3]; Albrecht-Schmitt, T. E.  ^[2]; Altmaier, M. ^[1]; Geckeis, H. ^[1]

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1. Institute for Nuclear Waste Disposal, Karlsruhe Institute of Technology, Germany
2. Department of Chemistry and Biochemistry, Florida State University, USA
3. Los Alamos National Laboratory, USA

The interaction of Np(v) with borate was investigated in 0.1–5.0 M NaCl and 0.25–4.5 M MgCl₂ solutions with 7.2 ≤ pHm ≤ 10.0 (pHm = –log[H⁺]) and 0.004 M ≤ [B]_tot ≤ 0.16 M. Experiments were performed under an Ar-atmosphere at T = (22 ± 2) °C using a combination of under- and oversaturation solubility experiments, NIR spectroscopy, and extensive solid phase characterization. A bathochromic shift (≈5 nm) in the Np(v) band at λ = 980 nm indicates the formation of weak Np(v)–borate complexes under mildly alkaline pHm-conditions. The identification of an isosbestic point supports the formation of a single Np(v)–borate species in dilute MgCl₂ systems, whereas a more complex aqueous speciation (eventually involving the formation of several Np(v)–borate species) is observed in concentrated MgCl₂ solutions. The solubility of freshly prepared NpO₂OH(am) remained largely unaltered in NaCl and MgCl2 solutions with [B]_tot = 0.04 M within the timeframe of this study (t ≤ 300 days). At [B]_tot = 0.16 M, a kinetically hindered but very significant drop in the solubility of Np(v) (3–4 log₁₀-units, compared to borate-free systems) was observed in NaCl and dilute MgCl₂ solutions with pHm ≤ 9. The drop in the solubility was accompanied by a clear change in the colour of the solid phase (from green to white-greyish). XRD and TEM analyses showed that the amorphous NpO₂OH(am) “starting material” transformed into crystalline solid phases with similar XRD patterns in NaCl and MgCl₂ systems. XPS, SEM–EDS and EXAFS further indicated that borate and Na/Mg participate stoichiometrically in the formation of such solid phases. Additional undersaturation solubility experiments using the newly formed Na–Np(v)–borate(cr) and Mg–Np(v)–borate(cr) compounds further confirmed the low solubility ([Np(v)]_aq ≈ 10^–6–10^–7 M) of such solid phases in mildly alkaline pHm-conditions. The formation of these solid phases represents a previously unreported retention mechanism for the highly mobile Np(v) under boundary conditions (pH_m, [B]_tot, ionic strength) of relevance to certain repository concepts for nuclear waste disposal.

• Research Organization: Energy Frontier Research Centers (EFRC) (United States). Center for Actinide Science & Technology (CAST); Florida State Univ., Tallahassee, FL (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES)
• Grant/Contract Number: SC0016568
• OSTI ID: 1582687
• Alternate ID(s): OSTI ID: 1767699
• Journal Information: Dalton Transactions, Journal Name: Dalton Transactions Vol. 49 Journal Issue: 5; ISSN 1477-9226
• Publisher: Royal Society of Chemistry
• Country of Publication: United Kingdom
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
nuclear
defects
mechanical behavior
corrosion
charge transport
superconductivity
magnetism and spin physics
separations
geophysics/geochemistry
materials and chemistry by design
synthesis (novel materials)
synthesis (self-assembly)
synthesis (predictive)
synthesis (scalable processing)