Article pubs.acs.org/est
Effects of Titanium Doping in Titanomagnetite on Neptunium Sorption and Speciation E. Miller Wylie,† Daniel T. Olive,‡,§̧ and Brian A. Powell*,† †
Department of Environmental Engineering and Earth Sciences, Clemson University, Clemson, South Carolina 29634, United States Department of Chemistry, University of California, Berkeley, California 94720, United States §̧ Nuclear Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States ‡
S Supporting Information *
ABSTRACT: Neptunium-237 is a radionuclide of great interest owing to its long half-life (2.14 × 106 years) and relative mobility as the neptunyl ion (NpO2+) under many surface and groundwater conditions. Reduction to tetravalent neptunium (NpIV) effectively immobilizes the actinide in many instances due to its low solubility and strong interactions with natural minerals. One such mineral that may facilitate the reduction of neptunium is magnetite (Fe2+Fe 3+2O 4). Natural magnetites often contain titanium impurities which have been shown to enhance radionuclide sorption via titanium’s influence on the Fe2+/Fe3+ ratio (R) in the absence of oxidation. Here, we provide evidence that Tisubstituted magnetite reduces neptunyl species to NpIV. Titaniumsubstituted magnetite nanoparticles were synthesized and reacted with NpO2+ under reducing conditions. Batch sorption experiments indicate that increasing Ti concentration results in higher Np sorption/reduction values at low pH. High-resolution transmission electron microscopy of the Ti-magnetite particles provides no evidence of NpO2 nanoparticle precipitation. Additionally, X-ray absorption spectroscopy confirms the nearly exclusive presence of NpIV on the titanomagnetite surface and provides supporting data indicating preferential binding of Np to terminal TiO sites as opposed to FeO sites.
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INTRODUCTION
Using magnetite as a model mineral phase to understand neptunium mobility is attractive due to the large amount of thermodynamic and crystal-chemical data available on this iron oxide.19−22 Additionally, it is a common constituent of many igneous rocks and sediments derived from these ore bodies. Recent studies have examined the interaction of actinides with magnetite in terms of iron and actinide redox chemistry.11,23,24 Several factors influence these reactions including bulk stoichiometry (Fe2+/Fe3+ ratio, R), solution pH, the presence of secondary ions, surface electrons via electron hopping, and the diffusion of Fe2+ through the solid, among others.13 In addition, natural magnetites often contain impurities at the cation site that can influence reactivity. One common impurity is titanium, which can result in significant concentrations of titaniumsubstituted (titano-)magnetite in igneous rocks (10−100 g kg−1).25 Titano-magnetites have been observed at the U.S. Department of Energy (DOE) Hanford site with an average Ti content (x) of 0.15 in Fe3‑xTixO4 and they are also present in many potential repository locations as an accessory mineral in granite.26−28 Several studies have examined the synthesis of
Actinide speciation under environmental conditions has been the focus of a large body of research since the advent of the nuclear age, especially with respect to the long-term management of nuclear waste.1,2 A few long-lived (105−1010 years) isotopes of U, Np, Pu, Am, and Cm produced by nuclear fission are of considerable concern.3 Neptunium-237 is of particular interest due to its long half-life (2.14 × 106 years) and the relative mobility of the pentavalent ion as NpO2+ in surface and groundwater environments.4,5 Retardation of mobile actinyl ions such as NpO2+ and UO22+ can occur naturally by reduction to less mobile tetravalent species through microbial processes or reduction by redox-active species such as Fe2+, Cr3+, or S2−.6−8 As such, substantial research has examined the mechanisms of sorption and reduction of actinyl ions by structural Fe2+ in iron oxide minerals such as magnetite (Fe2+Fe3+2O4).9−11 A range of tetravalent actinide (An) species result from this reduction including AnO2 and AnIV phosphates, carbonates, and organometallics depending on the presence of complexing ligands and other local conditions.12 Under certain circumstances, AnO2 nanoparticles can precipitate.13,14 The formation of actinide nanoparticles or association of actinides with natural nanoparticles has the potential to increase the environmental mobility of tetravalent actinides.15−18 © XXXX American Chemical Society
Received: November 1, 2015 Revised: January 11, 2016 Accepted: January 12, 2016
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DOI: 10.1021/acs.est.5b05339 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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Environmental Science & Technology
nm, and the supernatant was retained for chemical analysis. Inductively coupled plasma mass spectrometry (ICP-MS) procedure details are provided in the SI. The oxidation state of neptunium was examined using acid extraction followed by LaF3 coprecipitation.31,32 This acidic extraction step has been previously used to quantitatively remove pentavalent and hexavalent actinides from iron oxide minerals.23,24,33,34 It is an inherently indirect measurement, and the assumption in the method is that any actinides not leached after adjustment of the pH to 1.5 are in the tetravalent state. Following leaching, the oxidation state of the aqueous actinides is determined (in the current work using LaF3 coprecipitation) and the total Np oxidation state distribution was determined as follows:
nanoparticulate titanomagnetite and its reactivity owing to the ability to systematically control R using titanium.29,30 A series can be produced by replacement of Fe3+ by Ti4+ in the unit cell yielding a complete solid solution of Fe3‑xTixO4 from magnetite (x = 0) to ulvospinel (x = 1). As Ti4+ replaces Fe3+, a reduction of the unit cell Fe3+ to Fe2+ occurs for charge balance. This results in a proportional increase in R-values greater than that of stoichiometric magnetite (0.5). The distribution of Fe2+ and Fe3+ between the A and B sites depends on x and results in a range of magnetic, electronic, and structural properties. Pearce et al. describes a series of titanomagnetite nanoparticle suspensions produced at room temperature under anoxic conditions with x values ranging from 0.00 to ∼0.40 according to the following:30 (1 + x)Fe2 + + (2 − 2x)Fe3 + + x Ti4 + + 8OH− ⇔ Fe3 − xTixO4 + 4H 2O
fNp,total = 1.0 = fNp(IV),aqueous + fNp(IV),leachate + fNp(IV),leached,solid
(1)
+ fNp(V/VI),leachate + fNp(V/VI),aqueous
These particles have been shown to exhibit a strong reductive capacity, effectively reducing the pertechnetate (TcO4−) ion29 to TcIV and UO22+ to UIV species13 under anaerobic conditions. The focus of the current contribution is to examine the reactivity of NpO2+ with this series of synthetic Ti-substituted magnetite nanoparticles under reducing conditions. Batch sorption experiments indicate that increasing Ti concentration results in higher Np sorption/reduction values at low pH. Highresolution transmission electron microscopy of the Ti-magnetite particles provides no evidence of NpO2 nanoparticle precipitation. Additionally, X-ray absorption spectroscopy confirms the nearly exclusive presence of NpIV subsequent to reaction and provides insight about the coordination environment of Np on the surface of the particles. This study indicates that titanomagnetite nanoparticles are an effective reductant for pentavalent neptunium under anaerobic conditions and also demonstrates the use of Ti doped nanoparticles for the study of potentially preferential binding of actinides to TiO sites.
where the NP fraction remaining on the leached solid phase ( f Np(IV),leached,solid) is calculated by subtracting the fractions of all other measured aqueous and leachate species. The total fractions of each Np oxidation state can be determined by summing the fractions observed in the aqueous and leached solutions. Since the LaF3 coprecipitation step does not differentiate between pentavalent and hexavalent actinides, the oxidized fraction is reported as Np(V/VI). However, based on the reducing conditions and the initially added Np(V) state, it is reasonable to assume any Np in the oxidized fraction is present as Np(V). Aliquots of the suspensions were acidified to pH ≈ 1.5 using 1 M HCl and mixed in the anaerobic chamber for 1 h to leach the Np from the nanoparticles. The aliquots were then centrifuged for 20 min at 8000 rpm and 1 mL of supernatant was removed for the coprecipitation step. One mL of a lanthanum fluoride stock (0.25 M H2SO4, 0.8 M HNO3, 1 mM KMnO4, and 10 mM La(NO3)3) and concentrated HF (0.1 mL) was added to the supernatant and the solution was mixed for 3 min followed by centrifugation. The supernatant was diluted into 2% HNO3 for ICP-MS analysis. The LaF3 coprecipitates tetravalent and trivalent actinides. Therefore, any remaining aqueous Np was present as Np(V/VI). Samples for X-ray absorption spectroscopy (XAS) analysis were prepared in 2 mL microcentrifuge tubes. Titanomagnetite suspensions (∼90 mg) were added to each tube (∼45 g L−1 titanomagnetite total suspension) then spiked with Np(V) stock solution (1.5 × 10−3 M) to achieve a loading value of 400 mg Np kg−1 titanomagnetite. The pH of each reactor was adjusted to ∼6 using NaOH and HCl. The reactions were then placed on an end-over-end shaker. After a period of 2 days, the samples were removed from the shaker, centrifuged, and the supernatant was retained for further analysis. The wet paste was deposited directly into a 20 mm3 aluminum sample cell and sealed with Katpton tape for XAS analysis. All XAS spectra were collected using a liquid nitrogen-cooled cryostat engineered by Los Alamos National Laboratory to minimize beam-induced reduction of the sample. Neptunium LIII-edge (17,610 keV) X-ray absorption spectroscopy measurements were collected at the Stanford Synchrotron Radiation Light Source (SSRL) beamline 11−2 using a Si(220) double-crystal monochromator (ϕ = 0 crystal set) for energy selection and calibrated using the first inflection point of the K-edge of a yttrium reference foil (17 038.4 ± 0.3 eV).35 The data were collected in fluorescence mode using a 100 element Ge detector, with deadtime corrections applied using SixPack software.36 Standard extended X-ray absorption finestructure (EXAFS) analysis37 was carried out using the software
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MATERIALS AND METHODS Titanium magnetite nanoparticles (∼10 nm) were synthesized under anoxic conditions using methods described by Pearce et al.30 Characterization of these particles indicates that Fe and Ti are incorporated into Fe3‑xTixO4 ranging from 0 < x < 0.46 with possible minor TiO2 at x > 0.35 (Supporting Information, SI). Solids were separated by centrifugation or magnetically and washed three times prior to use. Methods used to measure Fe and Ti concentrations and Fe2+/Fe3+ (R) are provided in the SI. All reaction procedures were performed under anaerobic conditions (18.2 MΩ·cm resistivity). A 4.2 × 10−6 M 237Np(V) working solution (prepared via dilution of the stock solution into ultrapure water) was spiked into 0.1 g L−1 titanomagnetite suspensions to achieve a final Np(V) concentration of 2.1 × 10−8 M in each batch reactor. The pH of the suspensions was adjusted to 3, 5, and 7 using NaOH and HCl. The reactions were then placed on an end-over-end shaker. After a period of 30 days, the samples were removed from the shaker, centrifuged to achieve a suspended particle size 0.25) the monomeric U(IV) species is dominant.13 Thus, the potential preferential binding of U(IV) to TiO sites may have prevented precipitation of UO2. In this work, no NpO2 precipitates were identified using high resolution TEM (Figure 3B,D) or demonstrated via observation of an NpNp distance in EXAFS spectra. Furthermore, the Np solid phase concentration was too low to detect Np using STEM-EDS. Thus, it appears that
Figure 1. Total neptunium fraction associated with solid titanomagnetite as a function of Ti molar substitution at each experimental pH. Error represents standard deviation between multiple experiments and triplicate measurements obtained from ICP-MS analysis.
sorbed was high (>∼80%) at pH > 3 and all x values. At pH 3, a marked increase was observed between x = 0 (pure Fe3O4) and x ≈ 0.4 (Fe2.6Ti0.4O4) with fraction sorbed approaching 70% in samples with the highest titanium substitution. At pH 3, some magnetite dissolution is expected. Here, we observed dissolution values of Fe and Ti on the order of 10−25% for pH 3 and 5 (SI). With aqueous concentrations of Np > 80% in the low Ti samples (x = 0, 0.1, 0.2), it is likely that dissolution is not the primary factor accounting for the variation in Np sorption data observed across our Ti doping range. Measurements of the Np oxidation state distribution indicated that the fraction of the aqueous Np from the sorption experiments correlated with the fraction of aqueous and leached Np(V,VI) from the LaF3 coprecipitation step at pH 3 as shown in Figure 2. The predominance of Np(V) in the aqueous phase indicates that even if mineral dissolution is occurring, dissolved iron is not facilitating reduction of Np(V) to Np(IV). Thus, the observation of aqueous Np(V) and the correlation between reduced and sorbed Np indicates that reduction of Np(V) to Np(IV) in these systems is surface mediated, which is consistent with previous studies examining Pu reduction on magnetite.23,24 The strong sorption of Np under all conditions is indicative of Np(V) reduction to Np(IV) which was verified using the XAS measurements described below. The
Figure 3. High-resolution TEM (A, B, D) and STEM (C) of the titanium magnetite nanoparticles loaded for XAS measurements (x = 0.4, pH = 6, loading = 400 mg Np·kg−1 titanomagnetite). The absence of neptunium colloid was confirmed by lattice measurements (D) that indicate the exclusive presence of magnetite planes. C
DOI: 10.1021/acs.est.5b05339 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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Figure 4. Fourier transformed EXAFS, offset for clarity.
the Np is sufficiently distributed (i.e., present as a monomeric surface complex) to prevent detection of NpO2 which would provide a sufficiently high EDS signal after concentration of the Np into the NpO2 nanoparticle. It is noteworthy that the solid phase concentrations of Np used in this work (∼400 ppm) are significantly lower than those used to study U interactions with titanomagenetite (∼20 000 ppm based on solution conditions provided). Potential implications of this are discussed below. EXAFS analysis of the samples indicated that Np(V) has been reduced to Np(IV) in all cases, and there is a systematic shift in the scattering distance near 3 Å (Figure 4). The edge region of the five samples and an NpO2 standard is shown in Figure 5. Edge
Table 1. Absorption Edge Positions as Determined by First Derivative, and White Line Peak Positions are Listed for the Titanomagnetite Samples as Well as NpO2 and Np(V) (aq) Standards for Comparisona sample
absorption edge, eV
absorption maximum, eV
“Ti” peak amplitude
TM0 (x = 0.00) TM1 (x = 0.12) TM2 (x = 0.23) TM3 (x = 0.34) TM4 (x = 0.46) NpO2 Np(V)
17612.5 ± 0.5 17612.9 ± 0.5 17612.5 ± 0.5 17612.5 ± 0.5 17612.4 ± 0.5 17613.0 ± 0.5 17612.1 ± 0.5
17616.8 ± 0.5 17617.4 ± 0.5 17616.4 ± 0.5 17616.7 ± 0.5 17616.7 ± 0.5 17618.1 ± 0.5 17615.8 ± 0.5
N/A 1.3 ± 1.0 2.2 ± 1.5 2.4 ± 1.1 4.4 ± 1.7 N/A N/A
a
Amplitude of the peak at 3.6 Å, correlated with Ti concentration and tentatively assigned to NpTi scattering, as determined by EXAFS fitting is also listed.
An initial fit of the pure magnetite sample using the model of NpO2 gave reasonable agreement to the first shell neptunium− oxygen distance,49 but with no indication of NpNp scattering, suggesting a lack of NpO2 precipitation. An additional NpFe scattering path was found to describe the data. The lack of NpO2 precipitation on magnetite is not consistent with observations of UO2 formation on magnetite by Latta.13 However, the relatively high uranium surface loading of approximately 20 000 mg U kg−1 magnetite compared with the neptunium surface loading of 400 mg Np kg−1 magnetite used in this work may explain this discrepancy. While the scatter patterns of the Ti-doped samples also show no evidence of NpO2 precipitation, the data cannot be fit with the same parameters as the pure magnetite sample. Unfortunately, iron- and titanium-Np scattering are quite similar, so it is difficult to directly distinguish between the two and sorption could be occurring on either FeO or TiO sites. While it may be possible that Ti substitution is causing a sufficient number of tetrahedral iron sites to open accounting for the increased Np sorption, we do not have any direct evidence of Fe2+ occupying tetrahedral sites. As such, the authors propose that Np simply preferentially sorbs to Ti−O sites in this system. This behavior is consistent with the observation of increasing Np sorption with increasing titanium substitution in the nanoparticles and
Figure 5. Normalized X-ray absorption spectra from samples, and NpO2 and Np(V) (aq) standards and k-space EXAFS (inset). The similarity of the titanomagnetite samples’ absorption edges to the Np(IV) standard, note the lack of “neptunyl” shoulder, and lack of NpNp scattering peak at 3.7 Å, shows reduction of Np(V) without precipitation.
and white line positions, Table 1, are characteristic of Np(IV), as well as lack of the characteristic neptunyl shoulder from oxygen double bonds in Np(V) indicate reduction of the Np(V) when sorbed to the titanomangnetite nanoparticles.44−46 Reduction of Np(V) to Np(IV) on magnetite is consistent with previous studies also conducted under anaerobic conditions.47,48 D
DOI: 10.1021/acs.est.5b05339 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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Environmental Science & Technology Table 2. EXAFS Fitting Resultsa “Fe” path
oxygen
“Ti” path
sample
ΔE0 (eV)
N
r (Å)
N
r (Å)
N
r (Å)
TM0 (x = 0.00) TM1 (x = 0.12) TM2 (x = 0.23) TM3 (x = 0.34) TM4 (x = 0.46)
2.0 ± 0.7 1.6 ± 0.6 0.4 ± 0.8 −0.7 ± 0.9 0.4 ± 1.0
9.2 ± 0.4 9.8 ± 0.4 9.7 ± 0.5 8.2 ± 0.4 10.2 ± 0.6
2.38 ± 0.01 2.35 ± 0.01 2.34 ± 0.01 2.33 ± 0.01 2.33 ± 0.01
5.4 ± 0.5 3.2 ± 0.6 3.5 ± 0.8 2.8 ± 0.6 3.8 ± 0.9
3.47 ± 0.01 3.47 ± 0.02 3.45 ± 0.02 3.40 ± 0.02 3.44 ± 0.03
N/A 1.3 ± 1.0 2.2 ± 1.5 2.4 ± 1.1 4.4 ± 1.7
N/A 3.65 ± 0.08 3.66 ± 0.06 3.60 ± 0.04 3.63 ± 0.04
The sample spectra were fit using an oxygen single scattering path, an oxygen multiple scattering path parameterized using values from the single scattering path, a short Np-metal path (Fe), and a longer Np-metal path (Ti). The change in E0 was allowed to vary for each sample, but the same value was used for each path in a given sample. The Debye-Waller factors for each type of path were refined but held constant across samples. The best-fit values are 0.016 ± 0.001, 0.015 ± 0.003, and 0.012 ± 0.007 Å2 for the oxygen, iron, and titanium paths, respectively. All five samples were fit simultaneously in R-space, from 1.1 to 4.0 Å, using 34 variables and 63 independent points, with an overall R-factor of 0.005. Uncertainties reported by IFEFFIT, from inversion of the covariance matrix. a
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supported by the shift in the EXAFS scattering pattern near 3 Å. The bond distances, which are tentatively identified with “Np Fe” bonds and “NpTi” bonds average 3.45 and 3.64 Å, respectively, are reported in Table 2. The “NpFe” bond distance of 3.44 Å in this work is similar to 3.47 and 3.50 Å reported for NpFe by Law et al. for Np(IV) under differing biogeochemical conditions in which no nanoparticle precipitation was noted.50
Corresponding Author
*Phone: 864-656-1004; e-mail:
[email protected] (B.A.P.). Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
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Notes
ENVIRONMENTAL SIGNIFICANCE Whether the mechanism is direct Ti−O-Np sorption, an indirect influence of titanium substitution, or both, it is clear that increased titanium concentration in these particles results in increased Np sorption at low pH and under anaerobic conditions. Under environmental conditions, Np concentrations will likely be low enough for monomeric ion sorption models to be valid in virtually all locations outside of a small zone proximal to the source, where NpO2 nanoprecipitates may dominate. However, XAS sample preparation generally requires high loading concentrations to accurately resolve the spectra. These loading concentrations can lead to the formation of precipitates in many instances as an unintended consequence of the high surface concentrations required for analysis. In this study, sample loadings were low enough to prevent the formation of high concentrations of NpO2 nanoprecipitates. As such, these findings represent a more accurate depiction of the actinide loading concentrations typically observed under environmental conditions. Many previous studies examining U(VI) reduction to U(IV) have demonstrated that the formation of ternary U(VI) CO3Ca and U(VI)CO3Mg complexes can prevent sorption and reduction. Despite the fact that Np(V) generally forms weaker carbonate complexes than U(VI), this phenomenon should be less significant but may still influence Np(V) reduction in these systems. As these data demonstrate, there is a shift in the spectra and sorption behavior with respect to the Ti substitution level. Future experiments will attempt to elucidate the specific reactivity of Ti within the mineral surface as well as the influence of Np speciation especially upon examination of pure iron and titanium systems.
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AUTHOR INFORMATION
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research is funded by the Office of Basic Energy Sciences of the U.S. Department of Energy as part of the Heavy Element Chemistry Program (DE-SC0010355). Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.5b05339. ICP-MS data, UV−vis spectroscopy, nanoparticle synthesis and characterization (PDF) E
DOI: 10.1021/acs.est.5b05339 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.est.5b05339 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
