Evidence of multiple sorption modes in AFm phases using Mo as structural probe

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Evidence of multiple sorption modes in AFm phases using Mo as structural probe

Abstract

Layered double hydroxides (LDHs) have been considered as effective phases for the remediation of aquatic environments, to remove anionic contaminants mainly through anion exchange mechanisms. Here, a combination of batch isotherm experiments and X-ray techniques was used to examine molybdate (MoO42-) sorption mechanisms on CaAl LDHs with increasing loadings of molybdate. Advanced modelling of aqueous data shows that the sorption isotherm can be interpreted by three retention mechanisms, including two types of edge sites complexes, interlayer anion exchange, and CaMoO4 precipitation. Meanwhile, Mo geometry evolves from tetrahedral to octahedral on the edge, and back to tetrahedral coordination at higher Mo loadings, indicated by Mo K-edge X-ray absorption spectra. Moreover, an anion exchange process on both CaAl LDHs was followed by in-situ time-resolved synchrotron-based X-ray diffraction, remarkably agreeing with the sorption isotherm. This detailed molecular view shows that different uptake mechanismsedge sorption, interfacial dissolution-reprecipitationare at play and control anion uptake under environmentally-relevant conditions, which is contrast to the classical view of anion exchange as the primary retention mechanism. This work puts all these mechanisms in perspective, offering a new insight into the complex interplay of anion uptake mechanisms by LDH phases, by using changes in Mo geometry as powerful molecular-scale probe.

Introduction

Layered double hydroxides (LDHs) are lamellar mixed hydroxides with a generic chemical formula [MII1−xMIIIx·(OH)2]x+(An-)x/n·mH2O, where species under brackets form the hydroxide layer, those at the right form the interlayer, MII and MIII represent divalent and trivalent cations, respectively, in the positively charged layers, and An- denotes interlayer anions[174]. Owing to LDH anion exchange capacity, abundant studies have investigated their uptake ability towards anionic contaminants (e.g., As, Se, Cr, B, halide species, etc.) from polluted water[175-178] without explicitly pointing out the field of LDH stability. Alumina ferric oxide monosulfate (so-called AFm) phases ([Ca4Al2(OH)12]2+·SO42-·mH2O), are minor but highly reactive phases present in hydrated cement, and belong to the CaAl LDHs family[38]. AFm-Cl2 ([Ca4Al2(OH)12]2+·(Cl-)2·mH2O), of which Friedel’s salt is one of the polytypes, is also an important cement hydration product [38] when concrete is subjected to Cl- corrosion (e.g., when concrete is in contact with mixing water, sea water, or deicing salt, or when Cl-bearing aggregate and admixtures are used to produce it)[50, 51]. It is considered as an effective adsorbent for oxyanions due to their easy exchange for inter-layer Cl-, if compared to interlayer divalent oxyanions such as SO42-. AFm phases are therefore potential scavengers of toxic soluble anions[39, 58, 59, 61] via both surface adsorption and anion exchange processes.
Conversely, oxyanions present in LDHs which contain Ca as MII and Al as MIII (CaAl LDHs) can be a threat to the environment and public health. Coal fly ash (CFA) and bottom ash are two of the main coal combustion products (CCPs), which are generated over 750 million tonnes per year on a global basis [57]. In contact with water, LDH types of minerals (e.g., CaAl LDHs[52, 53] and MgAl LDHs[179]) are widely detected or predicted to occur thermodynamically in bottom ash and Class C CFA[54-56]. Although extensively used as raw material or additive in the cement industry and in road embankments[180-182], coal ash is often considered as an environmental pollutant on a global scale due to its load of various potentially toxic trace elements[183], which include some oxyanionic species such as As, B, Cr, Mo, Sb, Se, V and W. According to recent investigations[184], many coal ash disposal sites are located close to residential areas, with a direct threat to local people. However, very little is known about the chemical stability, mobility and bioavailability of these oxyanions, which depend on the mechanism by which they are retained by the LDHs.
Molybdenum (Mo), considered a double-edged sword trace element for human’s health, is present at high toxic concentrations in CFA particles and their leachates[55, 57], as well as in irrigation drainage[185] and industrial discharge waters[186]. Furthermore, 99Mo is widely used as a precursor of the radiopharmaceutical 99mTc[187] and 93Mo (t1/2 = 4,000 years) is an activation product of the spent nuclear fuel[188]. These radionuclides should be disposed in nuclear waste repositories, in which cementitious based materials are abundant[189]. Mo(VI) oxyanions (MoO42-) are very mobile at the near-neutral to alkaline pH values, with their speciation dominated by tetrahedral molybdate. In highly acidic solutions, the speciation is dominated by distorted octahedral molybdate present in clusters (e.g., [Mo7O24]6-, [Mo8O26]4-, and [Mo12O37]2-)[190]. Cementitious phases are expected to act as effective barriers retarding the mobility of anionic Mo species, and thus preventing their release in the environment.
By analogy with clay minerals, LDH should have different sorption sites (high and low energy edge sites, interlayer exchange sites – see [191]) whose affinity for trace elements should depend on pH and ionic strength. This study aimed at validating this analogy by using a combination of chemical and physical methods, including sorption experiments, laboratory and synchrotron X-ray diffraction, and Mo K-edge X-ray absorption spectroscopy (XAS). These methods allowed producing for the first time a model for the local geometry of different Mo active adsorption sites on CaAl LDH.

Materials and methods

Materials and Chemicals.

Degassed Milli-Q water (18.2 MΩ·cm) was used for all solutions and all the chemicals, including Na2MoO4·2H2O (purity >99%), were purchased from Sigma Aldrich and were analytical grade. All experiments were carried in a N2-filled glove box (O2<2 ppm, using NaOH as the CO2 trap) to prevent possible CO2 contamination.
Three types of CaAl LDHs, including monosulfoaluminate (AFm-SO4), Friedel’s salt (AFm-Cl2), and monocarboaluminate (AFm-CO3), were synthesized. AFm-SO4 was synthesized by suspending a 0.7:1 molar mixture of CaSO4·2H2O and tricalciumaluminate (C3A), which was obtained by heating the stoichiometric mixture of CaCO3 and Al2O3 for 72 h at 1300 °C[61], at a liquid-to-solid (L/S in kg/kg) ratio of 10 and was subsequently aged at room temperature for 42 days with constant magnetic stirring. After aging, the slurry was vacuum filtered through a Millipore® 0.22-μm membrane filter and dried in the glove box. A slight deficit (30%) of sulfate with respect to the AFm-SO4 stoichiometry was kept in order to avoid small amounts of ettringite, which were present in previous work[36]. As a result, 22 at% katoite [Ca3Al2O6·6H2O] was detected by XRD (Figure S1). AFm-Cl2 and AFm-CO3 were prepared by mixing stoichiometric amounts of C3A with CaCl2·6H2O or CaCO3, respectively, at the same L/S ratio.

Adsorption Experiments.

Kinetic experiments were performed in order to determine the equilibrium time of CaAl LDHs’ dissolution and molybdate sorption. After 6 h and 12 h, the concentration of dissolved ions from AFm-SO4 and AFm-Cl2 became maximum and stable, respectively. Molybdate sorption finished within a few hours. Based on these results, an equilibrium time of 48 h after the dissolution equilibrium was used for sorption experiments.
Batch experiments were performed at room temperature in the glove box as a function of initial molybdate concentration. This ranged from 0.001 to 20 mM at an L/S ratio of 500 at pH 12.4, which was reached by adding tiny 2 M NaOH solution and equal to the pH value of the CaAl LDHs synthesis suspension. Thus the molar ratio of added MoO42- to inter-layer SO42-/(Cl-)2 was between 0.0002 and 4. The low-end of molybdate concentrations (Reactor ClC and SC) was chosen to keep a molar Mo concentration comparable to the surface site density and also to avoid the formation of Ca-bearing precipitates, according to thermodynamic calculations. Besides, high MoO42- concentrations were also chosen to be similar to the stoichiometric amounts of inter-layer anions (Reactor ClF and SF), to provide enough MoO42- for anion exchange. Besides, even higher loadings were introduced to supersaturate the suspension by CaMoO4(s). All suspensions were equilibrated on an end-over-end shaker. At each sampled time, three aliquots of 4 mL of suspension each were taken from each reactor at different times and filtered through Chromafil® 0.22-μm syringe filters. The first sample was taken after dissolution equilibrium to measure the initial Ca, Al, S, and Cl concentrations. The second sample was collected immediately once molybdate was added. The last one was taken after the slurry reached equilibrium, following kinetic experiments. The solution pH was measured with a combined glass pH electrode (Metrohm 6.0233.100) connected to an Orion (525A) pH meter, that was previously calibrated using four pH reference solutions. Total Ca, Al, S, and Mo concentrations in the filtrate were analysed by inductively coupled plasma optical emission spectrometry (ICP-OES) with a Varian 720-ES apparatus and the concentration of released Cl- was determined by ion chromatography (IC, Metrohm 883).
Six solid samples for each CaAl LDH were selected to be studied by synchrotron based X-ray spectroscopic methods, which were denoted by Cli (Cl represents AFm-Cl2) and Si (S represents AFm-SO4), with i = A (0.001 mM), B (0.005 mM), C (0.02 mM), D (0.25 mM), E (0.5 mM), and F (5 mM), respectively.

 Aqueous Data Modelling.

The PHREEQC code[192] and the latest ThermoChimie database[193] was used to model all the sorption stages of MoO42- on CaAl LDHs, and to calculate the SI value of each possible formed mineral phase. Regarding the sorption sites on surfaces, parameters for MoO42- surface complexation were fitted using a non-electrostatic model because of a lack of experimental constraints that would be necessary to build a more complex surface complexation model. Note that, the value of the specific surface area has no influence on the calculation results in the framework of a non-electrostatic model. As MoO42- loading increasing, the uptake amount was well modelled by the precipitation of a new LDH phase (AFm-MoO4) and of CaMoO4 phase further.

In-situ X-ray Diffraction.

In situ X-ray Diffraction (XRD) experiments were performed by circulating a mineral suspension through a capillary cell, as a function of different [Mo]initial. A multifunctional reactor was used to control the pH, oxygen concentration, stirring and pumping speed. First, the CaAl LDHs were equilibrated with water at pH 12.4 under constant stirring, and the circulation through the capillary cell was started. The slurry went through the connected kapton capillary, which was sealed with PEEK nuts and ferrule fittings (IDEX Health & Science). Then, the concentration of molybdate was increased in the reactor step by step to repeat the six selected concentrations used in batch experiments, until the ratio of Mo to Cl2/SO4 was over 1:1. For each step, the measurement lasted from one to two hours, to assure equilibrium was reached.
The in-situ experiments were performed at the CRISTAL beam line (SOLEIL synchrotron). Diffraction patterns were recorded on a MAR345 image plate detector with an exposure time of 5 s at the wavelength of 0.978 Å (∼12.8 keV). The wavelength and detector-to-sample distances were calibrated using a NIST LaB6 standard. With the addition detector’s read-out time, the time resolution of data acquisition was 120 s. After converted into one-dimensional XRD patterns using the program Fit2D, background was subtracted.

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Solid Phase Characterization.

The synthesized CaAl LDHs were identified with powder X-ray diffraction. The samples were introduced in a humidity chamber CHC plus+ from Anton Paar to control the hydration state at a RH of 5% (approximately equal to the relative humidity in the glove box) and room temperature. They were ground using an agate mortar and dispersed in ethanol. Oriented mounts were prepared by drying the resulting suspension onto glass slides in the glove box. XRD patterns were recorded on a Bruker D8 powder diffractometer equipped with a SolX Si(Li) solid state detector from Baltic Scientific Instruments using Cu Kα radiation at 1.5406 Å.
The nitrolysis method was used to quantify the elemental composition of the fresh CaAl LDHs. 10 mg of solid phase were digested in 2% HNO3 in triplicates and the ratios between Ca, Al, and Cl/S were estimated by measuring the elemental concentration using ICP-OES and IC. The katoite impurity content and the amounts of each of the inter-layer anions were deduced from the Ca/Al stoichiometry.
Morphological images of the freshly synthesized CaAl LDHs were collected using a scanning electron microscope (SEM, Hitachi S-4800). The specific surface area was measured by the Brunauer–Emmett–Teller (BET) N2-adsorption method. TEM was performed using a Philips CM20 operated at 200 kV and having a line resolution of 1.4 Å. Prior to observations, the samples were crushed, dispersed using an antistatic gun, and then embedded in resin. After the resin was polymerized, it was cut in slices having a thickness of 80 nm which were deposited on copper grids.
TGA/DTG were performed with a Mettler Toledo TGA/DSC 2 apparatus to determine the structural water content. Nitrogen was used as the reactive gas at a rate of 50 mL min-1. The weight loss was measured from 60 to 1000 °C with a heating rate of 10 °C min-1, after preheating at 60 °C for 1 h to remove physically sorbed water.
Reacted solid phases from the six selected low-loading reactors were analysed by PDF methods with synchrotron based XRD to determine the phase structure after molybdate sorption. Diffraction experiments were performed using the 2circle diffractometer of CRISTAL beamline at SOLEIL the French Synchrotron Facility. Two setups were used during these experiments: i) For PDF measurements, a high energy beam (wavelength of 0.257Å) was selected and patterns recorded on a MAR345 Image plate detector with exposure time of 20 min. One-dimensional diagrams were obtained by integration of the images using fit2D software [194]. PDF were generated using the PDFgetX3 code [195]; ii) For the other measurements, a step by step rotation of a two dimensional hybrid pixel detector (XPAD) was employed to obtain a better angular resolution and to cover a large Q range. ImageReducer, a local program, was used to perform azimuthal integration and to regroup data from multiple images into a one-dimensional diagram. A wavelength of 0.436 Å was selected to collect these data. For each configuration, beam energy and detectors orientations were calibrated using a LaB6 (NIST- SRM660) standard powder sealed in an Ø2mm capillary.
Molybdenum K-edge X-ray absorption spectroscopy (XAS) measurements were conducted at the Core Level Absorption & Emission Spectroscopy (CLÆSS, BL22) beam line with a source of multipole wiggler at the Spanish synchrotron ALBA-CELLS, Barcelona, Spain, and at the SpLine Spanish CRG Beamline (BM25A) at the ESRF, Grenoble, France. The double crystal monochromator (DCM) with a pair of Si(311) crystals was used and the energy was always calibrated with a Mo(0) foil in parallel with measurement. The BM25A beamline are equipped with a -70 °C ethanol cooled double Si(111) crystal, which gives an energy resolution of ΔΕ/Ε = 1.5×10-4. All samples were sealed using Kapton tape mounted on a sample holder, and measured in fluorescence mode, except for the references and samples with high Mo concentration (E and F), which were prepared as pellets by diluting the solids in boron nitride and measured in transmission mode. Samples were always protected under N2 atmosphere before being transferred into the vacuum experimental chamber, in which the temperature was lowered to 77 K within a liquid N2 cryostat to minimize the effects of thermal disorder due to atomic vibrations, An Amptek Cadmium Telluride (CdTe) fluorescence detector (at the CLÆSS) and Sirius liquid nitrogen cooled Si(Li) 13-multi-element solid state X-Ray detector from e2v (at the BM25A) were used for data collection. The Demeter software package was used for the data integration and reduction of XANES (Athena), as well as the data fitting of the EXAFS (Artemis)[196]. Radial distribution functions were obtained by FT of k3-weighted EXAFS oscillations (k-range: 4.0-12.5 Å-1) using a Kaiser-Bessel window. FEFF8.4[197] was used to calculated the theoretical backscattering paths to perform the fit in back-transformed reciprocal space (k), and it was also employed to obtain the calculated Mo K-edge XANES spectra. Several models of surface complexes were created using Materials Studio 4.0 (Accelrys Inc.) to generate scattering paths for the EXAFS fitting of the Mo-reacted CaAl LDHs.

Table of contents :

Chapter 1. Introduction-Barriers around geologic nuclear waste repositories
1.1. Geological nuclear waste disposal
1.2. Multi-barrier systems
1.3. Sorption behavior of radionuclides in potential barriers
1.3.1. On iron phases existing in canisters and steel reinforcements
1.3.2. On hydrated cement phases existing in backfills and tunnels
1.3.3. On clays
1.3.4. On granitic minerals
1.4. Objectives of the thesis
Chapter 2. Evidence of multiple sorption modes in AFm phases using Mo as structural probe
2.1. Introduction
2.2. Materials and methods
2.2.1. Materials and Chemicals.
2.2.2. Adsorption Experiments.
2.2.3. Aqueous Data Modelling.
2.2.4. In-situ X-ray Diffraction.
2.2.5. Solid Phase Characterization.
2.3. Results and discussion
2.3.1. Wet Chemistry Experiments.
2.3.2. In-situ Time Resolved X-ray Diffraction.
2.3.3. Local Geometry of Adsorbed Molybdate.
2.3.4. Mo Distribution in Reacted LDHs Particles.
2.3.5. Complexation environment around adsorbed Mo.
2.3.6. Estimation of edge sites density.
2.3.7. Environmental implications.
Supporting Information
Chapter 3. Selenite uptake by AFm phases: a description of intercalated anion coordination geometries
3.1. Introduction
3.2. Materials and methods
3.2.1. Materials and chemicals.
3.2.2. Adsorption experiments.
3.2.3 Aqueous data modelling.
3.2.4 In-situ time-resolved XRD.
3.2.5 Solid phase characterization.
3.3. Results and discussion
3.3.1 Batch sorption isotherm.
3.3.2. In-situ time resolved XRD.
3.3.3. Sorbed Se coordination.
3.3.4. Sulfur K-edge EXAFS results.
3.3.5. Linear relationship between basal spacing and hydrated anions’ radii.
3.3.6. Structural stability of AFm-SeO3.
3.3.7. Environmental Implications.
Supporting Information
Chapter 4. Determination of redox potentials imposed by steel corrosion products in cement-based media
4.1. Introduction
4.2. Materials and methods
4.2.1. Materials and chemicals.
4.2.2. Synthesis and characterization of Fe-bearing phases.
4.2.3. Batch sorption experiments.
4.2.4. XANES spectroscopy.
4.2.5. PDF spectroscopy.
4.3. Results and discussion
4.3.1. Aqueous phase results.
4.3.2. Surface Se and Sb species.
4.3.3. Surface U and Mo species.
4.3.4. PDF analysis of reacted NZVI.
4.3.5. “In-situ” experimental Eh values.
4.3.6. Environmental implications.
Supporting Information
Chapter 5. RNs (i.e., U, Se, Mo, and Sb) sorption behavior on hydrated Fe-bearing CEM-V/A cement
5.1. Introduction
5.2. Materials and methods
5.2.1. Materials and chemicals.
5.2.2. Preparation of powder samples of hydrated Fe-bearing cement products.
5.2.3. Synthesis of each separated CEM-V/A cement hydration product.
5.2.4. Kinetics experiments.
5.2.5. Batch sorption experiments.
5.2.6. Preparation of RNs-containing Fe0-bearing cement cores.
5.2.7. Polishing of RNs-containing cement cores and Micro-probe analysis.
5.2.8. Sulfur K-edge, Selenium K-edge, and Molybdenum K-edge XANES-EXAFS spectroscopy
5.2.9. PDF analysis.
5.3. Results and discussion
5.3.1. XRD characterization and dissolution kinetics of Fe-bearing hydrated cement.
5.3.2. U(VI) sorption on hydrated CEM-V/A cement particles.
5.3.3. Se(IV) sorption on hydrated CEM-V/A cement particles.
5.3.4. Mo(VI) sorption on hydrated CEM-V/A cement particles.
5.3.5. Sb(V) sorption on hydrated CEM-V/A cement particles.
5.3.6. Microprobe mapping on Fe0/cement interface in presence of RNs
5.4. Conclusions
Chapter 6. The influence of surface impurities on the reactivity of pyrite toward aqueous U(VI)
6.1. Introduction
6.2. Materials and methods
6.2.1 Materials
6.2.2 Batch sorption experiments
6.2.3 Spectroscopic and Microscopic Analyses
6.3. Results and discussion
6.3.1 Pyrite Characterization
6.3.2 Aqueous phase analysis for Type I pyrite suspension
6.3.3 Aqueous phase analysis for Type II pyrite suspension
6.3.4. XPS Surface features
6.3.5. XAS Characterization on uranium speciation
6.3.6. SEM surface morphology
6.3.7. Influencing factors on pyrite reactivity.
6.4. Conclusions
General conclusions
Retention of MoO42- on AFm phases.
SeO32- uptake by CaAl LDH.
Determination of Eh values imposed by steel corrosion products in cement-based media.
RNs sorption behaviors on hydrated Fe-bearing CEM-V/A cement.
The influence of surface impurities on the reactivity of pyrite toward aqueous U(VI).
Perspectives
Dissolution of spent nuclear fuel (UO2) in silica-rich fluids.
Uranium(VI) colloidal nanoparticles in cement leachate.
Sorption behaviors of RN oxyanions on LDHs’ surface.
Intercalated anion’s size vs. LDHs’ basal spacing.
Sorption competition of RNs on corrosion products of reinforced Fe0.
Synchrotron-based X-ray analysis of Fe0-reinforced cement cores.
U(VI) retention on synthetic trace elements doped pyrite.
Thermodynamic predictions of secondary phases in cementitious repositories.
References 

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