Application of layered double hydroxides for 99Tc remediation

Applied Clay Science 176 (2019) 1–10

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Research Paper

Application of layered double hydroxides for a,b,⁎,1

c

b

99

Tc remediation

c

d

a,e

N. Daniels , C. Franzen , G.L. Murphy , K. Kvashnina , V. Petrov , N. Torapava , A. Bukaemskiyb, P. Kowalskib, H. Sib, Y. Jib, A. Hölzera, C. Walthera

T

a

Institute für Radioökologie und Strahlenschutz, LUH, 30419 Hannover, Germany Institute of Energy and Climate Research (IEK-6), Nuclear Waste Management and Reactor Safety, Forschungszentrum Jülich GmbH, 52425 Jülich, Germany Institute of Resource Ecology, Helmholtz Zentrum Dresden Rossendorf, 01314 Dresden, Germany d Institut für Mineralogie, LUH, 30167 Hannover, Germany e ALS Scandinavia AB, 97775 Luleå, Sweden b c

ARTICLE INFO

ABSTRACT

Keywords: Technetium Layered double hydroxides Uptake Disposal

The present study investigates possible use of Layered Double Hydroxides (LDH) for Tc(VII) remediation. Mg/ Al– and Mg/Fe–LDH were obtained by a hydrothermal route and thermally activated at 450 °C, which was shown to significantly improve the Tc(VII) removal efficiency. Based on XRD investigation of Tc–LDH phases, the uptake Tc(VII) from the solution follows the restoring of a LDH structure. X-ray absorption spectroscopy demonstrates that Tc ions interact solely via the TceO bond, leaving no evidences of farther atomic interactions with, e.g., layers of LDH. The presence of competing anions, like NO3−, or CO32– in the solution decreases Tc (VII) uptake by LDH. Maximum uptake capacity of thermally activated Mg0.67/Al0.33-LDH and Mg0.75/ Fe0.25–LDH were derived from fitting the experimental data into the modified Langmuir equation and correspondingly comprise 0.227 mol/kg and 0.213 mol/kg. In agreement with these findings, theoretical simulations predicted incorporation energies for Mg0.67/Al0.33–LDH and Mg0.75/Fe0.25–LDH of −128 kJ/mol and − 110 kJ/ mol, respectively. Investigation of stability of Tc–LDH in different aqueous solutions demonstrated a rather low release Tc(VII) in contact with diluted solutions containing Cl− and OH−, however, in a high saline solution, like Q-brine a rather fast release of TcO4− occurs due to anion exchange with Cl−.

1. Introduction Technitium (99Tc) is a high yield fission product of uranium (235U) and due to its long life-time (T1/2 = 2.13·105 y) is of considerable concern in the safety assessment of a final disposal site for nuclear waste. In the spent nuclear fuel (SNF), 99Tc is present in different chemical forms, like metallic and oxide precipitates (Kleykamp, 1988; Ewing, 2015). During SNF corrosion, 99Tc is expected to release and stabilized in the form of pertechnetate anion, Tc(VII)O4−, which is well soluble, poorly sorbs on mineral surfaces and hence, can be transported at nearly groundwater velocity (Rand et al., 1999; Dickson et al., 2014). A transport retardation of radionuclides stabilized in anion form, such as 99TcO4−, 129I−, 79SeO42− or 14CO32−, has been identified as an actual challenge of nuclear waste disposal (Curtius et al., 2004; Allada et al., 2005; Curtius et al., 2008; Das et al., 2007; Aimoz, 2012; Baston et al., 2012). A number of methods were recently proposed for remediation of 99Tc, for instance transformation of Tc into a low-soluble

form, e.g. TcO2 or TcS2 (Mezer et al., 1991; Liu et al., 2008), incorporation into iron-alloys (Taylor, 2011), iron oxides and oxy-hydroxides (Um et al., 2012; Smith et al., 2015) and uptake by layered double hydroxides (Wang and Gao, 2006; Gu et al., 2018), functionalized (Rajec et al., 2015) and customized materials (Wang et al., 2012; Sheng et al., 2017; Li et al., 2018; Zhao et al., 2018). Furthermore, layered double hydroxides (LDH) have received much attention in the past decades due to their properties and potential application in a water treatment technology (Xu et al., 2010; Zhao et al., 2010). Due to their high stability in oxidizing environmental conditions and relatively simple and inexpensive synthesis, LDH were also shown to be one of the most promising candidates for quick sequestration of TcO4− from aqueous solutions. Typically, LDH belong to the group of mixed hyx+ [(An-)x/n·mH2O]xdroxides with the general formula [MII(1-x)MIII x (OH)2] , where MII and MIII are divalent and trivalent cations and An- is an anion, compensating the excess charge, x+. LDH consist of planes of positively charged brucite-type M(II, III) sheets that are separated by

Corresponding author at: Institute für Radioökologie und Strahlenschutz, LUH, 30419 Hannover, Germany. E-mail address: [email protected] (N. Daniels). 1 Current address: Institute of Energy and Climate Research (IEK-6), Nuclear Waste Management and Reactor Safety, Forschungszentrum Jülich GmbH, 52425 Jülich, Germany. ⁎

https://doi.org/10.1016/j.clay.2019.04.006 Received 22 November 2018; Received in revised form 1 April 2019; Accepted 8 April 2019 0169-1317/ © 2019 Elsevier B.V. All rights reserved.

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interlayers occupied with anions (e.g. NO3−, CO32−) and some water molecules. An interaction of TcO4− with LDH may occur via different mechanisms, i.e. (i) adsorption on the outer surface, (ii) intercalation by anion exchange and (iii) intercalation through reconstruction. Direct anion exchange is usually characterized by a different selectivity of LDH towards different anions and therefore can significantly limit Tc uptake, particularly if LDH are in their thermodynamically more stable carbonate-form, i.e. when intercalated with CO32– ions (Bontchev et al., 2003; Gillman et al., 2008). An exchange capacity for TcO4− may be further improved by thermal “activation”, in which all initial intercalating anions are eliminated by calcination. The resulting product is an amorphous mixed oxide phase, which is able to restore the LDH structure in contact with aqueous solution. Extended anion sequestration by thermally activated LDH was already demonstrated for ClO4−, F−, I− and IO3− (Ma et al., 2011; Aimoz et al., 2012; Yang et al., 2012; Iglesias et al., 2016). Thus, an uptake of TcO4− during LDH reconstruction is expected to be an effective uptake mechanism. This work focuses on understanding the interaction of 99Tc(VII) with Mg/Al– and Mg/Fe–LDH. In particular, a quantitative assessment of TcO4− uptake by these LDH on different conditions was carried out in batch experiments. The effects of thermal treatment and aqueous solution composition, as well as impact of CO2 from ambient atmosphere on the 99Tc(VII) uptake by LDH were evaluated. An insight into the atomic scale environment of 99Tc when adsorbed on LDH was obtained by means of X-ray absorption spectroscopy and complementary density functional theory-based simulations. A stability of LDH–Tc(VII) was verified in batch leaching tests using solutions of low and high salinity.

hydrogel. Afterwards, the solution was transferred into an autoclave where it was subjected to hydrothermal synthesis at 120 °C for 10 h. When the synthesis was completed, the resulting white suspension was filtered by a paper filter (blue line), washed with 200 mL of deionized water and dried at 80 °C until the mass did no longer change. The final product was denoted as HTC-80. Pyroaurite, Mg/Fe–LDH, was prepared according to Kang et al. (2013): 9.1 g of NaOH and 0.17 g of Na2CO3 were dissolved in 70 mL of DW. In a separate vial 19.8 g of Mg(NO3)2∙6H2O and 10.6 g of Fe (NO3)3∙9H2O were dissolved in 87.15 mL of DW. Afterwards, under constant stirring, FeeMg solution was added dropwise (0.1 mL/min) to the alkali‑carbonate solution. Here, the formation of a dark-red suspension was observed. Stirring was continued for one hour more after the mixing of the solution was completed for better aging of Mg/Fe hydrogel. Subsequently, the suspension was filtered by a paper filter (blue line) and washed with threefold volume of DW to remove remaining NaOH. The dark-red precipitate was dried at 80 °C until mass did no longer change, and the sample denoted as PyA-80. In following, these obtained samples of HTC-80 and PyA-80 were divided into two batches: one was directly used for Tc(VII) uptake experiments, another was calcined at 450 °C for 4 h in order to remove intercalated anions (NO3− and OH−) and H2O. The final calcined products, correspondingly denoted as HTC-450 and PyA-450, were used in Tc(VII) uptake experiments along with as-synthesized HTC-80 and PyA-80. The content of Al, Mg, and Fe concentration in the synthesized LDH solids was determined by ICP-OES using a Thermo Scientific instrument (iCAP 6000 Series): known amounts of HTC and PyA were dissolved in 20 mL of concentrated HClO4 at 80 °C until complete dissolution and consequent dilution of the aqueous solutions was observed. Nitrate anions in the solids were analysed by spectrophotometry after separation by ion chromatography (Ion Chromotograph ICS-2000 from DIONEX with DIONEX RFIC IonPac AS20 2 × 250 mm chromatographic column). The amount of H2O in as-synthesized LDH was estimated from the mass balance. The concentration of CO32– anions was calculated accounting for the general structure of LDH [MII1III x+ [Am-1 x/m∙nH2O] and the electro-neutrality rule. For dexMx (OH)2] termination of the structure of calcined LDH, it was also assumed that interlayer H2O, NO3− and OH– anions are completely removed at 450 °C (Yang et al., 2002; Iglesias et al., 2016), whereas Mg, Al and Fe are in the oxide form: MgO, Al2O3 and Fe2O3. The maximum loading capacity of LDH, Qmax (charge-eq/g) was calculated using the following equation:

2. Experimental 2.1. Materials Deionized water (DW) was used for synthesis. For uptake and leaching experiments with 99Tc, DW water was boiled for 3 h to remove dissolved O2 and CO2. Reagents for synthesis of LDH were of analytical grade and obtained from Merk: Mg(NO3)2∙6H2O, Al(NO3)3∙9H2O, NaOH and Na2CO3. Concentrated NH4OH (30%) was purchased from Aldrich. Re(VII) stock solution (2.22∙10−2 M) was prepared by dissolution of 0.31 g of NaReO4 (Roth, Germany) in 50.86 mL of DW. A stock solution of 99Tc was prepared by dissolution of 51 mg K99TcO4 in 25 mL of MilliQ water (18.2 MΩ) so the final concentration of stock solution comprised 10 mmol/L. This solution was used for uptake experiments. The 99 Tc concentration in the filtrate was determined by liquid scintillation counting (LSC) with a TriCarb 2500TR/AB instrument (CanberraPackard) and Ultima Gold XR scintillation cocktail (Perkin Elmer).

Qmax =

n(MIII ) Mr III

(1)

where n(M ) of Fe or Al is an excess positive charge of LDH sheets respectively in PyA-80 or HTC-80, in charge-eq/mol, defined as a corresponding mol fraction of Fe or Al, Mr. is a molar mass of LDH estimated from the elemental analysis, g/mol. Powder X-ray diffraction (XRD, Brucker D4 Endeavor diffractometer) was used to characterize the synthesized LDH solids as well as Tc(VII)–LDH. The XRD patterns of LDHs were recorded with CuKα radiation (λ = 1.5418 Å) at ambient temperature in the 2Θ-range from 7 to 70° with 0.02° step size and 2 s recording time for every step. The powder XRD data collected for two as-synthesized LHD (HTC-80 and PyA-80) were analysed by pattern matching (Lebail method) using the program FullProf. These were modelled using hexagonal models (space group R3m ). The peak shapes were modelled using a pseudoVoigt function and the background using a shifted Chebyshev function. The zero point, lattice parameters and peak profile parameters were refined together. The refined lattice parameters were used to extract the Miller indices. Due to the similarities in the XRD-patterns for as-synthesized LDH and LDH reconstructed in the Tc(VII)-containing solution, the extracted Miller indices were further used for calculation of lattice parameters, a and c in other samples, i.e. Tc-HTC-450, Tc-HTC-

2.2. Synthesis and characterization of LDH Syntheses of LDH phases were performed in a teflon reactor to avoid any additional mineral cross-contamination. The design of the reactor vessel was adopted from (Rozov et al., 2010), yet without a pH controller and automated titrator. Instead, a peristaltic pump was used to provide a constant flow of the feed solution. The setup allows control of pH (Metrohm) and redox potential (Metrohm). For syntheses on CO2free conditions the solutions were constantly bubbled with N2. Additionally, temperature control (heating) and constant stirring is possible. Two types of LDH, Mg/Al–LDH and Mg/Fe–LDH, were synthesized for Tc uptake tests. Hydrotalcite, Mg/Al–LDH, was prepared according to the method described in Bontchev et al. (2003): 9.8 g of Mg (NO3)2∙6H2O and 4.7 g of Al(NO3)3∙9H2O were dissolved in 150 mL of DW. At 25 °C, under constant stirring and purging of N2 gas (to avoid CO2 incorporation), concentrated NH4OH was added dropwise to the Mg/Al solution until pH reached 8.5. White milky suspension of Al (OH)3·Mg(OH)2∙nH2O was stirred for another hour for aging the 2

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450(CO2), Tc-PyA-450 and Tc-PyA-450(CO2). For that the relationship between interlayer distances (dhkl), the Miller indices and lattice parameters, a and c (for hexagonal lattice) was applied (Shaskolskaya, 1984; De Graef and McHenry, 2012). In particular, reflexes (003) and (006) were used for calculation of c, whereas the cell parameters a = b were determined from the (012), (015), (018) and (110) reflexes. Accounting for more than one reflex in calculation of cell parameters also allowed for uncertainty estimation. The interlayer spacing d was calculated from relationship d = c/3. 2.3. Uptake tests with

99

(EXAFS) measurement. For 99Tc(VII) adsorbed on HTC-450 and PyA-450 phases, XANES) and EXAFS spectra were acquired in fluorescence mode at the Tc K-edge (21,044 eV) at the Rossendorf Beamline at ESRF (ROBL), Grenoble, France. The energy of the Si(111) double-crystal monochromator was calibrated using a Mo foil (edge energy 20,000 eV). During the measurement, the samples were kept at 15 K in a closed-cycle He cryostat to prevent photon-induced changes of oxidation state and to improve spectral quality by reducing thermal disorder (Debye–Waller effect) (Kirsch et al., 2011). The EXAFS data were fitted with EXAFSPAK using theoretical backscattering amplitudes and phase shifts calculated with FEFF 8.2. The XANES spectra were compared with a reference spectra of Tc(VII)O4– (Lukens et al., 2005).

Tc(VII) and Re(VII)

Two different effects on Tc(VII) uptake on LDH were investigated in this work: (1) the effect of intercalated anions and (2) the effect of CO2. In the experiment (1) Re(VII), an inactive chemical analogue of 99Tc (VII), was used. For that ca. 50 mg of fresh synthesized and calcined LDH were mixed with 45 mL of standard Re(VII) solution (5∙10−5 mol/ L) and left for equilibration at room temperature and ambient atmosphere. Kinetics of Re(VII) uptake was determined by repeated sampling of the solution, filtering through 0.2 μm membrane filters and concentration determination of dissolved Re(VII) by ICP-OES. In the experiment (2), the influence of CO2 on 99Tc(VII) uptake on HTC-450 and PyA-450 was investigated by performing parallel experiments on air and N2 atmosphere (using a glove box). Similar to experiment (1) ca. 50 mg of calcined LDH were mixed with 30 mL of 99 Tc(VII) standard solution (5∙10−5 mol/L). Kinetics of Tc(VII) uptake were determined by regular sampling of the solution and measuring the fraction of dissolved 99Tc by LSC. To improve the separation of solution and LDH-particles, the samples were filtered through 0.2 μm membrane filters. pH and Eh of the solutions were controlled in the beginning (t0) and each time the samples were probed. The Tc(VII)-uptake capacity of HTC and PyA were evaluated from the sorption isotherms. The calcined LDH (ca. 50 mg) were equilibrated with the 99Tc(VII) solutions of different concentrations ranging from 1∙10−6 to 1∙10−3 mol/L at constant pH 10.3 ± 0.2 on CO2-free conditions. After final equilibration (equilibration time was preliminary defined in experiment (2)), the concentration of dissolved Tc(VII) was determined by LSC. Equilibrium adsorption of Tc on LDH (Ads, g/kg) was calculated by equation. (2):

Ads =

(A 0

2.5. Desorption study Batch-leaching experiments were conducted using 0.5 g powder samples of synthesized and calcined LDH phases with adsorbed Tc(VII). These were contacted with 40 mL of leaching solution so that final solid-to-solution ratio was 12.5 g/L. The effect of CO2 was studied by conducting parallel experiments under N2 and ambient atmosphere. Leaching behaviour of 99Tc was studied in three different types of leaching solution: 1∙10−4 M NaOH (pH 10), 0.1 M TRIS buffer with pH 8.1 (Goldberg et al., 2002) and Q-brine with pH 5.5 (Schuessler et al., 2001). After contacting the solids with the leaching solutions, the pH was monitored and systems were left for equilibration under continuous shaking. For each of the systems an aqueous sample (ca. 1 mL) was periodically collected, filtered with 0.2 μm membrane filters and submitted for analysis of dissolved 99Tc using LSC. In case of high saline solutions, a correlation of measured count rates with quenching factor was made. The time-dependent release fraction of Tc (Rft) was calculated according to equation (3):

Rft =

2

100%

(3)

where m0 is the initial amount of Tc bonded to the LDH phase in mg, mt is the amount of 99Tc released into the solution at a time-interval, t, in mg. 2.6. Computational methodology

(2)

Simulations of Tc(VII) incorporation in LDH were performed using the plane-wave Quantum-ESPRESSO package based on density functional theory (Giannozzi et al., 2009). The plane-wave energy cutoff was set to 30 Ryd and the core electrons were represented by ultrasoft pseudopotentials (Vanderbilt, 1990). The calculations were performed with PBE (Perdew et al., 1996) and PBEsol (Perdew et al., 2008) exchange correlation functionals. The PBEsol functional was selected as it by design results in much better description of the structures. The LDH phases were represented by 3 0.25+ layers thick, periodically repeated slabs of [Mg0.75MIII 0.25(OH)2] composition (where MIII represents either Fe or Al). The structural refinement was performed based on the model and parameters reported in Radha et al. (2007). The reference interlayer anion was OH– (one per elemental unit Mg3MIII of 3R1 polytype) which was subsequently replaced by a TcO4− anion. The reference super-cell structure contained 66 atoms and the structures with 1 TcO4− anion contained 69 atoms. The pairs of cations (Mg/Al) and (Mg/Fe) were randomly distributed in the cation layers keeping the 1:3 ratio in each layer. In order to better reproduce the experimental conditions the 2:1 LDH structure [Mg0.66Al0.33(OH)2]0.33+ was computed as well. The reference supercell structure (Mg2Al per elemental unit) in this case consisted of 48 atoms. To achieve convergence in the Brillouin space, a 4x4x1 MethfesselPaxton k-points grid was applied (Methfessel and Paxton, 1989). The

where A0 is the 99Tc specific activity of the blank solution without LDH in Bq/mL, At the equilibrium 99Tc specific activity of the solution in Bq/ mL, T1/2 the half-life time of 99Tc (6.65∙1012 s), M is the molar mass of 99 Tc in g/mol, msol a mass of aqueous solution in contact with LDH in mL, mLDH is a mass of solid LDH phase (freshly synthesized or calcined) in kg. 2.4. X-ray absorption spectroscopy of

mt mt

99

At ) T1 M m sol ln 2 NA mLDH

m0

99

Tc-LDH

The samples of Tc–HTC and Tc–PyA were prepared by equilibration of approximately 300 mg of LDH phase (preliminary calcined at 450 °C) with 40 mL of 99Tc(VII) standard solution (5∙10−4 M). In total four samples were prepared for X-ray absorption spectroscopy (XAS) measurement: two in ambient atmosphere and two more in N2 atmosphere (using a glove box). After an equilibration time of 3 weeks, the samples were centrifuged (at 3000 g for 30 min) to separate Tc–LDH solids from the solution, the supernatant was decanted and analysed to determine the degree of Tc(VII) uptake. In all cases, the uptake was of the order of 1.3∙106 Bq/sample (or ca. 6600 ppm). The solid phases were subsequently separated by centrifugation, put into double confined sample holders and flash-frozen in liquid N2 to avoid any structural changes during transport and set up of the samples for X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure 3

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disappearance of the main reflexes, except for three: at d 2.549, 2.1108 and 1.4919 Å. This is consistent with the XRD pattern of MgO (periclase) and a minor mount of spinel Al2MgO4. Earlier observations by Kanezaki (1998), Yang et al. (2002) and Kang et al. (2013), who studied the evolution of XRD patterns of HTC as a function of temperature by in situ HTXRD, demonstrated that annealing of LDH at 450 °C causes a removal of interlayer water, OH– groups and intercalated anions (in our case it is NO3− and CO32−) and results in collapsing of layered brucitelike structure. A complete removal of NO3− anions agrees well with the results of elemental analysis of calcined phases given in Table 1, however, the calculations of stoichiometry of thermally treated HTC450 and PyA-450 phases indicated that some traces of CO32– anions might be still present. According to Kanezaki (1998) a complete decarbonization of HTC occurs in the temperature region between 405 °C and 580 °C and may result in a mixed oxide of MgO and Al2O3. The XRD pattern of PyA-450 demonstrated three distinct reflections at d 2.4494, 2.1118 and 1.4933 Å, matching well with the XRD pattern of MgO (periclase) and mixed oxide; no reflections typical for spinel MgFe2O4 could be identified. A slow removal of CO32- anions from PyA was observed by Fernandez et al. (1998) in the temperature interval from 400 °C to 500 °C, whereas further annealing of PyA at temperatures higher than 500 °C results in the formation of the stable spinel phase MgFe2O4 and MgO. As such annealing of LDH at a temperature of 500 °C and higher likely results in the irreversible formation of spinel phases, this is considered to be unfavorable for the ability of LDH to reconstruct the layered structure again in contact with aqueous solution and consequently, lowering their uptake capacity.

Table 1 Determined concentrations of Mg, Al, Fe and NO3- anion in the as-synthesized and thermally treated LDH (in g/g). Mg/Al–LDH (hydrotalcite) Phase

Mg

Al

NO3−

HTC-80 HTC-450

0.1871 0.3383

0.1044 0.1779

0.1833 2.2∙10−3

Mg/Fe–LDH (pyroaurite)

PyA-80 PyA-450

Mg

Fe

NO3−

0.2492 0.3413

0.1555 0.2283

0.0509 4.2∙10−3

entropies needed for estimation of the reaction standard free energy were acquired from Shock et al. (1997). For solids, thermodynamic parameters were estimated from parameters acquired from Latimer (1951) and Spencer (1998). 3. Results and discussions 3.1. Characterization of bare LDH phases The results of elemental analysis and LDH compositions by ion chromatography are summarized in Table 1. Based on the results of elemental analysis, the stoichiometric formulae of as-synthesized LDH and thermally treated products were found:

HTC

80: [Mg 0.67Al 0.33 (OH)1.3 ](NO3 )0.3 (CO3 )0.04 1.01H2 O.

HTC

450: [Mg 0.68Al 0.32O1.33 ](CO3 ) 0.08 .

PyA

80: [Mg 0.78Fe 0.22 (OH)1.57 ](NO3 )0.06 (CO3 ) 0.13 0.42H2 O.

PyA

450: [Mg 0.78Fe 0.22 O1.11 ](CO3 ) 0.06 .

3.2. Tc(VII) uptake on LDH The process of reconstruction of LDH after calcination is not well understood. Particularly, whether thermally activated LDH restores the layered brucite-like structure in the presence of Tc(VII)O4– anion and consequently accommodate it as an intercalated anion. Should LDH remain in the solution as mixed oxide (or hydroxide) phase, anion incorporation takes place to a much lesser extent. To elucidate these processes, XRD patterns of Tc–HTC-450 and Tc–PyA-450 were recorded in ambient and CO2-free atmosphere. As a reference, the structure of thermally activated LDH equilibrated with DW was investigated. Corresponding patterns of Tc-bearing LDH and reference bare LDH are presented in Fig. 1A and B. XRD patterns collected for the reference phases PyA-80 and HTC-80 were analysed using pattern matching as implemented in program FullProf and fitted against hexagonal models (space group R3m ). This is consistent with the results of Radha et al. (2007). Both Tc–HTC samples reveal patterns having (003) and (006) reflections, which is typical for layered LDH structure of 3R1 polytype and confirms the restoring of the brucite-like layers after annealing. The a parameter of the hexagonal lattice (Table 3) is observed to vary only subtly for all three phases, whereas the c parameter increases from reference HTC to Tc–HTC(CO2) and Tc–HTC. This corresponds to a slight shrinkage in the stacking direction, when a CO32– anion is intercalated as compared to a TcO4− anion. In case of identically-charged anions, the spacing between the layers is affected by the anion size (Costa et al., 2012; Costantino et al., 2014). In our case, the electrostatic interaction between the layers and an anion (i.e. the charge of anion) most likely plays a more significant role in the spacing parameter change. The larger tetrahedral TcO4− anion of radius 2.41 Å (Williams and Carbone, 2015) carries less negative charge than the flat doubly charged CO32– anion of radius 1.89 Å (Roobottom and Jenkins, 1999). Additionally, the spacial orientation of the CO32– anion parallel to the basal planes of LDH allows all three oxygens atoms to interact with the hydroxyl groups of the brucite-layers. Hence, the charge compensation of positively-charged brucite layers becomes more effective and, as a consequence, the layers are subjected to stronger attractive forces, exhibiting less space in between the stacking planes.

Elemental analysis revealed the MII/MIII ratios to be around 2.0 for HTC and 3.5 for PyA phases. Although synthesis of LDH was carried out under constant flow of N2, both low-temperature phases, HTC-80 and PyA-80, were found to have NO3- and CO32- as intercalated anions. Higher amounts of CO32- in PyA compared to HTC are due to the synthesis route with use of some Na2CO3. Results of XRD examination summarized in Table 2 demonstrate that both low-temperature phases HTC-80 and PyA-80 have typical layered structure. Reflexes of HTC-80 and PyA-80 obtained by powder XRD were found to be in a good agreement with the XRD pattern of respective LDH phases reported earlier by Fernandez et al. (1998), Kanezaki (1998), and Rozov et al. (2010). The Miller indices, extracted from the refinement with the program FullProf are consistent with the pattern of the 3R1 polytype (Radha et al., 2007). Elemental analysis of the calcined phases revealed complete removal of interlayer water and NO3−and partial removal of CO32– anions. Calcination of HTC at 450 °C results in almost complete Table 2 Indices and peak positions on the XRD patterns of as-synthesized LDH phases. LDH

HTC-80 PyA-80

d, Å (003)

(006)

(012)

(015)

(018)

(110)

(113)

7.705 7.936

3.874 3.971

2.585 2.631

2.307 2.345

1.963 1.999

1.528 1.557

1.501 1.528

4

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Fig. 1. XRD patterns of as synthesized bare LDH (a), thermally activated at 450 °C (b), LDH equilibrated with Tc(VII) solution on CO2-free conditions (c) and on ambient conditions (d): A – HTC, B – PyA. Table 3 Calculated lattice parametersa of reference HTC-80 and Tc–HTC-450, obtained by reconstruction routine on inert and ambient (in presence of CO2) conditions.

Table 4 Calculated lattice parametersa of reference PyA-80 and Tc–PyA-450, obtained by reconstruction routine on inert and ambient (in presence of CO2) conditions.

Parameter

HTC-80

Tc–HTC-450

Tc–HTC-450(CO2)

Parameter

PyA-80

Tc–PyA-450

Tc–PyA-450(CO2)

a, Å c, Å da, Å

3.0634 ± 0.0004 23.282 ± 0.004 7.761 ± 0.001

3.071 ± 0.007 23.73 ± 0.04 7.91 ± 0.013

3.061 ± 0.003 23.54 ± 0.06 7.85 ± 0.02

a, Å c, Å d, Å

3.089 ± 0.001 23.288 ± 0.004 7.763 ± 0.001

3.108 ± 0.006 24.10 ± 0.10 8.03 ± 0.03

3.112 ± 0.003 24.25 ± 0.09 8.08 ± 0.003

a

a

Calculated based on the hexagonal lattice.

In agreement with the previous discussion, the c parameter for Tc–HTC-450(CO2) lies between HTC-80 and Tc–HTC-450. Reflexes (003) and (006) on the Tc–HTC-450(CO2) pattern are also located between those of reference HTC-80 and Tc–HTC-450, hinting at an averaged pattern, when both TcO4− and CO32– anions are intercalated simultaneously. Similar findings were made for PyA samples. As it is seen from Fig. 1B, reflexes typical for LDH structure can be recognized in all three XRD patterns, corroborating the hypothesis of the layered structure being formed by reconstruction. Similarly, for the case of PyA, the a parameter, shown in Table 4, changes only marginally with intercalation of the TcO4− anion. The difference is not statistically significant. The c parameter of both solids containing the TcO4− anion was found to be higher as compared to the as-synthesized phase intercalated only with the CO32– anion, in good agreement with the trend obtained for HTC solids. However, in contrast to HTC, the shrinkage in spacing for PyA intercalated with both CO32– and TcO4− anions was not observed; this solid revealed the largest c parameter among PyA samples.

Calculated based on the hexagonal lattice.

Consequently, the c parameters increase in the following order: PyA80 < Tc-PyA-450 < Tc-PyA-450(CO2). In the preliminary uptake tests with Re(VII), acting as a stable (inactive) analogue of 99Tc(VII), it was demonstrated, that the background electrolyte and thermal treatment both affect significantly the efficiency of Re(VII) removal, oiunting out at a similar issue for Tc(VII) uptake. The results, described in Fig. 1 of the Supplementary Information (SI), indicate a systematically lower removal of Re(VII) by “as-synthesized solids” compared to the “thermally activated” LDH. In most cases, a prior removal of intercalated anions by high temperature treatment significantly improved Re(VII) (and presumably Tc(VII)) sequestration from the solution. Moreover, the Re(VII) uptake by thermally treated LDH, i.e. HTC-450 and PyA-450, from DW was systematically higher than from the solution containing 1 mM NaNO3. These results indicate a competition for interaction with LDH may occur for NO3− and TcO4− anions, if they are simultaneously present in the solution and or NO3− is intercalated in LDH. For that reason the subsequent experiments on Tc(VII) uptake were carried out only with 5

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Fig. 2. Uptake kinetics of Tc(VII) on LDH from DW on ambient and N2 atmosphere: C(Tc)tot = 5·10−5 M, C(LDH) = 2 g/L, pH 10, contact time 604 h.

“thermally activated” LDH: HTC-450 and PyA-450, and in the DW as an aqueous media. A rather strong effect of ambient atmosphere (in particular CO2) on Tc(VII) uptake by LDH was observed. As it is shown in Fig. 2, for both HTC-450 and PyA-450 the uptake is significantly higher in CO2-free atmosphere. In the absence of CO2, 99Tc(VII) uptake efficiency is comparable for both LDH phases (white bars). On the ambient conditions, CO2 dissolves in the solution, producing CO32– anions, and the latter compete with TcO4− for sorption on LDH (black bars). Moreover, on ambient atmosphere, the Tc(VII) uptake on HTC-450 exceeds the one for PyA-450 by a remarkably high factor of four. The exact reason for this behavior is not immediately clear. It seems that while restoring its structure in the solution, PyA-450 retains a stronger affinity to intercalate CO32– anions, compared to HTC-450. Consequently, the effect of competing CO32– on Tc(VII) is more pronounced for PyA-450. Following on from the discussed result, the synthesis of Cl− or NO3− intercalated LDH can be considered more appropriate for improving Tc(VII)-uptake efficiency. As it was mentioned above, additional calcination would significantly increase a removal performance of LDH towards 99Tc(VII). The maximum sorption capacity of Tc(VII) on thermally activated LDH was measured by means of sorption isotherms and evaluated using two different equations: Langmuir and Freundlich isotherms, represented by equations (4)) and (6), respectively. The Langmuir isotherm represents the relationship between the number of active sites of the surface undergoing adsorption and the dissolved equilibrium concentration of sorbate.

Ads = Ads max

whereas the intercept equals 1/(Adsmax∙Kads), enabling evaluation of maximum sorption (Adsmax) and sorption constant (Kads) (Limousine et al., 2007). The Freundlich isotherm is an empirical relation between the equilibrium of sorbate concentration in the solution and on the surface: 1/n Ads = KF Ceq

Where Ads is an amount of adsorbed Tc(VII) in mol/kg, KF is a constant for relative adsorption capacity in L/kg, n is an affinity constants at a particular temperature and related to energy of adsorption (Limousine et al., 2007). In our case the process was evaluated only at room temperature. The n-values between 1 and 10 indicate a favourable sorption of TcO4−. This equation can be also written as:

logAds = log KF +

(4)

Where Ads is the amount of adsorbed Tc(VII) in mol/kg, Adsmax is the maximum amount of adsorbed Tc(VII) in mol/kg, Kads is an adsorption constant related to the sorption-desorption energy and to the affinity binding sites for TcO4− in mol/L, Ceq is the equilibrium concentration of Tc(VII) in the solution in mol/L. This equation can be rewritten in the following way:

Ceq Ads

=

Ceq 1 + Ads max KAds Adsmax

1 log Ceq n

(7)

Similar to equation (4), a linear dependence of log(Ads) vs. log(Ceq) complies with sorption obeying the Freundlich's law. The sorption data were fitted with equations (5) and (7) in order to estimate the character of Tc(VII) sorption on thermally activated HTC450 and PyA-450 (Fig. 3A and B respectively). The data were compared to the processed results on Tc(VII) sorption on as synthesized HTC in CO32– obtained by Curtius et al. (2004). As it can be seen from the Fig. 3A, the data fitted into the Langmuir law are plotted as linear dependences with a confidence limit of 98% in case of HTC-450 and 96% for PyA-450. Fitting of experimental data from (Curtius et al., 2004) resulted in 78% confidence (summarized in Table 5), which strongly suggest that sorption does not obey the Langmuir's law. For that reason fitting parameters for data of Curtius et al. (2004) were not evaluated. The maximum capacity, Adsmax, for Tc(VII) uptake, evaluated from the modified Langmuir equation was 0.227 mol/kg for HTC-450 and slightly lower for PyA-450, 0.213 mol/kg. The same trend was observed for the adsorption constant, Kads: it is higher for HTC-450, signifying that Mg/Al–LDH has a higher affinity towards TcO4− incorporation than Mg/ Fe–LDH, if no other competing anions are available. This is however due to the higher MIII to MII ratio in HTC, which provides for a higher layer charge density (Forano et al., 2013). It should to be noted that none of isotherms in this work, and in Curtius et al. (2004), reached a sorption plateau, i.e. fitting the sorption data with Langmuir's law may not be appropriate. For that reason additional fitting of sorption data was performed using Freundlich's law, which is a better option for the isotherms, where the clear limit of sorption is not reached (Limousine et al., 2007). The derived sorption data were also compared to the results of Tc(VII)

KAds Ceq 1 + KAds Ceq

(6)

(5)

This form of the Langmuir equation is used for fitting an isotherm in order to verify whether the sorption process obeys Langmuir's law, i.e. adsorption occurs in a monolayer with no interaction of neighbouring adsorption sites. For this reason Ceq/Ads is linearly proportional to Ceq. If so, the slope of the linear dependence will be equal to 1/Adsmax, 6

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Fig. 3. Fitting of sorption isotherms for Tc(VII) on LDH with modified Langmuir (A) and Freundlich (B) equations. Table 5 Fitting parameters for Tc(VII) sorption on LDH according to the Langmuir (Eq. (5)) and Freundlich's equations (Eq. (7)). Fitting parameters

HTC PyA HTCa a

Langmuir's law

Freundlich's law 2

Adsmax, mol/kg

Kads, mol/L

R

2.27∙10−1 2.13∙10−1 n.d.b

9.13∙104 5.13∙104 n.d.

0.98 0.96 0.78

KF, L/kg

n

R2

9.2∙103 5.2∙103 3.1∙102

1.17 1.22 1.24

0.98 0.99 0.97

Data of Curtius et al. (2004); bnot determined.

interaction with hydrotalcite reported by Curtius et al. (2004). The linear plots in the modified Freundlich coordinates, i.e. log(Ads) vs. log(Ceq), shown in Fig. 3B, demonstrated higher confidence limits, than for Langmuir fit. Values of KF and n calculated from the slope and intercepts of linear plots are summarized in Table 5. Parameter KF, which is related to the sorption capacities, for thermally activated HTC-450 and PyA-450 is at least an order of magnitude higher than that of as-synthesized HTC, reported by Curtius et al. (2004). Obviously such a difference originates from the removal of intercalated anions during thermal treatment, which decreases the amount of competing anions, such as CO32– and simplifies an uptake for Tc(VII). However, in the investigation by Curtius et al. (2004), as-synthesized LDH phase was used in CO32– form. Since LDH in general have a higher affinity towards the CO32– anion, these incorporated anions are competing strongly with TcO4− anions and thus, hindering the incorporation. The values of n in all cases are close to 1, which is considered to be favourable for sorption of TcO4− on LDH (Das et al., 2007). An important point of characterization of LDH uptake capacity can be a comparison of evaluated maximum capacity, Adsmax, with an estimated theoretical loading capacity, Qmax (charge-eq/g). The latter can be calculated as the maximum amount of monovalent cations that can be intercalated by 1 g of LDH. The values of 3.87∙10−3 and 2.78∙0.7−3 charge-eq/g were respectively obtained for HTC-80 and PyA-80. It clearly shows that the parameter Qmax is generally higher for HTC, as LDH has higher MIII/MII ratio. The sheets of HTC carry higher positive charge, which correspondingly requires higher amounts of counter charged ions to neutralize the system. Comparison of these values with experimentally evaluated Adsmax, shown in Table 5, demonstrates that substitution of interlayer anions with TcO4− in HTC-80 is only possible up to 5.8% of available anionic capacity. Similar calculations for PyA80 resulted in 7.7%. It should be noted, that a direct comparison of loading capacity of as synthesized LDH and thermally treated LDH, where the structure has been restored in solution, may not be entirely

Fig. 4. Technetium K-edge XANES for Tc-bearing LDH phases obtained by Tc (VII) sorption onto thermally activated HTC-450 and PyA-450 on ambient and CO2–free (inert) conditions.

correct as the uptake mechanisms in these two cases are different, namely a substitution of anions in case of HTC-80 and PyA-80 and reconstruction of LDH structure and intercalation in case of HTC-450 and PyA-450. 3.3. Characterization of Tc-LDH structure and stability The Tc XANES spectra are plotted in Fig. 4 and the edge positions are listed in Table 6. Typically, the oxidation state is determined by comparing the edge positions, which in case of Tc(VII) and Tc(IV) are roughly 7 eV apart: spectra of reference compounds yield edges of 21,027.9 and 21,035.0 eV respectively (Almahamid et al., 1995; Wharton et al., 2000). For Tc it is rather difficult to estimate the exact position of the edge peak, therefore Tc(VII) was determined by a distinct pre-edge peak in the region of 21,045–21,050 eV (Saeki et al., 2012; Marshal et al., 2014), which does not appear in the Tc(IV) Table 6 Average bond distances, d, Å, coordination number, N, and Debye-Waller, σ2, factors in the EXAFS studies of Tc(VII)–LDH.

7

Sample

Bond

N.

d, Å

σ2, Å2

S02

∆E

F, %

Tc–HTC-450 inert Tc–HTC-450 amb. Tc–PyA-450 inert Tc–PyA-450 amb.

Tc Tc Tc Tc

4 4 4 4

1.727(16) 1.719(16) 1.717(16) 1.716(16)

0.0023 0.0012 0.0018 0.0025

0.86 0.85 0.87 0.89

−0.7 0.7 2.3 2.0

61 24 27 29

– – – –

O O O O

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Table 7 The TceO bond lengths and standard thermodynamic data for TcO4− incorporation into Mg/Al– and Mg/Fe–LDH, according to equation (8). LDH structure

Tc–O bond length, (Å)

ΔH°r, (kJ/ mol)

ΔG°r, (kJ/mol)

(Mg0.75/Fe0.25): 1 TcO4 (Mg0.75/Al0.25): 1 TcO4 (Mg0.66/Al0.33): 1 TcO4

1.720 1.720 1.720

−159 −155 −178

−110 −105 −128

(−202 kJ/mol) were taken into account (Smith, 1977; Williams and Carbone, 2015). The computed TceO bond lengths are given in Table 7. These values match well to those measured by XAS, and hence, support the mechanism of TcO4− incorporation in the interlayer space of LDH. The obtained incorporation energies, indicate that incorporation of Tc (VII) into Mg0.66/Al0.33–LDH is more favourable than into Mg0.75/ Fe0.25–LDH, which is in agreement with the experimentally determined higher uptake of Tc(VII) on HTC compared to PyA. Simulations also demonstrate a decrease in incorporation free energy (i.e. higher stability of Tc–LDH) with increasing of MIII/MII ratio, which is in agreement with an increased layer charge density (Forano et al., 2013). Comparison of the Al/Mg– and Fe/Mg–LDH with the same MIII/MII ratio, though, indicates a slightly higher stability of Tc(VII) – Fe /Mg-LDH. Similar calculations of complete exchange of OH– for TcO4− resulted in the large positive ΔG° values, which indicate impossibility of such scenario. Only partial substitution of interlayer anions with TcO4− is thermodynamically favourable. This conclusion is also in line with our experimental results on maximum capacity (Adsmax) and its comparison to Qmax. Adsmax for HTC-450 and PyA-540 respectively comprise 6% and 8% of theoretically estimated maximum capacity Qmax.

Fig. 5. EXAFS function of pertechnetate (TcO4−) containing LDH. Open symbols – experimental data, solid line – the best fit; the EXAFS data are offset by +10.

3.4. Desorption study The stability of Tc-LDH phases at different conditions, e.g., ambient or N2 atmosphere, and in leaching solutions different in composition and pH, was investigated in batch leaching tests. Here only thermally activated LDH were used to prepare parent Tc– and Re–LDH for leaching tests. The leaching of Re(VII) was found to proceed fairly fast compared to Tc(VII) (not shown). Almost complete release of Re(VII) from LDH occurs after a week of leaching already in diluted NaOH (1∙10−4 M), whereas the released fraction of Tc(VII) within this timeframe was below 0.05%. This remarkable differences in Re and Tc desorption behaviour clearly demonstrates that using inactive chemical analogues of radionuclides may be misleading. No further leaching experiments were carried out with Re(VII) for simulation of Tc(VII) behaviour. Fig. 7 demonstrates the effect of ambient atmosphere on 99Tc release from LDH. Tc(VII) release is slightly higher in an ambient atmosphere, although does not exceed 0.1% during three months of leach testing. This observation can be considered as advantageous property of LDH capable of retaining 99Tc(VII) even at high alkaline pH values and ambient CO2 partial pressure. Furthermore, a slightly higher stability of Tc–PyA-450 compared to Tc–HTC-450 was systematically observed on ambient and CO2–free conditions, which is in line with the outcomes of theoretical simulations, presented above. As shown in Fig. 8 the results at pH 8.1 (TRIS buffer) as well as at pH 10 demonstrate nearly no effect of pH on leaching of Tc(VII) from LDH. In contrast to that, almost complete release of Tc(VII) occurs in contact with high saline solution (Q-brine), comprised mostly of MgCl2. Similarly, Curtius et al. (2004) reported rather quick Tc(VII) substitution by Cl− anions already in diluted MgCl2 solution, attributing this to the higher affinity of LDH to Cl− anions compared to TcO4−. However, a quick substitution of TcO4− by Cl− in LDH can also be driven by differences in chemical potentials for Cl− anions in the solution and in the solid phase.

Fig. 6. Fourier transforms of pertechnetate (TcO4−) containing LDH. Open symbols – experimental data, solid line – the best fit; the FT data are offset by +2.

spectrum. As shown in Fig. 4, Tc in all LDH phases remains in the form of Tc(VII), i.e. as TcO4− anion. Background subtracted k3-weighted EXAFS spectra and associated Fourier transforms for the Tc-LDH are shown in Figs. 5 and 6 respectively. The results of the data fitting, summarized in Table 6, demonstrate that only Tc – O interactions were detected. The best fit was obtained for coordination number 4 and no interactions beyond the oxygen coordination environment (presumably of TcO4− anion) were identified by EXAFS. The coordination number (4) and interatomic distances (ca. 1.72 Å) are in a very good agreement with those reported earlier by XAS (Saeki et al., 2012). Additionally, analysis of the EXAFS data showed no contributions at longer distances, indicating fairly weak (presumably electrostatic) interaction of TcO4− anion with positively charged layers of LDH. Complementary to the XAS examination, an insight into atomic scale incorporation of Tc(VII) into LDH phases was obtained using density functional theory-based simulations. The thermodynamic characteristics (i.e. standard enthalpy (ΔH°r) and the standard free energy (ΔG°r) of incorporation of TcO4− between the LDH layers were computed for the following reaction:

LDH

nOH + n TcO4

LDH

n TcO4 + nOH

(8)

As we are interested in the relative differences in the incorporation energies between LDH of different composition, in addition to the total energies of the LDH structures we computed the energies of OH– and TcO4− anions in the gas phase. In order to calculate the enthalpy of reaction (8) the hydration enthalpies of OH– (−460 kJ/mol) and TcO4− 8

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rather low Tc(VII) release in diluted solutions containing Cl− and OH−, however, in a high saline solution a rather fast substitution of TcO4− by Cl− occurs. The parallel investigation of Tc(VII) and Re(VII) interaction with LDH revealed a rather different leaching behavior of TcO4− and ReO4−, suggesting that typical application of Re as an inactive chemical analogue of 99Tc may lead to incompatible results. Acknowledgements The research leading to these results received funding from the Federal Ministry of Education and Research (BMBF) under project agreement no. 02NUK021G, the Conditioning project. We also gratefully thank Dr. Victor Vinograd for scientific support in the interpretation of powder XRD results. Appendix A. Supplementary data 99

Fig. 7. Distribution of Tc between LDH phase (filled bars) and solution (open bars) after 104 days of leaching in 1∙10−4 M NaOH: the effect of ambient atmosphere.

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.clay.2019.04.006. References Aimoz, L., Wieland, E., Taviot-Gueho, C., Dähn, R., Vespa, M., Churakov, S.V., 2012. Structural insight into Iodine uptake by AFm phases. Envir. Sci. Technol. 46, 3874–3881. Allada, R.K., Pless, J.D., Nenoff, T.M., Navrotsky, A.M., 2005. Thermochemistry of hydrotalcite-like phases intercalated with CO32−, NO3−, Cl−, I−, and ReO4−. Chem. Mater. 17, 2455–2459. Almahamid, I., Bryan, J.C., Bucher, J.J., Burrell, A.K., Edelstein, N.M., Hudson, E.A., Kaltsoyannis, N., Lukens, W.W., Shuh, D.K., Nitsche, H., Reich, T., 1995. Electronic and structural investigations of technetium compounds by X-ray absorption spectroscopy. Inorg. Chem. 34, 193–198. Baston, G.M.N., Marshall, T.A., Otlet, R.L., Walker, A.J., Mather, I.D., Williams, S.J., 2012. Rate and speciation of volatile carbon-14 and tritium releases from irradiated graphite. Mineral. Mag. 76 (8), 3293–3302. Bontchev, R.P., Liu, S., Krumhans, J.L., Voigt, J., Nenoff, T.M., 2003. Synthesis, characterization and ion exchange properties of hydrotalcite Mg6Al2(OH)16(A)x(A)´2− − − and NO3−, 2≥x≥0) derivatives. Chem. Mater. 15, x∙4H2O (A, A´= Cl , Br , I 3669–3675. Costa, D.G., Rocha, A.B., Souza, W.F., Chiaro, S.S.X., Leitão, A.A., 2012. Comparative structural, thermodynamic and electronic analyses of Zn-Al-an− hydrotalcite-like compounds (An- = Cl−, F−, Br−, OH−, CO32− or NO3−): an ab initio study. Appl. Clay Sci. 56, 13–22. Costantino, U., Vivani, V., Bastianini, M., Costantino, F., Nocchetti, M., 2014. Ion exchange and intercalation properties of layered double hydroxides towards halide anions. Dalton Trans. 43, 11587–11596. Curtius, H., Wellens, D., Odoj, R., 2004. Sorption of Technetate on Mg-Al-layered double hydroxide. In: Qaim, S.M., Coenen, H.H. (Eds.), Advances in Nuclear and Radiochemistry. Extended Abstracts of NRC 6, pp. 593–595. Curtius, H., Paparigas, Z., Kaiser, G., 2008. Sorption of selenium on Mg-Al and Mg-Al-Eu layered double hydroxides. Radiochim. Acta 96, 651–655. Das, J., Sairam, B., Baloarsigh, N., Parida, K.M., 2007. Calcined Mg-Fe-CO3 LDH as an adsorbent for removal of selenite. J. Coll. Int. Sci. 316, 216–223. De Graef, M., McHenry, M., 2012. Structure of Materials: An Introduction of Crystallography, Diffraction and Symmetry. Dickson, J.O., Harsh, J.B., Flurz, M., Lukens, W.W., Pierce, E.M., 2014. Competitive incorporation of perrehnate and nitrate into solidate. Envir. Sci. Technol. 48 (21), 12851–12857. Ewing, R.C., 2015. Long-term storage of spent nuclear fuel. Nat. Mater. 14, 252–257. Fernandez, J.M., Ulibarri, M.A., Labajos, F.M., Rives, V., 1998. The effect of iron on the crystalline phase formed upon thermal decomposition of Mg-Al-Fe hydrotalcites. J. Mater. Chem. 8 (11), 2507–2514. Forano, C., Constantino, U., Prevot, V., Taviop Gueho, C., 2013. Layered double hydroxides (LDH). In: Bergaya, F., Lagay, G. (Eds.), Handbook for Slay Science. vol 5. pp. 745–758 chapter 14.1. Giannozzi, P., Baroni, S., Bonini, N., Calandra, M., Car, R., Cavazzoni, C., Ceresoli, D., Chiarotti, G.L., Cococcioni, M., Dabo, I., Dal Corso, A., de Gironcoli, S., Fabris, S., Fratesi, G., Gebauer, R., Gerstmann, U., Gougoussis, C., Kokalj, A., Lazzeri, M., Martin-Samos, L., Marzari, N., Mauri, F., Mazzarello, R., Paolini, S., Pasquarello, A., Paulatto, L., Sbraccia, C., Scandolo, S., Sclauzero, G., Seitsonen, A.P., Smogunov, A., Umari, P., Wentzcovitch, R.M., 2009. QUANTUM ESPRESSO: a modular and opensource software project for quantum simulations of materials. J. Phys. Condens. Matter 21 (39), 395502–395521. Gillman, G.P., Noble, M.A., Raven, M.D., 2008. Anion substitution of nitrate-saturated layered double hydroxide of Mg and Al. Appl. Clay Sci. 38, 179–186. Goldberg, R.N., Kishore, N., Lennen, R.M., 2002. Thermodynamic quantities for the ionization reactions of buffers. J. Phys. Chem. Ref. Data 31 (2), 357. Gu, P., Zang, S., Li, X., Wang, X., Wen, T., Jehan, R., Alsaedi, A., Hayat, T., Wang, X., 2018. Recent advances in layered double hydroxides-based nanomaterials for the

Fig. 8. Distribution of 99Tc between LDH phase (filled bars) and solution (open bars) after 104 days of leaching on ambient atmosphere: the effect of pH and leaching solution composition.

4. Conclusions The present work discusses application of anionic clays, Mg/Al– and Mg/Fe–LDH, for remediation of 99Tc. Formation of a typical LDHstructure using a hydrothermal route at 120 °C was confirmed by powder XRD. The interaction of Tc(VII) with Mg/Al– and Mg/Fe–LDH was investigated in batch sorption experiments. The removal of Tc(VII) was observed to improve significantly after LDH are thermally “activated” at 450 °C. Based on XRD investigation of Tc–LDH phases, the incorporation of Tc(VII) in this case is following the restoration of the LDH layered structure. EXAFS results demonstrated that Tc atoms participate in only one type of interactions, Tc – O (with TceO bond of 1.72 Å), leaving no evidences of farther atomic interactions with components of LDH layers, e.g., Mg, Al or Fe. The presence of other anions in the solution, like NO3−, or CO32– (if interaction occurs on ambient conditions), inhibits Tc(VII) uptake by competing for binding with LDH. Presently investigated HTC-450 revealed the maximum adsorption capacity of 0.227 mol/kg, which is slightly higher than that of PyA-450 (0.213 mol/kg). In agreement with this are results of theoretical simulations: incorporation energies for Mg0.67/Al0.33–LDH (−128 kJ/mol) and Mg0.75/Fe0.25–LDH (−110 kJ/mol). Simulation also predicts a higher stability of Tc(VII) incorporated in the Mg/Fe–LDH compared to Mg/Al–LDH with an equal MIII/MII ratio. Investigation of Tc–HTC-450 and Tc-PyA-450 stability in different leaching media demonstrated 9

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