**4. Features of the sorption of various substances vapors by activated forms of natural zeolite**

The comparative ease of zeolites chemical modification opens up wide possibilities for controlled changes in their structure and properties. This makes them very convenient objects for studying the nature of sorption interactions, molecular-sieve effects. The vast majority of these studies were conducted with synthetic zeolites. The natural forms of such minerals which are multicomponent systems complex and variable according to composition are much less known. Their physicochemical properties essentially depend on the content of zeolitic phase in the rock, such as cation exchange form and nature of impurities. Even with the same content of the zeolitic phase in the rock a mismatch in the properties of individual samples may be observed. Natural zeolites are biporous systems; the primary porosity of them is determined by the micropores of crystals, and the secondary one – by the transitional pores and macropores. The latter are the main transport arteries for the supply of a substance; they determine the absorption of relatively large molecules and play an important role for a number of sorption and catalytic processes.

270 Ion Exchange Technologies

l; = 30 min.

**Figure 6.** Degree of Cu2 + by natural zeolite as a function of the pH. CСu2+=0,25 g/l; V = 50 ml; m = 0,25 g /

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**рН**

t, min

**Figure 7.** Degree Pb2+ (1); Cu2+ (2); Co2+ (3); Ni2+ (4) by natural zeolite as a function of the process

**4. Features of the sorption of various substances vapors by activated** 

The comparative ease of zeolites chemical modification opens up wide possibilities for controlled changes in their structure and properties. This makes them very convenient objects for studying the nature of sorption interactions, molecular-sieve effects. The vast majority of these studies were conducted with synthetic zeolites. The natural forms of such minerals which are multicomponent systems complex and variable according to

Thus, the suggested sorbent effectively recovers copper ions from model solutions within short periods of time. Cheap natural zeolites may be used in the processes which do not provide regeneration of ionites. The opportunity of its one-time use with subsequent burial provides application thereof in such spheres where synthetic zeolites or ion-exchange

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polymers cannot be used under economical reasons.

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Hydrophilic inorganic sorbents, which include zeolites, are practically useless for adsorption of many organic substances from aqueous solutions since water molecules interact with the OH - groups to form strong hydrogen bonds (5-10 kcal/mol) energy of which exceeds the energy of adsorption of most organic molecules [10]. In this connection, it is important to study the sorption of vapours of various substances on the surface of such minerals.

We used acidoactive (processing for twenty four hours by 3 and 12 N hydrochloric acid solution) air-dried and dehydrated (600ºС) natural zeolite samples as the adsorbate. Sorption was determined by the increase in mass of the sorbent (initial sample 1 g) placed in open weighing cup over liquid adsorbate in a desiccator under normal conditions.

Porous crystals of zeolites are of great interest as highly specific adsorbents both on the molecular sieve effect and also on the nature of their channels' surface. Adsorption peculiarities are connected with the fact the delicacy of the crystal structure creates a large adsorption volume (up to 0,54 cm3/g for faujasites), and its geometry determines the molecular sieve properties. The presence of acceptor centers (cations, the Lewis acid centers) firmly holding the electron donors, or OH - groups, strongly binding bases (Bronsted centers), causes strong interaction of adsorbed molecules with the adsorbent.

Depending on the nature of these molecules, various types of interactions may occur between them and zeolites. The specificity of zeolites with respect to the molecules of this adsorbate is determined by the values of the interaction energies of different types. In case of the heteropolar adsorbent, the polarization of the adsorbate molecules occurs by its electrostatic field. High adsorption energy of zeolite by molecules capable for specific interaction with cations having a peripheral dipoles (water, aether), the large quadrupoles and -electron bonds (nitrogen, olefins, benzene) are typical for zeolites. The energy of dispersive attraction makes decisive contribution to the specificity of the process. For zeolites, the prevalence of the energy may be due to the dense environment of adsorbate molecules by the atoms of oxygen frame. Therefore, it is very high energy of adsorption of organic molecules with peripheral functional groups having atoms with free electron pairs and -bonds for them.

The ratio of Si/Al in natural zeolites can be improved by chemical treatment of the crystals, resulting to the removal of part of aluminum from the frame. Herewith, dealumination can occur without significant changes in their characteristic structural features. A simple option

thereof is the treatment of crystals with acid solutions. The result of this chemical action is the artificial increase of the adsorption volume and the effective pores' size.

The curves of the kinetics organic solvents vapours sorption on dehydrated and nondehydrated zeolite samples are given in the Figure 8. For each of them, they are close to each other, and according to the value of weight gain of the solvent they are in the following order: acetonetoluenecarbon tetrachloride. This range correlates with the size of Vander Waals molecules. Not less important role is played by well-known sorption activity of the polar solvent (acetone). For molecules of hydrophobic solvents water sorbed and bound with cations in the crystallographic positions of the mineral blocks the entrance windows in its cavity. It is shown [11] that the sorption of carbon tetrachloride vapours occurs only on the outer surface of both natural and activated clinoptilolite, and sorption of acetone – in the micropores of the latter. If we assume that water molecules are completely desorbed in zeolite pores dehydrated at 6000C, then the process of sorption of these compounds on the active sites of sorbent runs with displacement of pre-sorbed water.

The results of acetic acid and water vapours sorption on zeolites treated with 3 and 12 N hydrochloric acid are given in the Figure 9. A slight difference in the values of weight gain of the substance on the original forms of mineral is shown in this Figure; a small difference in weight gain of water and organic acid on non-dehydrated and dehydrated zeolites treated with 3 N solution of mineral acid is due to insufficiently developed micro porosity of the mineral. Specific adsorption of water molecules by zeolite is mainly represented by interaction of free electron pairs of oxygen atoms with cations of zeolite cavities surface. The polar molecules of water penetrating into its micropores are sorbed, including as a result of ion-dipole interaction with the active centers (cations, compensating the excess charge of zeolite frame). Water molecules bond with the cations and the frame is predominant in the case of natural zeolites [12]. The adsorbate-adsorbent interaction completely determines the process.

**Figure 8.** Degree of the mass gain of solutions as a function of the process duration t. sample №2: **dehydrated zeolite 1 –** toluene; 3- acetone; 5- carbon tetrachloride. **air-dry zeolite** 2- toluene; 4- acetone; 6- carbon tetrachloride.

process.

6- carbon tetrachloride.

**mass gain, %**

thereof is the treatment of crystals with acid solutions. The result of this chemical action is

The curves of the kinetics organic solvents vapours sorption on dehydrated and nondehydrated zeolite samples are given in the Figure 8. For each of them, they are close to each other, and according to the value of weight gain of the solvent they are in the following order: acetonetoluenecarbon tetrachloride. This range correlates with the size of Vander Waals molecules. Not less important role is played by well-known sorption activity of the polar solvent (acetone). For molecules of hydrophobic solvents water sorbed and bound with cations in the crystallographic positions of the mineral blocks the entrance windows in its cavity. It is shown [11] that the sorption of carbon tetrachloride vapours occurs only on the outer surface of both natural and activated clinoptilolite, and sorption of acetone – in the micropores of the latter. If we assume that water molecules are completely desorbed in zeolite pores dehydrated at 6000C, then the process of sorption of these compounds on the

The results of acetic acid and water vapours sorption on zeolites treated with 3 and 12 N hydrochloric acid are given in the Figure 9. A slight difference in the values of weight gain of the substance on the original forms of mineral is shown in this Figure; a small difference in weight gain of water and organic acid on non-dehydrated and dehydrated zeolites treated with 3 N solution of mineral acid is due to insufficiently developed micro porosity of the mineral. Specific adsorption of water molecules by zeolite is mainly represented by interaction of free electron pairs of oxygen atoms with cations of zeolite cavities surface. The polar molecules of water penetrating into its micropores are sorbed, including as a result of ion-dipole interaction with the active centers (cations, compensating the excess charge of zeolite frame). Water molecules bond with the cations and the frame is predominant in the case of natural zeolites [12]. The adsorbate-adsorbent interaction completely determines the

**Figure 8.** Degree of the mass gain of solutions as a function of the process duration t. sample №2: **dehydrated zeolite 1 –** toluene; 3- acetone; 5- carbon tetrachloride. **air-dry zeolite** 2- toluene; 4- acetone;

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t, h

the artificial increase of the adsorption volume and the effective pores' size.

active sites of sorbent runs with displacement of pre-sorbed water.

**Figure 9.** Degree of the mass gain of acetic acid and water as a function of the process duration t. **acetic acid: (**sample №2) 1 - dehydrated zeolite; 2 - air-dry zeolite; (sample №3) 5 - dehydrated zeolite; 6 - airdry zeolite; **water:** (sample №2) 3 - dehydrated zeolite; 4 - air-dry zeolite

Large scale polarizability (in comparison with water) and low dielectric permeability of the organic acid that increase the binding energy of the cation-fixed ion create preconditions for more effective development of the dispersion forces which play an important role in the mechanism of weak organic acid absorption by ionites.

Acid treatment of zeolite samples with 3 N hydrochloric acid under normal conditions practically does not lead to significant changes in the structural properties of the sorbents, as it is shown above (Table 2). The values of specific surface area for them according to BET and t-graphic have small differences [4]. However, increase of acid concentration under comparable conditions leads to an increase of micro- and mesopores volume. Therefore, zeolite activated by 12 N acid has a higher sorption capacity. Such treatment fosters the expansion of the input windows of mineral channels, and water allocation on ignition contributes to the displacement of cations inside and improvement of primary porosity which depends on the nature of zeolite. This effect seems to be the result of changes in the chemical nature of adsorption sites and porous structure of the mineral in the process of activation. This condition is caused by changes in relative positions of aluminum-oxygen and silicate-oxygen tetrahedrons of the core.

It is shown by example of acetic acid (Figure 9, 10) that there is an evident divergency between the curves of sorption of vapours of the studied substance on non-dehydrated and dehydrated zeolite. Study of the kinetics process showed that for 200 h there is a saturation of natural mineral and sorption volume, is 47 and 89 mmol/g, accordingly.

Increasing of activating acid concentration leads to development of micro-cavities and channels interconnected within the frame of zeolite free of exchangeable cations that promotes the growth of the volume of adsorbed acetic acid. The latter circumstance indicates that the activation process in comparison with the amorphization of zeolite prevails under these conditions; this characterizes the studied mineral as an acid-resistant.

**Figure 10.** Degree of the sorbtion S of acetic acid vapors by natural zeolite as a function of the process duration t. 1**-** dehydrated zeolite**;** 2**-** air-dry zeolite

The results of spectroscopic (Figure 11) and X-ray studies of natural zeolite activated by 3 and 12 N hydrochloric acid solutions give evidence of preservation of mineral crystal structure due to acid resistance thereof.

**Figure 11.** IR spectra of zeolites. 1 - natural zeolite;2 - activated 3 N hydrochloric acid; 3 - activated 12 N hydrochloric acid; 4 - after the sorption of acetic acid vapors.

The absorption bands at 464, 640, 776, 950 cm-1 typical for Si-O-Al intratetrahedral deformative and symmetric valence vibrations are prescribed in original zeolite spectrum. When treating it by 3 and 12 N hydrochloric acid, all the characteristic frequencies become more apparent. The position of the peak at 464 cm-1 is kept and there is a shift of the absorption bands at 640 and 776 to the long-wave area (672; 680 and 800;832 cm-1, accordingly) without changing the intensity of the spectrums. This gives evidence of preservation of samples crystallinity degree after activation. In the spectrums of zeolite saturated by acetic acid vapours frequencies of adsorbate molecules methyl groups vibrations appear at 1380 cm-1 and the absorption band broadens at 1640 cm-1 due to overlap of the deformation vibrations of water molecules and asymmetric COO— valent groups.

274 Ion Exchange Technologies

duration t. 1**-** dehydrated zeolite**;** 2**-** air-dry zeolite

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**Figure 10.** Degree of the sorbtion S of acetic acid vapors by natural zeolite as a function of the process

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, cm-1

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The results of spectroscopic (Figure 11) and X-ray studies of natural zeolite activated by 3 and 12 N hydrochloric acid solutions give evidence of preservation of mineral crystal

**Figure 11.** IR spectra of zeolites. 1 - natural zeolite;2 - activated 3 N hydrochloric acid; 3 - activated 12 N

The absorption bands at 464, 640, 776, 950 cm-1 typical for Si-O-Al intratetrahedral deformative and symmetric valence vibrations are prescribed in original zeolite spectrum. When treating it by 3 and 12 N hydrochloric acid, all the characteristic frequencies become more apparent. The position of the peak at 464 cm-1 is kept and there is a shift of the absorption bands at 640 and 776 to the long-wave area (672; 680 and 800;832 cm-1, accordingly) without changing the intensity of the spectrums. This gives evidence of preservation of samples crystallinity degree after activation. In the spectrums of zeolite Dealumination is accompanied by rupture of Si-O-Al bonds and the resulting shift of asymmetric valent intratetrahedral vibrations frequency from 1040 to 1056 and 1100 cm-1.

The studies showed that an increase in acid concentration leads to an increase in sorption ability of mineral connected with an increase of mineral porosity in the cation removal process. Thus, depending on the nature of adsorbate and adsorbent type, the appropriate modes of activation can be selected.
