In-situ dehydration study of the Sr-, Cd- and Pb-exchanged natrolite

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.

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/)

In-situ dehydration study of the Sr-, Cd- and Pb-exchanged natrolite

aDecommissioning Technology Research Division, Korea Atomic Energy Research Institute (KAERI), Daejeon, 34057, South Korea

bGlobal Ocean Research Center, Korea Institute of Ocean Science & Technology, Busan, 49111, South Korea

cDepartment of Oceanography, Pusan National University, Busan, 46241, South Korea

dDepartment of Earth and Environmental Sciences, Chonnam National University, Gwangju, 61186, South Korea

eDepartment of Geological Sciences, Pusan National University, Busan, 46241, South Korea

A R T I C L E I N F O

Keywords:

Dehydration

In-situ X-ray diffraction

Natrolite

Rietveld refinement

A B S T R A C T 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 Pb- exchanged 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− 6 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− 4 K− 1 up to 300 ◦C with a 2.9% volume contraction and − 5.3 × 10− 4 K− 1 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

1 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, adsorpprecipita-tion, reverse osmosis, solvent extracprecipita-tion, 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)

Contents lists available at ScienceDirect Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso

https://doi.org/10.1016/j.micromeso.2022.112277

Received 8 July 2022; Received in revised form 3 October 2022; Accepted 8 October 2022

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 four-

coordinated 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 Pb- exchanged natrolites (NAT) and their thermal behaviors during dehydration

2 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 1 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.)

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 1

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)

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

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 1 and 2, and selected bond distances and angles are listed in supporting Tables 3 and 4

3 Result and discussion

The synchrotron powder X-ray diffraction (PXRD) patterns of Cd-

and 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 1 and 2, and Supporting Figs 3, 4, and 5 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

Table 1

Chemical composition of the fully exchanged K-, Sr-, Cd-, and Pb-natrolites.a

aValues are normalized based on 16 aluminum atoms per unit cell

b Results from Energy dispersive X-ray Spectroscopy (EDS)

cThe water contents in wt% Weight loss by Thermogravimetric analysis (TG) up to ca 400 ◦C

dConfirmed from EDS and TG analysis Water contents are calculated from weight loss

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 whole- profile-fitting method (Supporting Figs 2 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− 6 K− 1 up to 200, 100,

Fig 2 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

Fig 3 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− 1 Estimated standard deviations are smaller than the size of each symbol

and 100 ◦C, respectively Beyond the abovementioned temperature, the

coefficients of Sr-NAT and Cd-NAT were determined as − 2.7(7) × 10− 4

K− 1 up to 300 ◦C with a 2.9% volume contraction and − 5.3 × 10− 4 K− 1

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− 5 K− 1 Notably,

Cd-NAT had the lowest coefficient among all the samples The thermal

expansion coefficients are listed in Supporting Table 2 and shown in

Supporting Fig 2

Fig 3b shows temperature-dependent changes in the

ortho-rhombicity of Sr-, Cd-, and Pb-NAT The orthoortho-rhombicity 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 3

and 4) Fig 4 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 five- coordinated 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 1 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 EFC- water cluster to sustain the flexible channel These environmental changes at 300 ◦C lead to an unstable coordination environment in Sr- NAT, 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 4 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.)

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 five-

coordinated 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 three-

coordinated 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 5 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 2 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 2 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 Sr- NAT 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 5 Changes in the chain rotation angle (ψ, degrees) and number of H2O contents per 80 framework oxygens

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

4 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 Cd-

and 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 (NRF-

2022R1F1A1074593, 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 Nradi-ational Synchrotron Light

Source (NSLS) at Brookhaven National Laboratory (BNL)

Appendix A Supplementary data

Supplementary data to this article can be found online at https://doi org/10.1016/j.micromeso.2022.112277

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