INVESTIGATION OF THE ADSORPTION OF BENZENE ON KONGYRTOG BENTONITE

ABSTRACT

The adsorption of benzene on Kongyrtog bentonite, located in the region of Kashkadarya (Uzbekistan), was studied in the article. According to the experimental results, isotherms of benzene on bentonite (according to IUPAC) can belong to class IV according to Brunauer, Deming, Deming and Teller (BDDT) classification. According to De Boer’s classification, the hysteresis loops of adsorption can be attributed to the H3 type, and it was determined that the bentonite of Kongyrtog is mainly slotted. According to the Langmuir and BET equations, the specific surface (S) of bentonite is equal to 58∙10^-3 m²/kg, and the volume of micropores (W₀) (according to Dubinin-Radushkevich) is equal to 23,19∙103 m³/kg. The minimum pore radius of Kongyrtog bentonite is 5,7 Å and the maximum pore radius is 94 Å.

АННОТАЦИЯ

В статье изучена адсорбция бензола на бентоните Конгыртог, расположенном в Кашкадарьинской области (Узбекистан). По результатам экспериментов изотермы бензола на бентоните (по ИЮПАК) могут относиться к классу IV по классификации Брунауэра, Деминга, Деминга и Теллера (BDDT). По классификации Де Бура петли гистерезиса адсорбции можно отнести к типу Н3, и было установлено, что бентонит Конгыртог в основном щелевой. По уравнениям Ленгмюра и БЭТ удельная поверхность (S) бентонита равна 58∙10^-3 м²/кг, а объем микропор (W0) (по Дубинину-Радушкевичу) равен 23,19∙ 10³ м³/кг. Минимальный радиус пор бентонита Конгыртог составляет 5,7 Å, а максимальный радиус пор составляет 94 Å.

Keywords: bentonite, monolayer capacity, saturation volume, porosity radius, isotherm, hysteresis loop, micropore, mesopore, surface area.

Ключевые слова: бентонит, емкость монослоя, объем насыщения, радиус пористости, изотерма, петля гистерезиса, микропора, мезопора, площадь поверхности.

Introduction. Currently, the demand for adsorbents (zeolite, activated carbon, (activated coal) silica gel, palygorskites, etc.) in various industries is increasing. In particular, bentonites, isolated from natural mineral sorbents, possess a certain importance owing to their highly polished surface area and adsorption capacity [5, P.60-63]. A number of scientific and practical works are being carried out in the world by the use of cheap and high-quality sorbents in important sectors such as chemistry, paint, oil and gas, pharmaceuticals, textile industries, and uribication waste water [11, P.184-191; 3, P.210-214]. When using adsorbents, their selectivity, quality, surface area, cost and regeneration properties are important. For proper use of natural sorbents, various methods of activation (acidic, thermal, hydro-thermal treatment) and modification (surfactants, polyelectrolytes) are widely used to regulate their surface area and pore volume [10, P.92-95]. Such methods are bonded with their mechanism of great effect on the pore volume and specific surface of adsorbents [6, P.112-116].

Experimental. Bentonite from the Kongyrtog mine, located in the north-west of the city of Karshi (Uzbekistan) [1; P. 265], was used as a research object. Its chemical composition is as follows: (mass %): SiO₂ -58,62; Al₂O₃-18,82; TiO₂-0,81; Fe₂O₃ -6,68; Na₂O -2,52; K₂O-2,56; CaO-0,80; MgO-1,20; P₂O₅-0,12; SO₂-0,35; It was found that SO₃-0,28 and the mass loss at heating is equal to -7,17. The chemical composition of bentonite was analyzed in the central laboratory of JSC ‘Uzbek Geology-Kidruv’ (ICPE-9820 Atomic Emission Spectrometer, Shimadzu, Japan). Kongyrtog bentonite was first crumbled into a powder, and then it was cleaned from additional impurities (sand fragments, quartz, sedimentary rocks, etc.) by boiling in distilled water for 24 hours.

Adsorption and desorption isotherms of benzene on brown bentonite were measured on the Mac-Ben device. Before the adsorption process, the adsorbate was purified under vacuum conditions until its saturated vapor pressures were the same as those reported in the literature. Adsorption results were analyzed using the BET, Langmuir, Freundlich and Dubinin-Radushkevich models.

Results and discussion. The adsorption and desorption isotherms of benzene on Kongyrtog bentonite are presented in Fig. 1. The isotherm of benzene adsorption is characterized by the fact that, the initial specific relative pressures sharply increased up to P/P_s=0,13 (Henry area) and then, slowly increased in the diapason P/P_s=0,13-0,42 and became equal to a=0,32 mol/kg. According to the experimental results, benzene adsorption isotherms on brown bentonite (according to IUPAC) belong to type IV according to the BDDT classification. Adsorption function a=f(P/P_s), [2, P.10] a hysteresis surface was formed in the adsorption isotherms at a=0,43-0,68 mol/kg. It is explained by the simultaneous occurrence of polymolecular adsorption and capillary condensation in this range. It was determined that canleatribted adsorption hysteresis loops were determined type H3 according to De Boer's classification. Adsorption hysteresis of this type is characterized by the fact that it occurs as a result of adsorption in slit like pores open from all sides [4, P.125]. Adsorption analyze plays an important role in the process of obtaining effective clay-modified adsorbents on the basis of Kongyrtog bentonite. [9, P.668-671]

Figure 1. Adsorption and desorption isotherms of benzene on Kongyrtog bentonite

In Fig. 2 changing the volume of micropores calculated on the basis equation of Dubinin-Radushkevichon the base of an isotherm adsorption and desorption of benzeneon brown bentonite has presented.

Figure 2. The volume of micropores calculated on the basis of the Dubinin-Radushkevich equation

According to the results, it is obvious that the micropore volume of micropores (W₀) was 0,23∙10³ m³/kg, and the mesopore volume (W_me) was 0,36∙10³ m³/kg. The adsorbent consisted of 38% micro- (W₀) and on 62% meso- (W_me) pores (Tab. 1).

Table 1.

Parameters of adsorption of benzene vapors on Kongyrtog bentonite

From table 1 it is show that the monolayer capacity of bentonite (a_m) is 0,22∙10³ mol/kg, the specific surface area (S) is 58∙10³ m²/kg and the saturation adsorption capacity (V_s) was found to be equal 60,3∙10³ m³/kg [8, P.712-716; 7, P.1569-1573].

The integral and differential distribution curves of adsorbent of pore volumes from their radiises are presented in Fig.3,4

Figure 3. Integral curve of dependence adsorption volume of Kongyrtog bentonite on pore size

Figure 4. Differential dependence curve of adsorption volume of Kongyrtog bentonite from pore size

According to the obtained results, the minimum pore radius of the bentonite is 5,7 Å, and the maximum pore radius is 94 Å. and the largest dimension of pores in the adsorbent was enabled at 20,3 Å. The differential curve of dependence and adsorption volume on porosity size was characterized by the fact that it has a wave air.

Figure 5. Benzene adsorption isotherm according to the Freyndlix equation

According to the results in coordinates of the isotherm of benzene adsorption on Kongyrtog bentonite, the tangent angle 1/n = 0,56 to n = 1,78 Freyndlix constant was equal K_F=1,18×10³ (Fig. 5).

Also, the results of benzene adsorption on Kongyrtog bentonite and the values of the calculations performed according to the Langmuir model are presented on Fig.6.

Figure 6. Results of adsorption of benzene vapors on Kongyrtog bentonite according to the Langmuir model

Table 2.

Adsorption parameters of benzene adsorption on Kongyrtog bentonite according to the Langmuir model

It can be seen from table 2 that the Langmuir constant (K_L) for benzene adsorption on bentonite was 15,62, the correlation coefficient (R²) was 0,9821 and the Gibbs energy (DG) was -6,671 kJ/mol. So, the process of adsorption of benzene is exothermic with heat release.

Conclusion. Based on these investigations, it can be concluded that according to the classification of isotherms of benzene adsorption on Kongyrtog bentonite, it can be attributed to type IV, the adsorption being simultaneous with both polymolecular and capillary condensation. Bentonite consists of open pores with slit-like openings. The maximum porosity of the adsorbent calculated according to the Freundlich model is 20,3 Å, n = 1,78. Gibbs energy of adsorption (DG) equals -6,671 kJ/mol which explains why the adsorption of benzene is physical by its nature.

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