Plutonium Desorption from Mineral Surfaces at Environmental Concentrations of Hydrogen Peroxide

Knowledge of Pu adsorption and desorption behavior on mineral surfaces is crucial for understanding its environmental mobility. Here we demonstrate that environmental concentrations of H₂O₂ can affect the stability of Pu adsorbed to goethite, montmorillonite, and quartz across a wide range of pH values. In batch experiments where Pu­(IV) was adsorbed to goethite for 21 days at pH 4, 6, and 8, the addition of 5–500 μM H₂O₂ resulted in significant Pu desorption. At pH 6 and 8 this desorption was transient with readsorption of the Pu to goethite within 30 days. At pH 4, no Pu readsorption was observed. Experiments with both quartz and montmorillonite at 5 μM H₂O₂ desorbed far less Pu than in the goethite experiments highlighting the contribution of Fe redox couples in controlling Pu desorption at low H₂O₂ concentrations. Plutonium­(IV) adsorbed to quartz and subsequently spiked with 500 μM H₂O₂ resulted in significant desorption of Pu, demonstrating the complexity of the desorption process. Our results provide the first evidence of H₂O₂-driven desorption of Pu­(IV) from mineral surfaces. We suggest that this reaction pathway coupled with environmental levels of hydrogen peroxide may contribute to Pu mobility in the environment

Reduction of Plutonium(VI) to (V) by Hydroxamate Compounds at Environmentally Relevant pH

Natural organic matter is known to influence the mobility of plutonium (Pu) in the environment via complexation and reduction mechanisms. Hydroxamate siderophores have been specifically implicated due to their strong association with Pu. Hydroxamate siderophores can also break down into di and monohydroxamates and may influence the Pu oxidation state, and thereby its mobility. In this study we explored the reactions of Pu­(VI) and Pu­(V) with a monohydroxamate compound (acetohydroxamic acid, AHA) and a trihydroxamate siderophore desferrioxamine B (DFOB) at an environmentally relevant pH (5.5–8.2). Pu­(VI) was instantaneously reduced to Pu­(V) upon reaction with AHA. The presence of hydroxylamine was not observed at these pHs; however, AHA was consumed during the reaction. This suggests that the reduction of Pu­(VI) to Pu­(V) by AHA is facilitated by a direct one electron transfer. Importantly, further reduction to Pu­(IV) or Pu­(III) was not observed, even with excess AHA. We believe that further reduction of Pu­(V) did not occur because Pu­(V) does not form a strong complex with hydroxamate compounds at a circum-neutral pH. Experiments performed using desferrioxamine B (DFOB) yielded similar results. Broadly, this suggests that Pu­(V) reduction to Pu­(IV) in the presence of natural organic matter is not facilitated by hydroxamate functional groups and that other natural organic matter moieties likely play a more prominent role

Effect of Natural Organic Matter on Plutonium Sorption to Goethite

The effect of citric acid (CA), desferrioxamine B (DFOB), fulvic acid (FA), and humic acid (HA) on plutonium (Pu) sorption to goethite was studied as a function of organic carbon concentration and pH using batch sorption experiments at 5 mg_C·L^–1 and 50 mg_C·L^–1 natural organic matter (NOM), 10^–9–10^–10 M ²³⁸Pu, and 0.1 g·L^–1 goethite concentrations, at pH 3, 5, 7, and 9. Low sorption of ligands coupled with strong Pu complexation decreased Pu sorption at pH 5 and 7, relative to a ligand-free system. Conversely, CA, FA, and HA increased Pu sorption to goethite at pH 3, suggesting ternary complex formation or, in the case of humic acid, incorporation into HA aggregates. Mechanisms for ternary complex formation were characterized by Fourier transform infrared spectroscopy in the absence of Pu. CA and FA demonstrated clear surface interactions at pH 3, HA appeared unchanged suggesting HA aggregates had formed, and no DFOB interactions were observed. Plutonium sorption decreased in the presence of DFOB (relative to a ligand free system) at all pH values examined. Thus, DFOB does not appear to facilitate formation of ternary Pu-DFOB-goethite complexes. At pH 9, Pu sorption in the presence of all NOM increased relative to pH 5 and 7; speciation models attributed this to Pu­(IV) hydrolysis competing with ligand complexation, increasing sorption. The results indicate that in simple Pu-NOM-goethite ternary batch systems, NOM will decrease Pu sorption to goethite at all but particularly low pH conditions

Kinetic Studies of the [NpO₂(CO₃)₃]^4– Ion at Alkaline Conditions Using ¹³C NMR

Carbonate ligand-exchange rates on the [NpO₂(CO₃)₃]^4– ion were determined using a saturation-transfer ¹³C nuclear magnetic resonance (NMR) pulse sequence in the pH range of 8.1 ≤ pH ≤ 10.5. Over the pH range 9.3 ≤ pH ≤ 10.5, which compares most directly with previous work of Stout et al., we find an average rate, activation energy, enthalpy, and entropy of <i>k</i>_ex²⁹⁸ = 40.6(±4.3) s^–1, <i>E</i>_a =45.1(±3.8) kJ mol^–1, Δ<i>H</i>^‡ = 42.6(±3.8) kJ mol^–1, and Δ<i>S</i>^‡ = −72(±13) J mol^–1 K^–1, respectively. These activation parameters are similar to the Stout et al. results at pH 9.4. However, their room-temperature rate at pH 9.4, <i>k</i>_ex²⁹⁸ = 143(±1.0) s^–1, is ∼3 times faster than what we experimentally determined at pH 9.3: <i>k</i>_ex²⁹⁸ = 45.4(±5.3) s^–1. Our rates for [NpO₂(CO₃)₃]^4– are also faster by a factor of ∼3 relative to the isoelectronic [UO₂(CO₃)₃]^4– as reported by Brucher et al. of <i>k</i>_ex²⁹⁸ = 13(±3) s^–1. Consistent with results for the [UO₂(CO₃)₃]^4– ion, we find evidence for a proton-enhanced pathway for carbonate exchange for the [NpO₂(CO₃)₃]^4– ion at pH < 9.0

Np(V) and Pu(V) Ion Exchange and Surface-Mediated Reduction Mechanisms on Montmorillonite

Due to their ubiquity and chemical reactivity, aluminosilicate clays play an important role in actinide retardation and colloid-facilitated transport in the environment. In this work, Pu­(V) and Np­(V) sorption to Na-montmorillonite was examined as a function of ionic strength, pH, and time. Np­(V) sorption equilibrium was reached within 2 h. Sorption was relatively weak and showed a pH and ionic strength dependence. An approximate NpO₂⁺ → Na⁺ Vanselow ion exchange coefficient (Kv) was determined on the basis of Np­(V) sorption in 0.01 and 1.0 M NaCl solutions at pH < 5 (Kv ∼ 0.3). In contrast to Np­(V), Pu­(V) sorption equilibrium was not achieved on the time-scale of weeks. Pu­(V) sorption was much stronger than Np­(V), and sorption rates exhibited both a pH and ionic strength dependence. Differences in Np­(V) and Pu­(V) sorption behavior are indicative of surface-mediated transformation of Pu­(V) to Pu­(IV) which has been reported for a number of redox-active and redox-inactive minerals. A model of the pH and ionic strength dependence of Pu­(V) sorption rates suggests that H⁺ exchangeable cations facilitate Pu­(V) reduction. While surface complexation may play a dominant role in Pu sorption and colloid-facilitated transport under alkaline conditions, results from this study suggest that Pu­(V) ion exchange and surface-mediated reduction to Pu­(IV) can immobilize Pu or enhance its colloid-facilitated transport in the environment at neutral to mildly acidic pHs

Plutonium(IV) and (V) Sorption to Goethite at Sub-Femtomolar to Micromolar Concentrations: Redox Transformations and Surface Precipitation

Pu­(IV) and Pu­(V) sorption to goethite was investigated over a concentration range of 10^–15–10^–5 M at pH 8. Experiments with initial Pu concentrations of 10^–15 – 10^–8 M produced linear Pu sorption isotherms, demonstrating that Pu sorption to goethite is not concentration-dependent across this concentration range. Equivalent Pu­(IV) and Pu­(V) sorption <i>K</i>_d values obtained at 1 and 2-week sampling time points indicated that Pu­(V) is rapidly reduced to Pu­(IV) on the goethite surface. Further, it suggested that Pu surface redox transformations are sufficiently rapid to achieve an equilibrium state within 1 week, regardless of the initial Pu oxidation state. At initial concentrations >10^–8 M, both Pu oxidation states exhibited deviations from linear sorption behavior and less Pu was adsorbed than at lower concentrations. NanoSIMS and HRTEM analysis of samples with initial Pu concentrations of 10^–8 – 10^–6 M indicated that Pu surface and/or bulk precipitation was likely responsible for this deviation. In 10^–6 M Pu­(IV) and Pu­(V) samples, HRTEM analysis showed the formation of a body centered cubic (bcc) Pu₄O₇ structure on the goethite surface, confirming that reduction of Pu­(V) had occurred on the mineral surface and that epitaxial distortion previously observed for Pu­(IV) sorption occurs with Pu­(V) as well