Powder XRD Structural Study of Ba 2 + Modiﬁed Clinoptilolite at Di ﬀ erent Stages of the Ion Exchange Process Conducted at Two Temperature Regimes—Room Temperature and 90 ◦ C

: Partial and almost complete barium exchange on clinoptilolite is performed and structurally studied for di ﬀ erent 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 H 2 O 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 di ﬀ erent occupations of the positions of incoming Ba 2 + and outgoing cations (Na + , Ca 2 + , K + , Mg 2 + ) 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.


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.
The specific microporous structure of these minerals is characterized by SiO 4 4− and AlO 4 5− tetrahedra forming framework with channels and cages in which cations compensating the framework charge are occupying specific sites and are coordinated by H 2 O 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][2][3][4][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  Selected powder XRD patterns of Ba-clinoptilolite at different exchange regimes are shown on Figure 2. The sample exchanged at 90 °C for 24 h displays lowest intensity of 020 peak compared to the other two Ba-exchanged samples. The sample at 90 °C for 2 h and at RT for 22 days show almost equal intensity of this peak. Exact intensity values of 020 and 220 calculated by profile fitting [22] and normalized to the intensity of the complex peak at 2θ = 22.39 (d = 3.97 Å) are given in Table 1.  Selected powder XRD patterns of Ba-clinoptilolite at different exchange regimes are shown on Figure 2. The sample exchanged at 90 • C for 24 h displays lowest intensity of 020 peak compared to the other two Ba-exchanged samples. The sample at 90 • C for 2 h and at RT for 22 days show almost equal intensity of this peak. Exact intensity values of 020 and 220 calculated by profile fitting [22] and normalized to the intensity of the complex peak at 2θ = 22.39 (d = 3.97 Å) are given in Table 1. The performed EDS analyses give numeric data (Tables 2 and 3) for barium content during the applied exchange procedures, which describe specific trends of exchange ( Figure 3).  The performed EDS analyses give numeric data (Tables 2 and 3) for barium content during the applied exchange procedures, which describe specific trends of exchange ( Figure 3).  These trends correspond to certain stages of ion exchange and practically last until the 22nd day (528 h) when 3 Ba 2+ 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  These trends correspond to certain stages of ion exchange and practically last until the 22nd day (528 h) when 3 Ba 2+ 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.

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 Å 3 (sample 90 • C, 2h) to 2107.9 Å 3 (sample 90 • C, 22 d). The difference powder XRD plot of sample Ba-clinoptilolite (90 • C, 22 d) after the final refinement stage is shown on Figure 4. The difference powder XRD plot of sample Ba-clinoptilolite (90 °C, 22 d) after the final refinement stage is shown on Figure 4. After the final stage of the refinements we obtained values for the structural parameters, which are reported in Table 5 for Ba-clinoptilolite (90 °C, 22 d) and Ba-clinoptilolite (90 °C, 2h) together with the interatomic distances reported in Table 6. After the final stage of the refinements we obtained values for the structural parameters, which are reported in Table 5 for Ba-clinoptilolite (90 • C, 22 d) and Ba-clinoptilolite (90 • C, 2h) together with the interatomic distances reported in Table 6.

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 Ba 2+ 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 structural factor F 2 020 value related to different compositions of the exchangeable cationic complex in clinoptilolite, modelling the content with Li + , Na + , Ca 2+ , K + , Ba 2+ , Cs + , and Tl + ions in the plane of symmetry of the structure.

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 ( Figure 3) at positions Ba1 and Ba2 determined by the above authors.
After a number of refinement cycles of sample Ba-clinoptilolite (90 • C, 22 d) ( Figure 5) 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, Figure 5) we found a third position, Ba(K) ( Table 5) 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) ( Table 6) 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 H 2 O sites were additionally refined in the channels of Ba-clinoptilolite with varying occupancies, with some of them coordinating the barium cations ( Figure 5, Table 5). The two dimensional channel system A and B parallel to (010) is crossed by channel C formed by eight-member rings. This is presented in Figure 5, where the cations (Ba in this case) appearing in different channels (A and B) but in channel C they are disposed in a continuous raw, observed in yz projection.
According to Kirov et al. [17] there are two channels C and C`, which cross channels A and B. In the present paper, we constructed a scheme for better representing the positioning cationic sites in the gallery structure of Ba-clinoptilolite ( Figure 6). The free 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. Four framework oxygen atoms (O1) from the 72 ones (per unit cell) are bridge atoms between the neighbouring "layers-like" configuration, which explains the perfect (010) cleavage of the crystals. In the space between the "layers" the cation positions (lying on the ac symmetric plane) and the H2O molecules are situated, and this space can be described as a gallery space [17]. Kirov et al. [17] state that this gallery space can be decomposed into four systems of parallel cages, notated on Figure 6 AC, BC, AC`, and BC`.
The above specified representation of the gallery-type structure for HEU group of minerals shows that each cation position belongs to gallery space and is located simultaneously in three lanes (channels). In such case, each cation position can be defined within two cages.
For cations in positions M1, M3, and M4 it is appropriate to be represented in cage AC` and position M2-in cage BC. From this perspective it cannot be proposed that the exchange of cations takes place along the channels. In fact, the movement of H2O molecules and exchangeable cations The two dimensional channel system A and B parallel to (010) is crossed by channel C formed by eight-member rings. This is presented in Figure 5, where the cations (Ba in this case) appearing in different channels (A and B) but in channel C they are disposed in a continuous raw, observed in y-z projection.
According to Kirov et al. [17] there are two channels C and C', which cross channels A and B. In the present paper, we constructed a scheme for better representing the positioning cationic sites in the gallery structure of Ba-clinoptilolite ( Figure 6). The two dimensional channel system A and B parallel to (010) is crossed by channel C formed by eight-member rings. This is presented in Figure 5, where the cations (Ba in this case) appearing in different channels (A and B) but in channel C they are disposed in a continuous raw, observed in yz projection.
According to Kirov et al. [17] there are two channels C and C`, which cross channels A and B. In the present paper, we constructed a scheme for better representing the positioning cationic sites in the gallery structure of Ba-clinoptilolite ( Figure 6). The free 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. Four framework oxygen atoms (O1) from the 72 ones (per unit cell) are bridge atoms between the neighbouring "layers-like" configuration, which explains the perfect (010) cleavage of the crystals. In the space between the "layers" the cation positions (lying on the ac symmetric plane) and the H2O molecules are situated, and this space can be described as a gallery space [17]. Kirov et al. [17] state that this gallery space can be decomposed into four systems of parallel cages, notated on Figure 6 AC, BC, AC`, and BC`.
The above specified representation of the gallery-type structure for HEU group of minerals shows that each cation position belongs to gallery space and is located simultaneously in three lanes (channels). In such case, each cation position can be defined within two cages.
For cations in positions M1, M3, and M4 it is appropriate to be represented in cage AC` and position M2-in cage BC. From this perspective it cannot be proposed that the exchange of cations takes place along the channels. In fact, the movement of H2O molecules and exchangeable cations The free 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. Four framework oxygen atoms (O1) from the 72 ones (per unit cell) are bridge atoms between the neighbouring "layers-like" configuration, which explains the perfect (010) cleavage of the crystals. In the space between the "layers" the cation positions (lying on the ac symmetric plane) and the H 2 O molecules are situated, and this space can be described as a gallery space [17]. Kirov et al. [17] state that this gallery space can be decomposed into four systems of parallel cages, notated on Figure 6 AC, BC, AC', and BC'.
The above specified representation of the gallery-type structure for HEU group of minerals shows that each cation position belongs to gallery space and is located simultaneously in three lanes (channels). In such case, each cation position can be defined within two cages.
For cations in positions M1, M3, and M4 it is appropriate to be represented in cage AC' and position M2-in cage BC. From this perspective it cannot be proposed that the exchange of cations takes place along the channels. In fact, the movement of H 2 O molecules and exchangeable cations through the "channel" system is not restricted by (Si, Al)O 4 barriers and in this sense, the entire channel space represents a different space configuration [17].
The negative charge of the framework comes mainly from the bridging oxygen atoms O1 of tetrahedron T2, keeping the extra-framework cations in the plane of symmetry and the preferred sites are occupied by particular cations depending on their charge and size. In clinoptilolite, the extra-framework cations occupy four positions (Figures 5 and 6). Positions M1 and M2 accommodate cations with a radius of around 1.0 Å (Na + , Ca 2+ ). Position M3 is occupied by cations with larger ionic radii such as K + (1.31 Å) and Ba 2+ (1.35 Å). Position M4 is the only one which is not coordinated by framework oxygens and is occupied by Mg 2+ , surrounded only by H 2 O molecules. The cation positions are usually partially occupied.
In the case of Ba exchanged clinoptilolite for 22 days at 90 • C, barium ions prefer the position M3-described in this paper as split Ba2 and Ba(K), where 2.24 (per unit cell) cations dispose. The other portion of Ba 2+ is distributed as 0.52 cations in Ba1(M1) and 0.29 in Ba(Ca)·(M2), respectively. A significant difference between clinoptilolite and heulandite is uncovered in the Al occupancy of the tetrahedral position T5, which is almost three times bigger in heulandites [17]. This is clearly illustrated by the ion exchange of the sample Ba-clinoptilolite (90 • C, 22 d), where the occupation of position Ba1 decreases sharply compared to that of natural barium heulandite from Norway [16].
The refined structure of Ba-clinoptilolite (90 through the "channel" system is not restricted by (Si, Al)O4 barriers and in this sense, the entire channel space represents a different space configuration [17]. The negative charge of the framework comes mainly from the bridging oxygen atoms O1 of tetrahedron T2, keeping the extra-framework cations in the plane of symmetry and the preferred sites are occupied by particular cations depending on their charge and size. In clinoptilolite, the extraframework cations occupy four positions (Figures 5 and 6). Positions M1 and M2 accommodate cations with a radius of around 1.0 Å (Na + , Ca 2+ ). Position M3 is occupied by cations with larger ionic radii such as K + (1.31 Å) and Ba 2+ (1.35 Å). Position M4 is the only one which is not coordinated by framework oxygens and is occupied by Mg 2+ , surrounded only by H2O molecules. The cation positions are usually partially occupied.
In the case of Ba exchanged clinoptilolite for 22 days at 90 °C, barium ions prefer the position M3-described in this paper as split Ba2 and Ba(K), where 2.24 (per unit cell) cations dispose. The other portion of Ba 2+ is distributed as 0.52 cations in Ba1(M1) and 0.29 in Ba(Ca)·(M2), respectively.
A significant difference between clinoptilolite and heulandite is uncovered in the Al occupancy of the tetrahedral position T5, which is almost three times bigger in heulandites [17]. This is clearly illustrated by the ion exchange of the sample Ba-clinoptilolite (90 °C, 22 d), where the occupation of position Ba1 decreases sharply compared to that of natural barium heulandite from Norway [16].
The refined structure of Ba-clinoptilolite (90 °C, 22 d) is almost totally exchanged with barium cations (3.02 per unit cell) distributed in four cationic sites: Ba1, Ba2, Ba(K), and Ba(Ca) (Figures 5 and  6). Each site is with specific coordination including oxygen atoms from the tetrahedral framework and H2O molecules.

Movement of H 2 O Molecules and Ba 2+ Cations in the Clinoptilolite Structure During Barium Exchange
The coordinating oxygen and H 2 O 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 to Koyama and Takeuchi [7]) is coordinated by framework oxygens O2 × 2 and H 2 O molecules O12, O16, and O15 × 2. With the time of barium exchange, sodium ions leave the structure and H 2 O in position O15 is attracted to the coordination group of position Ba1 and O15 changed its atomic coordinates during the exchange process (Table 5,   to Koyama and Takeuchi [7]) is coordinated by framework oxygens O2x2 and H2O molecules O12, O16, and O15 × 2. With the time of barium exchange, sodium ions leave the structure and H2O in position O15 is attracted to the coordination group of position Ba1 and O15 changed its atomic coordinates during the exchange process (Table 5,   On the basis of the above comments on the behaviour of the H2O molecules when coordinating the exchangeable cations, we may conclude that during the exchange process these molecules are attracted by incoming barium cations, thus making moves like H2O positions O15 and O18 but other like O16 lower their occupation, change their position, and finally do not participate in the coordination of the incoming cations (Ba 2+ in our case). In addition, the opposite case is observed. Position O17 is not a coordinating one in sample Ba-clinoptilolite (90 °C, 2 h) but later during the barium exchange it increases its occupancy and participates in the coordination of position Ba1. At the same time, a new 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 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, and 22 d at 90 °C and RT, respectively were also performed. The important data at the final stage of refinements is the obtained number of atoms per cationic position (Tables 7 and 8), which well correlate with the data of EDS (Tables 2 and 3). On the basis of the above comments on the behaviour of the H 2 O molecules when coordinating the exchangeable cations, we may conclude that during the exchange process these molecules are attracted by incoming barium cations, thus making moves like H 2 O positions O15 and O18 but other like O16 lower their occupation, change their position, and finally do not participate in the coordination of the incoming cations (Ba 2+ in our case). In addition, the opposite case is observed. Position O17 is not a coordinating one in sample Ba-clinoptilolite (90 • C, 2 h) but later during the barium exchange it increases its occupancy and participates in the coordination of position Ba1. At the same time, a new H 2 O position (O19) appears in the coordination of position Ba(K).
All these changes in the positions of H 2 O molecules during the ion exchange process indicate 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, and 22 d at 90 • C and RT, respectively were also performed. The important data at the final stage of refinements is the obtained number of atoms per cationic position (Tables 7 and 8), which well correlate with the data of EDS (Tables 2 and 3). 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 lowering of the number of all original cations is observed (Figure 9, right). Up to 72 h this exchange is 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 them decrease rapidly their content and then leave the structure after different exchange times (Figure 9, right) except calcium, which preserves 0.12 cations per unit cell even after 22 days of exchange (Table 7).  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 lowering of the number of all original cations is observed (Figure 9, right). Up to 72 h this exchange is 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 them decrease rapidly their content and then leave the structure after different exchange times (Figure 9, right) except calcium, which preserves 0.12 cations per unit cell even after 22 days of exchange (Table 7).   (Figure 9, left). The other barium positions (Ba1, Ba(K), and Ba(Ca)) are occupied slightly at the first hours of the exchange process and with time there is only negligible increase of the occupancies. The calcium position Ba(Ca) (M2) starts to accept 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 proceeds similar to the exchange process at 90 °C (reaching 1.00 Ba 2+ per unit cell). However, in the following time intervals up to 72 h, the exchange proceeds very slowly and reaches 1.18 Ba 2+ , which   (Figure 9, left). The other barium positions (Ba1, Ba(K), and Ba(Ca)) are occupied slightly at the first hours of the exchange process and with time there is only negligible increase of the occupancies. The calcium position Ba(Ca) (M2) starts to accept 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 proceeds similar to the exchange process at 90 • C (reaching 1.00 Ba 2+ per unit cell). However, in the following time intervals up to 72 h, the exchange proceeds very slowly and reaches 1.18 Ba 2+ , which is more than twice as low as the sample at 90 • C. Then the barium exchange continues slowly (like the sample at 90 • C) reaching 1.64 Ba 2+ at the 22nd day.   On Figure 10 (right) it is seen that the outgoing cations leave the structure quickly up to the first 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).
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 exchange, predominantly realized in position Ba2. The outgoing cations (Na + , Ca 2+ , K + , and Mg 2+ ) do not show any tendency of preferable leaving-they all together intensively leave the structure during the first 2 h and then this process is slowed down following the tendency of barium take up by clinoptilolite. This indicates that the exchangeable cations movement in the structure is realized easily without preferable directions, which shows that the diffusion process proceeds isotropically [16]. This conclusion fits well with the theory of gallery space in the HEU-type structure [17].

Conclusions
Clinoptilolite sample subjected to barium exchange for a period up to the 22 days at room temperature and 90 °C shows that the exchange process proceeds all the time with different speed. This is confirmed both by EDS analyses and Rietveld XRD structure refinement at different exchange times.
The starting Rietveld refinement was performed on the maximum Ba exchanged clinoptilolite (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 populated. 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.
The behaviour of the H2O molecules when coordinating the exchangeable cations during the 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 these changes in the positions of H2O molecules during the ion exchange process indicate their diffusion accompanied by the exchangeable cations' movements.
The barium cations during the ion exchange at 90 °C eagerly enter the clinoptilolite structure during the first 2 h. In the same interval, a profound lowering of the number of all original cations is observed. Then the exchange process is gradually slower up (especially after 72 h) to the 22nd day. The tendency for the outgoing cations is the opposite-their contents lower accordingly. The barium exchange on clinoptilolite at room temperature for the first 2 h proceeds similar to the exchange process at 90 °C and then is slowed up to the 22nd day.  On Figure 10 (right) it is seen that the outgoing cations leave the structure quickly up to the first 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).
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 exchange, predominantly realized in position Ba2. The outgoing cations (Na + , Ca 2+ , K + , and Mg 2+ ) do not show any tendency of preferable leaving-they all together intensively leave the structure during the first 2 h and then this process is slowed down following the tendency of barium take up by clinoptilolite. This indicates that the exchangeable cations movement in the structure is realized easily without preferable directions, which shows that the diffusion process proceeds isotropically [16]. This conclusion fits well with the theory of gallery space in the HEU-type structure [17].

Conclusions
Clinoptilolite sample subjected to barium exchange for a period up to the 22 days at room temperature and 90 • C shows that the exchange process proceeds all the time with different speed. This is confirmed both by EDS analyses and Rietveld XRD structure refinement at different exchange times.
The starting Rietveld refinement was performed on the maximum Ba exchanged clinoptilolite (90 • C, 22 d). After refinement of unit-cell parameters, atomic occupancies and coordinates of exchangeable cationic sites and H 2 O molecules we determined four barium positions differently populated. A number of 9 H 2 O sites were additionally refined in the channels of Ba-clinoptilolite with varying occupancies, with some of them coordinating the barium cations.
The behaviour of the H 2 O molecules when coordinating the exchangeable cations during the exchange process shows that these molecules are attracted by incoming barium cations, thus making intraspace moves. Some of the H 2 O positions change their occupation and atomic coordinates. All these changes in the positions of H 2 O molecules during the ion exchange process indicate their diffusion accompanied by the exchangeable cations' movements.
The barium cations during the ion exchange at 90 • C eagerly enter the clinoptilolite structure during the first 2 h. In the same interval, a profound lowering of the number of all original cations is observed. Then the exchange process is gradually slower up (especially after 72 h) to the 22nd day. The tendency for the outgoing cations is the opposite-their contents lower accordingly. The barium exchange on clinoptilolite at room temperature for the first 2 h proceeds similar to the exchange process at 90 • C and then is slowed up to the 22nd day.
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 Ca 2+ 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.