In situ Spectroscopic Identification of Neptunium(V) Inner-Sphere Complexes on the Hematite–Water Interface

Abstract

Hematite plays a decisive role in regulating the mobility of contaminants in rocks and soils. The Np(V) reactions at the hematite-water interface were comprehensively investigated by a combined approach of in situ vibrational spectroscopy, X-ray absorption spectroscopy and surface complexation modeling. A variety of sorption parameters such as Np(V) concentration, pH, ionic strength, and the presence of bicarbonate was considered. Time-resolved IR spectroscopic sorption experiments at the iron oxide-water interface evidenced the formation of a single monomer Np(V) inner-sphere sorption complex. EXAFS provided complementary information on bidentate edge-sharing coordination. In the presence of atmospherically derived bicarbonate the formation of the bis-carbonato inner-sphere complex was confirmed supporting previous EXAFS findings.1 The obtained molecular structure allows more reliable surface complexation modeling of recent and future macroscopic data. Such confident modeling is mandatory for evaluating water contamination and for predicting the fate and migration of radioactive contaminants in the subsurface environment as it might occur in the vicinity of a radioactive waste repository or a reprocessing plant.

Uranium and neptunium retention mechanisms in Gallionella ferruginea/ferrihydrite systems for remediation purposes

“…The same is valid for abiotic ferrihydrite as shown in Fig. 5, as well as for hematite (Müller et al 2015). The different intensities may be due to different film heights prepared on the ATR crystal.…”

Neptunium Reactivity During Co-Precipitation and Oxidation of Fe(II)/Fe(III) (Oxyhydr)oxides

“…This resulted in Np-O interactions at 1.86 Å and 2.36 Å for the oxidised magnetite samples and 1.88 Å and 2.39 Å for the oxidised green rust samples ( Table 2). These distances are consistent with axial Np-O distances in Np(V) and both equatorial Np(V) and Np-O distances in Np(IV) [4,7,16,19,23,25]. Multiple scattering paths were present in both oxidised samples, which arise as a result of scattering within the linear Np(V) dioxygenyl actinyl ions, again confirming the presence of Np(V) dioxygenyl in the samples [47][48][49].…”

“…Previous studies demonstrate that Np(V) can interact with phases such as ferrihydrite [18][19][20], lepidocrocite [21], goethite [19,20,22], hematite [19,20,23] and magnetite [12,16,20,24]. Here, for Fe(III) containing minerals, Np(V) typically forms inner sphere Np(V) complexes or, in the presence of carbonate, Np(V)-carbonate surface complexes [19,23,25]. Additionally, recent work highlights that during transformation of ferrihydrite to hematite, Np(V) may be incorporated into hematite [19].…”

Neptunium and manganese biocycling in nuclear legacy sediment systems

“…Neptunyl sorption has been observed in to a similar extent in earlier studies using comparable sediment systems (Law et al, 2010) and has been attributed to sorption to negatively charged mineral surfaces (e.g. Fe(III)-or Mn(IV) -bearing minerals; Combes et al, 1992;Nakata et al, 2002;Arai et al, 2007;Müller et al, 2015;Wilk et al, 2005). In microbially active microcosms prior to the onset of Mn ingrowth to porewaters (0 -7 days), 43.0 ± 1.9 % of the added Np was removed from the groundwater ( Figure 1A).…”

“…Removal of Np from solution in both the oxic and nitrate reducing systems at circumneutral pH is occurring and is likely due to Np(V) sorption to Fe or Mn mineral surfaces (e.g. Combes et al, 1992;Nakata et al, 2002;Wilk et al, 2005;Arai et al, 2007;Law et al, 2010;Müller et al, 2015). By contrast, the XANES spectra for the progressive Mn-reducing microcosm, and the poised Mn(IV)-, Fe(III)-, and SO4 2--reducing systems showed Np(IV)-like features with a loss of the multiple scattering resonance structure due to the loss of the two neptunyl dioxygenyl oxygen backscatterers (Figure 2).…”