Powder XRD Structural Study of Ba2+ Modified Clinoptilolite at Different Stages of the Ion Exchange Process Conducted at Two

Article Powder XRD Structural Study of Ba2+ Modiﬁed Clinoptilolite at Diﬀerent Stages of the Ion Exchange Process Conducted at Two Temperature ◦ Regimes—Room Temperature and 90 C

Louiza Dimowa 1,*, Yana Tzvetanova 1 , Ognyan Petrov 1, Iskra Piroeva 2 and Filip Ublekov 3

1 Institute of Mineralogy and Crystallography, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria; [email protected] (Y.T.); [email protected] (O.P.) 2 Academician Rostislav Kaishev Institute of Physical Chemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria; [email protected] 3 Institute of Polymers, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria; [email protected] * Correspondence: [email protected]

Received: 30 September 2020; Accepted: 20 October 2020; Published: 22 October 2020

Abstract: Partial and almost complete barium exchange on clinoptilolite is performed and structurally studied for different durations (2 h, 24 h, 72 h, 168 h, 12 d, 22 d) at room temperature and 90 ◦C of the ion exchange process. Continuing ion exchange up to the 22nd day is proved by EDS analyses data and powder XRD (intensity changes of 020 and 200 peaks). Rietveld structure refinement was first performed on the maximum Ba exchanged clinoptilolite at 90 ◦C for 22 days (3.04 atoms per unit cell). Four barium positions and 9 H2O sites were refined. The split positions Ba2 and BaK (around M3 site in channel C) were found mostly occupied by 2.23 atoms per unit cell. The rest of refined samples showed different occupations of the positions of incoming Ba2+ and outgoing cations (Na+, Ca2+,K+, Mg2+) during ion exchange, describing extra-framework cationic movements, which are released easily without preferable directions. The exchanges at 90 ◦C and room temperature were found proceeding similarly up to the 2nd hour, but then at room temperature the process is slowed and at 22nd day 1.64 barium atoms per unit cell are structurally refined.

Keywords: clinoptilolite; Ba-exchange; powder XRD; Rietveld; crystal structure

1. Introduction Natural zeolites are one of the most interesting mineral groups in the mineralogical classifications with more than 70 species. Some of them like clinoptilolite, mordenite, and chabazite form huge deposits in many countries including Bulgaria, which are of economic value and are a challenge for scientific investigations. Clinoptilolite forms about 90% of the world deposits of natural zeolite material used in different utilization procedures. 4 5 The specific microporous structure of these minerals is characterized by SiO4 − and AlO4 − tetrahedra forming framework with channels and cages in which cations compensating the framework charge are occupying specific sites and are coordinated by H2O molecules. This structural configuration is a prerequisite of unique properties of natural zeolites—ion exchange, sorption, adsorption, catalytic activity, surface modification, reversible dehydration-rehydration, etc. [1–5]. The crystal structure of clinoptilolite was first determined by Alberti [6] using single crystal XRD analysis. The author proved that clinoptilolite is isostructural with heulandite displaying HEU-type structure according to the recent structural classification of natural zeolites. Three cationic positions

Minerals 2020, 10, 938; doi:10.3390/min10110938 www.mdpi.com/journal/minerals Minerals 2020, 10, 938 2 of 16 were proved in the channels with one of them new, compared to heulandite. A little later Koyama and Tacheuchi [7] performed a new structural analysis of clinoptilolite crystals proving the same structure but finding a fourth cationic position, which they attributed to magnesium occupation. Using powder XRD data, Petrov et al. [8] refined the structure of sedimentary microcrystalline clinoptilolite and also proved the monoclinic structure (space group C2/m) as the previous authors and found increased Mg2+ content in position M4 proposed by Koyama and Tacheuchi [7]. Over the years, there have been many research papers published reporting results for ion-exchange of clinoptilolite with large spectrum of cations (alkaline, alkaline-earth, metals, etc.). However, studies on ion-exchange with large bivalent cations like Ba2+ (ionic radius 1.35 Å) are rare [9,10]. Hawkins and Ordonez [11] first prepared Ba-exchanged clinoptilolite and observed substantial decrease in the intensity of 020 diffraction line. Later, Petrov et al. [12] refined the structure of Ba-exchanged sedimentary clinoptilolite using powder XRD data and proved the distribution of barium ions in the plane of symmetry of the structure at around the potassium position (site M3 according to Koyama and Tackeuchi [7]) replacing the K+ ions. Cerri et al. [13] performed an interesting study on solid-state transformation of (NH4, Ba)-clinoptilolite upon heat treatment. The authors found increased thermal stability of Ba-dominated clinioptilolite—the higher the barium content, the higher the amorphization temperature. Naturally occurring Ba-clinoptilolite has still not been found or proved. However, Barium containing clinoptilolite with Ba contents up to 1.83 wt% BaO (0.33 Ba2+ per unit cell) was discussed [14] on a series of samples in Miocene sediments from the Yamato Basin of the Japan Sea—ODP Site 797. Similar content (0.44 Ba2+ per unit cell) was reported for clinoptilolite sample from Beli plast deposit, Bulgaria [15]. However, for the heulandite, the isostructural counterpart of clinoptilolite, it was proved to have natural representative discussed by Larsen et al. [16]. The authors described this find in detail including single crystal XRD refinement of the Heulandite-Ba new zeolite species in the heulandite series, occurring as an accessory mineral in hydrothermal veins of the Kongsberg silver deposit, Buskerud County, Norway. They found that Heulandite-Ba is monoclinic, C2/m. Their sample is almost with the same Ba concentration (2.20 Ba per unit cell) with the Ba-exchanged clinoptilolite (2.54 Ba per unit cell) of Petrov et al. [12]. In both samples all Ba is accumulated in the centre of the C ring distributed among three cation sites. In order to find additional structural specificity of distribution of barium ions in clinoptilolite structure during different regimes of ion-exchange, namely temperature and duration, we specify the purpose of the present paper to study Bulgarian microcrystalline clinoptilolite from Beli Plast deposit during barium exchange processes applying Rietveld powder XRD structural control. The results may shed light on specific site occupation of Ba2+ ions, probably governed by its large ionic radius rather than its bivalence character. This data may answer also to the found higher thermal stability of Ba-clinoptilolte. The changes in the occupancy of the cationic positions during the ion exchange process can contribute to the elucidation of the logistic and transport possibilities of barium and outgoing cations in the structure of clinoptilolite at room temperature (RT) and 90 ◦C. The successive changes in the populations of the cationic positions during the ion exchange process are connected with the movement of the cations in clinoptilolite structure. This is studied by the methods of X-ray diffraction analysis. We apply specific interpretation of Ba-clinoptilolite based on the gallery model proposed by Kirov et al. [17]. The authors show that the HEU structural type as well as the structures of other pseudo-layered zeolites, differ substantially from both the channel type structures (MOR, LTL, MAZ, LAU, etc.) and the cage type ones (CHA, FAU, LTA, etc.). The channel model of the porous space applied to HEU-type zeolites gives a topologically imprecise idea of the architecture of the porous space and misleads in the evaluation of its “logistical” capabilities—movement and positioning of exchangeable cation [17]. Consideration of the partially exchanged barium structures in our study up Minerals 2020, 10, 938 3 of 16 to the almost completely exchanged one is presented associated with the intracrystalline diffusion of cations. Understanding this process can be facilitated by adopting the gallery space model. At last, the aim of this study is to obtain new structural and crystal chemical data about the positioning of exchangeable large bivalent cation like Ba2+, which may help for structural characterization of naturally occurring Ba-clinoptilolite if found analogous to the already proved natural Ba-heulandite.

2. Materials and Methods

2.1. Materials The studied material is clinoptilolite tuﬀ from Beli Plast deposit (Eastern Rhodopes, Bulgaria) rich in clinoptilolite (80 wt%, proved by quantitative XRD analysis). After grinding and sieving, the fraction with grain size 0.032–0.016 mm (richest in clinoptilolite) was used for further experiments. Additional mineral phases present in the selected fraction were removed by heavy liquid separation giving only clinoptilolite and small amount (6–7 wt%) of opal-C. The last stage was the removal of opal-C using chemical treatment with solution of NaOH. All the above stages were described in detail by Dimowa et al. [18].

2.2. Cation Exchange The cation exchange experiments were performed with 8 g of clinoptilolite using 240 mL 0.5 M solution of BaNO3 for temperature regimes—room temperature (RT) and 90 ◦C. The solution was freshened at 48 h, 96 h, and 192 h. Samples for analyses were taken at 2 h, 24 h, 72 h, 168 h, 12 days, and ﬁnally at the 22nd day. Each separated sample was carefully washed with hot distilled water.

2.3. SEM/EDS Chemical analysis of each Ba exchanged clinopilolite sample was done with JEOL JSM6390 (JEOL technics Ltd., Tokyo, Japan) scanning electron microscope (SEM) coupled with an Oxford Instruments (Oxford Instruments Analytical Limited, High Wycombe, Buckinghamshire, UK) energy-dispersive X-ray (EDX) analyzer, with an acceleration voltage of 20 kV.

2.4. X-ray Diffraction Studies Powder XRD investigations were performed using Bruker D8 Advance ECO (Bruker AXS GmbH, Karlsruhe, Germany) scanning diffractometer, operating at 40 kV and 25 mA in Bragg–Brentano geometry with Ni-filtered Cu Kα radiation and a LynxEye-XE (Bruker AXS GmbH, Karlsruhe, Germany) scanning detector over the 2θ range of 8–100◦ with a scanning rate of 0.01◦/1 s. Structural refinements of unit-cell parameters, atomic positions, and site occupancies of cations and H2O molecules in clinoptilolite structure at different stages of barium exchange were performed using Rietveld based [19,20] refinement with Topas software (version 4.2). [21]. The background was fitted by a Chebyshev polynomial with 20 coefficients and the pseudo-Voigt peak function was applied. For all Ba-exchanged clinoptilolite samples, the refinement started with the model for Ba-heulandite of Larsen et al. [16] (ICSD #152331) and for the original Na, Mg positions and some H2O molecules the clinoptilolite structural model of Koyama and Takeuchi [7] (ICSD #100096) was used.

3. Results

3.1. Powder XRD and EDS Data of Ba Exchanged Clinoptilolite The clinoptilolite sample subjected to ion exchange with barium cations for a period up to the 22 days (at RT and 90 ◦C) undergoes exchange, which proceeds all the time with different speed confirmed from both the EDS analyses and the marked intensity changes of some peaks in the powder XRD patterns of samples selected at different exchange times (Figure1). On Figure1 the peaks 020 and 220 Minerals 2020, 10, 938 4 of 16 are indicated, which are most influenced by the degree of barium exchange. Comparing the patterns of theMinerals material 2020, 10 from, x FOR 2 PEER h of REVIEW exchange and the next taken at 24 h, 72 h, 168 h, 12 days, and 22 days, 4 of 17 we notice significant intensity changes in the above peaks in the diffraction patterns—020 decreases while days, we notice significant intensity changes in the above peaks in the diffraction patterns—020 220 increases (Table1). decreases while 220 increases (Table 1).

FigureFigure 1. Powder1. Powder XRD XRD patterns patterns of of the the puriﬁed purified clinoptilolite clinoptilolite (bottom)(bottom) andand thethe Ba-exchangeBa-exchange sequence of

clinoptiloliteof clinoptilolite at 90 at◦C( 90 left°C ()left and) and room room temperature temperature (RT) (RT) (right (right) (from) (from second second patternpattern toto top—2 h, h, 24 24 h, 72 h, 16872 h, h, 168 12 h, days, 12 days, and and 22 days, 22 days, of exchange). of exchange).

TableTable 1. 1.Integral Integral Intensities Intensities of 020 020 and and 220 220 peaks peaks of Ba-clinoptilolite of Ba-clinoptilolite for different for diﬀ erentexchange exchange times and times andtemperatures. temperatures.

Ion Exchange Integral Integral Intensities Ion Exchange Duration (h)/Temperature (◦C) Duration (h)/ Intensities 020 220 Temperature (°C) 020 220 initial 125.54 13.26 2 90initial 125.54 13.26 84.97 25.07 2 RT2 90 84.97 25.07 93.82 22.53 24 902 RT 93.82 22.53 66.1 42.51 72 9024 90 66.1 42.51 63.11 48.76 16872 90 90 63.11 48.76 61.17 52.36 288168 90 90 61.17 52.36 57.21 53.04 528 90 56.89 58.55 288 90 57.21 53.04 528 RT 82.22 28.7 528 90 56.89 58.55 528 RT 82.22 28.7 Selected powder XRD patterns of Ba-clinoptilolite at different exchange regimes are shown on Figure2Selected. The sample powder exchanged XRD patterns at 90 of◦ CBa-clinoptilolite for 24 h displays at different lowest intensityexchange ofregimes 020 peak are comparedshown on to theFigure other two2. The Ba-exchanged sample exchanged samples. at 90 The °C for sample 24 h displays at 90 ◦C lowest for 2 h intensity and at RT of for020 22peak days compared show almost to equalthe intensityother two of Ba-exchanged this peak. Exact samples. intensity The sample values at of 90 020 °C and for 2202 h and calculated at RT for by 22 profile days show fitting almost [22] and normalizedequal intensity to the of intensity this peak. of Exact the complex intensity peakvalues at of 2θ 020= 22.39and 220 (d calculated= 3.97 Å) areby profile given fitting in Table [22]1. and normalized to the intensity of the complex peak at 2θ = 22.39 (d = 3.97 Å) are given in Table 1.

Minerals 2020, 10, 938 5 of 16 Minerals 2020, 10, x FOR PEER REVIEW 5 of 17

FigureFigure 2. 2.Selected Selected powderpowder XRD patterns patterns of of Ba-exchanged Ba-exchanged clinoptlolite. clinoptlolite.

TheThe performed performed EDS EDS analyses analyses givegive numeric data data (Tables (Tables 22 andand 3)3) for for barium barium content content during during the the applied exchange procedures, which describe specific trends of exchange (Figure 3). appliedMinerals exchange 2020, 10, x FOR procedures, PEER REVIEW which describe speciﬁc trends of exchange (Figure3). 6 of 17

Table 2. Chemical composition (EDS, wt%) and structural formulae of Ba-clinoptilolite (90°) based on TableTable 2. Chemical 3. Chemical composition composition (EDS, (EDS, wt%) wt%) andand structuralstructural formulae of of Ba-clinoptilolite Ba-clinoptilolite (RT) (90 based◦) based on on 72 oxygen atoms. 72 oxygen72 oxygen atoms. atoms. Samples Samples Oxides2 h 2 h 24 h 24 h 72 h 72 h 168 168 h h 12 12 dd 22 d 22 d SamplesOxides 2 h 24 h 72 h 168 h 12 d 22 d OxidesSiO2 SiO2 66.24 66.2460.39 60.3960.49 60.4960. 60.8181 59.2259.22 59.5059.50 AlSiO2O23 Al2O3 11.4368.93 11.4310.5855.06 10.5810.8569.12 10.85 10.9960.71 10.99 10.8210.8268.26 10.8810.8862.72 MgOAl2O3 MgO12.310.22 0.220.199.77 0.1912.210.16 0.1610.770.0 0.033 12.19 –– – 11.09– CaOMgO CaO2.290.30 2.291.130.23 1.130.730.29 0.730.720.22 0.72 0.440.440.24 0.200.200.21 NaCaO2O Na2O0.722.74 0.720.462.08 0.460.122.48 0.122.15– – 2.35 –– – 2.13– NaK2O2O K2O0.930.78 0.930.770.62 0.770.30.705 0.350.0.5833 0.33 0.250.250.51 0.060.060.21 BaOK2O BaO6.811.32 6.819.730.99 9.7312.921.23 12.92 13.0.95 13.8585 14.6514.650.93 15.6815.680.67 BaOΣ Σ 88.645.86 88.64 83.255.02 83.25 85.626.76 85.62 86.736.53 86.73 85.38 85.388.24 86.32 86.328.42 Σ 92.24 73.77AtomsAtoms per formula 92.79 unit unit (apfu) (apfu)81.91 92.72 85.45 Si 29.91 29.84Atoms per formula29.71 unit (apfu)29. 68 29.62 29.62 Si 29.91 29.84 29.71 29.68 29.62 29.62 AlSi 29.746.08 29.776.16 29.796.28 29.776.32 29.746.38 29.796.38 Al 6.08 6.16 6.28 6.32 6.38 6.38 MgAl 0.156.26 0.146.22 0.126.20 0.06.232 6.26– 6.21– Mg 0.15 0.14 0.12 0.02 – – CaMg 1.110.19 0.600.18 0.380.18 0.380.16 0.240.16 0.110.15 Ca 1.11 0.60 0.38 0.38 0.24 0.11 Ca 1.27 1.20 1.14 1.13 1.10 1.08 Na Na0.63 0.630.44 0.440.11 0.11– – –– – – Na 0.65 0.65 0.58 0.55 0.43 0.19 K K0.54 0.540.485 0.4850.22 0.220.20 0.20 0.160.16 0.040.04 K 0.72 0.68 0.67 0.59 0.52 0.41 Ba Ba1.20 1.201.88 1.882.49 2.492.6 2.655 2.872.87 3.063.06 Ba 0.99 1.06 1.14 1.25 1.41 1.57

FigureFigure 3. 3.Exchanged Exchanged Ba Ba2+2+ forfor didifferentﬀerent duration time time and and temperature temperature (EDS (EDS data). data).

These trends correspond to certain stages of ion exchange and practically last until the 22nd day (528 h) when 3 Ba2+ cations per unit cell are exchanged at 90 °C (Figure 3). At RT the ion-exchange process is starting slowly and up to the 22nd day the maximum barium content is below 1.6 per unit cell, which is lower than the value (1.88) for the sample at 90 °C for 24 h. In the case of the high temperature Ba-exchange, the process is rapid and at 72 h the barium content is already high—2.49 per unit cell.

3.2. Rietveld Structural Analyses of Ba Exchange on Clinoptilolite with Time and Temperature The starting Rietveld structural refinement was performed on the maximum Ba exchanged clinoptilolite—Ba-clinoptilolite (90 °C, 22 d) in order to fix the final Ba positions when all the original cations are exchanged. The refinement of Ba-clinoptilolite finally converged at acceptable reliability factors (Table 4). Table 4 shows that the unit cell parameters of all studied Ba-exchanged samples are very similar except slight increase of the volume, V, from 2106.7 Å3 (sample 90 °C, 2h) to 2107.9 Å3 (sample 90 °C, 22 d).

Minerals 2020, 10, 938 6 of 16

Table 3. Chemical composition (EDS, wt%) and structural formulae of Ba-clinoptilolite (RT) based on 72 oxygen atoms.

Samples Oxides 2 h 24 h 72 h 168 h 12 d 22 d

SiO2 68.93 55.06 69.12 60.71 68.26 62.72 Al2O3 12.31 9.77 12.21 10.77 12.19 11.09 MgO 0.30 0.23 0.29 0.22 0.24 0.21 CaO 2.74 2.08 2.48 2.15 2.35 2.13 Na2O 0.78 0.62 0.70 0.58 0.51 0.21 K2O 1.32 0.99 1.23 0.95 0.93 0.67 BaO 5.86 5.02 6.76 6.53 8.24 8.42 Σ 92.24 73.77 92.79 81.91 92.72 85.45 Atoms per formula unit (apfu) Si 29.74 29.77 29.79 29.77 29.74 29.79 Al 6.26 6.22 6.20 6.23 6.26 6.21 Mg 0.19 0.18 0.18 0.16 0.16 0.15 Ca 1.27 1.20 1.14 1.13 1.10 1.08 Na 0.65 0.65 0.58 0.55 0.43 0.19 K 0.72 0.68 0.67 0.59 0.52 0.41 Ba 0.99 1.06 1.14 1.25 1.41 1.57

These trends correspond to certain stages of ion exchange and practically last until the 22nd day 2+ (528 h) when 3 Ba cations per unit cell are exchanged at 90 ◦C (Figure3). At RT the ion-exchange process is starting slowly and up to the 22nd day the maximum barium content is below 1.6 per unit cell, which is lower than the value (1.88) for the sample at 90 ◦C for 24 h. In the case of the high temperature Ba-exchange, the process is rapid and at 72 h the barium content is already high—2.49 per unit cell.

3.2. Rietveld Structural Analyses of Ba Exchange on Clinoptilolite with Time and Temperature The starting Rietveld structural refinement was performed on the maximum Ba exchanged clinoptilolite—Ba-clinoptilolite (90 ◦C, 22 d) in order to fix the final Ba positions when all the original cations are exchanged. The refinement of Ba-clinoptilolite finally converged at acceptable reliability factors (Table4). Table4 shows that the unit cell parameters of all studied Ba-exchanged samples are very similar except 3 3 slight increase of the volume, V, from 2106.7 Å (sample 90 ◦C, 2h) to 2107.9 Å (sample 90 ◦C, 22 d).

Table 4. Reliability factors (Rexp, Rwp, Rp, GOF, RB) and reﬁned unit cell parameters of Ba-clinoptilolite from the ion exchange sequence at 90 ◦C.

Samples Parameters 2 h 24 h 72 h 168 h 12 d 22 d Rexp 2.06 2.10 2.10 2.10 2.17 2.10 Rwp 5.51 5.47 5.16 5.33 5.13 5.32 Rp 4.06 3.99 3.80 3.87 3.78 3.89 GOF 2.67 2.61 2.46 2.54 2.36 2.54 RB 2.03 2.15 1.66 1.77 1.79 1.83 α (Å) 17.6784(7) 17.6845(9) 17.6848(11) 17.6861(8) 17.6869(10) 17.6872(6) b (Å) 17.9478(9) 17.9507(10) 17.9516(10) 17.9507(9) 17.9542(9) 17.9527(8) c (Å) 7.4074(3) 7.4000(4) 7.3973(4) 7.4002(3) 7.3954(3) 7.3958(2) β (◦) 116.317(2) 116.221(3) 116.186(3) 116.227(3) 116.169(3) 116.158(2) V (Å3) 2106.7(2) 2107.3(2) 2107.4(2) 2107.5(12) 2107.7(2) 2107.9(2)

The difference powder XRD plot of sample Ba-clinoptilolite (90 ◦C, 22 d) after the final refinement stage is shown on Figure4. Minerals 2020, 10, x FOR PEER REVIEW 7 of 17

Table 4. Reliability factors (Rexp, Rwp, Rp, GOF, RB) and refined unit cell parameters of Ba- clinoptilolite from the ion exchange sequence at 90 °C.

Samples 2 h 24 h 72 h 168 h 12 d 22 d Parameters Rexp 2.06 2.10 2.10 2.10 2.17 2.10 Rwp 5.51 5.47 5.16 5.33 5.13 5.32 Rp 4.06 3.99 3.80 3.87 3.78 3.89 GOF 2.67 2.61 2.46 2.54 2.36 2.54 RB 2.03 2.15 1.66 1.77 1.79 1.83 α (Å) 17.6784(7) 17.6845(9) 17.6848(11) 17.6861(8) 17.6869(10) 17.6872(6) b (Å) 17.9478(9) 17.9507(10) 17.9516(10) 17.9507(9) 17.9542(9) 17.9527(8) c (Å) 7.4074(3) 7.4000(4) 7.3973(4) 7.4002(3) 7.3954(3) 7.3958(2) β(°) 116.317(2) 116.221(3) 116.186(3) 116.227(3) 116.169(3) 116.158(2) V (Å3) 2106.7(2) 2107.3(2) 2107.4(2) 2107.5(12) 2107.7(2) 2107.9(2)

Minerals 2020The, 10 difference, 938 powder XRD plot of sample Ba-clinoptilolite (90 °C, 22 d) after the final7 of 16 refinement stage is shown on Figure 4.

FigureFigure 4. Di4. ﬀDifferenceerence plot plot of of the the powder powder XRD XRD experimentalexperimental and and calculated calculated pattern pattern of ofBa-clinoptilolite Ba-clinoptilolite (90 °C, 22 d). (90 ◦C, 22 d).

AfterAfter the the ﬁnal final stage stage of of the the reﬁnements refinements wewe obtainedobtained values for for the the structural structural parameters, parameters, which which are reported in Table 5 for Ba-clinoptilolite (90 °C, 22 d) and Ba-clinoptilolite (90 °C, 2h) together with are reported in Table5 for Ba-clinoptilolite (90 ◦C, 22 d) and Ba-clinoptilolite (90 ◦C, 2h) together with thethe interatomic interatomic distances distances reported reported in in Table Table6 .6.

Table 5. Wyckoﬀ positions (Wp), atomic coordinates, and occupancy factors of samples Ba-clinoptilolite

(90 ◦C, 2h) and Ba-clinoptilolite (90 ◦C, 22 d).

Site Atom Wp x y z Occ 2 h Occ 22 d Si/Al Si4+ 8 j 0.1793(8) 0.1685(7) 0.0956(2) 1 1 Si/Al Si4+ 8 j 0.2885(8) 0.0898(7) 0.5002(2) 1 1 Si/Al Si4+ 8 j 0.2921(8) 0.3092(6) 0.2828(2) 1 1 Si/Al Si4+ 8 j 0.4356(9) 0.2012(7) 0.5887(2) 1 1 Si/Al Si4+ 4 h 0.0 0.2132(2) 0.0 1 1 2 O1 O − 4 i 0.3047(2) 0.0 0.5494 (4) 1 1 2 O2 O − 8 j 0.2684(2) 0.3798(9) 0.3889(3) 1 1 2 O3 O − 8 j 0.3171(2) 0.3492(9) 0.1177(3) 1 1 2 O4 O − 8 j 0.2385(2) 0.1047(2) 0.2545(3) 1 1 2 O5 O − 4 g 0.5 0.1733(2) 0.5 1 1 2 O6 O − 8 j 0.0821(2) 0.1586(2) 0.0636(3) 1 1 2 O7 O − 8 j 0.3745(2) 0.2660(2) 0.4498(3) 1 1 2 O8 O − 8 j 0.0086(2) 0.2677(9) 0.1850(2) 1 1 2 O9 O − 8 j 0.2115(2) 0.2531(2) 0.1753(2) 1 1 2 O10 O − 8 j 0.3839(2) 0.1275(9) 0.5997(2) 1 1 Na(M1) Na+ 4 i 0.1497(34) 0.0 0.6335(26) 0.157(34) 0 Ca(M2) Ca2+ 4 i 0.4584(30) 0.0 0.8014(65) 0.282(21) 0.031(12) K(M3) K+ 4 i 0.2182(37) 0.5 1.0039(50) 0.136(13) 0 Mg(M4) Mg2+ 2 c 0 0 0.5 0.072(24) 0 Ba1 Ba2+ 4 i 0.3550(25) 0.5 0.3237(66) 0.085(15) 0.131(10) Ba2 Ba2+ 4 i 0.2492(14) 0.5 0.0716(37) 0.156(48) 0.444(28) Ba(Ca) Ba2+ 4 i 0.4818(59) 0 0.7820(42) 0 0.072(32) Ba(K) Ba2+ 4 i 0.2275(37) 0.5 0.9952(31) 0.073(31) 0.114(29) 2 O11 O − 4 j 0.601(16) 0.0889(47) 0.941(14) 0.145(20) 0.450(21) 2 O12 O − 4 i 0.4099(52) 0.0 0.7478(26) 0.520(30) 0.243(27) 2 O13 O − 8 j 0.5774(32) 0.0816(21) 0.9728(39) 0.68(2) 0.31(3) 2 O14 O − 2 b 0 0.5 0 1 0.92(12) O15 2 h 8 j 0.4753(38) 0.4111(36) 0.5290(34) O2 0.41(3) 0.556(34) O15 22 d − 4 h 0.5 0.3989(26) 0.5 O16 2 h 0.4410(26) 0.1802(41) O2 4 i 0.5 0.44(2) 0.30(1) O16 22 d − 0.4790(21) 0.4033(20) O17 O2 4 i 0.4016(43) 0.0 0.0029(16) 0.320(50) 0.476(30) − − O18 2 h 0.3808(41) O2 4 i 0.5 0.4710(38) 0.154(4) 0.448(59) O18 22 d − 0.3858(39) 2 O19 22 d O − 4 i 0.4017(38) 0.5 0.2630(67) 0 0.142(43) Minerals 2020, 10, 938 8 of 16

Table 6. Interatomic distances of the extra-framework cations and H2O molecules of samples Ba-clinoptilolite (90 ◦C, 2 h) and Ba-clinoptilolite (90 ◦C, 22 d) (nonindicated sample cells correspond to both samples).

Sample Atom 1 Atom 2 d(Å) Sample Atom 1 Atom 2 d(Å) Ba1 Na 0.36(23) K O11 2 2.49(12) × O18 0.978(16) O13 2 2.81(11) × Ba2 1.972(54) O4 2 3.010(31) × 2 h O16 2.19(10) O3 2 3.130(10) × 22 d O16 2.73(12) Ca O13 2 2.411(51) × Mg 2.301(12) O13 2 2.506(62) × Ba(K) 2.481(12) O1 2.524(27) K1 2.530(23) O14 2.642(45) 2 h O15 2 2.553(32) Ca 2.643(48) × 22 d O15 2 2.937(22) O10 2 2.731(38) × × O2 2 2.808(14) O11 2 2.767(78) × × O12 2.847(38) O17 2.875(10) O11 O13 0.576(45) O3 2 3.033(29) O8 3.105(77) × Ba2 Ba(K) 0.52(12) O13 3.11(12) K 0.55 (19) 011 3.19(13) Ba1 1.972(41) O12 O18 1.88(13) Na 2.12(12) O17 1.921(73) 22 d O19 2.432(36) 2 h O16 2.462(62) 2 h O18 2.839(21) 22 d O16 2.65(2) O11 2 2.849(23) 2 h O15 2 2.858(33) × × 22 d O18 2.875(19) 2 h O16 3.004(54) O3 2 2.919(15) O17 3.020(68) × O17 2.972(35) O6 2 3.14(10) × O2 2 3.096(22) O13 O13 2.93(11) × 2 h O16 3.102(47) O13 2.94(12) Ba(Ca) Ca 0.491(22) O10 3.042(16) O13 2 2.209(49) O8 3.068(22) × O14 2.24(11) O14 O5 3.110(34) O11 2 2.483(240 2 h O15 O15 1.13(10) × Ca 2.781(20) 2 h O18 2.21(10) O10 2 2.826(42) 2 h O16 2.564(30) × O1 2.827(16) 2 h O12 2.857(22) O13 2 2.868(47) 2 h O16 2.861(49) × Ba(Ca) 3.000(49) 2 h O18 3.002(31) Ba(K) K 0.201(21) 2 h O7 3.062(23) O11 2 2.62(11) 22 d O15 O16 2 1.926(21) × × Na 2.64(19) 22 d O19 2 2.59(16) × 22 d O19 2.83(11) 22 d O18 2 2.653(40) × O4 2.88(15) 22 d O7 3.166(54) O13 2.97(14) 2 h O16 O17 1.217(62) O3 3.065(20) 2 h O18 2.78(16) O17 3.07(10) 2 h O18 3.08(11) 2 h Na O18 0.72(20) 22 d O16 O16 1.288(16) Mg 2.38(13) 22 d O19 1.30(15) O16 2.521(16) 22 d O18 1.92 (56) O12 2.542(17) 22 d O19 2.431(18) O15 2.554(14) 22 d O12 2.652(21) O2 2.64(13) 22 d O17 2.690(11) K 2.66(28) O17 O6 2.917(34) O1 3.07(11) O4 3.004(16) 2 h Mg O15 1.693(32) 22 d O17 O19 1.96(17) O18 2.02(10) 2 h O18 O2 2.811(16) O16 2.125(44) 22 d O18 O2 2.868(17) O19 1.681(27) O12 2.914(44) 22 d O19 O3 3.051(13) Minerals 2020, 10, 938 9 of 16

4. Discussion

4.1. Powder XRD and EDS Data Interpretation of Ba Exchanged Clinoptilolite The EDS data confirms that the barium exchange on the studied clinoptilolite proceeds up to the 22nd day with different speed. Powder XRD analyses show marked intensity changes especially for peaks 020 and 200. These intensity changes may be interpreted according to Petrov et al. [12]. The authors refined the structure of almost fully Ba2+ exchanged clinoptilolite using powder XRD data. They comment the profound intensity changes in the powder XRD pattern. Later, Petrov [23] performed detailed crystal chemical calculations for the intensity change of 020 peak, theoretically showing the change of the 2 structural factor F 020 value related to different compositions of the exchangeable cationic complex in clinoptilolite, modelling the content with Li+, Na+, Ca2+,K+, Ba2+, Cs+, and Tl+ ions in the plane of symmetry of the structure.

4.2. Rietveld Structural Interpretation of Ba Exchange on Clinoptilolite with Time and Temperature The studied samples were taken at different hours of the ongoing ion exchange. It was proved that the cationic positions change their occupancy with time—from a partially to more fully occupied ones. The structural analysis of their population at different stages of the ion exchange process with respect to the extra-framework cations shows how the leaving of the outgoing cations takes place. From the presented structural models one can get an idea whether the cations leave more or less simultaneously, whether they remain in the structure until the final stages of ion exchange, and whether they leave it completely or small amounts of them remain for up to 22nd day. In addition, a more detailed idea of how barium positions are filled during ion exchange is obtained. The changes in the occupancy of the cationic positions during the ion exchange process can contribute to the elucidation of the logistic and transport possibilities of barium and outgoing cations in the structure of clinoptilolite at both temperature conditions. The designed initial model of the Ba-clinoptilolite structure was based on the data of Larsen et al. [16] for natural Ba-heulandite (monoclinic system, space group C2/m). Specific step was to model the barium populations—about 3 atoms per unit-cell according to EDS data (Figure3) at positions Ba1 and Ba2 determined by the above authors. After a number of refinement cycles of sample Ba-clinoptilolite (90 ◦C, 22 d) (Figure5) we found that the occupancy of position Ba1 lowers from 0.359 to 0.131 in our case and vice versa the occupancy of position Ba2 increases from 0.190 to 0.444, which correlates with the data of Petrov et al. [12]. The coordinates of these two barium positions in our Ba-clinoptilolite stay very close to those of Ba-heulandite [16]. In a cluster in the C ring (K + Ba1, Ba2, Figure5) we found a third position, Ba(K) (Table5) with occupancy 0.114, closely situated near the original potassium M3 position of Koyama and Takeuchi [7]. This position is slightly overlapping with Ba2 (at 0.52 Å) (Table6) and their simultaneous occupation is forbidden. Additionally, we found a fourth slightly occupied (0.073) barium position noted Ba(Ca), this time placed in the 8-member ring of channel B near M2 one, which is originally calcium position (Koyama and Takeuchi [7]). A number of 9 H2O sites were additionally refined in the channels of Ba-clinoptilolite with varying occupancies, with some of them coordinating the barium cations (Figure5, Table5). Minerals 2020, 10, 938 10 of 16

Minerals 2020, 10, x FOR PEER REVIEW 11 of 17

Figure 5. Structure representation of Ba-clinoptilolite (90 °C, 22 d). Left—channel A, middle—channel Figure 5. Structure representation of Ba-clinoptilolite (90 ◦C, 22 d). Left—channel A, middle—channel B, right—channelB,Figure right—channel 5. Structure C. C. representation of Ba-clinoptilolite (90 °C, 22 d). Left—channel A, middle—channel B, right—channel C. TheThe two two dimensional dimensional channel channel systemsystem A and B parallel parallel to to (010) (010) is is crossed crossed by by channel channel C formed C formed byby eight-member eight-memberThe two dimensional rings. rings. ThisThis channel is presented system in inA Figure Figureand B 5, parallel5 where, where theto the (010) cations cations is crossed (Ba (Ba in this inby thiscase)channel case) appearing C appearing formed in in didifferentbyﬀ eight-membererent channels rings. (A (A and and This B) B) isbut butpresented in inchannel channel in CFigure they C they 5,are where aredisposed disposed the cations in a incontinuous a(Ba continuous in this raw, case) raw,observed appearing observed in y-in in z projection. y-zdifferent projection. channels (A and B) but in channel C they are disposed in a continuous raw, observed in y- According to Kirov et al. [17] there are two channels C and C`, which cross channels A and B. In z projection.According to Kirov et al. [17] there are two channels C and C‘, which cross channels A and B. the present paper, we constructed a scheme for better representing the positioning cationic sites in In the presentAccording paper, to Kirov we constructed et al. [17] there a scheme are two for channels better representingC and C`, which the cross positioning channels cationic A and B. sites In in the gallerypresent structure paper, we of constructedBa-clinoptilolite a scheme (Figure for 6). better representing the positioning cationic sites in the gallery structure of Ba-clinoptilolite (Figure6). the gallery structure of Ba-clinoptilolite (Figure 6).

Figure 6. Gallery representation of the structure of Ba-clinoptilolite: distribution of O1 atoms in the plane 020, with directions of lanes A, B, C, and C` and location of the extra-framework cation positions FigureFigure 6. Gallery6. Gallery representation representation of of the the structurestructure of Ba-clinoptilolite: distribution distribution of ofO1 O1 atoms atoms in the in the and Ba positions. planeplane 020, 020, with with directions directions of of lanes lanes A, A, B, B, C,C, andand C` C‘ and location location of of the the extra-framework extra-framework cation cation positions positions and Ba positions. andThe Ba free positions. space in HEU type structure [17] is formed as a unitary space by four lanes (or channels A and B parallel to [001], C to [100], and C` [100]) divided by pillaring alumosilicate diortho-groups. TheThe free free space space in in HEU HEU type type structure structure [[17]17] is formed as as a a unitary unitary space space by by four four lanes lanes (or (or channels channels Four framework oxygen atoms (O1) from the 72 ones (per unit cell) are bridge atoms between the A andA and B parallelB parallel to to [001], [001], C C to to [100], [100], andand C‘C` [100])[100]) divided by by pillaring pillaring alumosilicate alumosilicate diortho-groups. diortho-groups. neighbouring “layers-like” configuration, which explains the perfect (010) cleavage of the crystals. In FourFour framework framework oxygen oxygen atoms atoms (O1)(O1) fromfrom thethe 72 ones (per (per unit unit cell) cell) are are bridge bridge atoms atoms between between the the theneighbouring space between “layers-like” the “layers” configuration, the cation whichpositions explains (lying the on perfect the ac (010)symmetric cleavage plane) of the and crystals. the H 2InO neighbouring “layers-like” configuration, which explains the perfect (010) cleavage of the crystals. moleculesthe space betweenare situated, the “layers”and this thespace cation can bepositions described (lying as aon gallery the ac space symmetric [17]. Kirov plane) et andal. [17] the stateH2O In the space between the “layers” the cation positions (lying on the ac symmetric plane) and the H O thatmolecules this gallery are situated, space can and be this decomposed space can be into described four systems as a gallery of parallel space cages, [17]. Kirovnotated et al.on [17]Figure state 6 2 molecules are situated, and this space can be described as a gallery space [17]. Kirov et al. [17] state AC,that BC,this AC`,gallery and space BC`. can be decomposed into four systems of parallel cages, notated on Figure 6 thatAC, this BC,The gallery AC`, above and space specified BC`. can be representation decomposed intoof the four gallery-type systems of structure parallel cages,for HEU notated group on of Figure minerals6 AC, BC,shows AC‘,The andthat above BC‘.each cation specified position representation belongs to of gallery the gallery-type space and is structure located simultaneously for HEU group in ofthree minerals lanes (channels).showsThe abovethat Ineach specifiedsuch cation case, representationposition each cation belongs position of to the gallery can gallery-type be space defined and structure within is located two for simultaneously cages. HEU group of in minerals three lanes shows that(channels). eachFor cation cations In positionsuch in case,positions belongs each M1,cation to galleryM3, position and space M4 can andit beis isdefinedappropriate located within simultaneously to twobe represented cages. in three in cage lanes AC` (channels). and In suchpositionFor case, M2—incations each cationincage positions BC. position From M1, this can M3, beperspective and defined M4 it within itis cannot appropriate two be cages.proposed to be represented that the exchange in cage of AC` cations and takespositionFor place cations M2—in along in cage positionsthe channels.BC. From M1, Inthis M3, fact, perspective and the M4movement it it is cannot appropriate of Hbe2O proposed molecules to be that represented and the exchangeable exchange in cage of cations AC‘ and positiontakes M2—inplace along cage the BC. channels. From this In perspective fact, the movement it cannot of be H proposed2O molecules that theand exchange exchangeable of cations cations takes

Minerals 2020, 10, 938 11 of 16

place along the channels. In fact, the movement of H2O molecules and exchangeable cations through theMinerals “channel” 2020, 10 system, x FOR PEER is not REVIEW restricted by (Si, Al)O4 barriers and in this sense, the entire channel 12 of space 17 represents a different space configuration [17]. throughThe negative the “channel” charge system of the is framework not restricted comes by (Si, mainly Al)O from4 barriers the bridgingand in this oxygen sense, atomsthe entire O1 of tetrahedronchannel space T2, keepingrepresents the a different extra-framework space configuration cations in [17]. the plane of symmetry and the preferred sites areThe occupied negative by charge particular of the cationsframework depending comes mainly on their from charge the bridging and size. oxygen In clinoptilolite, atoms O1 of the extra-frameworktetrahedron T2,cations keeping occupy the extra-framework four positions cations (Figures in the5 and plane6). Positionsof symmetry M1 and and the M2 preferred accommodate sites cationsare occupied with a radius by particular of around cations 1.0 depending Å (Na+, Ca on2+ ).their Position charge M3 and is size. occupied In clinoptilolite, by cations the with extra- larger ionicframework radii such cations as K+ (1.31 occupy Å) four and Bapositions2+ (1.35 (Figures Å). Position 5 and M4 6). is Positions the only oneM1 andwhich M2 is accommodate not coordinated cations with a radius of around 1.0 Å (Na+, Ca2+).2 +Position M3 is occupied by cations with larger ionic by framework oxygens and is occupied by Mg , surrounded only by H2O molecules. The cation radii such as K+ (1.31 Å) and Ba2+ (1.35 Å). Position M4 is the only one which is not coordinated by positions are usually partially occupied. framework oxygens and is occupied by Mg2+, surrounded only by H2O molecules. The cation In the case of Ba exchanged clinoptilolite for 22 days at 90 C, barium ions prefer the position positions are usually partially occupied. ◦ M3—describedIn the case in of this Bapaper exchanged as split clinoptilolite Ba2 and Ba(K), for 22 wheredays at 2.2490 °C, (per barium unit cell)ions cationsprefer the dispose. position The other portion of Ba2+ is distributed as 0.52 cations in Ba1(M1) and 0.29 in Ba(Ca) (M2), respectively. M3—described in this paper as split Ba2 and Ba(K), where 2.24 (per unit cell) cations· dispose. The otherA significant portion of diBaff2+erence is distributed between as clinoptilolite0.52 cations in and Ba1(M1) heulandite and 0.29 is in uncovered Ba(Ca)·(M2), in the respectively. Al occupancy of the tetrahedralA significant position difference T5, between which is clinoptilolite almost three and times heulandite bigger is in uncovered heulandites in the [17 ].Al This occupancy is clearly illustratedof the tetrahedral by the ion position exchange T5, ofwhich the sampleis almost Ba-clinoptilolite three times bigger (90 in◦C, heulandites 22 d), where [17]. the This occupation is clearly of positionillustrated Ba1 decreasesby the ion sharplyexchange compared of the sample to that Ba-clinoptilolite of natural barium (90 °C, heulandite 22 d), where from the Norway occupation [16 of]. positionThe refined Ba1 decreases structure sharply of Ba-clinoptilolite compared to that (90 of◦C, natural 22 d) barium is almost heulandite totally exchanged from Norway with [16]. barium cationsThe (3.02 refined per unit structure cell) distributed of Ba-clinoptilolite in four (90 cationic °C, 22 sites: d) is almost Ba1, Ba2, totally Ba(K), exchanged and Ba(Ca) with (Figuresbarium 5 andcations6). Each (3.02 site per is withunit cell) specific distributed coordination in four includingcationic sites: oxygen Ba1, atomsBa2, Ba(K), from and the Ba(Ca) tetrahedral (Figures framework 5 and 6). Each site is with specific coordination including oxygen atoms from the tetrahedral framework and H2O molecules. and H2O molecules. Position Ba1 is coordinated by four oxygen atoms (Figure7) and four H 2O molecules (O12, O15 2, O17)Position with interatomic Ba1 is coordinated bond distances by four oxygen from 2.81atoms Å (Figure to 3.03 7) Å. and Position four H Ba22O molecules is also coordinated (O12, O15 by × × 2, O17) with interatomic bond distances from 2.81 Å to 3.03 Å. Position Ba2 is also coordinated by four oxygen atoms (Figure7) and four H 2O molecules (O11 2, O17, and O18) with interatomic bond four oxygen atoms (Figure 7) and four H2O molecules (O11 ×× 2, O17, and O18) with interatomic bond distances from 2.85 Å to 3.10 Å. Position Ba(K) is coordinated by four oxygen atoms (O3 2, O4 2) distances from 2.85 Å to 3.10 Å. Position Ba(K) is coordinated by four oxygen atoms (O3 × 2,× O4 × 2)× (Figure7) and four H 2O molecules (O13 2, O17, O19) with interatomic bond distances from 2.83 Å to (Figure 7) and four H2O molecules (O13× × 2, O17, O19) with interatomic bond distances from 2.83 Å 3.07 Å. Position Ba(Ca) is coordinated by three oxygen atoms (O1, O10 2), (Figure7) and two H 2O to 3.07 Å. Position Ba(Ca) is coordinated by three oxygen atoms (O1, O10 × 2), (Figure 7) and two H2O molecules (O13 2) with interatomic bond distances from 2.83 Å to 2.87 Å. Positions Ba1, Ba2, and molecules (O13× × 2) with interatomic bond distances from 2.83 Å to 2.87 Å. Positions Ba1, Ba2, and Ba(K)Ba(K) are are similarly similarly coordinated coordinated by by four four oxygenoxygen atoms from from the the framework framework and and four four H2 HO2 molecules.O molecules. AccordingAccording to to the the performed performed Rietveld Rietveld structure structure refinementrefinement position, position, Ba(Ca) Ba(Ca) appears appears less less coordinated. coordinated.

FigureFigure 7. Coordination7. Coordination spheres spheres of ofthe the bariumbarium positions in in the the structure structure of of Ba-clinoptilolite Ba-clinoptilolite (90(90 °C, ◦22C, d) 22 d)

4.3. Movement of H2O Molecules and Ba2+ Cations in the Clinoptilolite Structure During Barium Exchange

The coordinating oxygen and H2O positions around the sites of the outgoing cations in the structure of Ba-clinoptilolite (90 °C, 2 h) are given on Figure 8. The Na+ position (M1-Na) according

Minerals 2020, 10, 938 12 of 16

2+ 4.3. Movement of H2O Molecules and Ba Cations in the Clinoptilolite Structure During Barium Exchange

The coordinating oxygen and H2O positions around the sites of the outgoing cations in the + structureMinerals of2020 Ba-clinoptilolite, 10, x FOR PEER REVIEW (90 ◦C, 2 h) are given on Figure8. The Na position (M1-Na) according 13 of 17 to Koyama and Takeuchi [7]) is coordinated by framework oxygens O2 2 and H O molecules O12, O16, × 2 andto O15 Koyama2. Withand Takeuchi the time of[7]) barium is coordinated exchange, by sodiumframework ions oxygens leave the O2x2 structure and H2 andO molecules H O in positionO12, × 2 O15O16, is attracted and O15 to× 2. the With coordination the time of groupbarium of exchange, position Ba1sodium and ions O15 leave changed the structure its atomic and coordinates H2O in duringposition the exchange O15 is attracted process to (Table the coordination5, 2 h and 22 group d). At of the position same timeBa1 and position O15 changed O16 also its changes atomic its coordinates during the exchange process (Table 5, 2 h and 22 d). At the same time position O16 also coordinates, lowers its occupancy, and ﬁnally does not coordinate position Ba1 in the case of sample changes its coordinates, lowers its occupancy, and finally does not coordinate position Ba1 in the case Ba-clinoptilolite (90 C, 22 d). Only position O12 preserves its coordinates around position Ba1. The of sample Ba-clinoptilolite◦ (90 °C, 22 d). Only position O12 preserves its coordinates around position H2O positions around the original position M3 (K) (Figure8) are preserved as coordinating Ba2 and Ba1. The H2O positions around the original position M3 (K) (Figure 8) are preserved as coordinating Ba(K) in the almost completely exchanged sample Ba-clinoptilolite (90 C, 22 d). For the original Ba2 and Ba(K) in the almost completely exchanged sample Ba-clinoptilolite◦ (90 °C, 22 d). For the positionoriginal M2 position (Ca) the M2 coordination (Ca) the coordination is preserved is preserved for O1 and for O13 O1 and but O13 O14 but with O14 the with time the of time ion exchange of ion becomesexchange forbidden becomes for forbidden the occupation for the occupation sphere of sphere position of position Ba(Ca) Ba(Ca) in sample in sample Ba-clinoptilolite Ba-clinoptilolite (90 ◦C, 22 d)(90 (Table °C, 226 d), 2 (Table h and 6, 22 2 d).h and 22 d).

FigureFigure 8. 8.Outgoing Outgoing cationscations coordination (2h (2h of of exchange). exchange).

OnOn the the basis basis of of the the above above comments comments onon thethe behaviourbehaviour of of the the H H2O2O molecules molecules when when coordinating coordinating thethe exchangeable exchangeable cations, cations, we we may mayconclude conclude thatthat during the the exchange exchange process process these these molecules molecules are are attractedattracted by by incoming incoming barium barium cations, cations, thus thus makingmaking moves like like H H2O2O positions positions O15 O15 and and O18 O18 but but other other likelike O16 O16 lower lower their their occupation, occupation, change change their position, their position, and finally and do finally not participate do not participate in the coordination in the 2+ of thecoordination incoming of cations the incoming (Ba2+ in cations our case). (Ba In in addition, our case). the In oppositeaddition, casethe opposite is observed. case Positionis observed. O17 is Position O17 is not a coordinating one in sample Ba-clinoptilolite (90 °C, 2 h) but later during the not a coordinating one in sample Ba-clinoptilolite (90 ◦C, 2 h) but later during the barium exchange it barium exchange it increases its occupancy and participates in the coordination of position Ba1. At increases its occupancy and participates in the coordination of position Ba1. At the same time, a new the same time, a new H2O position (O19) appears in the coordination of position Ba(K). H2O position (O19) appears in the coordination of position Ba(K). All these changes in the positions of H2O molecules during the ion exchange process indicate All these changes in the positions of H O molecules during the ion exchange process indicate intraspace diffusion, together with exchangeable2 cations movements. intraspace diffusion, together with exchangeable cations movements. Rietveld refinements of the structures of the rest Ba-exchanged samples for 24 h, 72 h, 168 h, 12 d,Rietveld and 22 d refinementsat 90 °C and ofRT, the respectively structures were of the also rest performed. Ba-exchanged The important samples for data 24 at h, the 72 h,final 168 stage h, 12 d, andof 22 refinements d at 90 ◦C is and the RT, obtained respectively number were of atoms also performed.per cationic Theposition important (Tables data 7 and at the8), which final stagewell of refinementscorrelate with is the the obtained data of numberEDS (Tables of atoms 2 and per 3). cationic position (Tables7 and8), which well correlate with the data of EDS (Tables2 and3).

Minerals 2020, 10, 938 13 of 16

MineralsTable 2020 7. Number, 10, x FOR of PEER atoms REVIEW per unit cell in the cationic positions of initial and Ba-exchanged clinoptilolite 14 of 17 (Cpt) at 90 C (after structural reﬁnement). Table 7.◦ Number of atoms per unit cell in the cationic positions of initial and Ba-exchanged clinoptiloliteM1 (Cpt) at 90 M2°C (after structural ~M3 refinement). ~M3 M1 M2 M3 M4 Total Sample 2+ Ba1M1 BaM2 (Ca) (Ba2) Ba(K)~M3 NaM1 M2 Ca M3 K Mg BaTotal Sample ~M3 Ba2 M4 Mg initial Cpt Ba1– Ba (Ca) – –Ba(K) – 1.0Na 1.84Ca 0.92K 0.38Ba – 2+ initial2 h Cpt 0.34– – – 0.63– 0.29– 0.631.0 1.121.84 / 0.920.54 0.150.38/ 1.26– 242 h h 0.350.34 – – 1.220.63 0.370.29 0.440.63/ 1.12/0.55/ 0.540.47 / 0.15/0.13/ 1.941.26 7224 h h 0.370.35 0.23– 1.481.22 0.400.37 0.160.44// 0.55/0.40/ 0.47/0.22 / 0.13/0.08/ 2.481.94 16872 h h 0.380.37 0.23 0.27 1.511.48 0.420.40 0.16/ – 0.40/ 0.38 0.22/ 0.18 0.08/ 0.03/ 2.582.48 12168 d h 0.420.38 0.27 0.28 1.681.51 0.460.42 –– 0.38 0.24 0.18 0.14 0.03/ – 2.842.58 2212 d d 0.520.42 0.28 0.29 1.771.68 0.460.46 –– 0.24 0.12 0.14 – – 3.042.84 22 d 0.52 0.29 1.77 0.46 – 0.12 – – 3.04 Table 8. Number of atoms per unit cell in the cationic positions of initial and Ba-exchanged clinoptilolite (Cpt)Table at RT 8. (afterNumber structural of atoms reﬁnement). per unit cell in the cationic positions of initial and Ba-exchanged clinoptilolite (Cpt) at RT (after structural refinement). M1 M2 ~M3 ~M3 M1 M2 M3 M4 Total Sample M1 M2 ~M3 M1 M2 M3 M4 Total Sample Ba1 Ba (Ca)~M3 (Ba2) (Ba2) Ba(K) Na Ca K Mg Ba2+ Ba1 Ba (Ca) Ba(K) Na Ca K Mg Ba2+ initialinitial Cpt Cpt – –– – – –– 1.01.0 1.84 1.84 0.92 0.92 0.38 0.38 – – 2 h 2 h0.24 0.24– – 0.520.52 0.240.24 0.640.64 1.32 1.32 0.71 0.71 0.16 0.16 1 1 24 h 24 h0.29 0.29– – 0.560.56 0.240.24 0.620.62 1.22 1.22 0.69 0.69 0.16 0.16 1.09 1.09 72h 72h0.33 0.33– – 0.600.60 0.250.25 0.560.56 1.16 1.16 0.68 0.68 0.15 0.15 1.18 1.18 168h168h 0.34 0.34– – 0.640.64 0.250.25 0.550.55 1.15 1.15 0.62 0.62 0.14 0.14 1.23 1.23 12d 12d0.36 0.360.08 0.08 0.810.81 0.260.26 0.440.44 1.04 1.04 0.56 0.56 0.13 0.13 1.48 1.48 22d 22d0.4 0.40.12 0.12 0.860.86 0.260.26 0.340.34 0.98 0.98 0.45 0.45 0.12 0.12 1.64 1.64

The barium cations during the ion exchange at 90 °C eagerly enter the clinoptilolite structure The barium cations during the ion exchange at 90 ◦C eagerly enter the clinoptilolite structure during the first 2 h reaching 1.26 barium per unit cell (Figure 9, left). In the same interval, a profound during the first 2 h reaching 1.26 barium per unit cell (Figure9, left). In the same interval, a profound lowering of the number of all original cations is observed (Figure 9, right). Up to 72 h this exchange lowering of the number of all original cations is observed (Figure9, right). Up to 72 h this exchange is is still significant reaching about 2.50 cations and then continues more slowly up to the 22nd day. The still significant reaching about 2.50 cations and then continues more slowly up to the 22nd day. The tendency for the outgoing cations is the opposite—their contents lower accordingly. After 2 h all of tendencythem decrease for the outgoing rapidly their cations content is the and opposite—their then leave the contents structure lower after accordingly. different exchange After 2 times h all of them(Figure decrease 9, right) rapidly except their calcium, content andwhich then preserves leave the 0.12 structure cations after per diunitfferent cell exchangeeven after times 22 days (Figure of 9, right)exchange except (Table calcium, 7). which preserves 0.12 cations per unit cell even after 22 days of exchange (Table7).

FigureFigure 9. Extent9. Extent of of barium barium exchange exchange (left (left),), barium barium sites sites (middle (middle)) and and outgoing outgoing cations cations ( right(right)) atat 9090◦ C. °C. Figure9 (middle) shows that the most populated position Ba2 (M3) is following the occupation trend ofFigure the total 9 (middle) barium shows exchange that (Figure the most9, left).populated The other position barium Ba2 (M3) positions is following (Ba1, Ba(K), the occupation and Ba(Ca)) aretrend occupied of the slightly total barium at the exchange ﬁrst hours (Figure of the 9, left). exchange The other process barium and positions with time (Ba1, there Ba(K), is only and Ba(Ca)) negligible increaseare occupied of the occupancies. slightly at the The first calcium hours of position the exchange Ba(Ca) process (M2) startsand with to accepttime there barium is only cations, negligible noticed afterincrease 72 h of of exchange the occupancies. and slightly The calcium changes position its atomic Ba(Ca) coordinates (M2) starts (Table to accept5). barium cations, noticed after 72 h of exchange and slightly changes its atomic coordinates (Table 5). The barium exchange on clinoptilolite at room temperature (RT) (Figure 10, left) for the ﬁrst 2 h The barium exchange on clinoptilolite at room temperature (RT) (Figure 10, left) for the first 2 h proceeds similar to the exchange process at 90 C (reaching 1.00 Ba2+ per unit cell). However, in the proceeds similar to the exchange process at 90 ◦°C (reaching 1.00 Ba2+ per unit cell). However, in the following time intervals up to 72 h, the exchange proceeds very slowly and reaches 1.18 Ba2+, which is following time intervals up to 72 h, the exchange proceeds very slowly and reaches 1.18 Ba2+, which

Minerals 2020, 10, 938 14 of 16

Minerals 2020, 10, x FOR PEER REVIEW 15 of 17 more than twice as low as the sample at 90 C. Then the barium exchange continues slowly (like the is more than twice as low as the sample at ◦90 °C. Then the barium exchange continues slowly (like 2+ samplethe sample at 90 ◦ atC) 90 reaching °C) reaching 1.64 Ba1.64 Baat2+ the at the 22nd 22nd day. day.

FigureFigure 10. 10.Extent Extent of of barium barium exchange exchange ((leftleft),), barium sites sites ( (middlemiddle) )and and outgoing outgoing cations cations (right (right) at) RT. at RT.

ConcerningConcerning the the occupancies occupancies of of thethe bariumbarium positions during during ion ion exchange exchange at atroom room temperature temperature (RT)(RT) (Figure (Figure 10 ,10, middle, middle, Table Table8) 8) again again position position Ba2 Ba2 (around (around M3)M3) isis mostly exchanged exchanged with with Ba Ba2+ but2+ but at 22 days is lower occupied (0.86 per unit cell) compared to the exchange at 90 °C. The trend of at 22 days is lower occupied (0.86 per unit cell) compared to the exchange at 90 ◦C. The trend of occupationoccupation of of position position Ba2 Ba2 with with timetime ofof exchangeexchange is is similar similar to to the the total total barium barium exchange exchange at room at room temperaturetemperature (Figure. (Figure. 10, 10, left). left). The The otherother positionspositions are are slowly slowly occupied occupied during during the the same same time. time. OnOn Figure Figure 10 10 (right) (right) it it is is seen seen that that thethe outgoingoutgoing cations cations leave leave the the structure structure quickly quickly up upto the to thefirst first 2 h of the process and then their outgoing is slowed but at the 22nd day all of them are found to be 2 h of the process and then their outgoing is slowed but at the 22nd day all of them are found to be present in small amounts (Table 8). present in small amounts (Table8). As a summary of the discussed above specificity of the barium exchange on clinoptilolite both As a summary of the discussed above specificity of the barium exchange on clinoptilolite both at room temperature and 90 °C we may state that during the first 2 h we notice intensive barium at roomexchange, temperature predominantly and 90 realized◦C we in may position state Ba2. that The during outgoing the firstcations 2 h (Na we+, noticeCa2+, K intensive+, and Mg2+ barium) do + 2+ + 2+ exchange,not show predominantly any tendency of realized preferable in leaving—they position Ba2. all The together outgoing intensively cations leave (Na the, Ca structure,K , and during Mg ) dothe not first show 2 h any and tendency then this ofprocess preferable is slowed leaving—they down following all together the tendency intensively of barium leave take the structureup by duringclinoptilolite. the first 2This h and indicates then this that process the exchangeable is slowed cations down followingmovement the in the tendency structure of is barium realized take easily up by clinoptilolite.without preferable This indicates directions, that which the exchangeable shows that the cations diffusion movement process proceeds in the structure isotropically is realized [16]. This easily withoutconclusion preferable fits well directions, with the whichtheory showsof gallery that space the diinff theusion HEU-type process structure proceeds [17]. isotropically [16]. This conclusion fits well with the theory of gallery space in the HEU-type structure [17]. 5. Conclusions 5. Conclusions Clinoptilolite sample subjected to barium exchange for a period up to the 22 days at room temperatureClinoptilolite and sample90 °C shows subjected that the to exchange barium exchangeprocess proceeds for aperiod all the time up to with the different 22 days speed. at room temperatureThis is confirmed and 90 ◦bothC shows by EDS that analyses the exchange and Rietveld process XRD proceeds structure all refinement the time with at different different exchange speed. This is confirmedtimes. both by EDS analyses and Rietveld XRD structure refinement at different exchange times. TheThe starting starting Rietveld Rietveld refinement refinement waswas performedperformed on on the the maximum maximum Ba Ba exchanged exchanged clinoptilolite clinoptilolite (90 °C, 22 d). After refinement of unit-cell parameters, atomic occupancies and coordinates of (90 ◦C, 22 d). After refinement of unit-cell parameters, atomic occupancies and coordinates of exchangeable cationic sites and H2O molecules we determined four barium positions differently exchangeable cationic sites and H2O molecules we determined four barium positions differently populated. A number of 9 H2O sites were additionally refined in the channels of Ba-clinoptilolite with populated. A number of 9 H O sites were additionally refined in the channels of Ba-clinoptilolite with varying occupancies, with 2some of them coordinating the barium cations. varying occupancies, with some of them coordinating the barium cations. The behaviour of the H2O molecules when coordinating the exchangeable cations during the The behaviour of the H O molecules when coordinating the exchangeable cations during the exchange process shows that2 these molecules are attracted by incoming barium cations, thus making exchange process shows that these molecules are attracted by incoming barium cations, thus making intraspace moves. Some of the H2O positions change their occupation and atomic coordinates. All intraspacethese changes moves. in Some the positions of the H of2O H positions2O molecules change during their the occupation ion exchange and processatomic coordinates. indicate their All thesediffusion changes accompanied in the positions by the of Hexchangeable2O molecules cations’ during movements. the ion exchange process indicate their diffusion accompaniedThe barium by the cations exchangeable during the cations’ ion exchange movements. at 90 °C eagerly enter the clinoptilolite structure duringThe bariumthe first cations2 h. In the during same theinterval, ion exchangea profound at lowering 90 ◦C eagerly of the number enter the of clinoptiloliteall original cations structure is duringobserved. the first Then 2 h.the In exchange the same process interval, is gradually a profound slower lowering up (especially of the number after 72 of h) all to original the 22nd cations day. is observed.The tendency Then for the the exchange outgoing process cations isis graduallythe opposite—their slower up contents (especially lower afteraccordingly. 72 h) to The the barium 22nd day. Theexchange tendency on for clinoptilolite the outgoing at cationsroom temperature is the opposite—their for the first contents2 h proceeds lower similar accordingly. to the exchange The barium exchangeprocess on at clinoptilolite90 °C and then at is room slowed temperature up to the 22nd for the day. first 2 h proceeds similar to the exchange process at 90 ◦C and then is slowed up to the 22nd day. Minerals 2020, 10, 938 15 of 16

The performed barium exchange on clinoptilolite up to 22 days was chosen to study the process for a relatively long time. The expectation was to reach full exchange but at the end of the period the plateau is still not reached and 0.12 Ca2+ still remain in the structure. With time, probably total exchange can be reached or the remaining calcium cations will be blocked for further exchange. The latter needs further investigation.

Author Contributions: Conceptualization, L.D.; methodology, L.D. and O.P.; software, L.D., Y.T., O.P., I.P., and F.U.; investigation, L.D., Y.T., O.P., I.P., and F.U.; writing—original draft preparation, L.D. and O.P.; writing—L.D., Y.T., and O.P.; visualization, L.D. and Y.T.; funding acquisition, L.D., Y.T., and O.P. All authors have read and agreed to the published version of the manuscript. Funding: This work was financially supported by K2-2020 of IMC–BAS. Acknowledgments: The authors of this paper are thankful to Project BG05M2OP001-1.002-0005 PERIMED, K2-2020 of IMC–BAS, and to the reviewers for their constructive comments and suggestions, which helped us to improve considerably the manuscript. Conflicts of Interest: The authors declare no conflict of interest.

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