Interaction of Np(v) with borate in alkaline, dilute-to-concentrated, NaCl and MgCl2 solutions

Abstract

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 ≤ pH_m ≤ 10.0 (pH_m = –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 pH_m-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 pH_m ≤ 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 pH_m-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.