Article pubs.acs.org/est
In situ Spectroscopic Identification of Neptunium(V) Inner-Sphere Complexes on the Hematite−Water Interface Katharina Müller,*,† Annett Gröschel,†,‡ André Rossberg,† Frank Bok,† Carola Franzen,† Vinzenz Brendler,† and Harald Foerstendorf† †
Helmholtz-Zentrum Dresden − Rossendorf, Institute of Resource Ecology, Bautzner Landstr. 400, 01328 Dresden, Germany Dresden University of Applied Sciences, Friedrich-List-Platz 1, 01069 Dresden, Germany
‡
S Supporting Information *
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. Timeresolved 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.
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oxides (FHO) was intensively studied.1,10−21 The vast majority of these studies presents results of batch experiments and modeling approaches. Recently, Np sorption onto hematite, magnetite, goethite and ferrihydrite were reviewed by Li and Kaplan.22 The neptunium sorption capacities of the substrates and the impact of specific parameters, such as pH, ionic strength, competing ions, ambient and inert gas atmosphere, on the macroscopic sorption behavior are described. In general, the macroscopic results showed a typical S-type Np(V) sorption edge, indicating no sorption at low pH, gradually increasing sorption capacity at pH > ∼4 and complete sorption at pH > ∼8. The sorption was found to strongly depend on the type of FHOs: goethite > ferrihydrite > hematite, magnetite. No reduction of Np(V) to Np(IV) was observed upon sorption to hematite. The presence of carbonate was found to decrease the sorption capability.13−15,17,23−25 However, from critical review of published data, confidence in predictive modeling of contaminant transport is restrained by several contradictory sets of Np(V) surface species proposed in the literature, namely ≡Fe−OH−NpO2+, ≡Fe−O−NpO20, ≡Fe−O−NpO2(OH)−, (≡Fe−O)2−NpO2−.26
INTRODUCTION Neptunium (Np) is one of the most important components of nuclear waste, because of its high toxicity and the long half-life of its isotope 237Np, making it a major contributor of the total radiation in 10 000 years. The geochemistry and the migration behavior of Np must therefore be considered for the long-term safety assessment of repository sites.1−3 The migration of such contaminants in an aqueous environment is strongly affected by their molecular reactions at the solid−water interface, for example, sorption onto mineral phases, surface precipitation, and colloid formation. 4 These processes are primarily determined by the metal’s oxidation state, ranging from +III to +VII for Np, and the distribution of appropriate species in aqueous solution. The fully hydrated pentavalent neptunyl(V) ion, NpO2+, dominates its aqueous speciation within a wide range of environmental conditions.2 Thus, the reactions of Np(V) at the solid−water interface must be understood at a fundamental level in order to reliably predict its hydrogeochemistry.5,6 Among the various components of geological materials, iron oxides play a crucial role in regulating the mobility of contaminants, due to their widespread presence in geology, high sorption capacity and tendency to form coatings on mineral surfaces.7 In case of nuclear waste disposal, their presence as corrosion products in the technical barriers is to be considered.8,9 In recent years, the sorption behavior of Np(V) onto synthetic and naturally occurring iron hydroxides and © 2015 American Chemical Society
Received: Revised: Accepted: Published: 2560
October 23, 2014 January 16, 2015 January 16, 2015 January 16, 2015 DOI: 10.1021/es5051925 Environ. Sci. Technol. 2015, 49, 2560−2567
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Environmental Science & Technology
water (D2O). Because of the isotopic shift of the D2O absorption band, the detection limit for the NpO2+ ion can be reduced by 3 orders of magnitude to concentrations in the middle micromolar range (10−100 μM). Specifics on the preparation and characterization of the deuterated 0.021 M NpO2Cl2 stock solution are given in.29 For further details on sample dilution, adjustment of ionic strength, pH and carbonate concentration, and the monitoring of prevalent oxidation state and absence of colloidal phases, the reader is referred to the Supporting Information (SI) SI1. Characterization of Hematite. Hematite purchased from US Research Nanomaterials Inc. (Stock No. US3160) was used for the sorption experiments in this study. Identification of the bulk material was performed by X-ray diffraction (XRD; D8 Bruker-AXS diffractometer using Cu Kα radiation (k = 1.5406 Å), operating in diffraction mode at 40 kV and 40 mA and equipped with a graphite secondary monochromator), Raman spectroscopy (Horiba Jobin Yvon LabRAM Aramis) and room temperature 57Fe Mössbauer spectroscopy (Wissenschaftliche Elektronik GmbH). In comparison to the ICDD reference data (00−033−0664) the hematite sample was identified as pure hematite (Figure SI 1A). This was also confirmed by the obtained Raman spectral data (Figure SI 1B)34 and by Mössbauer spectroscopy (Figure SI 1C). For further characterization, the morphology and particle size were examined by scanning electron microscopy (SEM; Hitachi, S-4800). The particles are homogeneous around 70 nm (Figure SI 1D). The specific surface area (SSA) was determined to be 41.1 m2 g−1 by the N2−BET technique (Beckman Coulter analyzator SA 3100). The isoelectric point (iep) was found at pH 8.5 by zeta potential measurements (Malvern Zetasizer Nano-ZS) (Figure SI 1E). In situ ATR FT-IR Spectroscopic Sorption Studies. In this work, in situ ATR FT-IR spectroscopic measurements of sorption processes are based on the principle of reactioninduced difference spectroscopy. IR single beam spectra of a mineral film (stationary phase) are continuously recorded while it is rinsed by a flushing blank solution for equilibration and subsequent actinide solution for induced sorption (mobile phase). Since the acquisition time of each spectrum is about 30 s, the progress of the sorption process can be monitored with a time resolution in the subminute time range. This procedure allows the detection of spectral features showing very small absorption changes (optical density ≥10−5). A detailed description of the preparation of the hematite film, the flow cell setup, acquisition and evaluation of spectral data is given in SI 2 and elsewhere.30 EXAFS Spectroscopic Characterization. The EXAFS spectrum of one batch sorption sample was collected at the Rossendorf Beamline (BML20) at the European Synchrotron Radiation Facility (ESRF, Grenoble, France)35 equipped with two platinum coated mirrors used for rejection of higher harmonics and a water cooled Si(111) double crystal monochromator. For the sorption sample eight energy scans over the NpLIII-edge (17 610 eV) were collected at 15 K by using a closed cycle helium cryostat. For more details on sample preparation and experimental parameters the reader is referred to in SI 3 and SI 4. Surface Complexation Modeling. Surface complexation modeling (SCM) was applied to batch sorption experimental data from this work (SI 3) and literature.15,23,25 Ten different data sets were used (cf. Figure SI 4). All fits for all data sets were run with a consistent set of surface site density (SSD =
For reliable modeling of Np behavior and, thus, a better understanding of the sorption mechanisms, structural molecular information on the sorption complexes is crucial in addition to distribution coefficients (Kd values). Spectroscopic experiments are expected to provide such information. However, only a few studies of Np(V) sorption complexes on iron oxides have been performed to date. As an example, X-ray photoelectron spectroscopy (XPS) was applied to confirm the pentavalent oxidation state of the sorbing Np species on hematite.19 Since sorption reactions generally occur at the solid−water interface in the environment, and drying the sorption samples may cause alterations of the surface speciation, spectroscopic techniques providing an in situ characterization of the molecule complexes are of special interest.10 Extended X-ray absorption fine structure spectroscopy (EXAFS) was applied to characterize ternary Np(V) carbonate complexes on hematite and binary Np(V) sorption on goethite.1,10 However, XAS generally provides an averaged signal of all prevailing species. Although structural information can be extracted from the data by fitting to potential reference spectra, uncertainties increase with the number of species being present simultaneously. Furthermore, species in mineral pores can only hardly be identified and kinetic information is not obtained at all. For the in situ formation and identification of the surface complexes, attenuated total reflection Fourier-transform infrared (ATR FT-IR) spectroscopy is a very useful tool providing complementary molecular information.27 ATR FT-IR spectroscopy was recently applied to Np(V) surface reactions on oxides and hydroxides of Ti and Al.28,29 The surface species were elucidated by the characteristic frequencies of the stretching vibrations of the NpO2+ and CO32− ions. The interpretation was based on the comparison of results obtained from experiments of the appropriate aqueous species as well as from data of our previous works.28,30 The challenge of this work is the molecular characterization of micromolar Np(V) sorption complexes on hematite using in situ ATR FT-IR spectroscopy with a time resolution in the subminute range. The impact of pH, Np concentration, ionic strength and Np(V) loading on the formation of Np(V) surface complexes are investigated in detail for the first time. In addition, the formation of ternary surface species at the hematite surface in the presence of dissolved carbonate ions is considered. To complement the IR spectroscopic approach, EXAFS spectroscopy and surface complexation modeling of batch sorption data are applied and whenever possible compared to respective previous studies.1,15,31 The comprehensive molecular description of the processes at the hematite− water interface will improve the modeling of the Np migration behavior in the environment which, in turn, will lead to a more reliable risk assessment.
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EXPERIMENTAL SECTION Preparation and Characterization of Np(V) Solutions. The 237Np stock solution, in secular equilibrium with its daughter 233Pa was prepared as described previously.32 Apart from 233Pa, the 237Np solution was free of radioimpurities. The Np(V) oxidation state was achieved electrochemically using the procedure described elsewhere.33 The characteristic vibrational modes of the NpO2+ ion in solution and sorbed on mineral− water interfaces are generally observed in the spectral range below 850 cm−1 where strong interferences with modes from bulk water (H2O) occur.28,29 Therefore, the vibrational spectroscopic experiments were performed in deuterated 2561
DOI: 10.1021/es5051925 Environ. Sci. Technol. 2015, 49, 2560−2567
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Environmental Science & Technology 2.31 nm−1 as recommended by36) and surface protolysis constants (logKa1 = 8.4, logKa2 = −10.415). Values for the specific surface area were used specific to each data set as given in refs 15, 23, or 25 or as determined (cf. Table SI 2). The diffuse double-layer model (DDL) was applied using PHREEQC37 coupled with the software UCODE2005.38 The modeling of bidentate surface complexes is strongly discussed in the literature, since the used mass action expression can introduce errors if they are not carefully selected.39 In contrast to, for example, older versions of PHREEQC, the version 3.1.5−9113 used here already implements bidentate surface complexes. For more clarity, the definition within the UCODE_2005 template of the PHREEQC input file is given in Table SI 5.
indicates a sufficiently stable mineral film on the ATR crystal’s surface under the chosen conditions. The small bands between 1000 and 900 cm−1 can be most likely attributed to vibrational modes of the mineral. They represent changes in the molecular environment of functional groups of the Fe2O3 surface occurring during the equilibration step with the background electrolyte. At higher frequencies, small negative peaks at 1491 and 1356 cm−1 are observed. These bands may reflect a release of residual carbonate ions present at the solid phase which was stored under normal air conditions (pCO2 = 10−3.5 atm) prior to the experiment. Surface carbonate species typically show absorption maxima within the range 1600−1300 cm−1 and a similar behavior was already found during in situ sorption experiments using ferrihydrite.40,41 Upon prolonged conditioning, no further alterations of these bands are observed indicating that a steady state is obtained within 30 min of conditioning. The ATR FT-IR difference spectra calculated from the spectra recorded at the end of the conditioning step and after 5, 10, 30, and 60 min of the induction of Np(V) sorption are denoted as “Sorption” spectra in Figure 1A (middle black traces). These spectra exhibit broad absorption bands with maxima at 1040 and 790 cm−1. The time-resolved spectra are characterized by increasing band amplitudes reflecting Np(V) accumulation on the mineral’s surface with ongoing sorption time. The absorption band at 790 cm−1 is assigned to the antisymmetric stretching vibration υ3 of the Np(V) species sorbed on Fe2O3. The υ3 mode of the fully hydrated NpO2+ species is observed at 820 cm−1.42 The same species is dominant under the prevailing sorption conditions, that is, 0.1 M NaCl, D2O, pH 8, N2 (cf. Figure 3A).43 Upon sorption the υ3 mode is shifted to lower wavenumbers of about 30 cm−1. This is due to a decrease of the NpO force constant as a result of complexation with Fe2O3 units of the mineral phase in the equatorial plane.29,42 The in situ vibrational data of Np(V) sorbed onto oxides of Ti, Si and Zn may serve as a reference,29 as the experiments were performed under identical experimental conditions, that is, 0.1 M NaCl, D2O, pH 8, N2. A comparison of the data sets demonstrates that the frequency of the υ3 of the sorbed Np(V) species is observed at ∼790 cm−1 irrespective of the sorbing surface. The extent of the redshift of ∼30 cm−1 upon sorption of the Np(V) ion on these mineral oxide surfaces suggests a similar type of surface complexation that is an inner-sphere surface complex. A coordination of the actinyl ion by physisorption (electrostatic attraction) is not expected to shift the absorption frequency to such an extent.44 Similarly, innersphere sorption complexes of U(VI) on different mineral oxides, namely Fe2O3, ferrihydrite, TiO2 and Al2O3, were identified by considerable downshifts of the ν3(UO2) mode up to 50 cm−1.27,44−46 Additionally, the assumption of innersphere complexation is supported by the positive surface charge of Fe2O3 up to pH 8.5. In the absence of carbonate, electrostatic sorption of the positive NpO2+ would become more likely at pH values above the isoelectric point. The assignment to inner-sphere sorption is in agreement with previous EXAFS studies of Np(V) sorption on goethite.10 The progress of Np(V) sorption on the Fe2O3 surface can be monitored online by the time-dependent increase of the absorption band at 790 cm−1. The presence of one band showing a constant frequency maximum and bandwidth throughout the sorption time indicates the presence of only
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RESULTS AND DISCUSSION Infrared Spectroscopic Identification of Np(V) Sorption Complex. The course of an in situ sorption experiment at a total neptunyl(V) concentration of 50 μM, ionic strength of 0.1 M NaCl and pH 8 is illustrated by the spectra shown in Figure 1A.
Figure 1. In situ time-resolved ATR FT-IR spectra of Np(V) sorption on Fe2O3 (50 μM Np(V), D2O, 0.1 M NaCl, pH 8, N2, 0.4 mg Fe2O3 cm−2). The spectra of the conditioning, sorption and flushing process are recorded at different times as given (from bottom to top) (A). Variation of ionic strength at 50 μM Np(V) (B). Variation of initial Np concentration at 0.1 M NaCl (C). Only spectra obtained after 60 min of sorption are presented in B and C. Indicated values are in cm−1.
The spectra referred to as “Conditioning” reflect the equilibrium state of the hematite mineral film after flushing it with a blank solution for 10, 20, and 30 min (Figure 1A, lower red traces). Only small absorption changes are observed in the spectral region from 1600 to 700 cm−1. In the transmission spectrum of the hematite film strong absorption bands are observed below 900 cm−1 (cf. transmission spectra in Figure SI 2). Thus, the absence of these bands in the conditioning spectra 2562
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varying between 10 and 100 μM are shown in Figure 1C. As expected, the intensities of the vibrational modes related to the sorption complex are increased with increasing Np concentration because of higher accumulation of Np(V) at the Fe2O3 surface. However, no spectral shift was observed reflecting the formation of only one monomeric surface complex. This is in agreement with the time-resolved spectral data, where at lowest Np(V) loadings the frequency of υ3(NpO2) is not changed relative to intermediate and highest loadings (cf. Figure 1A). A single reaction stoichiometry under the applied conditions is in agreement with assumptions from XAS studies of Np(V) sorption on goethite and hematite.10,31 To gain further information on the surface structure, EXAFS experiments were carried out using one Np(V) batch sorption sample at pH 8. The spectra are shown in Figure 2 and fitted parameters are summarized in Table SI 1.
one type of surface species. After about 60 min, no further intensity increase was observed indicating that an equilibrium state at the interface was reached. The spectral region between 1200 and 900 cm−1 is characterized by strong overlapping bands due to vibrational modes of the solid phase. These bands possibly represent alterations at the mineral−water interface related to the reactions with the neptunyl ion. As these bands are generally less specific and cannot be accurately assigned to distinct molecular functional groups, a detailed interpretation may only be given in the future. Subsequent to sorption, “Flushing” of the Fe2O3 film with blank solution was performed in order to potentially provide information on surface species which can be easily removed. The difference spectra, shown in Figure 1A (upper blue traces) were calculated from the spectra recorded at the end of the sorption step and after 10, 20, and 30 min of flushing. No further changes in the spectra are observed after prolonged flushing (>30 min). In contrast to the sorption data, negative peaks showing significantly reduced intensities are observed between 1200 and 900 cm−1 and at 790 cm−1. The latter band can be attributed to Np(V) which is released from the mineral film. Recently, surface species of Se(VI) and U(VI) showed vibrational bands very similar in frequency and intensity during sorption and flushing steps and in comparison to aqueous species. These species showing a highly reversible sorption behavior during the vibrational experiments were identified as outer-sphere complexes.47,48 For the neptunyl ion, with respect to the strong 30 cm−1 shift of the υ3(NpO2) mode relative to the aqueous species and to the lower reversibility of the sorption process, a different interpretation becomes obvious and the formation of inner-sphere surface complexes is suggested. A similar sorption behavior was found in batch and modeling experiments for Np(V) interaction with goethite.20 IR and X-ray Absorption Spectroscopic Verification of the Single Np(V) Inner-Sphere Sorption Complex. For verification and further characterization of the formation of one single Np(V) inner-sphere surface complex on hematite, additional IR spectroscopic experiments changing ionic strength and Np(V) concentration and EXAFS measurements were performed. From recent in situ IR experiments, a correlation of the ionic strength with the amplitudes of the sorption spectra demonstrated the identification of outer-sphere complexes on mineral surfaces.47,48 The IR sorption spectra obtained from 10−1 and 10−2 M NaCl are shown in Figure 1B. No shift of the relevant absorption bands was observed. Even the intensity of the band assigned to υ3(NpO2) was highly reproduced demonstrating no impact of changes in ionic strength on Np(V) sorption. These spectroscopic findings confirm earlier macroscopic studies where ionic strength was varied in the range from 0.005 to 0.1 M NaClO4.15 Hayes et al. state, that the insensitivity of cation fractional adsorption to differences in ionic strength is consistent with model calculations assuming inner-sphere surface complex formation.49 In a previous study, the formation of Np(V) oligomers or precipitates occurring at the iron oxide−water interface was tentatively assumed.10 Recently, IR spectroscopy has been shown to be a capable tool to distinguish between sorption of monomers, oligomers and surface precipitation.27 The IR sorption spectra obtained at initial Np(V) concentrations
Figure 2. NpLIII-edge k3-weighted EXAFS spectrum (black, left) and corresponding Fourier- transform (FT) (black, right) of a batch sorption sample with a Np loading of 0.34 μmol m−2 (0.6 g hematite L−1, 10 μM Np(V), 0.1 M NaCl at pH 8.0). Shell fits with two structural models (red), estimated level of experimental error (blue horizontal line).
For the interpretation of the spectral features, the experimental error in the data must be considered (Figure 2, blue line; for explanation see SI). Three FT peaks show magnitudes that are significantly higher than the level of the experimental error: the scattering contributions of the two axial oxygen atoms (Oax) at 1.4 Å + ΔR, the oxygen atoms in the equatorial plane of the Np(V) (Oeq) at 1.95 Å + ΔR and a third scattering contribution at 3.28 Å + ΔR. The origin of the latter peak was previously explained as the scattering contribution of Fe atoms in a radial Np−Fe distance of 3.73−3.74 Å stemming from binary edge-sharing Np(V) sorption complexes.31 A fourth FT peak at 2.77 Å + ΔR with intensity only slightly higher than the error level is also observed. For the EXAFS structural analysis two different models have been fitted. For more details the reader is referred to SI 4. (1) The freely fitted structural parameters are in line with those observed for the binary edge-sharing Np(V) sorption complexes,31 especially the radial Np−Fe distance of 3.73 Å (Table SI 1, model 1). (2) If the small FT peak at 2.77 Å + ΔR is considered in the shell fit as an additional Np−Fe contribution then a Np−Fe(2) distance of 3.47 Å is resulting (Table SI 1, model 2). Because of the high correlation between the coordination number (CN) of the two Fe shells of 0.81 the inclusion of the additional shell induces a change of CNFe(1)/model 1 = 1.3 to CNFe(1)/model 2 = 1.9, which is unrealistic 2563
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Classical batch adsorption studies determined the extent of Np(V) accumulation at the hematite surface as a function of pH (Figure 3B). The results are in very good agreement with previously reported data for Np(V) sorption on different iron oxides.13,15,21,23 They confirm that Np(V) sorption starts at weak acidic conditions and increases to 100% sorption at pH 7. At alkaline conditions, the sorption remains complete in N2 atmosphere. The IR sorption spectra obtained in the pH range from 6 to 9 are shown in Figure 3C. No significant shifts of the bands are observed between pH 6 and 9 indicating the presence of the same type of sorption complex. However, the relative intensity of the band representing the υ3(NpO2+) at 790 cm−1 has considerably increased by a factor of approximately 4 in the spectrum recorded at pH 9 (Figure 3C). This reflects an enhanced uptake of Np(V) at higher pH values and is in agreement with the macroscopic studies (Figure 3B).15 The higher sorption capability can be explained by the decreasing surface charge that facilitates sorption of positive NpO2+ ions (cf. Figure SI 1E). SCM was applied to batch experimental data from this study as well as all other papers known to the authors for this system.15,23,25 Contrary to most other modeling efforts within that system, only one Np(V) surface complex was taken into account due to our spectroscopic evidence. The degree of hydrolysis of the surface species was accounted for using fit exercises (eq 1 with n varied between 0 and 3).
for an edge-sharing sorption complex. The Np−Fe distance of this shell decreases only slightly by 0.03 Å. All other structural parameters do not change significantly. The shorter Np−Fe(2) distance of 3.47 Å is similar to that observed in the presence of carbonate (3.43−3.46 Å).31 The impact of carbonate can be excluded, since all sample preparations and measurements were performed carefully under N2 atmosphere. Hence, the presence of the observed shorter Np−Fe(2) interaction may have different reasons, which cannot be clarified at the moment. In summary, due to the level of the experimental error, the discussion of this scattering contribution in model 2 has only a speculative character. As a consequence, model 1 is favored as shown in Figure SI 3 because of its simplicity, the inclusion of only significant scattering contributions, and the agreement with the IR spectroscopic data of this study and further EXAFS data.31 Modeling of Thermodynamic Sorption Data. The aqueous speciation pattern of a 50 μM Np(V) solution under inert gas atmosphere in the pH range from 2 to 10 was modeled based on the NEA TDB50 including its update43 using the code EQ3/6.51 This clearly shows that the fully hydrated neptunyl(V) ion, that is, NpO2+, is the predominant species within the whole pH range from 2 to 10 (Figure 3A). The
2≡FeOH 0 + NpO2+ + nH 2O ↔ (≡FeO)2 NpO2 (OH)n(n + 1) − + (n + 1)H+
(1)
In order to promote application of SCM data records in the modeling of more complex (natural and anthropogenic) scenarios, the rather simple Diffuse Double Layer model was selected. The computed mean residuals as measure for the quality of fit were nearly identical for all four tested hydrolyzed surface species (cf. Figure SI 4 and Table SI 4). For more clarity, the data sets distinguished by the molar ratio between Np and the available surface binding sites were more closely considered. The higher hydrolyzed species with the highest nominal negative charges are favored only at conditions with high surface loadings that actually do not favor bidentate complexes. Thus, it becomes clear that the most appropriate Np(V) surface species obviously has a stoichiometry according to eq 1 with n = 0, giving an overall charge of −1. The corresponding complex formation constant obtained is logK = −11.01 ± 0.1 (cf. Table SI 3). Using only the surface complex (≡FeO)2NpO2(H2O)3− identified as relevant within this work, the raw data fit turned out to be better than the fit with two surface species published in ref 15 This visualizes the benefit of thermodynamic SCM data sets that are based on a realistic surface speciation derived from spectroscopic experiments. The Impact of Present Bicarbonate. The IR spectroscopic data of binary sorption of carbonate onto Fe2O3 and its impact on the surface reactions of Np(V) are summarized in Figure 4. Generally, the trigonal planar CO32− anion exhibits in the IR spectrum a doubly degenerated stretching mode ν3 which is split into an antisymmetric (ν3,as) and a symmetric (ν3,s) mode when the symmetry is lowered, for example, upon coordination to a central metal atom or to surfaces.52 Hence, the IR spectrum of an aqueous solution of 0.1 M NaCO3, measured in D2O (pD 7.6) serves as a reference. The distinct
Figure 3. Speciation diagram of 50 μM Np(V) in aqueous solution at 0.1 M NaCl under N2 atmosphere (A). Np(V) batch studies onto Fe2O3 as a function of pH (10 μM Np(V), 0.1 M NaCl, N2, 8 g L−1 Fe2O3) (B), Mid-IR spectra of Np(V) sorption onto Fe2O3 at distinct pH (50 μM Np(V), D2O, 0.1 M NaCl, N2, 0.4 mg Fe2O3 cm−2, 60 min of sorption). Indicated values are in cm−1 (C).
dominance of NpO2+ up to pH 9 was confirmed by NIR spectroscopy in an earlier study.29 A contribution of NpO2Cl (aq) − due to the use of NaCl as background electrolyte − is negligible at an ionic strength of 0.1 M. Two hydrolysis products, that is, NpO2OH(aq) and NpO2(OH)2−, are predicted at pH ≥ 10 by the thermodynamic data, but only to a very small extent (
