Article pubs.acs.org/est
Formation of Neptunium(IV)−Silica Colloids at Near-Neutral and Slightly Alkaline pH Richard Husar,† Stephan Weiss,† Christoph Hennig,† René Hübner,‡ Atsushi Ikeda-Ohno,† and Harald Zan̈ ker*,† †
Institute of Resource Ecology, Helmholtz-Zentrum Dresden-Rossendorf, Bautzner Landstrasse 400, 01328 Dresden, Germany Institute of Ion Beam Physics and Materials Research, Helmholtz-Zentrum Dresden-Rossendorf, Bautzner Landstrasse 400, 01328 Dresden, Germany
‡
S Supporting Information *
ABSTRACT: The reducing conditions in a nuclear waste repository render neptunium tetravalent. Thus, Np is often assumed to be immobile in the subsurface. However, tetravalent actinides can also become mobile if they occur as colloids. We show that Np(IV) is able to form silica-rich colloids in solutions containing silicic acid at concentrations of both the regions above and below the “mononuclear wall” of silicic acid at 2 × 10−3 M (where silicic acid is expected to start polymerization). These Np(IV)−silica colloids have a size of only very few nanometers and can reach significantly higher concentrations than Np(IV) oxyhydroxide colloids. They can be stable in the waterborne form over longer spans of time. In the Np(IV)−silica colloids, the actinide oxygenactinide bonds are increasingly replaced by actinide oxygensilicon bonds due to structural incorporation of Si. Possible implications of the formation of such colloids for environmental scenarios are discussed.
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Peretroukhine et al.15 that silica can be incorporated into Pu(IV) and Th(IV) oxyhydroxide colloids under certain circumstances. We have recently published the first systematic investigations on the formation of An(IV) colloids in the presence of silicic acid, i.e., the formation of silica-containing U(IV) and Th(IV) colloids in near-neutral solutions only containing geogenic substances (carbonate, silicic acid, Na+).16,17 Our previous studies suggest that the higher the concentration of dissolved silicic acid and the pH are, the smaller and the more stable the An(IV) colloids of this type become. The isoelectric point of these nanoparticles was shifted toward lower pH values by the addition of silica. These particles can remain stable in aqueous suspension over years. A concentration of up to 10−3 M of colloid-borne U(IV) or Th(IV) was observed at near-neutral pH. Phenomenologically, the stabilization mechanism of these particles can be referred to as the so-called “sequestration” by silica which is well-known for trivalent metal ions such as iron(III)18,19 or curium(III)20 but has never been reported for An(IV) before. There are several sources of silicic acid in a nuclear waste repository which could supply the dissolved silicic acid needed for An(IV)−silica colloid formation. They include corroding
INTRODUCTION Most of the ecotoxicological risk caused by spent nuclear fuel stems from only a few chemical elements, among which the actinides plutonium (Pu) and neptunium (Np) prevail after a period of about 105 years.1,2 Since the reducing condition is expected in the unperturbed subsurface, Pu and Np are expected to be tetravalent in nuclear waste repositories and their surroundings in case of the failure of the nuclear waste containers. Due to the low solubility of tetravalent actinides (An(IV)), they are often assumed to be immobile in natural aquatic environments. However, An(IV) can also become mobile if they occur as colloids. For An(IV) oxyhydroxides this phenomenon has thoroughly been studied in the past.3−12 It has been shown that An(IV) oxyhydroxide colloids can increase the equilibrium concentration of An(IV) by a factor of about 100 at near-neutral and slightly alkaline pH as compared to the thermodynamic solubility of the An(IV) oxyhydroxides. For instance, concentrations of waterborne Th(IV) of about 10−5 M were observed due to the formation of Th(IV) oxyhydroxide colloids.3 However, the formation of An(IV) colloids can also be influenced by other components occurring in natural waters, such as low molecular mass organic acids, humic or fulvic acid, a combination of Al(III) plus Si(IV), etc. A comprehensive review on An(IV) colloid formation in aqueous solutions and their potential implication for the mobility of An(IV) under waste repository and disposal site conditions was recently given. 13 It has been reported by Yusov et al.14 and © 2014 American Chemical Society
Received: Revised: Accepted: Published: 665
August 8, 2014 November 13, 2014 November 17, 2014 November 17, 2014 dx.doi.org/10.1021/es503877b | Environ. Sci. Technol. 2015, 49, 665−671
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Article
Table 1. Colloid Characterization by Nephelometry, Ultracentrifugation (5 h, 100 000g) and Ultrafiltration (5-kDa)
1 The reference value is the count rate of pure water under the conditions applied. It was about 10 kcps. 2These values refer to the acidified sample (see text). n.a. = not added, n.m. = not monitored, UF = Ultrafiltration. UC = Ultracentrifugation.
nuclear waste glass, cement, bentonite, grout silica injected into the bentonite/bedrock interface, etc. Ingressing groundwater, too, contains 10−5 M to 10−3 M silicic acid.21 Given the presence of silicic acid, the formation of An(IV)−silica colloids in a nuclear waste repository cannot be ruled out. Possible processes of An(IV)−silica colloid formation may be the decrease of the alkalinity of solutions containing An(IV) carbonate complexes and silicic acid (e.g., by dilution), chemical or microbial reduction of An(V) or An(VI) in the presence of silicic acid and possibly the dissolution of nuclear fuel in the presence of silicic acid itself. For this reason, more studies are being required on An(IV)−silica colloids. In particular, studies on Pu(IV) and Np(IV) are essential, as the presence of these actinides in the waste repositories is critical in terms of radiation safety and associated environmental assessment. The objectives of this study are to elucidate the formation and the behavior of one of these critical silica-stabilized An(IV) colloids, i.e. of Np(IV)−silica colloids. On the basis of the obtained results, potential implication for the migration behavior of An(IV)−silica colloids at waste repositories is also discussed.
the Np oxidation state in the different samples of the experiments. The spectra of neptunium oxidation states III, IV, and V in acidic solution (cf. Figure S1 of the SI) were in accordance with corresponding spectra published by other researchers.24,31−36 From the spectra of the acidic “Np(IV) solutions” it can be deduced that a high purity of Np(IV) was reached by the two-step electrolysis (degree of purity ca. 95%). The missing of a peak of axial O atoms (Oax) at the Np ions at around R + Δ = 1.4 Å in the EXAFS spectra (see below) provides further evidence that pentavalent neptunium was absent and that the Np was tetravalent.30,32,37,38 Figure S2 of the SI gives an example of a UV-vis spectrum of Np(IV) in carbonate solution. Np(IV)−Silica Colloids above the “Mononuclear Wall” of Silicic Acid (MWSA). Neptunium(IV) carbonate solutions were mixed with solutions of silicic acid (produced from tetramethyl orthosilicate16) plus respective amounts of NaHCO3 (0.1 or 1 M) to reach Np concentrations of around 10−3 M and Si(IV) concentrations of >2 × 10−3 M. The latter concentration is also called the “mononuclear wall” of silicic acid, i.e., it is the limit above which silicic acid solutions are expected to contain polymers.39 This is of relevance for the reaction with actinides because the affinity of metal ions to polysilicic acid is much higher than the affinity to monosilicic acid.40,41 The formation of a solid phase (Np(IV) nanoparticles) is induced by decreasing the complexant (the carbonate) concentration when mixing the solutions. In contrast to our results of similar experiments with U(IV)16 and Th(IV),17 for Np(IV) at 10−3 M, we found that colloids formed immediately after mixing the solutions at slightly alkaline pH already (pH >8.5). For further details see the SI. Np(IV)−Silica Colloids below the MWSA. A silicic acid concentration above the MWSA isthough not impossible in naturerelatively far from typical environmental conditions. Therefore, we also conducted experiments at silicic acid concentrations below the MWSA. In such solutions only traces of polysilicic acid can be present.20 The Np(IV) concentration, too, was reduced in this case (3.1 × 10−4 M). In these experiments, the Np(IV)−silica colloids did not form immediately after mixing the reactants. However, such colloids formed after reducing the pH to 5.4 × 10−3 M silicic acid in the solutions, cf. Figure S4 of the SI). The shift indicates a change of the structure around the Np atoms at the particle surface. TEM. Figure S5a of the SI shows an example of a particle produced at the regime above the MWSA, as visualized by TEM. Particles produced at the regime below the MWSA are depicted in Figure S5b of the SI. The latter appear as assemblages of “primary particles” in a relatively loose arrangement having lateral dimensions of the micron size range. We assume that these micron-sized assemblages are formed due to aggregation during the deposition and drying of the particles on the copper grid. However, the sizes of both the individual particle in Figure S5a of the SI and the “primary particles” of the assemblage in Figure S5b of the SI correspond within a factor of 5 with the PCS result. More systematic investigations, which include studies into the time dependence of particle size (growth behavior, cf. section after next), would be necessary to obtain more specific information from a comparison between the results of TEM and the results of the noninvasive methods. Influences on Particle Size. In our studies on U(IV)− silica colloids16 and Th(IV)−silica colloids17 we have shown that silicic acid concentration and pH are crucial for the size of the silica-containing An(IV) particles produced. Figure S6 of the SI exhibits the dependency of SLI and particle size (PCS) on the initial silicic acid concentration for the Np(IV)−silica particles. As can be seen from this figure, the size of the Np(IV)−silica colloids depends on the silicic acid concentration in a similar way as in the case of U and Ththe higher the silicic acid concentration is, the smaller (and also the more stable in the waterborne state) the Np(IV) particles are whereby it seems that the MWSA (silicic acid concentration of 2 × 10−3 M) is a crucial value above which An(IV) particles start to be covered by a layer of polymerized SiO2 and to become exceptionally stable colloids as can be inferred from the behavior of Th(IV)−silica colloids.17 Figure S6 also demonstrates that the SLI can be a quite useful semiquantitative measure of the size of the particles in such a solution as discussed in more detail elsewhere.16 Particle Growth Behavior and Long-Term Stability. The time dependence of the light intensities scattered by colloidal systems that were generated from 10−3 M Np(IV) and silicic acid at different concentrations is shown in Figure 3. Three cases can be discerned in this figure. A silicic acid concentration of 1.3 × 10−3 M is not able to stabilize the nanoparticles resulting from 10−3 M Np(IV); fast sedimentation is observed in this case. A silica concentration of 3.2 × 10−3 M, too, results in slow sedimentation of the particles. Stabilization is found at 8.6 × 10−3 M silica. However, as can be seen in Figure 3, the colloids did not reach their final particle size immediately but showed a growth phase of several days. This is another analogy between the behavior of Np(IV)−silica colloids and the behavior of other An(IV)−silica colloids.16 It follows from Figure 3 that stabilization for at least 30 days is observed if enough silica is supplied. In another sample also
Sample 5 of Table 1 shows that 40 wt % of the colloidal particles must have had a size of ≥5 nm in this sample because they were removable by a 5 h centrifugation step at 100 000g (cf. Table 2 in Dreissig et al.16). Hence, about 40 wt % of the Np must have occurred as particles of 5 to 250 nm in the solution. The Np fraction not removable by the ultracentrifugation step may have comprised Np monomers (carbonate complexes) and Np oligomers. Ultrafiltration. The ultrafiltration experiments on the stable colloids of Samples 2 and 4 (cf. Table 1) using 5-kDa membranes indicate that about 80 wt % of the Np in these samples was in a colloid-borne form showing particle sizes between about 2 nm (see ultrafilter pore sizes given in Table 1 of Dreissig et al.16) and 250 nm (which follows from the absence of particle settling). Thus, the estimates based on both ultracentrifugation and ultrafiltration are in accordance with the particle size results of PCS. UV−Visible Spectroscopy. Figure 2 shows UV−vis absorption spectra of solutions containing Np(IV) species
Figure 2. UV−visible absorption spectra of soluble Np(IV) carbonate (1) ([Np] = 1.82 × 10−3 M, 1 M NaHCO3), colloidal Np(IV) (2) ([Np] = 1.72 × 10−3 M, [Si] = 3.2 × 10−3 M, 0.1 M NaHCO3) and the ultrafiltrate of colloidal Np(IV) silicate (3).
investigated after 24 h equilibration. Curve (1) represents the spectrum of a typical Np(IV) solution in 1 M NaHCO3. In Solution 2 (Curve 2) the carbonate concentration was only 0.1 M. At that concentration, carbonate is normally not able to stabilize >10−3 M Np(IV) in solution in the form of carbonate complexes and the Np(IV) precipitates as oxyhydroxide (see above). However, also silicate (above MWSA) was admixed to Solution 2 which prevented the Np(IV) from precipitation. A characteristic spectrum of colloidal Np(IV) is generated by this sample (Curve 2). This type of spectrum is, as mentioned, known for colloidal solutions of Np(IV) oxyhydroxide from the literature;6,42 light absorption is increased due to Np(IV) colloid formation and a broad peak with a maximum at 740 to 742 nm appears in such spectra. Furthermore, an increased SLI was observed for Solution 2 and PCS gave a particle size of 3 and the pH must not be lower than 7; and (ii) In the regime of silica above the MWSA, the initial Si/Np ratio must be >2.5 and the pH may be lower than 7. Chemical Composition of the Particles. It follows from the foregoing that the silica uptake of the Np(IV) particles depends on the supply of silicic acid during particle formation. Table 2 demonstrates this for samples of both the regime above and the regime below the MWSA of 2 × 10−3 M. The table is based on the differences between the samples’ Np and Si concentrations before and after ultrafiltration (5 kDa). It shows that the Si/Np ratio of the colloid particles does neither depend simply on the initial Si(IV) concentration of the solution nor simply on the initial Si/Np ratio of the solution. Note that the molar Si/Np ratio of the colloids given in Table 2 is an average over the particle radius. Our studies on Th(IV)−silica colloids showed that there may be significant variation of the Si/An(IV) ratio from the surface to the center of the particles.17 The correlation between the silicic acid concentration in the solution and the molar Si/Np ratio of the particles is obviously determined by the structural inclusion of silica into the particle inside (see next section) as well as by silica polymerization
Figure 4. Np L3 edge EXAFS spectra (left) and their corresponding Fourier transforms (right) of a colloid sample [(Np,Si)On(OH)4‑n· xH2O]4‑2n‑(4‑n) in aqueous solution with 1 mM Np(IV) and 3 mM Si in 0.1 M NaHCO3 (Sample “NpSi”), and a precipitate of [(Np)On(OH)4‑n·xH2O]4−2n‑(4‑n) under acidic conditions (Sample “NpOac”).
The Fourier transform (FT) of the EXAFS spectrum of sample NpOac shows an oxygen shell with a large Debye− Waller factor of 0.012 Å2 and a well developed Np peak. The large Debye−Waller-factor of the oxygen atoms indicates different NpO distances, although they cannot be resolved due to the limited k range. The spectrum is equivalent to that of similarly prepared Th(IV) oxyhydroxides17 and can be regarded as [(Np)On(OH)4‑n·xH2O]4‑2n‑(4‑n). The spectrum of the colloidal particles in sample NpSi exhibits a strong dampening of the EXAFS oscillation, related to a peak broadening in the FT and an even larger Debye− Waller factor. These spectral features indicate a significant structural disorder in the colloid structure. A small peak occurs in the FT which indicates silicon atoms with a NpSi distance of 3.11 Å. This short NpSi distance is characteristic of silica in bidentate coordination, while monodentate coordination would result in a significantly longer NpSi distance. A small but less specific peak at a larger distance seems to indicate NpNp interactions. It follows that, as for U(IV)−silica colloids and Th(IV)−silica colloids,16,17 also in the case of Np(IV)−silica colloids the An−O−An bonds are increasingly replaced by An−O−Si bonds, i.e., silica is included into the structure of the solid [(Np,Si)On(OH)4‑n·xH2O]4‑2n‑(4‑n) if the solution contains silicic acid. Implications for Potential Migration Behavior of An(IV). It has been proved that not only the “analogues” U(IV) and Th(IV), but also the highly radioactive Np(IV) itself is able to form silica-containing colloids. The assessment of Np behavior in the environment under reducing conditions should
Table 2. Molar Si/Np Ratio of the Np(IV)−Silica Colloids in Dependence on the Initial Si and Np Solution Concentrations [Si] and [Np] [Si] [10−3 M ]
[Np] [10−3 M ]
Si/Np in colloids [M/M]
8.6 7.3 5.4 3.2 1.3 0.7 1.5
1.04 1.05 1.05 1.05 1.04 1.02 0.25
3.6 3.5 2.6 1.7 0.8 0.4 1.3 669
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Table 3. EXAFS Fit Parametersa sample
scattering path
R [Å]
N
σ2 (Å2)
ΔEk=0
F
NpO NpSi NpNp NpO NpNp
2.28(1) 3.11(1) 3.75(2) 2.33(1) 3.83(1)
7.1(2) 1.3(1) 1.1(1) 7.2(3) 5.7(2)
0.017(4) 0.0091(2) 0.0098(1) 0.012(5) 0.0054(1)
−9.3
0.07
−7.6
0.33
NpSi
NpOac a
Errors in distances (R) are ±0.02 Å, errors in coordination numbers are ±15%. Standard deviations are given in parentheses.
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evaluate the role of such colloids since the formation of silicacontaining An colloids in the surroundings of a nuclear waste repository cannot be ruled out (see also the study by Kunze et al. on Cm(III)49). Np(IV)−silica colloids might prove to be relatively stable because both (i) a considerable repelling electrostatic force (i.e., a DLVO force) accompanied by a shift of the isoelectric point to lower pH values and (ii) significant non-DLVO forces due to the increased influence of silica in the colloids stabilize them.17,50 Special attention needs to be paid to decreases of the ionic strength due to intrusions of water poor in electrolytes (e.g., glacial melt waters after a possible future ice age51) because decreasing ionic strength increases colloid stability. Furthermore, transport of Np(IV)−silica colloids through the engineered barrier system (compacted bentonite) and through the fractures of crystalline host rock cannot be ruled out. Macromolecules of about 5 nm (lignosulfate, 30 kDa) were observed to diffuse through the pores of bentonite independently of the bentonite density.52 Humic substances and very small gold particles (2 nm), too, proved to be able to diffuse through compacted bentonite.53,54 However, slightly larger gold nanoparticles did not pass through compacted bentonite.54,55 The question whether or not Np(IV)−silica colloids are able to pass through bentonite or fractured host rock needs clarification. Further research is needed to elucidate the potential role of Np(IV)−silica colloids in environmental scenarios.
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ASSOCIATED CONTENT
S Supporting Information *
Descriptions of Np(IV) carbonate complex preparation, Np(IV)−silica colloid preparation and EXAFS spectroscopy. UV−vis spectra of Np(III), (IV) and (V) in HNO3 solution and of Np(IV) in carbonate solution; a comparison of the “colloid peak” of Np(IV) in the UV−vis spectrum of a solution at Si(IV) below the MWSA before and after ultrafiltration; the shift of a UV−vis “colloid peak” of Np(IV) due to the admixture of silica. TEM images of Np(IV)−silica colloids produced in solutions of Si(IV) above and below the MWSA. A diagram showing the dependence of Np(IV)−silica particle size on [Si(IV)]. A table describing the preparation of the EXAFS samples. This material is available free of charge via the Internet at http://pubs.acs.org/.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank C. Müller, A. Rumpel, and V. Brendler for technical assistance and fruitful discussions. 670
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