ISOTHERM AND DIFFERENTIAL ENTHALPY OF HYDROGEN SULFIDE ADSORPTION ON AN ACTIVATED SORBENT DERIVED FROM HYBRID TOMENTOSA WOOD

ABSTRACT

In the present article, the regularities of the adsorption isotherm and the differential enthalpy of hydrogen sulfide molecules on an activated carbon adsorbent obtained from the bark of the Paulownia tomentosa tree at a temperature of 303 K, depending on the adsorption value, as well as the mechanism of the sorption process, were investigated. It was experimentally established that the adsorption capacity of the studied adsorbent with respect to hydrogen sulfide at a pressure of P=588 torr is ~1.43 mmol/g. From the stepwise change of the adsorption isotherm and the differential enthalpy of hydrogen sulfide molecules on the activated carbon adsorbent from the bark of the Paulownia tomentosa tree, it was determined that the number of active sites with respect to hydrogen sulfide is 0.25 mmol/g, and the formation of a pentamer complex of adsorbate/adsorbent in the ratio of 5H₂S:adsorbent was established.

АННОТАЦИЯ

В настоящей статье исследованы закономерности изотермы адсорбции и дифференциальной энтальпии молекул сероводорода на активированном углеродном адсорбенте, полученном из коры дерева Павловния-Томентоза, при температуре 303 К в зависимости от величины адсорбции, а также выявлен механизм сорбционного процесса. Экспериментально установлено, что адсорбционная ёмкость исследуемого адсорбента по отношению к сероводороду при давлении P=588 torr составляет ~1,43 ммоль/г. Из ступенчатого изменения изотермы адсорбции и дифференциальной энтальпии молекул сероводорода на активированном углеродном адсорбенте из коры дерева Павловния-Томентоза определено, что количество активных центров по отношению к сероводороду равно 0,25 ммоль/г, и установлено образование пентамерного комплекса адсорбат/адсорбент в соотношении 5H₂S:адсорбент.

Keywords: adsorption, adsorbent, isotherm, relative pressure, enthalpy, microcalorimeter, hydrogen sulfide.

Ключевые слова: адсорбция, адсорбент, изотерма, относительное давление, энтальпия,  микрокалориметр, водород сульфид.

Introduction

In the world, a number of industrial sectors, in particular the chemical, metallurgical, oil and gas industries, are considered the main sources of waste gas emissions that contain toxic and hazardous impurities and pollute the atmosphere and the environment. In gas purification, adsorbents, including natural and synthetic zeolites, are widely used in various sectors of production [1–6].

In many branches of industry, in the production of different assortments of activated adsorbents, mainly carbonaceous materials with an initial carbon content above 76.0–86.0%, as well as stems and branches of plants, fruit pits recycled as waste (including apricot, peach, and walnut), clay minerals and other related raw material residues are used. In order to employ them as adsorbents, extensive scientific research is being carried out to improve their physicochemical properties and adsorption capacity, since the application of efficient adsorbents in various branches of industry is of great importance.

In Uzbekistan, the problem can be solved by activating wood wastes of trees grown in the country, which are considered a type of raw material that meets the requirements for adsorbents, by various methods, thereby enhancing their high adsorption capacity and studying their indicators that meet industrial demand for adsorbents [7–8].

During the activation process, when carbon monoxide and hydrogen are released from the system into the atmosphere, the activation process of thermally active adsorbents can be significantly accelerated, while a decrease in the rate of reaction of steam with carbon in these products is observed [9].

The transportation and storage of wood waste may cause many problems. Due to the high moisture content of wood waste, its direct use as fuel is not feasible. Therefore, the pyrolysis method is considered a more efficient way of processing them [10–14]. In addition, millions of tons of walnut shell waste accumulate worldwide. Walnut shell contains (in %): 25.8–51.9% cellulose, 16.8–47.6% lignin, 8.7–17.9% hemicellulose, 0.2–3.1% ash, and 3.8–10.41% moisture. When walnut shell is activated with H₃PO₄ at 170°C for 0.5–1.0 hours, the process is carried out in the presence of nitrogen and 12 other inert gases [15–23].

Methodology and Materials

In this article, the adsorption isotherm and differential enthalpy of hydrogen sulfide at 303 K on activated carbon derived from the bark of the Paulownia-Tomentosa tree are presented.

Activated carbon was obtained from the waste of Paulownia-Tomentosa tree, in particular from waste branches generated as a result of additional pruning three or four times a year and from the bark part of the tree, through a conventional activation method in two stages:

The first stage  consists of 1. pyrolysis of waste wood; 2. activation with steam. During the pyrolysis process, pieces of Paulownia-Tomentosa bark of 50–100 mm in size are placed into the pyrolysis unit. The unit is connected to an electric supply with a voltage of 60–65 V, and the temperature is set between 300°C and 800°C. Once the designated temperature is reached, the mass inside the unit is kept for 1.5–2 hours until a thermally active adsorbent is formed. During the thermal activation of the adsorbents, resinous tars and carbonaceous gases such as CO, CO₂, CH₄, and others are released in the range of 250–400°C.

In the second stage, the carbonaceous mass obtained after the pyrolysis process is activated with steam at 800°C for 1.5–2 hours to produce the final adsorbent.

The adsorption isotherm was measured using a universal high-vacuum apparatus. The principle of operation and characteristics of this device are fully described in the authors’ works [24–28]. This apparatus makes it possible to comprehensively determine the regular change of the sorption isotherm and differential enthalpy with respect to the adsorption capacity, the number, strength, and nature of active centers of the activated carbon adsorbent obtained from Paulownia-Tomentosa tree bark waste by conventional (thermal–pyrolysis and steam–gas conditions) methods, as well as to clarify the sorption mechanism.

Results and Discussion

The regular relationship between the adsorption isotherm and the differential enthalpy of hydrogen sulfide molecules on the activated carbon adsorbent obtained from the bark of the *Paulownia Tomentosa* tree at a temperature of 303 K, as well as the mechanism of the sorption process, was determined.

The logarithmic function isotherm of hydrogen sulfide adsorption obtained from the Paulownia Tomentosa tree is presented in Figure 1. At low saturation, the adsorption isotherm is equal to Ln(P/P₀)=–10 (P/P_s=4·10^-5, P=0.76 torr). This indicates weak sorption of hydrogen sulfide molecules on the adsorbent. The adsorption isotherm reached an adsorption capacity of 1.43 mmol/g at a relative pressure of P/P_s=0.033 (P=588 torr). The isotherm corresponds to the IV-type of Brunauer isotherms. Thus, hydrogen sulfide molecules are adsorbed in the pores of the adsorbent.

Figure 1. Adsorption isotherm of hydrogen sulfide molecules on an activated adsorbent obtained from the Paulownia tomentosa tree

Up to 0.25 mmol/g adsorption, the isotherm shows an initial linear change with Ln(P/P₀)=–6.6 (P/P_s=0.0029, P=25 torr); at 0.5 mmol/g adsorption and Ln(P/P₀)=–4.5 (P/P_s=0.01, P=180 torr), the isotherm exhibits a second linear change; at 0.75 mmol/g adsorption and Ln(P/P₀)=–4.27 (P/P_s=0.014, P=251 torr), the isotherm shows a third linear change with a sharp upward rise; and up to 1 mmol/g adsorption, a fourth linear change with partial bending is observed. This indicates that the number of active sites of the activated adsorbent derived from the bark of the Paulownia tomentosa tree with respect to hydrogen sulfide molecules is equal to 0.25 mmol/g. Thus, up to 1 mmol/g adsorption, four hydrogen sulfide molecules are sequentially adsorbed, forming a tetramer 4H₂S:adsorbent complex.

Starting from 1 mmol/g adsorption, the equilibrium relative pressure increases sharply, and at 1.25 mmol/g adsorption, a 5H₂S:adsorbent pentamer complex is formed. At a relative pressure of P/P_s=0.033 (P=588 torr) and adsorption amount of 1.43 mmol/g, the sorption process is completed.

The differential enthalpy of hydrogen sulfide molecule adsorption on the activated adsorbent obtained from the bark of the Paulownia tomentosa tree is presented in Figure 2.

Figure 2. Differential enthalpy of hydrogen sulfide molecule adsorption on an activated adsorbent obtained from Paulownia tomentosa tree

Based on differential enthalpy, the adsorption mechanism, i.e., the nature, strength, and number of zeolite active sites, corresponds to the sorption mechanism described above on the basis of the adsorption isotherm.

The differential enthalpy changes in a wave-like manner. This change corresponds to the complexes formed at the active sites of the adsorbent. At the initial stage, the differential enthalpy of adsorption equals ~33 kJ/mol at 0.02 mmol/g adsorption. With the saturation of sorption volume, at 0.22 mmol/g the enthalpy decreases almost linearly to 24.7 kJ/mol, forming the first minimum. This value corresponds to the sorption active site value (0.25 mmol/g) explained in the isotherm of the activated adsorbent obtained from Paulownia tomentosa bark, i.e., hydrogen sulfide molecules form a monomer 1H₂S:adsorbent complex with the activated adsorbent.

After 0.25 mmol/g adsorption, the increase of adsorption enthalpy up to 26.4 kJ/mol is related to the relocation of the initial hydrogen sulfide molecules, as well as the release of additional energy caused by van der Waals interactions between adsorbate molecules.

At 0.45 mmol/g adsorption, the differential enthalpy forms a step, and at 0.5 mmol/g adsorption it decreases to 22 kJ/mol, forming the second minimum and creating a 2H₂S:adsorbent dimer complex. With further saturation of the sorption volume, the enthalpy increases linearly up to 25 kJ/mol, forming a 3H₂S:adsorbent trimer at 0.75 mmol/g adsorption, and rises to 26 kJ/mol at 1 mmol/g adsorption, forming a 4H₂S\:adsorbent tetramer complex. During further adsorption of hydrogen sulfide molecules, the differential enthalpy first decreases to 22.5 kJ/mol, then increases to 23.5 kJ/mol at 1.25 mmol/g adsorption, forming a 5H₂S:adsorbent pentamer complex, thus completing the sorption process.

Conclusion

The isotherm and differential enthalpy of adsorption of hydrogen sulfide molecules on an activated adsorbent obtained from the bark of the Paulownia tomentosa tree were studied by the adsorption–calorimetric research method. In the interval from low saturations up to the experimental pressure (587 torr), the thermodynamics of the sorption process on an activated carbon adsorbent derived from local raw material, as well as the regularity of hydrogen sulfide molecules filling the volume of the adsorbent, were determined. It was experimentally established that the adsorption capacity of the studied adsorbent with respect to hydrogen sulfide at a pressure of P=588 torr is ~1.43 mmol/g. From the stepwise change of the adsorption isotherm and the differential enthalpy of hydrogen sulfide molecules on the activated carbon adsorbent from the bark of the Paulownia tomentosa tree, it was determined that the number of active sites with respect to hydrogen sulfide is 0.25 mmol/g, and the formation of a pentamer complex of adsorbate/adsorbent in the ratio of 5H₂S: adsorbent was established.

References:

1. Kai Qi, Lili Gao, Xuelian Li, and Feng He. Research Progress in Gas Separation and Purification Based on Zeolitic Materials. // Catalysts. - 2023. - 13(5). -PP. 855. [Electronic resource] URL: https://doi.org/10.3390/catal13050855  
2. Zeyu Tao, Yuanmeng Tian, Wei Wu, Zhendong Liu, Weiqi Fu, Chung-Wei Kung, Jin Shang. Development of zeolite adsorbents for CO₂ separation in achieving carbon neutrality. // npj Materials Sustainability. - 2024. - Vol.2. -№20.
3. Eduardo Pérez-Botella, Susana Valencia, and Fernando Rey. Zeolites in Adsorption Processes: State of the Art and Future Prospects. // Chemical Reviews. - 2022. - Vol.122. -№24. - PP.17647-17695.
4. Dinda S., Murge P., Chakravarthy Paruchuri B. A Study on Zeolite-Based Adsorbents for CO₂ Capture. // Bull. Mater. Sci. - 2019. - №42. - PP. 240. 
5. Murge P., Dinda S. Roy S. Zeolite-Based Sorbent for CO₂ Capture: Preparation and Performance Evaluation. // Langmuir. -2019. - №35. - PP. 14751–14760.
6. Gunawardene O.H.P., Gunathilake C.A., Vikrant K., Amaraweera S.M. Carbon Dioxide Capture through Physical and Chemical Adsorption Using Porous Carbon Materials: A Review. // Atmosphere. - 2022. - №13. - PP. 397.
7. Bogdanovich N.I., Chernousov Yu.I. Sorbents for the treatment of pulp and paper industry wastewater based on wood waste from wood processing. // Pulp. -1989. - №5. - PP. 41.
8. Kalinicheva O.A, Bogdanovich N.I., Dobele G.V. Pre-pyrolysis of wood raw materials by synthesis of activated carbon with NaOH. // Forestry Journal. -2008. №2. - PP.117-122.
9. Sultan Alam, Muhammad Sufid Khan, Whid Bibi, Ivar Zekker, Juris Burlakovs, Makrand M. Ghngrekr, Gourv Dhr Bhowmick, Nan Kllistov, Nikoli Pimenov and Muhammad Zhaoor Preparation of activted Carbon from the Wood of Paulownia tomentos as an Efficient adsorbent for the Removal of acid Red 4 and Methylene Blue Present in Wastewater. // Water. - 2021. - №13(11). - PP.1453. [Electronic resource] URL: https://doi.org/10.3390/w13111453
10. Pokonova Yu.V. Activation of the surface of carbon sorbents by radiation. // Chemical industry. - 2007. - №6(84). -  PP.291-295.
11. Savrasova Yu.A., Bogdanovich N.I., Makarevich N.A., Beletskaya M.G. Carbon adsorbents based on lignocellulosic materials // Forestry magazine. - 2012. - №1. - PP.107-112.
12. Masakatsu M., Harumi K., Akihiko S., Toyoji K., Kenji T. Rapid pyrolysis of wood block by microwave heating.  // Journal of Analytical and Applied Pyrolysis. - 2004. - №1(71). - PP.187-199.
13. Sultan A., Muhammad S.Kh., Whid B., Ivar Z., Juris B., Makrand M.G., Gourv D.B., Nikoli P., and Muhammad Zh. Preparation of activted Carbon from the Wood of Paulownia tomentos as an Efficient adsorbent for the Removal of acid Red 4 and Methylene Blue Present in Wastewater. // Water. - 2021. - №13(11). -PP.1453.
14. Chugunov A.D., Filatova E.G. Adsorption of petroleum products by modified and activated adsorbents. // News of universities. Applied chemistry and biotechnology. – 2021. - №2 (11). - PP. 318-325.
15. Salimov I.R., Murodova Yu.M., Murodov M.N., Tilloev L.I., Khaitov R.R. Determination of the optimal mode for obtaining activated carbon from fruit seed shells for the purification of alkanolamines. // Universum: Technical sciences: electronic. scientific journal. - Moscow. - 2020. - №. -7 (76). - PP. 77-81.
16. P.Ya.Bachurin, V.A.Smirnov. Technology of vodka and liquor products. ‘Processing of vodka sorting with activated carbon’ Physicochemical principles of processing with activated carbon. // NPP ‘Technofilter.’ - 2020. - PP. 5-13.
17. Jumaeva D., Raximov U., Toirov O., Ergashev O., Abdyrakhimov A. Basic thermodynamic description of adsorption of polar and nonpolar molecules on AOGW. // E3S Web of Conferences. -2023. – PP. 425.
18. Jumaeva, D., Toirov, O., Numonov, B., Raxmatullaeva, N., Shamuratova, M. Obtaining of highly energy-efficient activated carbons based on wood. // E3S Web of Conferences. -2023. – PP. 410.
19. Kouznetsova T.F., Kopysh E.A., Kulbitskaya L.V., Jumaeva D.J., Ivanets A.I. Textural properties of ordered nanoporous silica synthesized on mesogenic template. // Proceedings of the National Academy of Sciences of Belarus, Chemical Series. - 2023. - 59(2). - PP.125–138.
20. Jumaeva D., Toirov O., Okhunjanov Z., Raximov U., Akhrorova R. Investigation of the adsorption of nonpolar adsorbate molecules on the illite surface. // Journal of Chemical Technology and Metallurgy. - 2023. - 58(2). -PP. 353–359.
21. Namsivaym C. Appliction of coconut coir pith for the removal of sulfate and other anions from water /C.Namsivaym, D.Sngeeth. // Deslintion. – 2008. -vol.219. - iss.1–3. – PP. 3-13.
22. Zhumaeva D.Zh., Rakhmatullaeva N.T., Abdurakhimov A.Kh., Babaeva G.O., Khamrakulova K.Kh. Study and production of adsorbents from wood waste. //UNIVERSUM: Chemistry and biology: Electronic scientific journal. Issue №3(105). March. - 2023. - Part 1. [Electronic resource] URL: DOI-10.32743/UniChem.2023.105.3.15101
23. Nistratov A.V., Skaryukin A.S., Klushin V.N. Production and study of the porous structure of mineral-carbon adsorbents based on silica gel and polymer waste. // Sorption and chromatographic processes. - 2019. - Vol.19. - №2. - PP. 200-208.
24. Kh.Bakhronov, O.Ergashev, Gʻ.Оchilov, N.Esonkulova, A.Ganiev, N.Akhmedova, O.Ochilova Study of isotherm, thermodynamic characteristics and sorption mechanism of toluene adsorption on zeolite CsZSM-5 by adsorption-calorimetric method // Edelweiss Applied Science and Technology. - 2026. - Vol. 8. №6. - PP. 6959-6966. [Electronic resource] URL:  https://doi.org/10.55214/25768484.v8i6.3508
25. O.Ergashev, Kh.Bakhronov, M.Asfandiyorov, M.Kokhkharov, A.Ganiev, N.Akhmedova, Sh.Tulyaganova, K.Nazirov, N.Esonkulova Isotherm and main thermodynamic characteristics of adsorption of carbonyl sulfide molecules on zeolite KA (MSS-558). // Edelweiss applied science and sechnology. - 2025. -Vol. 9. - №6. - PP. 287-301. [Electronic resource] URL:  https://doi.org/10.55214/25768484.v9i6.7795
26. Reymov A., Raxmatkarieva F., Koxxarov M., Bakhronov Kh., Isotherm of ammonia adsorption in zeolite CaA (M-34). // Science and Education in Karakalpakstan. - 2024. - №3/2. - ISSN 2181-9203. - PP. 257-262.
27. Kokhkharov M., Rakhmatkarieva F., Bakhronov Kh., Rakhmatullaeva M, Absalyamova I., Karimov Y. Differential entropy and thermokinetics of ammonia molecule adsorption on CaA zeolite (M-22). // E3S Web of Conferences. ICESTE 2024. – PP. 563.
28. Kokhkharov M., Rakhmatkarieva F., Bakhronov Kh., Akhmedova N., Rakhmatullaeva M., Karimov Y.  Adsorption isotherm, differential heat, and sorption mechanism of ammonia on CaA zeolite.  // E3S Web of Conferences. Green Energy 2024. – PP. 587.