Bin Ma

Alejandro Fernandez-Martinez

Andrea Mancini

Barbara Lothenbach

Synchrotron-based spectroscopic study on structural incorporation pathways of Fe III in the zeolite framework

Fe III -containing aluminosilicate zeolites are present both in cement-relevant environments (e.g., degraded cement/clay interfaces, alkali-activated cements, and aged Roman cement) and in natural settings (e.g., volcanic tuffs). The presence of Fe III has an effect on radionuclide sorption in cement/clay-rich nuclear waste repositories, as it can impact radionuclide sorption, and for the efficiency of catalytic processes. Although Fe III is known to sit either in framework tetrahedral sites where it substitutes isomorphically Al III O 4 or in extra-framework octahedral sites as free Fe(H 2 O) 6 3+ , the structural incorporation process of Fe III into the two sites has received little attention. Here, we aim to discern feasible pathways of Fe III incorporation by using hydrothermal synthesis methods and synchrotron-based spectroscopic analyses. The results showed that introducing either Fe 3+ or Fe 2 O 3 ⸱xH 2 O initially during zeolite nucleation did not lead to Fe III incorporation into the representative zeolites but only to Fe 2 O 3 ⸱xH 2 O formation. Both Fe 3+ -paired anions (NO 3 -and Cl -) and HFO nanoparticles displayed a structure-directing effect, e.g., resulting in the formation of cancrinite and sodalite instead of the targeted zeolites. In contrast, a more feasible pathway to form the tetrahedral framework-Fe III O 4 was to introduce firstly cage-Fe III in faujasite-Y followed by a phase transformation to chabazite. This study will facilitate the comprehension of RNs sorption behavior on Fe-zeolites and the catalysis site optimization of existing Fe-zeolite catalysts.

INTRODUCTION

Zeolites are widely used in diverse areas (e.g., petrochemical industry, catalysis, agriculture, heat storage, and cement industry), which benefit from the microporous threedimensional framework structure and the exchangeable extraframework cation. Zeolites are also abundant in nature (e.g., in volcanic tuff [START_REF] Gelves | Mineralogical characterization of zeolites present on basaltic rocks from Combia geological formation, La Pintada (Colombia)[END_REF] ), with the aluminosilicate framework built with SiO 4 and AlO 4 tetrahedra primarily. [START_REF] Glauco | Natural Zeolites[END_REF] In cement industry, natural zeolites can be used as supplementary cementitious materials, which can reduce CO 2 emissions by partially replacing Portland cement (PC) clinkers. [START_REF] Ahmadi | Use of natural zeolite as a supplementary cementitious material[END_REF] On the other hand, zeolites have also been observed to form as secondary mineral phases in alkali-activated cements, [START_REF] Provis | Do Geopolymers Actually Contain Nanocrystalline Zeolites? A Reexamination of Existing Results[END_REF][START_REF] Shi | Stabilization/solidification of hazardous and radioactive wastes with alkali-activated cements[END_REF] aged Roman cements, [START_REF] Jackson | Phillipsite and Altobermorite mineral cements produced through lowtemperature water-rock reactions in Roman marine concrete[END_REF] and degraded PC/clay interfaces. [START_REF] Fernández | Geochemical constraints on the stability of zeolites and C-S-H in the high pH reaction of bentonite[END_REF][START_REF] Lothenbach | Zeolite formation in the presence of cement hydrates and albite[END_REF] Interfaces between cements and clays are present in many places, such as in underground concrete structures and cement/clay-rich nuclear waste repositories. [START_REF] Ma | Selenite Sorption on Hydrated CEM-V/A Cement in the Presence of Steel Corrosion Products: Redox vs Nonredox Sorption[END_REF] After many millennia, as cement hydrates degrade, the released alkalis together with aluminosilicate minerals from the adjacent clay would facilitate the formation of zeolites. [START_REF] Ochs | Cementitious Materials and Their Sorption Properties[END_REF] In the context of nuclear waste repositories, zeolites could be an important retarder for radionuclides (RNs) in the strongly altered cementitious near field. [START_REF] Ma | A review of the retention mechanisms of redoxsensitive radionuclides in multi-barrier systems[END_REF] Furthermore, iron (Fe) materials (e.g., embedded steels, waste iron-canisters, and iron-containing slags 9

) and their corro-sion/oxidation/dissolution products [START_REF] Ma | XANES-Based Determination of Redox Potentials Imposed by Steel Corrosion Products in Cement-Based Media[END_REF] could also be present in the formation zone of zeolite and participate in zeolite formation. Fe-containing zeolites could be beneficial to RNs retardation as these zeolites have the potential to immobilize both cationic and anionic RNs via inner-sphere complexation and/or reductive precipitation. [START_REF] Van Tendeloo | Chabazite: Stable Cation-Exchanger in Hyper Alkaline Concrete Pore Water[END_REF][START_REF] Van Tendeloo | Cation Exchange Properties of Zeolites in Hyper Alkaline Aqueous Media[END_REF][START_REF] Cama | Dissolution Kinetics of Synthetic Zeolite NaP1 and Its Implication to Zeolite Treatment of Contaminated Waters[END_REF] In fact, the existence of natural Fe-containing zeolites, such as heulandite, clinoptilolite, mordenite, chabazite, and phillipsite, has been widely reported in the literature. [START_REF] Gelves | Mineralogical characterization of zeolites present on basaltic rocks from Combia geological formation, La Pintada (Colombia)[END_REF][START_REF] Gelves | Activity of an iron Colombian natural zeolite as potential geo-catalyst for NH3-SCR of NOx[END_REF][START_REF] Roque-Malherbe | The state of iron in natural zeolites: A Mössbauer study[END_REF][START_REF] Marco | On the state of iron in a clinoptilolite[END_REF] Fe III -containing zeolites are not only considered as good absorbents for anionic contaminants but also widely used as catalysts to activate/oxidize CH 4 and NO. [START_REF] Cornell | The iron oxides: structure, properties, reactions, occurrences and uses[END_REF][START_REF] Fletcher | Violations of Löwenstein's rule in zeolites[END_REF] Typically, Fe III predominates over Fe II in zeolitic structures probably due to the size and charge similarities between Fe III and Al III . [START_REF] Marco | On the state of iron in a clinoptilolite[END_REF] Fe III could sit in framework tetrahedral sites as high spin Fe III or in extra-framework octahedral sites as free Fe(H 2 O) 6

3+

. [START_REF] Roque-Malherbe | The state of iron in natural zeolites: A Mössbauer study[END_REF] For the isomorphic substitution to occur, it is expected that Fe III adopts a tetrahedral coordination in oxygen lattices as its radius ratio to O 2-(r Me /r O2-) is equal to 0.35, i.e. within the range of 0.214 -0.4 for tetrahedral sites. [START_REF] Fricke | Incorporation of Gallium into Zeolites: Syntheses, Properties and Catalytic Application[END_REF] However, at high pH values Fe 3+ tends to form dihydroxo dimer rapidly and to precipitate further as ferric hydroxides, [START_REF] Zhu | Precipitation pathways for ferrihydrite formation in acidic solutions[END_REF] such as 2-line ferrihydrite (Fe 2 O 3 ⸱xH 2 O, HFO), which contains 20% tetrahedrally and 80% octahedrally coordinated Fe III . [START_REF] Michel | The Structure of Ferrihydrite, a Nanocrystalline Material[END_REF] Therefore, Fe-containing zeolites are usually found associated with ferric (oxyhydr)oxides, [START_REF] Gelves | Mineralogical characterization of zeolites present on basaltic rocks from Combia geological formation, La Pintada (Colombia)[END_REF][START_REF] Gelves | Activity of an iron Colombian natural zeolite as potential geo-catalyst for NH3-SCR of NOx[END_REF][START_REF] Roque-Malherbe | The state of iron in natural zeolites: A Mössbauer study[END_REF][START_REF] Marco | On the state of iron in a clinoptilolite[END_REF][START_REF] Rodriguez-Fuentes | Solid state multinuclear NMR study of iron species in natural and modified clinoptilolite from Tasajera deposit (Cuba)[END_REF] probably resulting in ambiguous Fe signals in insufficient characterizations.

Although the isomorphic substitution of Al III O 4 by Fe III O 4 is theoretically favorable, the occurrence of framework Fe III O 4 in natural zeolites has not been always observed and not clearly characterized. [START_REF] Marco | On the state of iron in a clinoptilolite[END_REF][START_REF] Rodriguez-Fuentes | Solid state multinuclear NMR study of iron species in natural and modified clinoptilolite from Tasajera deposit (Cuba)[END_REF] The conditions under which Fe III is incorporated in zeolites as well as the process by which Fe III is taken up into different zeolitic sites are not well understood. In this study, we aim to decipher the pathways of Fe III incorporation into the framework tetrahedral site and extra-framework octahedral site of zeolites by using hydrothermal synthesis methods and synchrotron-based spectroscopic analyses. Three different ways to introduce Fe III into a few common types of zeolites (i.e., faujasite, gismondine, Linda type A, and chabazite) in the relevant environment were designed and investigated, i.e., introducing i) Fe 3+ or ii) HFO in the initial synthesis gel of zeolites and iii) introducing cage-Fe III during phase transformation from faujasite to chabazite. We aim to elucidate the possibility of each Fe III incorporation pathway and to facilitate the comprehension of RNs sorption behavior on Fe III -containing zeolites and the rational modifications on the properties of active catalysis sites.

MATERIALS AND METHODS

Synthesis of zeolites without Fe III . Most materials and chemicals were purchased from Sigma-Aldrich; otherwise, specific information about their provenance is provided. Milli-Q water (18.2 MΩ•cm) was used in all the water-based experiments. By following the recipe (described in Table S1) that was explored in our previous study, [START_REF] Ma | Synthesis, X-ray diffraction and MAS NMR characteristics of nitrate cancrinite Na7.6[AlSiO4]6(NO3)1.6([END_REF] four types of highly pure Na-based zeolites, Faujasite-X (FAU-X), Faujasite-Y (FAU-Y), Linda type A (LTA), and Gismondine-P1 (GIS-P1), were synthesized successfully via hydrothermal methods. Typically, different amounts of alkali source (NaOH, pellets, >99% purity), Al source (NaAlO 2 , technical grade), Si source (either fumed SiO 2 (chemical grade, Aerosil 200) or Na 2 O⸱SiO 2 solution (reagent grade)) were mixed together in Teflon vessels and were heated in temperature-controlled ovens for crystallization. The obtained solids were recovered by, in sequence, vacuum filtration, Milli-Q water washing (until the filtrate pH was ~10), and drying in an oven at 80 °C (to remove excess water). Finally, the dried zeolites were equilibrated in a desiccator, with a saturated CaCl 2 solution and a CO 2 trap placed inside to maintain a constant 35% relative humidity and low CO 2 level, respectively.

Introduction of Fe III into the zeolites. The following three strategies were applied to introduce Fe III into selected zeolites:

Introduction of Fe 3+ via co-precipitation. Fe 3+ cation was introduced via co-precipitation by adding Fe(NO 3 ) 3 solution (from Fe(NO 3 ) 3 , 98% purity) into the initial synthesis gels of FAU-X, FAU-Y, LTA, and GIS-P1 (see Table S1 and Section 2.1). Different atomic ratios of Fe/Al, including 0.05, 0.1, 0.3, and 0.5, were applied in the initial gels. To neutralize the acid introduced by FeNO 3 , different extra amounts of NaOH were added into each reactor. Control experiments using FeNO 3 and FeCl 3 (from FeCl 3 ⸱6H 2 O, 97% purity) and their paired anions in Na-based forms were performed in the case of LTA, to discern the effect of the paired anions on directing zeolite formations. 24 A Fe/Al ratio of 0.5 was selected and the corresponding amounts of NO 3 -or Cl -were introduced with NaNO 3 (≥99.5% purity) or NaCl (≥99.5% purity) in solution, respec-tively. All the other synthesis conditions strictly followed Table S1. The same procedures as described in Section 2.1 were followed for sample filtration, washing, drying, and storage.

Introduction of HFO via co-precipitation. HFO was synthesized by adding 0.5 M NaOH solution into 0.1 M Fe(NO 3 ) 3 solution to bring the final pH to ~7.7. [START_REF] Cornell | The iron oxides: structure, properties, reactions, occurrences and uses[END_REF] After aging for 1 h, the brownish gel was centrifuged and washed for six times. Different amounts of fresh HFO gel was introduced into the initial synthesis gel of FAU-Y and GIS-P1 (see Table S1 and Section 2.1), to reach different Fe/Al atomic ratios of 0.01 and 0.03. After the crystallization, the solid phases were treated with the same procedures as described in Section 2.1.

Introduction of cage-Fe III during phase transformations. The phase transformation from acid faujasite-Y (FAU-YH) to chabazite (CHA), a process that can be accelerated and completed rapidly under the assistance of KOH solution, was investigated. [START_REF] Robson | Verified synthesis of zeolitic materials[END_REF] Two types of FAU-YH with Si/Al atomic ratios of 2.0 (denoted as LS-FAU-YH) and 2.6 (denoted as HS-FAU-YH) were studied. i) LS-FAU-YH was prepared by calcining (at 550 °C for 12 h) NH 4 -exchanged FAU-Y that was obtained via three-time cation exchange; for each time, 62.5 g L -1 FAU-Y was equilibrated in 3 M NH 4 NO 3 solution (from NH 4 NO 3 , 98% purity) at 80 °C for 48 h followed by vacuum filtration and washing. ii) HS-FAU-YH was obtained by calcining (at 550 °C for 12 h) one commercial FAU-YH from Alfa Aesar (Ref. 45868). To minimize Fe heterogeneity and limit the formation of oxide/hydroxide species (relative to aqueous exchange), the cage-Fe III was introduced into the two dried FAU-YH by diffusion impregnation of Fe(acac) 3 (acac = acetylacetonate, ≥99% purity) in ethanol (≥99.9% purity) solution (40 g L -1 FAU-YH). [START_REF] Snyder | The active site of low-temperature methane hydroxylation in iron-containing zeolites[END_REF][START_REF] Zecchina | Structure and nuclearity of active sites in Fezeolites: comparison with iron sites in enzymes and homogeneous catalysts[END_REF] Three Fe(acac) 3 concentrations of 0.001, 0.01, and 0.05 M in ethanol were applied and the impregnation performed at 50 °C for 48 h. After vacuum filtration and washing with pure ethanol, the Fe III -containing FAU-YH was calcined at 550 °C for 30 h to remove organic materials. To transform into CHA, the FAU-YH powders were mixed with 1.8 M KOH solution (from KOH pellets, >85% KOH basis) under an identical solid-to-liquid (S/L) ratio of 110 g L -1 . A transformation temperature of 95 °C or 150 °C was applied in the absence or presence of Fe III , respectively, for 96 h. After vacuum filtration, washing with Milli-Q water, and drying, two series of CHA, i.e., LS-CHA (from LS-FAU-YH) and HS-CHA (from HS-FAU-YH), with different Fe III loadings were obtained.

Solid phase characterization. X-ray diffraction (XRD) analysis was used to identify the zeolite framework structure and to check any possible competing crystalline phase. The dried zeolites were ground and passed through a 63-µm sieve before the measurement. XRD patterns were collected with a PANalytical X'Pert Pro diffractometer in a θ-2θ configuration. Oriented mounts were prepared and Co Kα radiation at 1.789 Å used. The step size, scan speed, and range were 0.017° 2θ, 0.54 s, and 5-90° 2θ, respectively.

The elemental fractions of Al, Si, Na, and K in the zeolites were determined by energy-dispersive X-ray spectroscopy (EDS) analysis that was supported on a field-emission scanning electron microscopy (FE-SEM, FEI Quanta 650 FEG ESEM). The pressed zeolite pellets with relatively flat surfaces were placed on carbon discs and were coated with 10 nm carbon (Sputterer Leica ACE200). The EDS measurement was performed under high vacuum mode (3 -4 × 10 -6 Pa) with an accelerating voltage of 15 kV. The accuracy of the EDS method had been verified in our previous work. [START_REF] Ma | Synthesis, X-ray diffraction and MAS NMR characteristics of nitrate cancrinite Na7.6[AlSiO4]6(NO3)1.6([END_REF] Furthermore, the composition and Fe contents in both LS-CHA and HS-CHA were determined by acid digestion method. 0.01 g of zeolite powder was sufficiently digested in 10 ml 5% HNO 3 and the undissolved Si phase was filtered out using 0.22 μm pore size membrane filters. The total concentrations of Fe, Al, Si, Na, and K in the filtrates were measured by inductively coupled plasma optical emission spectrometry (ICP-OES) with an Agilent ICP-OES 5110 apparatus.

Iron K-edge (7112 eV) X-ray absorption fine structure (XAFS) experiments were conducted at the B18 beamline (Diamond, Didcot, UK) and at beamline BL11-2 (Stanford Synchrotron Radiation Lightsource (SSRL), Menlo Park, USA). A Si(111) or Si(220) double-crystal monochromator was used at B18 or BL11-2, respectively. The instrumental energy calibration was conducted by assigning 7112 eV to the first inflection point of the Fe K-edge spectrum of an elemental Fe foil. All the XAFS samples were placed into Plexiglas sample holders, sealed with polyimide tape, and measured in fluorescence mode, except for Fe references that were measured in transmission mode using ionization chambers. The fluorescence signal was collected by a 9-element Ge solid-state detector (at B18) or a 100-pixel Ge solid-state monolith detector (at BL11-2). The extended X-ray absorption fine structure (EXAFS) spectra at Fe K-edge were recorded up to 12 Å -1 .

Aluminum K-edge (1559 eV) XAFS measurements were conducted at the Phoenix II, elliptical undulator beamline at the Swiss Light Source (SLS), Paul Scherrer Institute (PSI), Villigen, Switzerland. Energy selection was conducted using a highly performant grating monochromator and the incoming beam intensity (I 0 ) was monitored by measuring the drain current from a Ni covered thin polyethylene foil. An Al foil was used to calibrate the X-ray energy by setting the first derivative peak of the Al K white line at 1559.6 eV. All the samples were fine powders and were spread across indium foils that were attached to a copper sample holder. The sample measurement was conducted under high vacuum (~2.5 × 10 -4 mbar) and in both total electron yield (TEY) and fluorescence modes. The fluorescence signal was collected by a 4-element Vortex Si-drift diode detector. Since the presence of Si in zeolites can cut off Al K-edge EXAFS signal at ~ 1839 eV, the energy range collected for Al was from -150 to +300 eV (up to k ~8 Å -1 ).

Data integration, reduction, and correction (e.g., fluorescence self-absorption) of X-ray absorption near edge structure (XANES) spectra (Athena), as well as the data fitting of EXAFS spectra (Artemis), were performed using the Demeter software package. [START_REF] Ravel | HEPHAESTUS: data analysis for X-ray absorption spectroscopy using IFEFFIT[END_REF] A linear combination fit (LCF) was applied to the Fe K-edge XANES spectra to identify and quantify the Fe-containing species. For EXAFS analysis, radial structure functions were obtained by Fourier transforming (FT) the k 3 -weighted χ(k) spectra between 2.5 and 11.0 Å -1 using a Kaiser-Bessel window function. Theoretical backscattering paths were calculated by FEFF8.4, [START_REF] Ankudinov | Dynamic screening effects in x-ray absorption spectra[END_REF] to perform the fit in back-transformed reciprocal space (k). The goodness of the fit was evaluated based on the residual factor (R-factor).

The Fe 3+ co-precipitation samples were investigated by pair distribution function (PDF) analysis of high-energy X-ray scattering (HEXS). The HEXS measurements were performed at the ID31 beamline at the European Synchrotron Radiation Facility (ESRF), Grenoble, France. An incident X-ray radiation of E = 78 keV (λ = 0.1589 Å) was used. HEXS images were obtained using a Pilatus3 X CdTe 2M detector. Powder samples were loaded into Ø1 mm polyimide capillaries that were sealed after by epoxy glue. NIST certified CeO 2 powder sample was used for instrumental calibration and empty capillary with the same specification for background subtraction. The obtained images were integrated to one-dimensional diagrams using Fit2D software. [START_REF] Hammersley | Two-dimensional detector software: From real detector to idealised image or two-theta scan[END_REF] Data correction and generation of structure factors and PDFs were performed with the PDFGetX3 code. [START_REF] Juhas | PDFgetX3: a rapid and highly automatable program for processing powder diffraction data into total scattering pair distribution functions[END_REF] [START_REF] Ma | Synthesis, X-ray diffraction and MAS NMR characteristics of nitrate cancrinite Na7.6[AlSiO4]6(NO3)1.6([END_REF] These zeolites, having different framework structures and Si/Al ratios but similarly low crystallization temperatures, could form in the mild environments of underground repositories.

Different amounts of Fe(NO 3 ) 3 were added during synthesis while the Al/Si was kept constant and the products were characterized by XRD (Figure S1). At increasing Fe(NO 3 ) 3 loadings, cancrinite(NO 3 ) (CAN) was formed at the expense of FAU-X, LTA, and GIS-P1, whereas no such secondary phase occurred for FAU-Y. Cancrinite(NO 3 ) (Na 8 Al 6 Si 6 O 24 (NO 3 ) 2 •4H 2 O) is a zeolite-like feldspathoid with a Si/Al ratio of 1:1 (the same as LTA). The Si/Al ratio of 1:1 indicates alternately arranged Si-and Al tetrahedra in its framework. Cancrinite has also been reported to form during the reactions between leaked nuclear waste and primary subsurface minerals (e.g., quartz and feldspar). [START_REF] Bickmore | Nitrate-Cancrinite Precipitation on Quartz Sand in Simulated Hanford Tank Solutions[END_REF] Cancrinite is one of a few zeolitic minerals which has the potential to immobilize significantly mobile anionic species such as e.g., NO 3 -and CO 3 2-, on its anionic extra-framework sites. Figure 1. Experimental PDFs of (a) FAU-X and FAU-Y, of (b) LTA and GIS-P1, and of the zeolitic products with the addition of Fe(NO 3 ) 3 in the initial synthesis gels of the four zeolites. Simulated PDF references of HFO (HFO cal. ) and cancrinite(NO 3 ) (CAN cal. ) are also shown. We have chosen to display results with Fe/Al ratio of 0.3, except for the case of LTA with ratios of 0.1 and 0.5. T represents the tetrahedral Al or Si.

As from the XRD no direct information on the presence of disordered Fe-bearing precipitates was obtained, synchrotron PDF analysis was employed to check for disordered structures or nano-crystalline/amorphous phases in the zeolitic products. Since Al and Si are neighbours in the periodic system (Z = 13 and 14 for Al and Si, respectively) they are difficult to distinguish by X-ray scattering based analysis (e.g., PDF and EXAFS), and thus tetrahedral Al or Si are denoted together as "T". As shown in Figure 1, the existence of CAN was also confirmed in the PDFs by the characteristic correlations. The correlations at ~5.14, 6.40, and 7.75 Å attributed to the different T-T atomic pairs of CAN structure were also observed in the PDFs of Fe(NO 3 ) 3 -introduced FAU-X, LTA, and GIS-P1, but not for FAU-Y. The inhibition of CAN formation from the synthesis gel of FAU-Y could be due to the large mismatch between the Si/Al ratios of CAN (1:1) and FAU-Y (2:1). The characteristic correlation peaks from HFO can be detected for all zeolitic products where Fe was introduced, with increasing intensity as higher initial loadings of Fe(NO 3 ) 3 (Figure 1b, the case of LTA). The combined XRD and PDF results suggest that most of the introduced Fe 3+ precipitated as HFO instead of being incorporated into the zeolite framework. We hypothesize that the diffusion of Fe III into zeolitic cages is hindered by electrical charge repulsion due to the fact that under hyperalkaline condition (e.g., pH>13, see Table S2) the predominant aqueous Fe III species is anionic Fe(OH) 4 -(Figure A2). In the presence of Fe(NO 3 ) 3 , zeolites have a tendency to transform into CAN.

Effects of Fe 3+ -paired anions on zeolite formation. To discern whether Fe 3+ plays a role in the formation of cancrinite, control experiments were conducted where the same amount of NO 3 -was added in the form of NaNO 3 during LTA synthesis. In further experiments, FeCl 3 or NaCl was used instead. After hydrothermal treatment, the zeolitic products were characterized by XRD and PDF. As shown in Figure 2a, the product resulting from the NaNO 3 addition had the same XRD pattern as that from Fe(NO 3 ) 3 , indicating that in the presence of nitrate cancrinite is formed independent of the presence of Fe 3+ .

In the presence of Cl -, sodalite(Cl) (SOD, Na 8 Al 6 Si 6 O 24 Cl 2 ) was formed instead of LTA as shown by the XRD result, independent of which salt (FeCl 3 or NaCl) was used in the experiments, indicating a decisive role of anions on the zeolite structures formed. SOD often occurs together with CAN [START_REF] Reyes | Nucleation and growth process of sodalite and cancrinite from kaolinite-rich clay under low-temperature hydrothermal conditions[END_REF][START_REF] Barnes | The mechanism of the sodalite-to-cancrinite phase transformation in synthetic spent Bayer liquor[END_REF] and also possesses anionic sites (e.g., Cl -and OH -) that are expected to immobilize anionic contaminants. The PDF patterns of the solid products in the FeCl 3 and NaCl experiments are shown in Figure 2b. Differential PDF (d-PDF) for the FeCl 3 -introduced product was obtained by subtracting the PDF signal of NaCl-based product, which can be considered pure SOD, from that of FeCl 3 -based product. The first four main correlations in HFO were highlighted by vertical lines at ~1.98 (Fe-O pairs), ~3.03 (Fe-Fe pairs), ~3.45 (Fe-Fe pairs), and ~4.71 (Fe-O pairs), [START_REF] Manceau | Evaluation of the structural model for ferrihydrite derived from real-space modelling of high-energy X-ray diffraction data[END_REF] which matched well with the d-PDF patterns. This indicates that the initial introduction of Fe 3+ did not result in any difference in both ordered and disordered zeolitic structures but only resulted in HFO precipitation after hydrothermal reactions. The effective agents for the formation of CAN and SOD were only the paired anions, i.e., NO 3 -and Cl -, respectively, rather than the initially introduced Fe 3+ . The role of HFO on zeolite crystallization. Another possibility to incorporate Fe III into zeolite structures could be the reaction with HFO during zeolite crystallization; this process is susceptible to occur in nature when ferric (oxyhyr)oxides have precipitated before zeolite nucleation and crystallization. In such a case, the structure-directing effect of anions can be eliminated. The high purity of synthesized HFO was ascertained; only the typical broad two XRD peak occurred (Figure S3). The zeolitic products were synthesized with different loadings of HFO (Fe/Al ratios = 0.1 and 0.3) in the initial synthesis gels of FAU-Y and GIS-P1 and characterized by XRD.

As shown in Figure S4, with HFO loadings increasing, more CHA formed together with the target zeolite of FAU-Y while more FAU-X in the case of GIS-P1. As indicated by the LCF (Table S3) of Fe K-edge XANES-EXAFS spectra (Figure 3a), a small fraction of the initially added HFO has transformed to goethite for both FAU-Y and GIS-P1 products, with FAU-Y containing less goethite. No hematite was detected as goethite forms preferentially from HFO instead of hematite at pH 12-14 (i.e., the solution pH range after zeolite crystallization as shown in Table S2). [START_REF] Cornell | The iron oxides: structure, properties, reactions, occurrences and uses[END_REF] The experiments showed that amorphous HFO particles (size ~1-3 nm) and the released Fe(OH) 4 -affect zeolite nucleation, which resulted in the formation of chabazite or faujasite-X in the presence of HFO (Figure S4). Among the different ferric (oxyhyr)oxides, HFO is considered the most amorphous, metastable, and thus most reactive phase, i.e. the phase whose Fe III tetrahedra could most easily participate in the construction of the zeolitic framework together with Al 3+ tetrahedra. However, based on the XANES-EXAFS spectra it seems very likely that no or very little Fe III incorporation into zeolites has occurred. The reaction of HFO was also visible in the EXAFS data. Quantitative EXAFS fit (Figures 3b and3c, Table S4) indicated that the coordination number (CN) of the Fe-O shells of GIS-P1 product was with CN ~4.5 larger than that for FAU-Y product (CN ~3.9, almost the same as for the HFO reference 18, 19 ), indicating that the coordination symmetry of Fe III increased partially from tetrahedral (FeO 4 , occupying ~20% Fe III sites in HFO [START_REF] Michel | The Structure of Ferrihydrite, a Nanocrystalline Material[END_REF][START_REF] Manceau | Evaluation of the structural model for ferrihydrite derived from real-space modelling of high-energy X-ray diffraction data[END_REF] ) to octahedral (FeO 6 , occupying all the Fe III sites in goethite). The nucleation/growth of goethite is often retarded by foreign species, like SiO 3 2-and AlO 2 -, in solution due to competition with soluble Fe III species for sites on the subcritical nucleus or on the growing crystal. [START_REF] Cornell | The iron oxides: structure, properties, reactions, occurrences and uses[END_REF] The lower phase transformation from HFO to goethite for FAU-Y could be explained by the higher aqueous Al and Si concentrations (Tables S1 andS2) during the crystallization experiment compared with GIS-P1. During calcination, the 3-dimensional framework of LS-FAU-YH collapsed mostly as evidenced by the broad XRD peak envelopes (Figure S5a). In contrast, HS-FAU-YH was likely to keep the FAU framework intact, also after the introduction of Fe III in the cage using Fe(acac) 3 (Figure S5b). Thus, HS-FAU-YH should have more extraframework cationic sites available for Fe III in the cages, which was also confirmed indirectly by its higher Fe III contents (Table S5) in solid after diffusion impregnation. The acidic conditions allowed Fe III to diffuse into the faujasite cage, in contrast to the direct preparation at high pH values discussed in section 3.1, where no significant Fe uptake was observed. The uptake of Fe III in the cage did not affect the faujasite structure as verified by the XRD spectra.

After KOH treatment in a last step, the low-silica and highsilica FAU-Y having various Fe contents transformed to Kbased chabazite (Figure S5), with Si/Al ratios of LS-CHA and HS-CHA determined (from SEM-EDS) as ~1.8 and ~2.2, respectively. Fe K-edge XANES allows discerning between tetrahedral (Td) and octahedral coordination (Oh) of Fe III , mainly through the intensity of the pre-edge peak, the tetrahedral Fe III O 4 gives a more intense pre-edge peak. [START_REF] Cao | Origin of Magnetism in Hydrothermally Aged 2-Line Ferrihydrite Suspensions[END_REF][START_REF] Westre | A Multiplet Analysis of Fe K-Edge 1s → 3d Pre-Edge Features of Iron Complexes[END_REF] As shown in Figure 4a, the strong intensity of the pre-edge peak of both types of Fe-containing chabazites indicated the existence of tetrahedral Fe III O 4 .

The relatively high Fe III content in HS-CHA ensured a highquality EXAFS signal, allowing a further quantitative EXAFS fit, while the EXAFS signal quality of LS-CHA was too poor to make further investigation. Apparently, the oscillation frequency of the χ(k) function of HS-CHA-xFe was different from that of HFO (Figure 4b). After FT, a strong backscattering signal corresponding to the second coordination shell for HFO was observed (Figure 4c), attributing to the Fe-Fe atomic pairs at the distances of d Fe-Fe = 3.03 and 3.47 Å (Table 1). In contrast, the first FT peak of HS-CHA-xFe, corresponding to the first-shell O atoms, sharpened and shifted to a shorter distance compared with HFO. Quantitative EXAFS fits yielded the first shell of CN ∼4 O atoms with an identical bond length of 1.85 Å. Moreover, the second FT peak of HS-CHA-xFe was fitted well with the backscattering pair of Fe-T at a distance of d Fe-T = 3.25 Å, instead of any Fe-Fe backscattering pairs. Since Al and Si are neighbor elements, the tetrahedral AlO 4 and SiO 4 cannot be distinguished by X-ray backscattering signals and. Here, T represents tetrahedral Al or Si. It should be noted here that the CN of the second-shell Fe-T was determined as ~4 (the exact CN was fitted as ~3.1 probably due to the underestimated amplitude reduction factor (Ѕ 0 2 )), which is the exact coordination number of framework tetrahedra (e.g., Al and Si). This indicates that Fe III O 4 is most probably incorporated in the framework of HS-CHA-xFe during the phase transformation from faujasite. a CN: coordination numbers; R: atomic distances; σ 2 : Debye-Waller factors; ΔE 0 : shift of the threshold energy; R factor: goodness of fit. Ѕ 0 2 : amplitude reduction factor, 0.88, was obtained from the experimental EXAFS fit of goethite reference (Figure 3) by fixing the known CN of first-shell Fe-O and was applied for all the samples. Uncertainties are given by the number in brackets on the last digit(s), i.e., 9.4(3.2) represents 9.4 ± 3.2, and 3.0(3) means 3.0 ± 0.3. T represents the tetrahedral Al or Si.

Therefore, the spectrum of HS-CHA-xFe can be considered the reference to Fe III O 4 of chabazite. Although the strong intensity of pre-edge peaks of LS-CHA-xFe samples indicated a Td symmetry of Fe III , their edge peak shape was quite similar to HFO. By using HS-CHA-0.01Fe, HFO, goethite, and hematite as standards, LCF analysis of LS-CHA-xFe samples were conducted. It showed (see Figure 4a and Table 2) that all the LS-CHA-xFe samples consisted of two phases, i.e., Fe III O 4bearing chabazite and HFO. Besides, the proportion of HFO increased with initial loading of Fe(acac) 3 and that of Fe III O 4bearing chabazite decreased, correspondingly. Table 2. LCF results of Fe K-edge XANES of Fe III -containing LS-CHA. Also reported is the concentration sum (Ʃ), which is not constrained to 100% for samples, and the value of the reduced χ square (χ ν 2 ), both indicating good matches with the experimental spectra. The uncertainty is considered 10%. As illustrated by the broad peaks in Figure S5, LS-FAU-YH, the raw material before introducing Fe 3+ into the cage, had much less framework structures than HS-FAU-YH and thus less cation exchange capacity in positive cage sites. The limited Fe 3+ sorption sites in LS-FAU-YH, with the majority probably be surface sorption sites, were likely to be saturated at relatively low Fe(acac) 3 concentrations, which was verified by the little increase of solid Fe content in LS-FAU-Y-xFe (Table S5). After alkali treatment, the surface adsorbed Fe III could prefer to transform into HFO, resulting in the higher content of HFO under higher Fe(acac) 3 concentration (Table 2). The comparison between the cases of LS-CHA-xFe and HS-CHA-xFe provided a circumstantial evidence that diffusing into the zeolite cage is the first step to incorporate Fe III into the framework. A following phase transformation or dissolution/recrystallization of structure defects promotes then the isomorphic substitution of cage-M III (e.g., Fe III ) into the zeolite framework.

Al K-edge XAFS measurements on LS-CHA-xFe and HS-CHA-xFe were performed to detect any change of the local structure of the aluminosilicate framework. As shown in Figure 5, four absorption peaks can be observed, i.e., A (at 1566.2 eV from 1s → 3p transition), B (at 1569.2 eV from multiple scattering), C (at 1575.8 eV from 1s → 3d transition), and D (at 1582.4 eV from multiple scattering and/or 1s → 3d transition). [START_REF] Li | Al K-edge XANES spectra of aluminosilicate minerals[END_REF][START_REF] Geng | Nanometer-Resolved Spectroscopic Study Reveals the Conversion Mechanism of CaO•Al2O3•10H2O to 2CaO•Al2O3•8H2O and 3CaO•Al2O3•6H2O at an Elevated Temperature[END_REF] The major peak A at 1566.2 eV is considered the characteristic feature for the 4-fold connected AlO 4 tetrahedron of zeolites. In addition, the spectra with and without Fe III inclusion were almost the same for either low-silica or highsilica chabazite, suggesting that the surrounding local structure (typically the X-ray scattering domain is within ~5 Å) of center Al was not altered by the Fe III incorporation and thus -Al-O-Fe-bond did not form. Alternatively, the Fe contents in all the Fe-containing chabazites were too little to trigger any visible difference of Al K-edge XAFS signal. It is generally accepted that all zeolites obey Löwenstein's rule of "M 3+ avoidance" and that -Al-O-Al(Fe)bond formation is forbidden, although this rule is not completely applicable to several zeolites designed, e.g., for catalysis. [START_REF] Fricke | Incorporation of Gallium into Zeolites: Syntheses, Properties and Catalytic Application[END_REF][START_REF] Fletcher | Violations of Löwenstein's rule in zeolites[END_REF] Implications on Fe III incorporation pathways. The adopted experimental conditions were optimized to form pure target zeolites efficiently and to maximize Fe III incorporation. Consequently, the experimental conditions, mainly characterized by relatively high reaction temperature and alkalinity, were somewhat different from the actual environment of zeolitic phase evolution, e.g., degraded PC/clay interfaces and alkaliactivated cements, where zeolite formation occurs generally more slowly.

The following implications for zeolite formation under relevant environments can be deduced from the detailed analysis of different possible Fe III -uptake paths:

i) The intrusion of Fe III under alkaline conditions during zeolite nucleation does not result in a structural incorporation of Fe III into zeolites but leads to the precipitation of HFO. Under alkaline conditions, as suitable for zeolite formation, precipitation of ferric hydroxide occurs rapidly and results in low aqueous Fe III concentrations (Figure S2). In lab synthesis of zeolites, pH values used are typically higher than 10 in order to accelerate the nucleation and crystallization processes. [START_REF] Robson | Verified synthesis of zeolitic materials[END_REF] Above pH 9, the aqueous concentration of Fe III is dominated by Fe(OH) 4 -; with concentrations below 0.0001 mM at pH 8 which increase continuously up to ~ 3.7 mM at pH 14. Therefore, a chemical environment such as a degraded PC/clay interface with pH values between 8 and 10, would have even less chance to include Fe III in aluminosilicate structures due to the very low iron concentrations. In contrast, the presence of anions such as NO 3 -and Cl -in solution can lead to the formation of zeolites with anionic extra-framework sites such as cancrinite and sodalite.

ii) The intrusion of HFO alone could affect the original nucleation process of zeolite probably due to the interaction between AlO 2 -/SiO 3 2-and HFO nanoparticles and to the released Fe(OH) 4 -. However, no evidence did indicate any structural Fe III in zeolites, suggesting that the incorporation process is either not favorable or very slow once separate solid phases of ferric (oxyhyr)oxides and aluminosilicates formed.

iii) Fe III present in the cage of zeolites can change to tetrahedral framework sites via isomorphic substitution during zeolite phase transformations. Many zeolites are formed as metastable phases and often transform with time and/or changing conditions to more stable zeolites, especially in the presence of structure-directing agents or seeds. [START_REF] Van Tendeloo | Postsynthetic High-Alumina Zeolite Crystal Engineering in Organic-Free Hyper-Alkaline Media[END_REF][START_REF] Goel | Synthesis of Zeolites via Interzeolite Transformations without Organic Structure-Directing Agents[END_REF] Fe 3+ can slowly diffuse into the cage via cation exchange, which process could be more significant under slightly acidic and neutral conditions as common in geological media, where iron is not negatively charged. During zeolite crystallization and transformation, complex reactions, such as dissolution/reprecipitation, polymerization/depolymerization, and nucleation/crystallization, could occur, which gives good chances for the transformation of cage-Fe III to framework-Fe III . We assume that this could be the most feasible ways to incorporate tetrahedral framework-Fe III in zeolites. Under geochemical conditions, the diffusion of Fe III into the cages of evolving zeolites could be the first step towards framework-Fe III O 4 in more stable zeolites. For instance at the interface of the degraded PC with clay, the kinetically favored faujasite will transform into the more stable chabazite with time, a process by which Fe IIIcontaining chabazite could form. [START_REF] Lothenbach | Zeolite formation in the presence of cement hydrates and albite[END_REF] Consequently, the Fecontaining zeolite formation is beneficial to RNs retardation as these zeolites have the potential to affect redox conditions at such an interface and immobilize redox-sensitive anionic RNs that are more mobile. [START_REF] Ma | Selenite Sorption on Hydrated CEM-V/A Cement in the Presence of Steel Corrosion Products: Redox vs Nonredox Sorption[END_REF] Moreover, Fe II /Fe III -containing zeolites have also shown good performances on selective catalytic reduction of NO and on CH 4 hydroxylation. [START_REF] Snyder | The active site of low-temperature methane hydroxylation in iron-containing zeolites[END_REF][START_REF] Boroń | Influence of iron state and acidity of zeolites on the catalytic activity of FeHBEA, FeHZSM-5 and FeHMOR in SCR of NO with NH3 and N2O decomposition[END_REF]

CONCLUSIONS

In summary, we investigated three different reaction pathways with the higher possibility to form Fe III -containing zeolites in nature and in cement systems using hydrothermal synthesis methods and synchrotron-based analyses (XAFS and PDF). The results showed that introducing either Fe 3+ or HFO in the initial synthesis gels of zeolites does not result in structural incorporation of Fe III in zeolitic frameworks but lead to a main Fe III precipitate of HFO. Besides, the possible diffusion of aqueous Fe III into zeolitic extra-framework sites should be forbidden by electrical charge repulsion at high pH (e.g., pH>13) as the predominant aqueous Fe III species is anionic Fe(OH) 4 -as indicated by thermodynamic calculations. An obvious structure-directing effect was observed with Fe 3+paired anions of NO 3 -and Cl -, resulting in the formation of cancrinite and sodalite, respectively. In contrast, introducing firstly cage-Fe III in zeolites followed by a zeolitic phase transformation was proved to be the prerequisite to incorporate the tetrahedral framework-Fe III O 4 . The three proposed pathways for Fe III incorporation and the corresponding implication on

Figure 2 .

 2 Figure 2. (a) Experimental XRD patterns of LTA (without Fe) and of the zeolitic products with the addition of Fe(NO 3 ) 3 , NaNO 3 , FeCl 3 , and NaCl in the initial synthesis gel of LTA. (b) Experimental PDFs of LTA (without Fe) and the zeolitic products with the addition of FeCl 3 and NaCl in the initial synthesis gel of LTA. A Fe/Al ratio of 0.5 was selected and the same amount of the paired anion (i.e., NO 3 -or Cl -) introduced in the corresponding control experiment (i.e., with NaNO 3 or NaCl). Differential PDF (d-PDF) subtracting the PDF of NaCl-introduced product from that of FeCl 3introduced product is also shown together with the simulated PDF reference of HFO (HFO cal. ). T represents the tetrahedral Al or Si. A: LTA (Na 2 Al 2 Si 2 O 8 •4.5H 2 O, Ref. 01-073-2340), N: cancrinite(NO 3 ) (Na 8 Al 6 Si 6 O 24 (NO 3 ) 2 •4H 2 O, Ref. 00-038-0513), S: sodalite(Cl) (Na 8 Al 6 Si 6 O 24 Cl 2 , Ref. 01-088-2086).The role of HFO on zeolite crystallization. Another possibility to incorporate Fe III into zeolite structures could be the reaction with HFO during zeolite crystallization; this process is susceptible to occur in nature when ferric (oxyhyr)oxides have precipitated before zeolite nucleation and crystallization. In such a case, the structure-directing effect of anions can be eliminated. The high purity of synthesized HFO was ascertained; only the typical broad two XRD peak occurred (FigureS3). The zeolitic products were synthesized with different loadings of HFO (Fe/Al ratios = 0.1 and 0.3) in the initial synthesis gels of FAU-Y and GIS-P1 and characterized by XRD.As shown in FigureS4, with HFO loadings increasing, more CHA formed together with the target zeolite of FAU-Y while more FAU-X in the case of GIS-P1. As indicated by the LCF (TableS3) of Fe K-edge XANES-EXAFS spectra (Figure3a), a small fraction of the initially added HFO has transformed to

Figure 3 .

 3 Figure 3. Experimental Fe K-edge XAFS spectra of HFOintroduced (initial Fe/Al ratio = 0.1) zeolitic products of FAU-Y and GIS-P1, and of relevant ferric (oxyhyr)oxide references. (a) Normalized XANES spectra (in black) and the LCF results (in red). (b) k 3 -Weighted EXAFS oscillations. (c) Fouriertransformed (not corrected for phase shift) EXAFS signals. The black circles are the experimental data, and the red solid curves are quantitative fit results.The reaction of HFO was also visible in the EXAFS data. Quantitative EXAFS fit (Figures3b and 3c, TableS4) indicated that the coordination number (CN) of the Fe-O shells of GIS-P1 product was with CN ~4.5 larger than that for FAU-Y product (CN ~3.9, almost the same as for the HFO reference 18,19 ), indicating that the coordination symmetry of Fe III increased partially from tetrahedral (FeO 4 , occupying ~20% Fe III sites in HFO[START_REF] Michel | The Structure of Ferrihydrite, a Nanocrystalline Material[END_REF][START_REF] Manceau | Evaluation of the structural model for ferrihydrite derived from real-space modelling of high-energy X-ray diffraction data[END_REF] ) to octahedral (FeO 6 , occupying all the Fe III sites in goethite). The nucleation/growth of goethite is often retarded by foreign species, like SiO 3 2-and AlO 2 -, in solution due to competition with soluble Fe III species for sites on the subcritical nucleus or on the growing crystal.[START_REF] Cornell | The iron oxides: structure, properties, reactions, occurrences and uses[END_REF] The lower phase transformation from HFO to goethite for FAU-Y could be explained by the higher aqueous Al and Si concentrations (TablesS1 and S2) during the crystallization experiment compared with GIS-P1.

Figure 4 .

 4 Figure 4. Experimental Fe K-edge XAFS spectra of Fe III -containing LS-CHA and HS-CHA samples, and of HFO reference. (a) Normalized XANES spectra (in black) and the LCF results (in red). The pre-edge peak position is highlighted by the vertical dashed line. (b) k 3 -weighted EXAFS oscillations. (c) Fourier-transformed (not corrected for phase shift) EXAFS signals. The black circles are the experimental data, and the red solid curves are fit results. (d) A sketch describing the phase transformation of Fe IIIbearing zeolites. The cage cations (e.g., H + and K + ) and oxygen atoms are not displayed; units in blue and brown represent T sites (Al or Si) and Fe III sites, respectively.The role of cage-Fe III during phase transformation. A further possibility to incorporate Fe III into zeolite structures could occur based on zeolitic phase transformation (e.g., from faujasite to chabazite). Two types of FAU-YH with Si/Al atomic ratios of 2.0 (denoted as LS-FAU-YH) and 2.6 (denoted as HS-FAU-YH) were studied. In a first step, acid faujasites without Fe III were prepared by calcination to decompose and evacuate NH 4 + (for LS-FAU-YH) and organic resid-

Figure 5 .

 5 Figure 5. Experimental Al K-edge normalized XANES spectra of Fe III -containing (a) LS-CHA and (b) HS-CHA samples.Implications on Fe III incorporation pathways. The adopted experimental conditions were optimized to form pure target zeolites efficiently and to maximize Fe III incorporation. Consequently, the experimental conditions, mainly characterized by relatively high reaction temperature and alkalinity, were somewhat different from the actual environment of zeolitic phase evolution, e.g., degraded PC/clay interfaces and alkaliactivated cements, where zeolite formation occurs generally more slowly.The following implications for zeolite formation under relevant environments can be deduced from the detailed analysis of different possible Fe III -uptake paths:i) The intrusion of Fe III under alkaline conditions during zeolite nucleation does not result in a structural incorporation of Fe III into zeolites but leads to the precipitation of HFO. Under alkaline conditions, as suitable for zeolite formation, precipitation of ferric hydroxide occurs rapidly and results in low aqueous Fe III concentrations (FigureS2). In lab synthesis of zeolites, pH values used are typically higher than 10 in order to accelerate the nucleation and crystallization processes.[START_REF] Robson | Verified synthesis of zeolitic materials[END_REF] Above pH 9, the aqueous concentration of Fe III is dominated by Fe(OH) 4 -; with concentrations below 0.0001 mM at pH 8 which increase continuously up to ~ 3.7 mM at pH 14. Therefore, a chemical environment such as a degraded PC/clay interface with pH values between 8 and 10, would have even

  us a comprehensive understanding on the formation process of Fe III -containing zeolites both in nature and in lab experiments. This work could also provide an insight on the development of new catalytic routes and materials, and the optimization of existing zeolite catalysts.ASSOCIATED CONTENTSupporting Information Available. Additional materials referenced in the text are available free of charge.XRD patterns of HFO, Na-based zeolites, and zeolitic products with different initial loadings of Fe(NO 3 ) 3 . Solubility of amorphous Fe(OH) 3 as a function of pH. XRD patterns of FAU-Y and GIS-P1 and of zeolitic products with different initial loadings of HFO. XRD patterns of low/high-silica FAU-YH, HS-FAU-Y-xFe, and HS-CHA-xFe products. Hydrothermal synthesis conditions for Na-based zeolites and the aqueous results after zeolite crystallization. LCF results of Fe K-edge XAFS of HFO-introduced zeolite samples. Fe K-edge EXAFS refinement results. Solid Fe contents in FAU-Y-xFe and CHA-xFe.

  Al 2 Si 4 O 12 •8H 2 O), LTA (Na 2 Al 2 Si 2 O 8 •4.5H 2 O), and GIS-P1 (Na 6 Al 6 Si 10 O 32 •12H 2 O), were synthesized and fully characterized as detailed in our previous work.

			Simulated PDFs were obtained by
	PDFGui software. 33
	RESULTS AND DISCUSSION
	Fe 3+ co-precipitation with zeolites. Four highly pure zeo-
	lites,	FAU-X	(Na 2 Al 2 Si 2.5 O 9 •6.2H 2 O),	FAU-Y
	(Na 2

Table 1 .

 1 Local structure obtained from EXAFS refinement at Fe K-edge a .

	Sample	Atomic pair	CN	R (Å)	σ 2 (Å 2 •10 3 )	ΔE 0 (eV)	Ѕ0 2	R factor %
		Fe-O	1.0(4)	1.89(4)	9.4(3.2)
		Fe-O	3.0(3)	1.99(4)	8.9(1.0)
	HFO_Ref.					-1.6(9)	0.88(9)	3.1
		Fe-Fe	1.9(4)	3.03(1)	10.6(4.6)
		Fe-Fe	1.1(8)	3.47(3)	8.7(8.6)

ACKNOWLEDGMENTS

The authors would like to thank the following funding resources for this research: (a) Distinguished Senior Researcher Grant at Empa, (b) European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement number 754364 and (c) Nagra grant, grant number 16675. Erich Wieland and Rainer Dähn are thanked for their great help on XAFS measurements and scientific discussions. Agnieszka Poulain (ID31, ESRF), Thomas Huthwelker and Camelia Borca (Phoenix II, SLS), and Stephen Parry (B18, Diamond) are thanked for their assistance during synchrotron measurements. The synchrotron facilities are acknowledged for the allocation of beamtime (proposal number: SP21670, 20182181). Luigi Brunetti is acknowledged for measurements of the ion concentration of the solution, Biwan Xu and Zhenguo Shi for SEM-EDS measurements, Frank Winnefeld for XRD measurements, and Boris Ingold for the lab support.

AUTHOR INFORMATION

Corresponding Author *Phone: +33(0) 7 67 13 83 83. E-mail: bin.ma@empa.ch, mabinpku@gmail.com (B. MA).

Notes

The authors declare no competing financial interest.

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