Article pubs.acs.org/est
Sorption of Eu(III) on Granite: EPMA, LA−ICP−MS, Batch and Modeling Studies Keisuke Fukushi,†,* Yusuke Hasegawa,‡ Koushi Maeda,‡ Yusuke Aoi,‡ Akihiro Tamura,‡ Shoji Arai,‡ Yuhei Yamamoto,§ Daisuke Aosai,§,∥ and Takashi Mizuno§ †
Institute of Nature and Environmental Technology, Kanazawa University, Kakuma, Kanazawa, Ishikawa 920-1192, Japan Division of Earth and Environmental Sciences, Graduate School of Natural Science and Technology, Kanazawa University, Kakuma, Kanazawa, Ishikawa 920-1192, Japan § Japan Atomic Energy Agency, Mizunami, Gifu 509-6132, Japan ∥ Center for Membrane and Film Technology, Department of Chemical Science and Engineering, Kobe University, 1-1 Rokkodai-cho, Nada-ku, Kobe 657-8501, Japan ‡
S Supporting Information *
ABSTRACT: Eu(III) sorption on granite was assessed using combined microscopic and macroscopic approaches in neutral to acidic conditions where the mobility of Eu(III) is generally considered to be high. Polished thin sections of the granite were reacted with solutions containing 10 μM of Eu(III) and were analyzed using EPMA and LA−ICP−MS. On most of the biotite grains, Eu enrichment up to 6 wt % was observed. The Eu-enriched parts of biotite commonly lose K, which is the interlayer cation of biotite, indicating that the sorption mode of Eu(III) by the biotite is cation exchange in the interlayer. The distributions of Eu appeared along the original cracks of the biotite. Those occurrences indicate that the prior water−rock interaction along the cracks engendered modification of biotite to possess affinity to the Eu(III). Batch Eu(III) sorption experiments on granite and biotite powders were conducted as functions of pH, Eu(III) loading, and ionic strength. The macroscopic sorption behavior of biotite was consistent with that of granite. At pH > 4, there was little pH dependence but strong ionic strength dependence of Eu(III) sorption. At pH < 4, the sorption of Eu(III) abruptly decreased with decreased pH. The sorption behavior at pH > 4 was reproducible reasonably by the modeling considering single-site cation exchange reactions. The decrease of Eu(III) sorption at pH < 4 was explained by the occupation of exchangeable sites by dissolved cationic species such as Al and Fe from granite and biotite in low-pH conditions. Granites are complex mineral assemblages. However, the combined microscopic and macroscopic approaches revealed that elementary reactions by a single mineral phase can be representative of the bulk sorption reaction in complex mineral assemblages.
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INTRODUCTION Ascertaining how the elements migrate and accumulate in the geologic media in surface and subsurface environments is important for analyzing the safety of the disposal of hazardous trace elements such as radionuclides.1 The migration and accumulation of the trace elements are governed by sorption processes on the solid phases.2,3 Europium(III), a trivalent cation, has been used as a chemical analogue for trivalent actinides such as Am(III) and Cm(III), which are long-living radionuclides contained in high-level radioactive wastes.4−6 Sorption behavior of Eu(III) on single minerals has been studied extensively in the laboratory.5,7−16 The sorption mechanisms of Eu(III) on minerals have been identified from spectroscopic analyses.5,8−12,16 On the basis of the sorption mechanisms, sophisticated sorption modeling has been developed for the prediction of sorption behaviors.7,11−13,16 However, geologic media invariably comprise complex mineral assemblages, not a single mineral phase.17 Very few studies6,18 © 2013 American Chemical Society
have specifically examined the interaction of complex mineral assemblages with Eu(III). Granite, which is widespread in terrestrial environments, is regarded as a candidate host rock for future disposal sites of high-level radioactive wastes.6,19,20 Understanding the interaction between Eu(III) and granite provides a better prediction for the trivalent actinide behavior in the disposal environment.6,20,21 The solubility of the trivalent actinide is well recognized as low in high-pH conditions because of the formation of the hydroxides and carbonates.9,22 Furthermore, because mineral surfaces become negative with pH, cationic trivalent actinides in solution are removed by adsorption processes on any mineral surface at high pH conditions in the Received: Revised: Accepted: Published: 12811
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mL of the same pH adjusted electrolyte solutions containing 10 μM of Eu(III). Speciation calculations using REACT in the Geochemist’s Workbench 35 revealed that Eu(III) takes the Eu3+ form under pH 99%) from granite was conducted using Kyoto Fission-Track Co., Ltd. The separation procedures of the biotite from the granite were crushing, sieving, hand-magnet separation, heavy liquid separation, ultrasonic washing, and sieving again. The fraction of 63−180 μm was used for sorption experiments. The respective surface areas of granite and biotite powder, as measured using singlepoint BET method (Quantasorb Model QS-11; Quantachrome Instruments) were, respectively, 0.3 m2/g and 0.5 m2/g. The cation exchange capacities of granite and biotite powder, as measured using the standard method provided by Japanese Geotechnical Society38 were, respectively, 0.040 meq/g and 0.044 meq/g. All sorption experiments were conducted in a glovebox at room temperature under a N2 atmosphere to avoid formation of possible dissolved Eu(III)-carbonate complex in high pH conditions in sorption modeling. Teflon vessels were used for sorption experiments. First, 0.1 g of powder samples was added to the 100 mL solution. The solid concentrations used in all experiments were 1 g/L. The reaction time was 24 h, which was verified using sorption kinetic experiments (SI Figure S6). The sorption edges of Eu(III) on granite powder were obtained as a function of ionic strength and the initial Eu(III) concentration. The ionic strengths were I = 0.01, 0.1, or 1 M. The pH was adjusted to 2−6 by the addition of small volumes of standardized 0.1 M HCl or NaOH solutions. The Eu(III) concentrations were adjusted to 1 or 10 μM. The suspensions were stirred for 24 h using a magnetic stirrer. After the adsorption reaction, the suspension pH was measured. Then, the suspensions were filtered using a 0.2 μm membrane filter
absence of a substantial amount of dissolved organic matter.7,10−13,16 Therefore, the mobility of trivalent actinides must be limited considerably in high pH conditions. On the contrary, the mobility is expected to increase with decreased pH. Although groundwater pH in granitic rocks is reported to be slightly alkaline (pH 7−9),23,24 we are still unable to predict the perturbations of pH over long periods for the storage of radionuclide wastes. Reportedly, microorganisms inhabiting anaerobic fractures in granitic rocks produce a locally acidic environment.24 The understanding of trivalent actinide behavior in neutral-to-acidic pH in granite rocks presents a challenge and provides valuable information related to stability of the repository. Microscopic observations of the complex mineral assemblages reacted with trace elements offers the benefits of revealing host minerals for trace element sorption and these sorption characteristics.6,25−28 A macroscopic investigation including sophisticated sorption modeling of complex mineral assemblages provides useful information related to quantitative sorption behavior and sorption mechanisms.17,20,29−33 Nevertheless, few studies have combined microscopic and macroscopic approaches for trace element sorption on the complex mineral assemblages. In this study, the sorption of Eu(III) on granite at neutral to acidic conditions was studied microscopically using an electron microprobe analysis (EPMA) and laser ablation−inductively coupled plasma−mass spectrometry (LA−ICP−MS), and macroscopically using batch sorption experiments and mechanistic sorption modeling.
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MATERIALS A granite sample designated as 10MI26, which was collected from a borehole at 400 m depth, was obtained from the Mizunami Underground Research Laboratory (Japan Atomic Energy Agency) central Japan. The detailed geological setting of the granite was described by Nishimoto and Yoshida.34 The granite sample used in the study was visually fresh (Supporting Information, SI, Figure S1) and comprised mainly quartz (40%), plagioclase (21%), K-feldspar (30%), and biotite (9%). The X-ray diffraction pattern obtained from the bulk powdered sample showed no peaks indicating other minerals (SI Figure S2). EPMA analyses of the sample thin sections showed that Fe hydroxides and Ca minerals (fluorite and apatite) were associated with the biotite grains (SI Figures S3 and S4). EPMA analyses also showed that smectite, chlorite, garnet, and Ca minerals replaced the Ca-enriched parts of the plagioclase grain (SI Figure S5). As indicated by the absences of these phases from XRD patterns, the contribution of these secondary minerals are small.
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METHODS
Microscopic Observations and Analyses. All reagents were analytical grade. A stock solution of Eu(III) was prepared by dissolving its chloride salt. Granite samples were vacuumimpregnated with low-viscosity resin to avoid detachment of the clay fractions from the samples. Polished thin sections were prepared from the granite blocks and were used for analyses and observations using light microscopy, EPMA, and LA− ICP−MS. The thin sections were washed repeatedly in Teflon bottles with 0.01 M NaCl solutions of pH 4, 5, and 6 for 24 h. These procedures were necessary for fixing pH during sorption experiments. Then the thin sections were washed with distilled water and placed in the Teflon bottle with approximately 100 12812
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Figure 1. BSE and Eu distribution images of the area consisting quartz, K-feldspar, plagioclase, and biotite in the granite after reaction with Eu(III) at pH 4: Bt, biotite; Kf, K-feldspar; Pl, plagioclase.
Figure 2. BSE, K, Na, and Eu distribution images of the area showing quartz, K-feldspar, and biotite in granite before (a−d) and after (e−h) the reaction with Eu(III) at pH 4.
exhibiting little Eu enrichment were also observed (SI Figure S4). Figure 2 shows a BSE image and elemental distributions of K, Na, and Eu of an area containing biotite, K-feldspar, and quartz in the original thin section (Figure 2a−d) and after the reaction with the solution containing Eu(III) at pH 4 (Figure 2e−g). Almost no difference was found in the textures and compositions of K-feldspar and quartz between those before and after the reaction. Those of the biotite grains change after the reaction. The number and the width of cracks increases in the Eu(III) reacted sample. The distributions of Eu appear along the original cracks of the biotite, which existed before the sorption experiments, (Figure 2a,e−g), in which the distributions of K are disappeared. The Eu-enriched parts exhibit higher brightness in BSE and are correlated with Na most likely because of contact with the electrolyte solution. Point chemical analyses showed that the Eu2O3 contents in Eu enriched part in the biotite are 5−6 wt % (Table 1). The Eu2O3 contents in the affected parts at pH 4 are slightly lower than those at pH 5 and 6. The affected biotites contain detectable Na2O (approximately 1 wt %). In contrast, the K2O content of the affected parts are less than 1 wt %, whereas those of unaffected parts is 8.5 wt %. Although the SiO2 contents of the affected parts are slightly higher than those of unaffected parts, no marked differences for the other major components
(DISMIC-25AS; Advantec Toyo Ltd.). The Eu(III) concentrations in filtrate were measured using inductively coupled− plasma mass spectrometry (ICP−MS, X7; Thermo). The amount of the sorption was calculated from the differences of initial and final Eu concentrations. The adsorption edges of Eu(III) on biotite powder were also obtained for comparison. The ionic strengths and initial Eu(III) concentrations were, respectively, I = 0.01, 0.1, or 1 M and Eu(III) = 10 μM. The pH was adjusted to 2−6. The experimental procedure was identical to that used for granite.
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RESULTS
EPMA. Figure 1a shows a representative backscattered electron (BSE) image after reaction with Eu(III) at pH 4. This area includes four major mineral constituents: quartz, Kfeldspar, biotite, and plagioclase. The Eu distributions were observed in parts of biotite, but were not observed in quartz, Kfeldspar, or plagioclase (Figure 1b). The point chemical analyses of quartz, K-feldspar, and plagioclase also showed no detectable Eu contents (less than 0.1 wt % of Eu2O3). The Eu enrichments in biotite were observed also in other pH conditions (SI Figures S7). Although most of the biotite contained detectable Eu enrichment by EPMA analyses in the grains after the Eu(III) sorption experiment, biotite grains 12813
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was greater than 107 cps at the beginning of the ablation and increased after 10 s ablation. Subsequently, it decreased gradually with time (Figure 3c). More than 95% of Eu distributed up to 30 s, which was calculated to be approximately 30 μm depth from the top surface, which indicates that the Eu(III) penetrates into the biotite structure. The intensity of K with time in the biotite was correlated negatively with Eu. Batch Sorption. Figure 4a shows sorption edges of Eu(III) on granite as a function of ionic strength and Eu(III) concentrations. The amount of sorption became almost constant above pH 4. The amount of sorption decreased abruptly with decreased pH. The sorption of Eu(III) depends strongly on ionic strength above pH 4. The sorption of Eu(III) for I = 0.01 was 100%, whereas that for I = 1 was zero under 1 μM of Eu(III) above pH 4. Figure 4b shows the sorption edges of Eu(III) on biotite as a function of ionic strength. The sorption behavior closely resembled that of the granite. Strong ionic strength dependency of the sorption was found above pH 4. The amount of sorption decreased abruptly with decreased pH from 4 to 2. The sorption of Eu(III) for I = 0.01 was almost 100%, whereas those for I = 1 were zero under 10 μM of Eu(III) at pH higher than 4.
Table 1. Averaged Chemical Compositions of the Unaffected and the Affected Biotite by Eu(III) Obtained from Point Chemical Analyses Using WDSa unaffected biotite
affected biotite at pH 4
N = 16 SiO2 TiO2 AI2O3 FeO MgO CaO Na2O K2O EU2O3 total a
N=4
affected biotite at pH 5 N=8
affected biotite at pH 6 N=4
avg.
sd.
avg.
sd.
avg.
sd.
avg.
sd.
35.5 2.9 13.7 27.4 4.5 0.01 0.05 8.5 0.04 92.7
0.8 0.1 0.4 1.0 0.2 0.01 0.02 0.1 0.03 0.9
38.4 3.1 14.0 26.1 5.2 0.1 1.2 0.4 5.2 93.7
1.2 0.1 0.3 1.0 0.4 0.0 0.2 0.1 0.1 1.9
38.0 2.5 14.5 26.1 5.0 0.1 0.8 0.4 6.1 93.4
1.3 0.0 0.4 0.3 0.1 0.0 0.1 0.1 0.1 1.5
36.6 2.8 14.2 26.2 4.4 0.2 0.9 0.4 6.3 92.0
0.8 0.2 0.6 0.8 0.5 0.0 0.3 0.2 0.3 1.9
Total Fe is expressed as FeO.
were found between the affected and unaffected parts of the biotite. LA−ICP−MS. Figure 3 presents representative intensities of 26 Mg, 29Si, 39K, 42Ca, and 151Eu signals vs time (LA−ICP−MS spectra) in K-feldspar (a), plagioclase (b), and biotite (affected parts) (c) reacted with Eu(III). The representative LA−ICP− MS spectra of the same minerals before the reactions with Eu(III) are given in Figure S8. The ablation started from 40 s and stopped at 100 s at a single spot. The repetition rate was 5 pulse/s. The rate of material removal was approximately X: Na + + Eu 3 + ⇒ X3: Eu 3 + + 3Na +
(1)
where >X: denotes the exchangeable site. The selectivity coefficient of eq 1 is given as follows: KNa Eu =
3 + β> X :Eu3+aNa 3
β>3X:Na+a Eu3+
(2)
where a represents the aqueous activity, and where β represents the activity of a species complexed with an exchange site. Surface activity coefficients are not well-known quantities. Here we assume that the activity coefficients of the species with exchange site are unity, as in previous studies.1,13 By the Gaines−Thomas convention, βj is given in terms of the fraction of the total electrical equivalents of exchange capacity occupied by cation j, with charge k. βj =
meq > xk : j per g solid CEC
(3)
Therein, CEC is cation exchange capacity given in the unit of meq/g. For complete modeling, the exchange reaction of dissolved cationic species such as H+, Fe(II), Fe(III), Al(III), and other cationic species at low pH conditions (pH < 4) should be considered. However, it is impossible to quantify the contribution of Fe(II) and Fe(III) to total iron concentrations. Moreover, the regressions using several exchange reactions to single sorption data engender the ambiguous estimations of selectivity coefficients. Therefore, we did not consider the possible exchange reactions at low pH conditions. In the modeling, the experimental data were fitted using two adjustable parameters: KNa_Eu, and CEC (SI Table S2). The modeled curves can reasonably fit the macroscopic sorption data observed at pH > 4 (Figure 4a). The reasonable fit validates the sorption mechanism described above. The cation exchange reaction in the modified biotite dominates the Eu(III) on granite at least under the experimental condition in this
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ASSOCIATED CONTENT
S Supporting Information *
An optical image and a corresponding mineral distribution map of the granite examined in this study (Figure S1), XRD patterns of the bulk granite sample (Figure S2), results of EPMA analysis of biotite grain before reaction with Eu(III) (Figure S3), results of EPMA analysis of biotite grain after reaction with Eu(III) (Figure S4), BSE image and elemental distributions of alteration minerals formed in plagioclase grain (Figure S5), 12816
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sorption kinetics of Eu(III) on the granite (Figure S6), results of EPMA analysis of biotite grain after reaction with Eu(III) at pH 6 (Figure S7), LA−ICP−MS spectra of major constituent minerals in granite before Eu(III) sorption (Figure S8), results of leaching tests of Fe and Al from granite powder as a function of pH (Figure S9), operating conditions for the LA−ICP−MS method (Table S1), some measured properties and model parameters for Eu(III) sorption on granite and biotite (Table S2), aqueous reactions, and these constants used in the sorption modeling (Table S3). This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Phone: +81-76-264-6520, e-mail: fukushi@staff.kanazawa-u.ac. jp. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We greatly appreciate discussions with T. Iwatsuki, T. Murakami, and T. Saito. The manuscript benefited from the comments of three anonymous reviewers. This study was supported by grants from JAEA Cooperative Research Scheme (A) for Kanazawa Univ./JAEA collaborative work.
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