Structural Insight into Iodide Uptake by AFm Phases - ACS Publications

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Structural Insight into Iodide Uptake by AFm Phases Laure Aimoz,*,†,‡ Erich Wieland,† Christine Taviot-Guého,§,∥ Rainer Daḧ n,† Marika Vespa,⊥,# and Sergey V. Churakov† †

Laboratory for Waste Management, Paul Scherrer Institute, 5232 Villigen PSI, Switzerland Institute of Geological Sciences, University of Bern, Baltzerstrasse 1-3, 3012 Bern, Switzerland § Université Blaise Pascal, Clermont Université, Institut de Chimie de Clermont-Ferrand, BP 10448, F-63000 Clermont-Ferrand, France ∥ CNRS, UMR 6002, Laboratoire des Matériaux Inorganiques, F-63177 Aubière, France ⊥ Department of Chemistry, Division of Molecular and Nanomaterials, Catholic University of Leuven, Dutch-Belgium Beamline, European Synchrotron Radiation Facility, BP 220, 38043 Grenoble, France # European Commission, Joint Research Centre, Institute for Transuranium Elements, P.O. Box 2340, D-76125 Karlsruhe, Germany ‡

S Supporting Information *

ABSTRACT: The ability of cement phases carrying positively charged surfaces to retard the mobility of 129I, present as iodide (I−) in groundwater, was investigated in the context of safe disposal of radioactive waste. 125I sorption experiments on ettringite, hydrotalcite, chloride-, carbonate- and sulfatecontaining AFm phases indicated that calcium−monosulfate (AFm−SO4) is the only phase that takes up trace levels of iodide. The structures of AFm phases prepared by coprecipitating iodide with other anions were investigated in order to understand this preferential uptake mechanism. X-ray diffraction (XRD) investigations showed a segregation of monoiodide (AFm−I2) and Friedel’s salt (AFm−Cl2) for I−Cl mixtures, whereas interstratifications of AFm−I2 and hemicarboaluminate (AFm−OH−(CO3)0.5) were observed for the I−CO3 systems. In contrast, XRD measurements indicated the formation of a solid solution between AFm−I2 and AFm−SO4 for the I−SO4 mixtures. Extended X-ray absorption fine structure spectroscopy showed a modification of the coordination environment of iodine in I−CO3 and in I−SO4 samples compared to pure AFm−I2. This is assumed to be due to the introduction of stacking faults in I−CO3 samples on one hand and due to the presence of sulfate and associated space-filling water molecules as close neighbors in I−SO4 samples on the other hand. The formation of a solid solution between AFm−I2 and AFm−SO4, with a shortrange mixing of iodide and sulfate, implies that AFm−SO4 bears the potential to retard 129I.

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INTRODUCTION Deep disposal of low- and intermediate-level radioactive waste in a cementitious repository is foreseen in Switzerland. The cement barrier functions to retard the release of radionuclides from the near field of the repository into the far field host rock (Opalinus Clay in the case of Switzerland) by limiting solubilities of cationic radionuclides and providing a large sorption capacity in the host rock. Radioactive waste contains considerable amounts of 129I, a long-lived radionuclide (half-life of 1.57 × 107 years). Iodide (I−) is the thermodynamically stable aqueous species for iodine under the anoxic near-field conditions.1 However, the major solid phases in the engineered barriers foreseen for a deep geological repository (e.g., bentonite backfill and the main hydrated cement phases such as calcium silicate hydrates) have negatively charged surfaces and are unlikely to retard anions such as iodide. Due to its expected weak retention and long half-life, 129I is thus considered to be one of the dose-determining elements released from a repository for radioactive waste.1 Nonetheless, © 2012 American Chemical Society

hydrated cement has been reported to sorb iodide. Minor phases present in hydrated cement are thus likely to be responsible for this retention, which reduces the calculated release rate for 129I.2−4 However, to include such sorption processes in long-term predictions made for safety analysis, a thorough understanding of the iodide uptake mechanism is required, and in particular, the identity of the phases responsible for iodide retention need to be established. Previous studies showed that calcium silicate hydrate phases (C−S−H), the major component in hydrated cement, cannot entrap iodide.2 Minor minerals in hydrated cement carry positively charged surfaces and represent up to 15% of the mineral inventory in hydrated cement.5 These minerals are potential candidates for the retention of anionic species.6,7 Received: Revised: Accepted: Published: 3874

December 14, 2011 February 27, 2012 February 29, 2012 February 29, 2012 dx.doi.org/10.1021/es204470e | Environ. Sci. Technol. 2012, 46, 3874−3881

Environmental Science & Technology

Article

Milli-Q water (integral water purification system, Millipore). Tricalcium aluminate C3A [Ca3Al2O6] was synthesized by heating CaCO3 and Al2O3 for 24 h at 1400 °C.12 Hydrotalcite [Mg3Al(OH)8(CO3)0.5·2.5H2O] was prepared by the slow addition of 1 M Mg(NO 3 ) 2 ·6H 2 O and Al(NO3)3·9H2O solutions to a Na2CO3 solution under N2 flow. The pH was maintained constant at 11.4 (±0.2) by additions of NaOH when necessary.8,9 The suspension was centrifuged, and the extracted solid phase was rinsed with water and dried at 60 °C. The hydrotalcite was then resuspended at a liquid-to-solid ratio (L/S in kg/kg) of 100 in a fresh NaOH solution (pH 11.4). Hydrogarnet Ca3Al2O6·6H2O was synthesized by mixing C3A with boiling water at a L/S of 10 and stored in an oven at 105 °C during 10 days.13 The suspension was centrifuged and dried over a saturated CaCl2 solution. All subsequent experiments were performed in a glovebox under a N2 atmosphere (O2, CO2 < 2 ppm) at ambient temperature. The hydrogarnet was then resuspended in water at a L/S of 100. In addition, ettringite and AFm phase suspensions were synthesized at a L/S of 10. Ettringite was prepared by mixing stoichiometric amounts of CaO and Al2(SO4)3·18H2O. AFm phases Ca4Al2(OH)12·X·(2−6)H2O with X = 2 × Cl, CO3, SO4 or 2 × I were prepared by stoichiometrically mixing C3A with calcium salt (CaI2, CaCl2·2H2O, CaCO3, or CaSO4·2H2O). All suspensions containing the separate mineral phases were equilibrated on an end-over-end shaker over a period of one month. Coprecipitation and Competition Experiments. The aim of coprecipitation experiments was to check whether one AFm phase containing two different ions or two AFm phases, each containing one anionic species in its interlayers, would form. C3A and calcium salts (as CaI2 and CaCl2, CaCO3, or CaSO4) were mixed according to the stoichiometry of AFm phases and equilibrated over a one month period. Competition experiments were carried out to check whether recrystallization (with the neo-formation of a phase containing the competing anion) or an anionic exchange reaction (iodide being exchanged with the competing anion without major structural modification) would occur. Various amounts of KCl, K2CO3, or K2SO4, respectively, were added to a one month equilibrated suspension of AFm−I2 and further equilibrated for at least 48 h. AFm samples from coprecipitation or competition experiments can be represented by the following stoichiometry: (Ca4Al2(OH)12)x·I2·x· X2/ν·(1−x)·2−6H2O where ν = 1 or 2, for monovalent or divalent anions, respectively, and x is the mole fraction of “I2” (i.e., 2 × I−) in the AFm samples (eqs 1−3). nI x I2 in AFm−I2−Cl2 = nI + nCl (1)

Among these minor phases, there are the following: ettringite (AFt−SO4, 3(CaO)·Al2O3·3(CaSO4)·32H2O),8 hydrotalcite (Mg2−4(Al,Fe)(OH)6−12CO3(H2O)2−4), and AFm phases, namely, Friedel’s salt (AFm−Cl2, 3(CaO)· Al 2 O 3 ·(CaCl 2)·8H 2 O), 8 monocarbonate (AFm−CO 3 , 3(CaO)·Al2O3·(CaCO3)·11H2O),8 and monosulfate (AFm− SO4, 3(CaO)·Al2O3·(CaSO4)·12H2O).8 Ettringite is composed of Ca and Al hydroxide polyhedra forming channels filled with H2O and sulfate anions. Hydrotalcite and AFm phases belong to the LDH family and are composed of hydroxide layers of edge-sharing octahedra usually occupied by divalent and trivalent metal ions alternating with interlayers composed of water and charge compensating anions.5,6 Hydrotalcite is commonly composed of Mg2+−Al3+/Fe3+ hydroxides with carbonate anions in its interlayer space, whereas AFm phases are most commonly composed of Ca2+−Al3+/Fe3+ with chloride (AFm−Cl2), carbonate (AFm−CO3), and sulfate (AFm−SO4) anions. A general overview of the compounds mentioned in this paper and of their chemical formulas is summarized in Table S1 of the Supporting Information. The synthesis and characterization of pure AFm−I2, made with the intention of capturing radioiodine, was first reported by Brown and Grutzeck.9 Although the replacement of major anionic species (i.e., CO32−, SO42−, Cl−) in AFm phases by iodide could potentially reduce the mobility of 129I in the nearfield,10 an atomic-scale understanding of the uptake mechanisms is still missing. To the best of the author’s knowledge, only one study has been reported focusing on AFm phases containing a mixture of iodide with other anions.11 In this latter work, Kuzel found two distinct phases forming in the I−Cl system at 24 °C and at 220 °C. However, monoiodide (AFm− I2) was found to form a continuous solid solution series with monobromide (AFm−Br2) at 24 °C and at 150 °C, and also with AFm−SO4 at 100 °C. The goals of the present study were to quantitatively assess the uptake of iodide by the most common cement phases with positively charged surfaces (hydrotalcite, ettringite, AFm−Cl2, AFm−CO3, and AFm−SO4) and to determine the effect of anion competition on iodide incorporation. In a first series of experiments, AFm phases were synthesized by coprecipitation in the presence of iodide and the most important anions present in cement pastes, that is, chloride, carbonate, or sulfate, in order to investigate the extent of miscibility of iodide. In a second series of experiments, the effect of anion competition on the stability of AFm−I2 was investigated. The samples obtained were characterized using X-ray diffraction (XRD) to identify the potential of iodide to mix with chloride, carbonate, or sulfate and to check the solid solution formation in the I−SO4 system at ambient temperature in order to complement the previous observations made at 100 °C.11 The resulting compounds were also characterized using I K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy with the aim of developing a comprehensive understanding of iodide uptake mechanisms at long-range (XRD) and short-range (EXAFS) scales.

x I2 in AFm−I2−CO3 = n I

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MATERIALS AND METHODS Mineral Synthesis for Sorption Experiments. Sorption experiments on selected cement minerals were carried out with the aim of detecting which ones are promising for the retention of 129I; among them were ettringite, hydrotalcite, hydrogarnet, AFm−Cl2, AFm−CO3, and AFm−SO4. All experiments were performed using pro analysis grade chemicals and degassed

x I2 in AFm−I2−SO4 = n I 2

nI 2

+ nCO3

(2)

nI 2

+ nSO4

(3)

where n is expressed in moles. Note that the given stoichiometry does not necessarily imply the formation of a single phase containing both anions but can also result in a 3875

dx.doi.org/10.1021/es204470e | Environ. Sci. Technol. 2012, 46, 3874−3881


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segregation phenomenon with the formation of several phases (each containing only one anionic species in its interlayers). Calcium salt ratios and the amount of added potassium salts were chosen to reach a mole percent of iodide xI2 = 25%, 50%, and 75% in the “coprecipitation” and “competition” samples. These samples are hereafter referred as “cop-I-X” or “compet-IX”. Solid Phase Characterization. The samples were centrifuged, briefly washed with water, and thoroughly rinsed with degassed absolute ethanol in order to remove any unreacted salts. The samples were dried over a saturated solution of CaCl2 and stored in a glovebox. A mineralogical characterization (XRD) of the synthesized products was made using an X’Pert Pro Philips diffractometer equipped with a X’Celerator Scientific detector and a Cu anticathode (Kα1/ Kα2). The instrument was used in the θ/θ reflection mode, fitted with a nickel filter, 0.04 rad Soller slits, 10 mm mask, 1/4° fixed divergence slit, and 1/4° fixed antiscatter slit. The diffraction patterns were measured over the range 5−70° (2θ) with a step size of 0.02° and a counting time of 2 s/step. Hydrogarnet or ettringite impurities, respectively, were observed in some of the AFm samples. A phase transition at ∼130 K for pure AFm−I2 has been reported.14 In order to assess the extent of the modifications induced by this transition, the reduction of the unit cell volume between the high and low temperature structures induced by the phase transition was determined based on XRD data. Measurements were carried out at the powder diffraction station of the Swiss Norwegian beamline15 of the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. The wavelength was calibrated using the NIST silicon standard (640C) and a Rietveld refinement resulted in a λ value of 0.5020 Å. The AFm−I2 sample was mounted in a 0.7 mm capillary. The low temperature data were collected after 20 h of precooling using a liquid N2 flux set at 100 K. The positions of the 006 and 110 lines were determined from single-line profile analysis using a pseudo-Voigt peak shape model16 in order to estimate the a/b and c unit cell parameter variations between the high and low temperature polymorphs. The cop-I-SO4 and compet-I-SO4 samples were characterized by XRD with the aim of determining the evolution of the interlayer distance against the iodine content. The positions of the 006 reflections (i.e., at 2θ ∼ 20°) were determined from single-line profile analysis also using a pseudo-Voigt peak shape model.16 Furthermore, the elemental composition of the compounds was determined by chemical analysis. First, the Ca/Al ratio was estimated by digesting 100 mg of solid phase in 1% HNO3 and by measuring the Ca and Al elemental concentration using inductively coupled plasma optical emission spectrometry (ICP-OES). The ettringite impurity was deduced from the Ca/Al stoichiometry. Second, 100 mg of solid phase was placed in a 0.1 M K2CO3 solution. The SO42− and I− in the solid phase were quantitatively displaced into solution due to the large stability of AFm−CO3 and the high CO32− concentration in solution.17 The concentration of released SO42− and I− was determined by high-performance ion exchange chromatography (IC) and corrected for ettringite contamination in order to determine the actual iodide/sulfate ratio in the AFm phase (Table S2 in the Supporting Information). In addition, thermogravimetry (TG) measurements were carried out on the compet-I-SO4 sample with the

initial I content of xI2 = 50% in order to determine its structural water content, according to a procedure described elsewhere.18 Note that for a given preparation method (coprecipitation or competition) and a given mixture (I−Cl, I−CO3, or I−SO4), samples with different iodine contents resulted in the same mineralogy (Table S3 in the Supporting Information). Therefore, only XRD results for samples with an initial I content of xI2 = 50% are shown. 125 I Sorption Experiments. Sorption experiments were performed using 125I tracer onto hydrotalcite, hydrogarnet, ettringite, AFm−Cl2, AFm−CO3, and AFm−SO4. The 125I tracer, present as I− in slightly alkaline solutions, was purchased from Perkin−Elmer. The content of I−, expressed as a fraction of the total activity, was verified to be greater than 99% by measuring the activity after separation of the I− fraction by IC following procedures described elsewhere.19 A 125I-labeled 10−8 M KI solution was thereafter prepared. Aliquots of the 125I tracer solution were added to five duplicates of each mineral phase suspension (total 4 × 10−11 M I−). An additional 125I-free duplicate was used to check the mineralogy and purity of each sample by XRD, and five duplicates were included in the sorption experiments for five separate equilibration times: 1 day, 7 days, 28 days, 84 days, and 168 days. After the selected equilibration times, the samples were centrifuged (60 min at 95000 g). Triplicate aliquots were sampled from the supernatant and analyzed for 125I together with standards. Activities were measured with a γ-counter (Cobra, Canberra−Packard). Time correction was applied to compensate for radioactive decay (t1/2(125I) = 59.4 days). The sorbed iodide fraction was determined as follows: F=1−

A sample A std

(4)

where Asample (cps) is the measured 125I activity remaining in the supernatant and Astd (cps) is the measured activity in the standard corresponding to the activity initially added to the samples. The iodide fraction sorbed on the minerals, F, is assumed to be not detectable when the difference between Astd and Asample is within the uncertainty of the measurements. Calculated uncertainties in F take into account the uncertainties of standard and sample measurements and the estimated uncertainty of the L/S. Iodine K-Edge EXAFS Analysis. Iodine K-edge bulk EXAFS spectra were collected at the Dutch Belgium Beamline (DUBBLE, BM26) of the ESRF.20 The measurements were carried out according to a previous procedure18 (see details in the Supporting Information, Methodology S1), except that the He cryostat was set to 15 K to improve data quality. The selected k-range for the Fourier transform (FT) was identical for all samples (range 2.9−11.5 Å−1). Fits were performed in inverse Fourier transform (FT−1) using theoretical scattering paths calculated with FEFF8.4.21,22 Scattering potentials were determined using atomic configurations obtained in ab initio geometry optimization runs (see next section). The optimized structure of AFm−I2 was used for the EXAFS fitting of AFm− I2, I−Cl, and I−CO3 samples, whereas the optimized structure of AFm−I−(SO4)0.5 was used for the EXAFS fitting of I−SO4 samples. Note that, for a given preparation method (coprecipitation or competition) and a given mixture (I−Cl, I−CO3, or I−SO4), k3-weighted EXAFS spectra of the samples with different iodine contents revealed only slight differences, which resulted in 3876



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Figure 1. Experimental EXAFS spectra and corresponding fit models at the I K-edge of AFm samples containing Cl−, CO32−, or SO42− and an initial I content xI2 = 50% (with “I2” standing for 2 × I−, see definition in eqs 1−3) prepared by coprecipitation (cop) or by competition (compet) methods. (a) k3-weighted: (b) FT (modulus and imaginary part) with the closest fitted shell highlighted in yellow and the shells found at further distances highlighted in gray (c) FT−1.

ature of transition (at 100 K) due to the lowering of symmetry (Figure S3 in the Supporting Information). The a/b and c parameters were determined to decrease by 0.006 Å and 0.186 Å, respectively, by lowering the temperature. This corresponds to a very small shrinkage in the (a, b) plane and in the stacking direction. As a consequence, the differences in coordination numbers (CN) and distances (R) induced by the phase transition are expected to be negligibly small and within the uncertainty of the EXAFS technique (e.g., smaller than 0.01 Å for I−I and I−OH2O distances due to reduction of ∼a, and smaller than 0.03 Å for I−Ca, I−Al, I−OOH due to a reduction of ∼c/6). Therefore, the structural parameters (CN and R) determined at low temperature are considered to be within the uncertainties also representative of the ambient temperature polymorph. Nonetheless, by lowering the temperature, the thermal motion is constrained. Thus, the reported Debye− Waller factors from the low temperature measurement are expected to be smaller than the one representative of the ambient temperature structure. A strong local order of the coordination environment around iodine atoms is indicated by the large oscillations from 2 to 13 Å−1 (Figure 1a) in the k3-weighted EXAFS. Large amplitudes at k > 10 Å−1 suggest the presence of heavy (high Z) backscattering atoms, such as calcium and/or iodine atoms. Three main shells were observed in the corresponding FT, at R + ΔR = 2.9 Å, 4.5 Å, and 5.6 Å (Figure 1b). The first FT peak

similar fit models (Figure S1 and Table S4 in the Supporting Information). Therefore, only EXAFS results for samples with an initial I content of xI2 = 50% are shown. Ab Initio Structure Optimization. The structural position of water and ions in the interlayer of AFm−I2, with the composition Ca8Al4(OH)24 × I4 × (H2O)8, and AFm−I− (SO4)0.5, with the composition Ca8Al4(OH)24 × I2 × (SO4) × (H2O)i where i = [8, 10, 11, or 12], were obtained by ab initio geometry optimization based on a method described elsewhere (see details in the Supporting Information, Methodology S2).18 The equilibrium amount of water in the interlayer of AFm− I−(SO4)0.5 was estimated from the lattice energies of AFm−I− (SO4)0.5 containing different amounts of structural water (see details in the Supporting Information, Methodology S2).

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RESULTS AND DISCUSSION XRD and EXAFS Characterization of AFm−I2. A pure AFm−I2 sample, used as a reference compound in this study, was synthesized and characterized for comparison with coprecipitation and competition samples. EXAFS measurements carried out at 15 K were found to greatly improve the data quality (see Figure S2 in the Supporting Information). However, as these measurements were performed below the temperature of the phase transition, the structural modifications induced at low temperature had first to be estimated. Additional diffraction lines were observed below the temper3877



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Table 1. EXAFS Results for Samples Prepared by Coprecipitation (cop) and by Competition (compet) Methods Measured at the I K-Edge at 15 Ka

compet cop compet cop compet cop

AFm−I2 I−Cl I−Cl I−CO3 I−CO3 I−SO4 I−SO4

xI2

I−O1

initial

CN

R (Å)

CN

R (Å)

σ (Å2)

ΔE0 (eV)

R factor

100% 50% 50% 50% 50% 50% 50%

9.2(9) 9.2(8) 9.3(8) 9.8(9) 6.8(1.2) 8.1(9) 7.4(1.0)

3.50(3) 3.50(3) 3.50(3) 3.51(3) 3.50(1) 3.56(2) 3.57(3)

4.7(9) 4.8(8) 4.8(8) 4.2(9) − 6.2(1.0) 6.9(1.0)

3.84(3) 3.83(3) 3.84(3) 3.84(3) --3.84(2) 3.85(3)

0.011 0.011 0.012 0.012 0.009 0.010 0.009

−4.6(2.9) −4.3(2.5) −4.1(2.7) −4.4(2.6) −5.3(1.3) −1.4(3.7) −0.5(2.9)

0.03 0.03 0.04 0.02 0.03 0.07 0.09

I−O2

CN, R, σ2, ΔE0, and R factor: coordination number, interatomic distance, Debye−Waller factor, shift of the threshold energy, and “goodness of fit” as defined in IFEFFIT.29 S02 was fixed to 1.0. The Debye−Waller factor was constrained to be identical for all shells in each sample. CN(I − O1) + CN(I − O2) was constrained between 10 and 14 using a amplitude factor of 100.29 Uncertainties are given by the number in brackets for the last digits, i.e., 9.2(3.6) means 9.2 ± 3.6, and 3.50(3) means 3.50 ± 0.03. a

could be fitted with two I−O backscattering pairs at R ∼ 3.5 Å and R ∼ 3.8 Å (Table 1). Corresponding Debye−Waller factors were nonetheless still relatively high, indicating a non-negligible disorder of oxygen backscattering atoms. The shells at R + ΔR ∼ 4.6 Å and ∼5.6 Å found in pure AFm−I2 could be successfully fitted over a R range from 4.0 to 6.0 Å with I−Ca and I−I backscattering pairs, respectively. However, fitting simultaneously the lower and higher R ranges (i.e., from 2.3 to 3.9 Å and from 4.0 to 6.0 Å) using a common value for the energy shift (ΔE0) was not successful. Because the difference in ΔE0 between the two independent fits exceeded realistic physical values, we limited the EXAFS analysis to the R range from 2.3 to 3.9 Å for all samples (Table 1). XRD and EXAFS Characterization of Coprecipitation and Competition Samples. The diffraction patterns of coprecipitation and competition samples were measured and compared to the one of pure AFm−I2. Note that AFm phases containing different anions can be distinguished based on their interlayer distances (i.e., the larger the interlayer anion, the larger the interlayer distance), which is reflected in the positions of the 001 reflection series. The 001 lines characteristics of reflection of AFm−I2, AFm−Cl2, and AFm−SO4 are indicated by dashed lines in Figure 2; special features present in some samples and described hereafter are circled in gray. Pure AFm−I2 and AFm−SO4 are also plotted for comparison. Phase Segregation. Phase segregation has been observed for compet-I-CO3, compet-I-Cl, and cop-I-Cl. In the compet-ICO3 sample, 001 lines corresponding to AFm−CO3 and AFm− I2 were observed (Figure 2). The partial recrystallization of AFm−I2 into AFm−CO3 demonstrates the larger stability of AFm−CO3 compared to AFm−I2. In the compet-I-CO3 sample, the 001 line corresponding to AFm−I2 displays a right shoulder (observed for all xI2 contents), which can be attributed either to the presence of AFm−I2 in different hydration states, or more likely, to the formation of some degrees of interstratification of I and OH−(CO3)0.5. Nonetheless, the k3-weighted EXAFS spectrum shows a similar shape for compet-I-CO3 and pure AFm−I2 (Figure 1a). This indicates that the coordination environment of the compet-I-CO3 sample is almost identical to that of pure AFm−I2, thus, mainly reflecting the AFm−I2 phase present in the sample. Similarly to the compet-I-CO3 samples, two separate AFm phases were obtained in the cop-I-Cl and compet-I-Cl samples: AFm−I2 and AFm−Cl2 (Figure 2). AFm−Cl2 formed in the presence of AFm−I2. The partial recrystallization of AFm−I2 into AFm−Cl2 demonstrates the larger stability of AFm−Cl2

Figure 2. X-ray diffraction patterns of AFm samples containing Cl−, CO32−, or SO42− and an initial I content xI2 = 50% (with “I2” standing for 2 × I−, see definition in eqs 1−3) prepared by coprecipitation (cop) or by competition (compet) methods. Impurities of hydrogarnet are found in I−Cl and AFm−SO4 samples. Impurities of ettringite are found in cop-I-SO4 and AFm−SO4 samples. “i.s.” I and OH−(CO3)0.5 indicates interstratification of iodide and hemicarboaluminate in the AFm structure as AFm−(I−(OH−(CO3)0.5)); “s.s.” AFm−I2 with AFm−SO4 indicates solid solution between iodide and sulfate AFm end-members.

compared to AFm−I2. The k3-weighted EXAFS spectra show similar shapes for cop-I-Cl, compet-I-Cl, and pure AFm−I2 samples (Figure 1a), indicating a similar coordination environment of iodine. Hence, for the compet-I-CO3, cop-I-Cl, and compet-I-Cl samples, three main shells were observed in the corresponding FT, similarly to AFm−I2 (Figure 1b). Two I−O backscattering pairs could be fitted, as for AFm−I2 (Table 1). Therefore, the formation of AFm−Cl2 or AFm−CO3 in these samples did not disturb the local structure of AFm−I2, even at short-range distances around iodine atoms. 3878



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Interstratification Phenomena. Interstratification was observed in the cop-I-CO3 sample. Indeed, for this sample, a series of 001 lines was observed at 2θ values 10.40°, 15.64°, and 20.92° corresponding to d-spacings of 8.51, 5.67, and 4.25 Å, respectively. This finding suggests the formation of an AFm phase with an almost regular stacking of interlayer galleries alternatively filled with I− and (OH−−CO32−). Assuming that these three latter reflections correspond to 002, 003, and 004 reflections, respectively, an interlayer distance d001 = 17.01 ± 0.02 Å (2σ) was obtained. This value agrees well with interstratifications of AFm−I2 (d001 ∼ 8.83 Å15,18) and hemicarboaluminate (AFm−OH−(CO3)0.5 with d001 ∼ 8.20 Å) giving rise to a supercell with a d001 = 17.03 Å. This phenomenon, known as staging effect, is hardly ever encountered in LDH systems23 but has been reported for AFm phases containing mixtures of chloride and sulfate anions (Kuzel’s salt).24,25 The first 001 reflection is expected to appear with a very low intensity and can only be observed when the relative proportion of the two kinds of interlayer is exactly 50%.23 Here the 001 reflection (expected at 2θ ∼ 5.19°) does not appear, revealing the presence of some stacking disorder. The formation of an interstratified compound is promoted by the identical crystallographic space group of AFm−I2, and AFm−OH−(CO3)0.5 (R3̅), while for example, AFm−Cl2 and AFm−CO3 crystallize in different space groups, C2/c and P1, respectively. Nevertheless, AFm−OH−(CO3)0.5 has a distinctly smaller interlayer distance (8.20 Å) than AFm−I2 (8.83 Å), which hinders I and (OH, CO3) to mix within the same interlayers. The interstratifications of iodide and carbonate in the cop-ICO3 sample were found to strongly affect the short-range order of iodine. In the k3-weighted EXAFS spectra, the oscillations at high k values of the cop-I-CO3 sample were strongly damped compared to pure AFm−I2 (Figure 1a). A main shell was detected in the corresponding FT at R + ΔR = 2.9 Å, and limited structural information was observed at larger R + ΔR (Figure 1b). A single I−O backscattering pair was sufficient to obtain a good fit for the cop-I-CO3 spectrum (Table 1). In contrast, a second I−O pair was required to reproduce the experimental spectra of all other samples. The I−O1 distance of the cop-I-CO3 sample is similar to the one in pure AFm−I2. However, a significant reduction in the coordination number was observed for the cop-I-CO3 samples compared to AFm−I2, emphasizing differences in the coordination environment of iodide in these compounds. In contrast to pure AFm−I2, no FT peaks were observed after R + ΔR ∼ 4 Å. This indicates a local disorder of the coordination environment around iodide ions in the cop-I-CO3 sample. Such a feature in I K-edge EXAFS spectra was previously observed for iodide-containing Zn−Al LDH, where the local disorder of the coordination environment around iodide was related to the presence of microstructural faults such as stacking faults.18 The local disorder of iodide in the cop-I-CO3 sample is assumed to be due to the presence of a singular stacking of (OH−−CO32−) in the interlayers adjacent to the iodide ones. Solid Solution Formation. For cop-I-SO4 and compet-ISO4 samples, only one series of 001 diffraction lines was observed indicating the formation of a single phase. The 001 reflections were displaced toward higher 2θ values with increasing of the iodide content, which indicates a gradual decrease of the interlayer distance (Figure 3). Thus, one can assume an intercalation of both anions in the same interlayer space, as previously observed for chloride and carbonate anions

Figure 3. Evolution of the unit cell parameter c based on the position of the 006 line as a function of I content (as xI2, with “I2” standing for 2 × I−, see definition in eqs 1−3) in the AFm−(I−SO4) samples.

in AFm phases.26 Indeed, because AFm−I2 and AFm−SO4 crystallize in the same space group (R3̅) and because they have very similar interlayer distances, 8.83 Å16,21 and 8.93 Å,27 respectively, the formation of a solid solution between these two compounds is facilitated. Hence, the formation of a solid solution series between AFm−I2 and AFm−SO4 occurs, not only at 100 °C, as reported elsewhere,11 but also at 23 ± 2 °C as demonstrated in this study. As a consequence, the anionic exchange between iodide and sulfate anions is possible starting from both end-members (see compet-I-SO4 and 125I sorption experiments). In the I−SO4 samples, oxygen atoms around iodine do not only originate from hydroxide groups and water molecules coordinated to calcium atoms as in pure AFm−I2. Neighboring oxygen atoms can also belong, like in AFm−SO4, to spacefilling water molecules. The solid solution can thus be regarded as an anion exchange reaction involving one sulfate ion and at least one space-filling water molecule being replaced by two iodide ions. The amount of space-filling water molecules was estimated based on ab initio calculations for the optimization of an AFm− I−(SO4)0.5 structure with different water contents. The calculations suggest that, per formula unit containing one SO42− ion, AFm incorporates between 10.6 and 11 water molecules (see details in the Supporting Information, Methodology S2 and Figure S4). The results of the calculations are in good agreement with TG measurements of the compet-I-SO4 sample (measured xI2 = 49%). The mass loss corresponding to structural water yielded 11 water molecules per formula unit containing one SO42− ion. Consequently, the geometry with the formula [Ca8Al4(OH)24 × I2 × (SO4)1 × (H2O)11] (Figure 4) was used for calculating the models to fit the EXAFS spectra. The k3-weighted EXAFS spectra of the cop-I-SO4 and compet-I-SO4 samples are similar in shape but differ notably from pure AFm−I2 (Figure 1a). A dampening of the 3879



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investigated because it was present as an impurity in some AFm samples. No uptake of I− by hydrogarnet, hydrotalcite, ettringite, AFm−Cl2, and AFm−CO3 was detectable, even after equilibration times up to six months. AFm−SO4 was the only phase that took up trace-levels of iodide (4 × 10−11 M I−). The fact that the other AFm phases (AFm−Cl2 and AFm− CO3) did not take up iodide although they expose external surfaces with similar physicochemical properties, demonstrates the importance of the interlayer anion for iodide uptake by AFm phases. This finding is in good agreement with results from experiments carried out at high iodide loadings, which showed the mixing of iodide in the interlayer of AFm−SO4. The uptake of iodide by AFm−SO4 was, within experimental errors, identical regardless of equilibration times and equal to a sorbed fraction F = 0.76 ± 0.08 (Table S5 in the Supporting Information). The uptake was rapid, complete within 1 day, which is consistent with the assumption of anion exchange as reaction mechanism reported to occur within minutes in LDH systems.23 Nevertheless, this uptake corresponds to a relatively low Rd value of (2.6 ± 0.3) × 10−2 m3/kg. Although low, this Rd value may still lead to a significant decrease in the mobility of trace amounts of 129I in hydrated cement. Besides, a complementary study has been performed on the thermodynamic properties of the AFm−I2/AFm−SO4 solid solution.28 This study is in excellent agreement with the present data interpretation. A continuous solid solution behavior was found over a large range of iodide concentration with a linear sorption isotherm for an iodide concentration in solution between 10−11 M and 10−3 M. Implications for Radioactive Waste Disposal. This structural study shows that, at ambient temperature, a solid solution forms via anion exchange between AFm−I2 and AFm− SO4, with a short-range mixing of iodide and sulfate. Thus, it is expected that the presence of AFm−SO 4 , which is thermodynamically stable in cement systems, can act as a sink for 129I and therefore reduce the mobility of 129I in a cement-based repository for radioactive waste. Furthermore, results from the structural investigation carried out using high loadings of iodide are in excellent agreement with those from sorption experiments performed at trace levels of iodide, indicating that the uptake mechanism is operative over a wide concentration range of iodide in solution. This study further reveals that chloride and carbonate are strong competitors for I−. High concentrations of chloride or carbonate anions can induce the formation of Friedel’s salt (AFm−Cl 2) or monocarbonate (AFm−CO3) and reduce the iodide retention by destabilizing pure AFm−I2 and AFm−(I−SO4) solid solutions.

Figure 4. Coordination environment of iodine as obtained by geometry optimization based on ab initio simulations for AFm−I− (SO4)0.5. I atoms are represented by violet spheres. Oxygen atoms from water molecules in the interlayer are large red spheres bonded with two protons (gray). Oxygen atoms from OH groups (small red spheres bonded to a proton) form the corners of Ca and Al polyhedra (blue and light blue, respectively). A sulfate molecule is shown as yellow tetrahedron.

oscillations was observed at k above 8 Å−1. The corresponding FT reveals a local coordination environment of iodide very different from the one in AFm−I2 (Figure 1b). The splitting of the first shell between two maxima at R + ΔR = 2.9 Å and 3.4 Å in the FT is much more pronounced in comparison with the previously discussed samples. The amplitude of the peak at R + ΔR = 4.6 Å is considerably reduced in the cop-I-SO4 and compet-I-SO4 samples compared to AFm−I2, and no further shell was observed (Figure 1b). In contrast to the single main frequency observed for AFm−I2 in the FT−1, at least two superimposed frequencies can be visualized in the FT−1 spectra of cop-I-SO4 and compet-I-SO4 (Figure 1c). The results from the fit show similar coordination numbers for the cop-I-SO4 and compet-I-SO4 samples. However, the I−O1 distance at R ∼ 3.5 Å for the I−SO4 samples is shorter (∼3.50 Å) compared to all of the others (∼3.56 Å) (Table 1). These differences between pure AFm−I2 and AFm−(I−SO4) samples are attributed to the presence of sulfate ions and additional space-filling water molecules modifying the short-range environment surrounding iodide. These EXAFS results show that the atomic configuration of oxygen atoms around iodine is modified when SO42− is introduced, therefore indicating a short-range mixing of I− and SO42−. This finding strongly supports the idea of the formation of a solid solution between AFm−I2 and AFm−SO4. Uptake of Trace-Level Iodide by AFm−SO4. Iodide uptake experiments were carried out using 125I radiotracer to determine which thermodynamically stable calcium aluminate cement minerals (AFt, AFm) are able to bind iodide at trace level. Note that preliminary sorption experiments revealed that I− uptake by C−S−H phases, the most abundant hydration product in cement paste, is negligible. Hydrogarnet was also

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ASSOCIATED CONTENT

S Supporting Information *

Results from the 125I-labeled experiments including all equilibration times, detailed mineralogical identification of the solid samples, chemical composition of the cop-I-SO4 and compet-I-SO4 samples reported in Figure 3, EXAFS results for all I contents and at different temperature, and diffraction lines corresponding to the high and low temperature AFm−I2 polymorphs. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +41-56-310-5656; e-mail: [email protected]. 3880



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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Dr. Enzo Curti and Dr. Urs Mäder are greatly acknowledged for scientific advice and for revising the manuscript. The authors thank Dr. Barbara Lothenbach (Empa) for providing the C3A and for valuable advice during sample preparation. Dr. Konstantin Rozov is thanked for his assistance during the hydrotalcite synthesis. The beamline scientists of the DUBBLE and SNBL beamlines (ESRF, Grenoble, France) provided expert technical support. Dr. Urs Eggenberger and Dr. Christoph Wanner (UniBern) are acknowledged for assistance during the XRD measurements. Dr. Martin Glaus and Werner Müller (PSI) are thanked for the ion chromatography analysis, as well as Susan Cohr (PSI) for providing the iodine radiotracer. The simulations were performed in the Swiss Center of Scientific Computing, Manno. Financial support was granted by the Helmholtz virtual institute of advanced solidaqueous radio-geochemistry and by the Swiss Cooperative for the Disposal of Radioactive Waste (Nagra), Switzerland.

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REFERENCES

(1) Nagra Technical Report, NTB 02-05; Nagra: Wettingen, Switzerland, 2002. (2) Bonhoure, I.; Scheidegger, A. M.; Wieland, E.; Dähn, R. Iodine species uptake by cement and CSH studied by I K-edge X-ray absorption spectroscopy. Radiochim. Acta 2002, 90 (9−11), 647−651. (3) Pointeau, I.; Coreau, N.; Reiller, P. E. Uptake of anionic radionuclides onto degraded cement pastes and competing effect of organic ligands. Radiochim. Acta 2008, 96 (6), 367−374. (4) Rojo, I.; Colas, E.; Grive, M.; Rovira, M.; de Pablo, J.; Duro, L. Comparison of iodine and selenate retention mechanisms onto cementitious materials. Geochim. Cosmochim. Acta 2009, 73 (13), A1113−A1113. (5) Lothenbach, B.; Wieland, E. A thermodynamic approach to the hydration of sulphate-resisting Portland cement. Waste Manage. 2006, 26 (7), 706−719. (6) Baur, I.; Johnson, C. A. Sorption of selenite and selenate to cement minerals. Environ. Sci. Technol. 2003, 37 (15), 3442−3447. (7) Leisinger, S. M.; Lothenbach, B.; Le Saout, G.; Kagi, H.; Wehrli, B.; Johnson, C. A. Solid solutions between CrO4- and SO4-Ettringite Ca6(Al(OH)6)2[(CrO4)x(SO4)1‑x]3·26 H2O. Environ. Sci. Technol. 2010, 44 (23), 8983−8988. (8) Taylor, H. F. W. Cement Chemistry; Thomas Telford Publishing: London, 1997; p 459. (9) Brown, D. R.; Grutzeck, M. W. The synthesis and characterization of calcium aluminate monoiodide. Cem. Concr. Res. 1985, 15 (6), 1068−1078. (10) Atkins, M.; Beckley, A. N.; Glasser, F. P. Influence of cement on the near-field environment and its specific interactions with uranium and iodine. Radiochim. Acta 1988, 44 (5), 255−261. (11) Kuzel, H.-J. Zur Frage der Mischkristallbildung von Calciumaluminiumhydroxysalzen. Neues Jahrb. Mineral., Monatsh. 1971, 477−491. (12) Atkins, M.; Glasser, F. P.; Kindness, A. Cement hydrate phases solubility at 25-degrees-C. Cem. Concr. Res. 1992, 22 (2−3), 241−246. (13) Matschei, T.; Lothenbach, B.; Glasser, F. P. Thermodynamic properties of Portland cement hydrates in the system CaO-Al2O3SiO2-CaSO4-CaCO3-H2O. Cem. Concr. Res. 2007, 37 (10), 1379− 1410. (14) Renaudin, G.; Rapin, J. P.; Elkaim, E.; Francois, M. Polytypes and polymorphs in the related Friedel’s salt [Ca 2 Al(OH)6]+[X·2H2O]− halide series. Cem. Concr. Res. 2004, 34 (10), 1845−1852. (15) van Beek, W.; Safonova, O. V.; Wiker, G.; Emerich, H. SNBL, a dedicated beamline for combined in situ X-ray diffraction, X-ray 3881


Structural Insight into Iodide Uptake by AFm Phases - ACS Publications

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