XANES and EXAFS analysis of oxidation state and local structure of plutonium reacted with iron oxides under anoxic conditions

Iron minerals form as corrosion products of zero-valent iron and steel in the “near field”, are present in many "far field" barriers (clay or granite) and occur widely in natural sediments. Depending on redox conditions, ground water composition and microbial activity, iron(hydr)oxides such as goethite (-FeOOH), lepidocrocite (-FeOOH), magnetite (FeIIFeIII2O4), and maghemite (-Fe2O3) have been observed as corrosion products of steel [1,2]. The solubility and complexation behavior of plutonium in aqueous systems are highly oxidation state dependent. As Iron(hydr)oxides have been shown to undergo redox reactions with plutonium [3-5] and to form plutonium surface complexes [6], they are expected to control to a large extent the migration behavior of plutonium. For example, sorption of Pu(V) to hematite, goethite and magnetite was found to be accompanied by surface mediated reduction to Pu(IV) [3-5,7]. While many previous laboratory studies have been conducted under air, the intention of our work was to investigate redox reactions of Pu with common iron oxides under well controlled and anoxic conditions to better simulate deep underground conditions. We combined in-situ oxidation state analysis on the mineral surfaces using advanced XAS techniques with wet chemical characterization of redox conditions and thermodynamic modeling. The part of the joint work focusing on aqueous Pu chemistry and thermodynamic description of Pu redox state distribution is presented at Migration’11 by Fellhauer et al..
Synthetic hematite, goethite, maghemite and magnetite were allowed to react under anoxic conditions (O2 ≤ 10 ppmv) in carbonate free 0.1 M NaCl with aqueous 242Pu(III) and 242Pu(V). Pu-LIII-edge XANES and EXAFS spectra were collected after 40 days and six months at the Rossendorf Beamline at ESRF, France, to assess in-situ oxidation states and local structures of plutonium reaction products. All measurements were carried out at 15 K using a closed-cycle He cryostat to reduce thermal disorder in the samples and avoid beam-induced changes in oxidation state.
After reaction with hematite, Pu(V) was largely reduced to Pu(IV) while Pu(III) was oxidized to Pu(IV). For example, after 6 months of reaction with hematite at pH 7.5, 30 % Pu(V) and 70 % Pu(IV) were observed for the sample with Pu(V)initial. Under similar reaction conditions, Pu(III)initial yielded 20 % Pu(V) and 80 Pu(IV). Despite these similar oxidation state distributions of Pu associated with the solid phase, [Pu] concentrations in solution differ for most of the 6 months reaction period by about two orders of magnitude. Final [Pu] concentrations are 2×10 10 M for the Pu(III)initial and 6×10 9 M for the Pu(V)initial samples. The EXAFS spectra gave no evidence for the formation of a solid PuO2 phase.
In magnetite suspensions at pH 6 and pH 8, reduction of Pu(V) to Pu(III) and formation of a Pu(III)-magnetite surface sorption complex was observed. In this surface complex, situated on (111) surfaces with octahedral termination, one Pu atom is linked via three oxygen atoms to three edge-sharing FeO6-octahedra. Due to the tridentate nature of the complex, it is likely to be very stable and play an important role in controlling Pu-magnetite reactions and Pu mobility under reducing conditions. However, at a higher plutonium loading (1 Pu atom / 29 nm2 instead of 1 Pu atom / 58 nm2) and with Pu(V)initial, only 60 % of Pu is surface complexed Pu(III)ads while about 40 % is present as solid PuO2. After 6 months, solution concentrations for Pu(III)initial or Pu(V)initial were at or below the detection limit of ~5×10 11 M (242Pu measured with liquid scintillation counting or ICP-MS).
Reaction with maghemite at pH 6 yielded very similar oxidation state distributions and solution concentrations for Pu(III)initial and Pu(V)initial samples. Changes in Pu(III) / Pu(IV) ratios in the reaction products can be attributed to differing residual Fe(II) contents (maghemite was prepared by oxidation of magnetite). After six months of reaction, Pu was present as 80% Pu(IV). As with magnetite, iron backscatterering indicates formation of an inner sphere surface complex. Formation of a solid PuO2 phase does not occur.
These data highlight the importance of plutonium surface complexation on different iron oxides in controlling environmental [Pu] concentrations. In particular, conservation of non negligible amounts of Pu(V) with hematite (20 to 30 % after 6 months) and goethite (45 % Pu(V) after 40 days) contrasts with published data [5]. In addition, our results highlight the necessity to consider trivalent Pu(III) species in addition to tetravalent Pu(IV) species and PuO2(am,hyd) for risk assessment under reducing conditions.