The removal of Sr, Cd, and Pb from nuclear and industrial waste is important as these are harmful to living organisms and the environment. Immobilization of these ions in a zeolite framework is a simple and suitable method. However, zeolites are easily dehydrated at high temperatures.
Microporous and Mesoporous Materials 346 (2022) 112277 Contents lists available at ScienceDirect Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso In-situ dehydration study of the Sr-, Cd- and Pb-exchanged natrolite Junhyuck Im a, Jaewoo Jung b, Kiho Yang c, Donghoon Seoung d, **, Yongmoon Lee e, * a Decommissioning Technology Research Division, Korea Atomic Energy Research Institute (KAERI), Daejeon, 34057, South Korea Global Ocean Research Center, Korea Institute of Ocean Science & Technology, Busan, 49111, South Korea c Department of Oceanography, Pusan National University, Busan, 46241, South Korea d Department of Earth and Environmental Sciences, Chonnam National University, Gwangju, 61186, South Korea e Department of Geological Sciences, Pusan National University, Busan, 46241, South Korea b A R T I C L E I N F O A B S T R A C T Keywords: Dehydration In-situ X-ray diffraction Natrolite Rietveld refinement The removal of Sr, Cd, and Pb from nuclear and industrial waste is important as these are harmful to living organisms and the environment Immobilization of these ions in a zeolite framework is a simple and suitable method However, zeolites are easily dehydrated at high temperatures Therefore, the environmental changes around these adsorbed cations and water molecules in the zeolite framework must be explored for effective immobilization and waste removal In this study, we investigated the structural changes in fully Sr-, Cd-, and Pbexchanged natrolites (NAT) from room temperature to 350 ◦ C using in situ synchrotron X-ray powder diffraction and Rietveld analysis In the thermogravimetric analysis, Sr-NAT showed a gradual weight loss up to 210 ◦ C, whereas Cd- and Pb-NAT showed a two-step weight loss in the ranges 90–280 ◦ C and 100–180 ◦ C, respectively Sr-, Pb-, and Cd-NAT exhibited low thermal expansions with the thermal expansion coefficients of − 3(1) × 10− 6, − 1.0(7) × 10− 6, and 1(2) × 10− K− 1, respectively, at the initial stage of increasing the temperature During the dehydration process, the coefficients of Sr- and Cd-NAT were − 2.7(7) × 10− K− up to 300 ◦ C with a 2.9% volume contraction and − 5.3 × 10− K− up to 150 ◦ C with 2.7% volume contraction, respectively At high temperatures, structurally, the Sr2+ and Cd2+ cations had six- and seven-coordinated bonding with framework oxygens and extra-framework species, whereas Pb2+ cations had three- and five-coordinated bonding In contrast, the extra-framework water molecules in Sr-NAT had three to five bonds, Cd-NAT had five, and Pb-NAT had six The chain rotation angle of the secondary building units (T5O10) increased in all cases, indicating that the channel shape becomes more elliptical during dehydration Sr- and Pb-NAT were amorphized at 350 ◦ C and 150 ◦ C, whereas Cd-NAT remained intact We concluded that Sr- and Pb-NAT were not thermally stable owing to the order-disorder transition of Sr2+ and high-disorder distribution of Pb2+, respectively Our findings provide a fundamental understanding of the structural changes and mechanism of thermal stability in natrolites containing hazardous elements Introduction Strontium, cadmium, and lead are common hazardous elements in industrial waste In particular, the radioactive 90Sr, which is formed from β decay, is found in nuclear waste [1], whereas the heavy metals cadmium and lead are found in industrial wastewater If these elements are not properly disposed of or separated, they could be harmful to living organisms and the environment [2–5] The elements become untraceable if they are converted into ions in aqueous environment Therefore, numerous chemical methods were developed for immobi lizing these ions [6] For example, liquid-liquid extraction can separate strontium cation from the aqueous dissolution of spent fuel, which is accomplished by the formation of extractable metal–organic complexes [1] In addition, metal cations can be removed by chemical precipita tion, adsorption, reverse osmosis, solvent extraction, and ion exchange [7–10] Zeolites are typically used as absorbents, separators, or ion ex changers for removing pollutants owing to their high cation-exchange capability (CEC) and high efficiencies in uptaking trace quantities of radioactive or heavy metal cations in an ion exchange as well as low-cost ion exchangers [11,12] Therefore, zeolites are widely applied in various industries [13] * Corresponding author ** Corresponding author E-mail addresses: dseoung@jnu.ac.kr (D Seoung), lym1229@pusan.ac.kr (Y Lee) https://doi.org/10.1016/j.micromeso.2022.112277 Received July 2022; Received in revised form October 2022; Accepted October 2022 Available online 11 October 2022 1387-1811/© 2022 The Authors Published by Elsevier Inc This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) J Im et al Microporous and Mesoporous Materials 346 (2022) 112277 Structurally, zeolites are composed of a three-dimensional frame work of SiO4 or AlO4 tetrahedral structures linked by oxygen atoms Substituting Si4+ with Al3+ induces a negative charge in the zeolite framework To compensate for this charge unbalance, mono-, di-, or trivalent extra-framework cations (EFCs) are added inside the pores or channels and are usually coordinated with framework oxygens and water molecules [14] The chemical process for immobilization of harmful cations using zeolite involves the substitution of EFCs such as sodium, calcium, or potassium with the harmful cations During the cationic exchange, harmful cations will migrate to specific sites in the channel-void system and form new coordinate bonds in the zeolite framework [15,16] The properties of zeolitic water molecules such as desorption capacity are affected by the ion charges, ionic radii, and number of metal cations in the channel Moreover, investigation of the thermal behavior, or stability, of zeolites by temperature is crucial and fundamental for leaching of harmful elements since structurally altering phenomena of zeolites can be affected by elevated temperatures For example, calcination and dehydration may occur cell volume contrac tion Also, zeolites possibly transit to a metastable phase during high temperature reactions Lastly, bond breaking, distributional changes, and eventually, structural amorphization, may happen due to mentioned thermal processes [17] Therefore, the environmental changes around the adsorbed cations and water molecules must be investigated as their behavior is critical for dehydration or rehydration In this study, we used a common zeolite called natural natrolite (Na2Al2Si3O10⋅2H2O) The natrolite framework is formed by the linkages of T5O10 units among the ordered Si(Al)-tetrahedra (Fig 1) These linkages form elliptical channels along the c-axis The geometrical shape of the channel window on the ab-plane is determined by the rotation angle (Ψ) of the T5O10 unit; the higher the angle, the more elliptical is the shape of the channel window The monovalent EFCs, Na+, and water molecules are located in the middle (along the major axis) and wall (along the minor axis) of the two-dimensional plane of the channel (Fig 1a) In addition, the EFCs and water molecules have six- and fourcoordinated bonding with the framework oxygens and each other, respectively In contrast, the divalent EFCs and water molecules of scolecite are located at the center (along the major axis) and wall (along the major or minor axis) of the two-dimensional plane of the channel, respectively (Fig 1b) The EFCs and water molecules have seven-, three- , and five-coordinated bonding with the framework oxygens and each other, respectively The distribution of EFC and water molecules are ordered in both the cases The analogs of natrolite, mesolite (Ca2Na2(Al2Si3O10)3⋅8H2O) and scolecite (CaAl2Si3O10⋅3H2O), are also not solid solutions similar to natrolite, owing to their low CEC [18–20] In earlier work, however, we have reported that natrolite became to be fully exchangeable for alkali, alkaline earth, and heavy metal cations since new route was found by using disordered phase of K-exchanged natrolite [21] Natrolite has been shown to excel in exchange and capture of harmful cation by tempera ture treatment For example, Cs-exchanged natrolite was fully dehy drated upon heating at 100 ◦ C, and this phase was remained to be anhydrous and non-exchangeable after quenching and even exposing to aqueous condition In this study, we investigate the structural changes in Sr-, Cd- and Pbexchanged natrolites (NAT) and their thermal behaviors during dehydration Experimental method 2.1 Sample preparation In our previous reports, fully potassium-exchanged natrolite (K-NAT, K16Al16Si24O80⋅14H2O) has shown enhanced cation exchange capacity whereas natural natrolite (ideally Na16Al16Si24O80⋅16H2O) has limited exchange rate for divalent and heavy metal cations [21,22] In this study, we chose K-NAT as starting material The K-NAT was prepared using a fully saturated KNO3 (ACS reagent grade from Sigma-Aldrich) solution and a ground mineral natrolite (San Juan, Argentina from OBG International) in a 100:1 wt ratio The mixture was stirred at 80 ◦ C by minimizing the loss of water content in a closed system After 24 h, the solid was separated from the solution by vacuum filtration The dried powder was used for the second and third exchange cycles in the same conditions The final product was vacuum-filtrated and air-dried From the Energy-dispersive X-ray spectroscopy analysis (EDS, JEOL Ltd.), we confirmed K+ was fully exchanged Further cation-exchange of Sr2+, Cd2+ and Pb2+ was proceeded same solution-exchange method with K-NAT preparation Stirring the mixture of the powdered K-NAT and fully saturated M(NO3)2, (M = Sr2+, Cd2+ and Pb2+), solutions in a Fig Polyhedral representations of (a) Natrolite and (b) Scolecite viewed along [100] or [001] direction Filled balls represent the extra-framework cations (yellow: Na2+ and blue: Ca2+) and water molecule oxygens (red), respectively Striped blue (sky) tetrahedra illustrate an ordered distribution of Si (Al) atoms in the framework Channel opening geometry is defined by chain rotation angle (Ψ, degree) Major (minor) axis is defined by long (short) distance of two framework oxygens in 2-dimensional plane Coordinate numbers (C.N.) of extra-framework cations and water molecules are presented (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) J Im et al Microporous and Mesoporous Materials 346 (2022) 112277 1:100 wt ratio in a closed system at 80 ◦ C for 24 h Separating the solid from the solution by vacuum filtration, and then repeating the above two steps two more times We confirmed final products (Sr-NAT, Cd-NAT and Pb-NAT) were also fully exchanged using the Scanning Electron Microscopy (SEM-EDS, JSM-6701F) A Field emission SEM operating at 15 keV was used at Korea Institute of Ocean Science & Technology, Busan, Korea The prepared samples were totally air-dried for a day prior to carbon coating To determine the amount of H2O molecules in framework, Thermalgravimetric analysis (TG) was per formed at Pusan National University, Busan, Korea A heating range is from 25 to 400 ◦ C and a heating rate of 10 ◦ C/min under a nitrogen atmosphere Chemical analysis results were summarized in Table function proposed by Thompson et al was used to model the observed Bragg peaks [26], and a March-Dollase function [27] was used to ac count preferred orientation The structural models of the Sr-, Cd- and Pb-NAT at room temperature and their high-temperature forms were then established by Rietveld methods [25,28,29] To reduce the number of parameters, isotropic displacement factors were refined by grouping the framework tetrahedral atoms, the framework oxygen atoms, and the non-framework cations, respectively Geometrical soft-restraints on the T-O (T = Si, Al) and O–O bond distances of the tetrahedra were applied: the distances between Si–O and Al–O were restrained to target values of 1.620 ± 0.001 Å and 1.750 ± 0.001 Å, respectively, and the O–O dis tances to 2.646 ± 0.005 Å for the Si-tetrahedra and 2.858 ± 0.005 Å for the Al-tetrahedra The amounts of water molecules were calculated using the result of Rietveld refinement with OW1, OW2, and OW3 multiplicities and occupancies Difference Fourier syntheses have confirmed that the channels in the dehydrated materials are free from any meaningful residual electron densities from water molecules In the final stages of the refinements, all background and profile parameters, scale factor, lattice constants, 2θ zero, preferred orientation function, and the atomic positional and thermal displacement parameters were simultaneously refined whereas the weight of the soft-restrain was remained The final refined parameters are summarized in supporting Tables and 2, and selected bond distances and angles are listed in supporting Tables and 2.2 In-situ dehydration experiment In-situ high-temperature synchrotron X-ray powder diffraction ex periments were performed at the X14A beamline at the National Syn chrotron Light Source (NSLS) at Brookhaven National Laboratory (BNL) The primary white beam from the bending magnet was mono chromatized using a Si (111) crystal, and sets of parallel slits were used to create a monochromatic X-rays with a wavelength of 0.7297(1) Å Powdered natrolite samples were packed into 1.0 mm quartz capillaries, which were connected into a vacuum for the ease of dehydration K-type thermocouple (Omega Engineering, Inc.) was inserted into capillary to measure temperature The capillaries were then wrapped with a heating coil [23] Temperature was increased from RT to ca 350 ◦ C by 50 ◦ C increments For temperature calibration, NaCl powder was loaded to capillary and heated like abovementioned condition Using X-ray diffraction, unit cell volume of NaCl powder was measured by every 50 ◦ C increment up to ca 350 ◦ C Real temperature inside capillary is then calibrated by matching between calculated and our measured unit cell volume [24] A Si-strip detector prototype consisting of a monolithic array of 640 silicon diodes coupled to a set of BNL’s HERMES application-specific integrated circuits (D.P Siddons, Private commu nications) was used to collect high-resolution powder diffraction data (Δd/d ~ 10− 3) The Si-strip detector covered 3.2◦ in 2θ and was stepped in 2◦ intervals over the angular range of 3–30◦ with counting times of 10s per step The wavelength of the incident beam was determined from a LaB6 standard (SRM 660a) Result and discussion The synchrotron powder X-ray diffraction (PXRD) patterns of Cdand Pb-NAT are indexed in the orthorhombic Fdd2 space group, whereas Sr-NAT is in the monoclinic Cc space group at ambient conditions (Fig 2) The chemical structures of Sr-, Cd-, and Pb-NAT were derived from Rietveld refinements at room temperature (RT), as shown in sup porting Tables and 2, and Supporting Figs 3, 4, and Our models of Sr-, Cd-, and Pb-NAT are consistent with those reported in a previous study [22] In situ high-temperature synchrotron PXRD patterns of Sr-, Cd-, and Pb-NAT recorded up to 350 ◦ C are shown in Fig 2a, b, and 2c, respectively The reflections match with those of the RT models During the Thermogravimetric analysis (TGA) of Sr-NAT (Supporting Fig 1), gradual dehydration was observed up to ~210 ◦ C In contrast, two stages of dehydration were observed for Cd-NAT at 90 ◦ C and 280 ◦ C and for Pb-NAT, at 100 ◦ C and 180 ◦ C In the XRD patterns of Sr-NAT, there is hardly any noticeable peak shifting when the temperature increased up to 200 ◦ C, but from 200 ◦ C or more, most peaks in the XRD patterns tended to shift toward high 2-theta When the temperature reached 300 ◦ C, most of the peaks broadened This indicates that the structure of Sr-NAT started to collapse at 300 ◦ C and completely amorphized when 2.3 Structural analysis by rietveld refinement Temperature-dependent changes in the unit-cell lengths and volume were derived from a series of whole profile fitting procedures using the GSAS suite of programs [25] The background was fitted with a Che byshev polynomial with ≤20 coefficients, and the pseudo-Voigt profile Table Chemical composition of the fully exchanged K-, Sr-, Cd-, and Pb-natrolites.a Elements K-NAT Sr-NAT Cd-NAT Pb-NAT a b c d K Na Al Sr K Al Cd K Al Pb K Al Atomic percent (%)b Chemical Compositiond %H2Oc 11.99 0.00 11.22 5.93 0.00 12.41 6.39 0.00 12.07 6.63 0.00 12.74 10.39 0.00 10.88 5.62 0.00 12.49 6.19 0.00 12.9 6.11 0.00 11.47 10.87 0.00 10.89 6.36 0.00 12.95 6.38 0.00 12.42 6.58 0.00 12.54 10.91 0.00 10.87 5.96 0.00 11.96 6.3 0.08 11.99 6.28 0.00 12.85 11.26 0.00 10.68 5.76 0.00 11.37 6.52 0.14 12.72 6.24 0.01 12.24 10.53 K16.3Al16Si24O80•13.6H2O 12.32 Sr7.6Al16Si24O80•23.2H2O 10.56 Cd8.2K0.06Al16Si24O80•20.3H2O 8.75 Pb8.2K0.03Al16Si24O80•15.8H2O Values are normalized based on 16 aluminum atoms per unit cell Results from Energy dispersive X-ray Spectroscopy (EDS) The water contents in wt% Weight loss by Thermogravimetric analysis (TG) up to ca 400 ◦ C Confirmed from EDS and TG analysis Water contents are calculated from weight loss J Im et al Microporous and Mesoporous Materials 346 (2022) 112277 Fig in-situ synchrotron X-ray powder diffraction patterns of (a) Sr-NAT, (b) Cd-NAT, and (c) Pb-NAT as a function of temperature Selected (hkl) indices of ambient phases are shown Asterisk marks indicate peaks of the impurity the temperature reached 350 ◦ C (Fig 2a) Fig 2b shows the PXRD patterns of Cd-NAT with increasing temperatures The impurity of Cd-NAT at RT was also confirmed as a by-product according to previous work [22] The patterns exhibit changes at 150 ◦ C and 300 ◦ C, which are consistent with the TGA results that revealed two stages of weight loss during dehydration It is clear that most of the hkl peaks of Cd-NAT shift abruptly toward higher 2-theta values at 150◦ , suggesting a strong lat tice contraction (guide arrows in Fig 1b) These peaks subsequently shifted to lower 2θ values up to 350 ◦ C Therefore, dehydration may have been more dominant than thermal expansion at 150 ◦ C, and thermal expansion may have been more dominant over dehydration up to 350 ◦ C Cd-NAT maintained its crystallinity better than Sr-NAT even at 350 ◦ C, and the dehydrated phase was recovered after cooling down to room temperature For Pb-NAT, amorphization progressed rapidly at 150 ◦ C, indicating that it has a relatively poor thermal stability compared to those of Sr- and Cd-NAT, and an amorphous phase was observed without significant changes even if the temperature increased to 350 ◦ C (Fig 2c) In the case of Pb-NAT, all the diffracted reflections obtained between 150 and 300 ◦ C or the various transformed phases bearing different water molecule contents under various pressures and temperatures could not be attributed to natrolite [30,31] Next, temperature-dependent changes in the unit-cell parameters and volumes of Sr-, Cd- and Pb-NAT were analyzed using the wholeprofile-fitting method (Supporting Figs and 3a) The calculated thermal expansion coefficients of Sr-, Cd-, and Pb-NAT (Fig 3a) is as low as − 3(1) × 10− 6, − 1.0(7) × 10− 6, and 1(2) × 10− K− up to 200, 100, Fig Temperature-induced changes of (a) unit-cell volume (Å3) and (b) the orthorhombicity, 2(b–a)/(b + a), of Sr-, Cd-, and Pb-NAT after converting space group to Fd Open symbol represents the recovered phase and thermal expansion coefficients are given in Fig 3a with unit of ◦ C− Estimated standard deviations are smaller than the size of each symbol J Im et al Microporous and Mesoporous Materials 346 (2022) 112277 and 100 ◦ C, respectively Beyond the abovementioned temperature, the coefficients of Sr-NAT and Cd-NAT were determined as − 2.7(7) × 10− K− up to 300 ◦ C with a 2.9% volume contraction and − 5.3 × 10− K− up to 150 ◦ C with a 2.7% volume contraction, respectively For Sr- and Cd-NAT, apparent negative volume expansion refers that dehydration is dominant rather than thermal expansion The volume of Cd-NAT expanded up to 350 ◦ C with a coefficient of 4(1) × 10− K− Notably, Cd-NAT had the lowest coefficient among all the samples The thermal expansion coefficients are listed in Supporting Table and shown in Supporting Fig Fig 3b shows temperature-dependent changes in the ortho rhombicity of Sr-, Cd-, and Pb-NAT The orthorhombicity of Cd-NAT increases dramatically from 0.023(1) to 0.047(1) in the range 100–150 ◦ C, increasing by 104% These drastic structural changes originate from the temperature-dependent a-axis changes (Supporting Fig 2) The a-axis length of Cd-NAT decreased by 2.5% at 150 ◦ C, whereas those of the b- and c-axes decreased only slightly Compared to that of Cd-NAT, the values of orthorhombicity of Sr- and Pb-NAT are relatively constant in the range 0.017(1)–0.026(1) because a- and b-axis lengths of Sr- and Pb-NAT decreased by a similar ratio with increasing temperature (Supporting Fig 2a and b) To understand the volume changes owing to the loss of water mol ecules during dehydration, the water contents at selected temperatures are calculated using the Rietveld refinement (Fig 4, supporting Tables and 4) Fig shows the Rietveld refinement, structure models, chemical compositions, stoichiometric water contents, water migration, T5O10 chain rotation angles, coordination numbers of water molecules, and cation locations in the channels of Sr-, Cd-, and Pb-NAT at selected temperatures All structural models are oriented along the ab-plane to show the changes in the extra-framework species in the elliptical channels At room temperature, one cation site and three water molecule sites in the natrolite channel of Sr-NAT are fully occupied (Fig 4a, supporting Table 1) The array of extra-framework species in the channel of Sr-NAT is similar to that in scolecite (Ca2+ variant of natrolite) (Fig 1b) The Sr2+ cation (Sr1 site) is located in the middle of the channel, and two water molecules (OW1 and OW2 sites) are located along the minor axis (short axis) wall, whereas OW3 is located close to the major axis (long axis) wall in the channel Using the Rietveld refinement, the chemical formula of Sr-NAT was calculated to be Sr8Al16Si24O80⋅24H2O, which indicates that 24 water molecules are present per unit cell The number of water molecules is consistent with the results of the chemical analysis (Table 1) At room temperature, the central EFC (Sr2+) in the natrolite channel exhibits a seven-coordinated bonding with four framework oxygens and three water molecules The OW1 and OW2 sites have a fivecoordinated bonding with the surrounding three framework oxygens, one EFC, and each other The OW3 site has a three-coordinated bonding with two framework oxygens and one EFC Owing to the smaller number of bonds for OW3 than those for the other water molecules, it was easily dehydrated at 150 ◦ C, which decreased the occupancy to 66%, i.e., four water molecules were removed (Fig 4b) At 300 ◦ C, the OW3 site no longer existed, and the occupancy of the OW1 and OW2 sites also decreased to ~83%, i.e., to 13.7 water mole cules per unit cell (Fig 4c) In the refined structural model, the residual water molecules remain at 300 ◦ C while TG result shows the Sr-NAT is almost dehydrated (supporting Figs and 3d) The residual sites are observed by Fourier density calculation in the Rietveld refinement They are defined as residual sites of water molecules, OW1 and OW2, due to mismatch with framework atoms and extra-framework cations We expect that tight bonds of H2O-cation at 300 ◦ C interrupts to be dehy drated For example, H2O-cation bond length at RT, 150, and 300 ◦ C ranges 2.67(2)–2.86(2), 2.56(3)–2.95(4), and 2.42(9)–2.53(7), respec tively The ordered distribution of Sr2+ up to 150 ◦ C became disordered at two sites at 300 ◦ C, and its coordination number reduced to six similar to that of Na+ in natural natrolite (Fig 1a) A half occupancy becomes a void in the EFC, resulting in less bonding with the framework or EFCwater cluster to sustain the flexible channel These environmental changes at 300 ◦ C lead to an unstable coordination environment in SrNAT, making it difficult to maintain the NAT type structure, compared to that of the Sr-NAT model at room temperature (Fig 4c) The chemical formula of Cd-NAT at room temperature was calcu lated to be Cd8Al16Si24O80⋅16H2O, indicating that 16 water molecules are present per unit cell The Cd atoms reported are two equivalent positions in the center of the channel and has a seven-coordinated Fig Polyhedral representations of (a) starting material, K-NAT, (b) Sr-NAT-RT, (c) Sr-NAT-150C, (d) Sr-NAT-300C, (e) Cd-NAT-RT, (f) Cd-NAT-150C, (g) Cd-NAT-350C, (h) Pb-NAT-RT, and (i) Pb-NAT-100C, viewed along [100] or [001] direction Filled balls represent the extra-framework cations (violet: K+, green: Sr2+, pink: Cd2+ and grey: Pb2+) and water molecule oxygens (red), respectively Striped blue (sky) tetrahedra illustrate an ordered distribution of Si (Al) atoms in the framework (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) J Im et al Microporous and Mesoporous Materials 346 (2022) 112277 bonding with four framework oxygens and three water molecules (Fig 4d) The water molecule sites, OW1 and OW2, are located along the minor axis wall and the major axis wall, respectively; OW1 has a fivecoordinated bonding with three framework oxygens and two EFCs and OW2 has a five-coordinated bonding with four framework oxygens and one EFC All extra-framework species showed disordered distribution with an occupancy of 50%, and these atomic positions are similar to those in natrolite (Fig 1a) When the temperature was increased to 150 ◦ C, OW2 on the major axis was partially dehydrated, migrated along the a-axis and subsequently merged with OW1 Owing to the environ mental changes for OW2 as a result of dehydration, the occupancy of OW1 increased from 50 to 79%, and Cd-NAT lost 3.4 water molecules per unit cell at 150 ◦ C (Fig 4e) At 350 ◦ C, the occupancy of OW1 decreased from 79 to 32% owing to the loss of five water molecules per unit cell, whereas the Cd2+ and water molecule positions are nearly the same as those in the model at 150 ◦ C (Fig 4f) Fig 4g and h shows structures of Pb-NAT at room temperature and 100 ◦ C, respectively The calculated chemical formula of Pb-NAT was Pb8Al16Si24O80⋅16H2O, indicating that 16 water molecules are present per unit cell at room temperature Unlike the distribution of the cation sites in Cd-NAT, two Pb2+ sites on the major axis, Pb1 and Pb2, have the highest disorder and lowest occupancy (less than 35%) among our models The Pb1 site has a five-coordinated bonding with four frame work oxygens and one water molecule, whereas Pb2 has a threecoordinated bonding with two framework oxygens and one water molecule The water molecule site, OW1, is located along the minor axis wall and has a fully occupied distribution Although two cation sites moved at 100 ◦ C, occupancies and coordination of cations are main tained After the dehydration, the occupancy decreased to 58%, comprising 9.3 water molecules per unit cell In Pb-NAT, despite the presence of numerous coordination bonds among the framework oxy gens, cations, and water molecules, a higher number of water molecules (6.7 molecules) were dehydrated and even amorphized at the low temperature of 100 ◦ C This can be attributed to the highly disordered distribution of Pb2+ cation, which results in void spaces and loss of bonding with increasing temperature We found that dehydration behavior Fig shows the changes in the chain rotation angles and remaining number of water molecules in a unit cell with increasing temperature These two values usually have an inverse relationship because the T5O10 unit can be rotated to open the elliptical channels via the dehydration of water molecules Sr- and Cd-NAT lose water molecules according to the slope of the equations, y = − 0.04(1)x + 25.2(6) and y = − 0.04(1)x + 17 (1), respectively However, the rate of dehydration in Pb-NAT was more than two times that of Sr- and Cd-NAT (y = − 0.09x + 18.2) The large amount of water loss in Pb-NAT at 100 ◦ C is consistent with the increase in the chain rotation angle with the highest slope of 0.04 among all our models Moreover, the distance between the EFC and framework oxy gens in Pb-NAT increased from 2.42(2) to 2.87(1) Å at room tempera ture and from 2.10(1) to 2.99(1)Å at 100 ◦ C (supporting Tables and 4) Hence, the highly disordered distribution of Pb2+ in Pb-NAT enables it to collapse easily after partial dehydration For Sr-NAT, the interatomic distance between the Sr2+ site and framework oxygens increase from 2.67(1) to 2.86(1) Å at room temperature and from 2.46(3) to 2.76(3) Å at 300 ◦ C (supporting Tables and 4) Up to 150 ◦ C, the chain rotation angle increased according to the lowest slope of 0.01 among all our models, which indicates that the channel of Sr-NAT is well sustained by extra-framework species during dehydration However, the slope of the rotation angle increased at 300 ◦ C owing to the disordered distribution of Sr2+ At 350 ◦ C, distributional change of EFC can collapse the position of the water molecules and the entire framework For Cd-NAT, the rotation angle increased with a slope of 0.03 up to 150 ◦ C, which is quite large compared to the slopes of Pb-NAT and Sr-NAT However, the po sition and coordinated bonding of Cd2+ were maintained even after big changes such as dehydration and migration of water molecules This reliable environment prevented the collapse of the channel above 150 ◦ C, and the rotation angle was similar to that at 300 ◦ C The differences of phase transition during dehydration between SrNAT and Cd-, Pb-NAT seem to be induced by their structural differ ences at ambient condition The Sr-NAT, monoclinic Cc symmetry, shows one cation site with three water molecule sites in its channel with seven hydrogen bonding with water molecules and framework oxygen atoms However, Cd-NAT shows orthorhombic symmetry, Fdd2, with two cation sites and one or two water molecule sites in their channel, Fig Changes in the chain rotation angle (ψ, degrees) and number of H2O contents per 80 framework oxygens J Im et al Microporous and Mesoporous Materials 346 (2022) 112277 concomitant with seven hydrogen bonding between water molecules and framework oxygen atoms In case of the Pb-NAT, cation makes just three and five bonds with other oxygen atoms The changes of cation site such as migration or splitting between room and high temperature make structure more unstable For example, distribution of cation site in the Sr- and Cd-NAT at 300 and 350 ◦ C, respectively, is very similar How ever, the Sr-NAT is structurally amorphized subsequent temperature while the Cd-NAT is still stable In case of the Pb-NAT, distribution of two cation sites, Pb1 and Pb2, are highly disordered and low occupied in channel at room temperature Also, these two sites migrate along a- and b-axis at 100 ◦ C Therefore, structure of the Pb-NAT easily amorphized at comparatively lower temperature than that of the Sr-NAT Appendix A Supplementary data Supplementary data to this article can be found online at https://doi org/10.1016/j.micromeso.2022.112277 References [1] T.A Todd, T.A Todd, J.D Law, R.S Herbst, Cesium and Strontium Separation Technologies Literature Review, United States, 2004 [2] M Balsamo, F Di Natale, A Erto, A Lancia, F Montagnaro, L Santoro, Cadmium adsorption by coal combustion ashes-based sorbents—relationship between sorbent properties and adsorption capacity, J Hazard Mater 187 (2011) 371–378 [3] B Bayat, Comparative study of adsorption properties of Turkish fly ashes: I The case of nickel(II), copper(II) and zinc(II), J Hazard Mater 95 (2002) 251–273 [4] K.S Hui, C.Y.H Chao, S.C Kot, Removal of mixed heavy metal ions in wastewater by zeolite 4A and residual products from recycled coal fly ash, J Hazard Mater 127 (2005) 89–101 [5] L.-N Shi, Y Zhou, Z Chen, M Megharaj, R Naidu, Simultaneous adsorption and degradation of Zn2+ and Cu2+ from 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in small pore natrolite with divalent extra-framework cations, Chem Mater 27 (2015) 3874–3880 Conclusion In this study, we demonstrated the structural behaviors of Sr-, Cd-, and Pb-NAT during the dehydration process At room temperature, the structure of Sr-NAT is similar to that of scolecite, whereas those of Cdand Pb-NAT are similar to that of natrolite For Sr-, Cd-, and Pb-NAT, very low thermal expansion coefficients were observed before 200, 100, and 100 ◦ C, respectively The negative thermal expansion values for Sr- and Cr-NAT indicated a contraction of volume during dehydra tion The structure of Sr- and Cd-NAT collapsed beyond 300 and 100 ◦ C, respectively, whereas that of Cd-NAT was stable up to 350 ◦ C The chain rotation angle and structural stability with increasing temperature depend on the amount of water loss and distributional changes of the EFCs in the framework, respectively Our findings further our under standing of the structural changes and mechanism of thermal stability in natrolites containing hazardous elements CRediT authorship contribution statement Junhyuck Im: Writing – review & editing, Writing – original draft, Visualization Jaewoo Jung: Writing – original draft, Software, Inves tigation, Funding acquisition Kiho Yang: Writing – original draft, Methodology, Investigation, Funding acquisition Donghoon Seoung: Writing – review & editing, Writing – original draft, Visualization, Methodology, Investigation, Funding acquisition Yongmoon Lee: Writing – review & editing, Writing – original draft, Software, Meth odology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper Data availability No data was used for the research described in the article Acknowledgement This work was supported by the National Research Foundation (NRF2022R1F1A1074593, NRF-2020R1C1C1013642, NRF-2019K1A3A7A 09101574, NRF-202006710003) of the Ministry of Science and ICT of Korean Government, and Chonnam National University Research Grant, 2017, the Korea Atomic Energy Research Institute (KAERI) [Grant No 521240-22, South Korea] This research was a part of the project titled ‘Selection of prospective mining area for Co-rich ferromanganese crust in western Pacific seamounts: 3-D resource estimation and environ mental impact evaluation’, funded by the Korean Ministry of Oceans and Fisheries, Korea (No 20220509) Experiments using synchrotron radi ation were supported by X14A beamline at National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory (BNL) ... determined by the rotation angle (Ψ) of the T5O10 unit; the higher the angle, the more elliptical is the shape of the channel window The monovalent EFCs, Na+, and water molecules are located in the middle... (along the major axis) and wall (along the minor axis) of the two-dimensional plane of the channel (Fig 1a) In addition, the EFCs and water molecules have six- and fourcoordinated bonding with the. .. oxygens and each other, respectively In contrast, the divalent EFCs and water molecules of scolecite are located at the center (along the major axis) and wall (along the major or minor axis) of the