Enhancement of CO2 capture in post combustion process using active carbon modified by amino acids

Enhancement of CO2 capture in post combustion process using active carbon modified by amino acids | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Enhancement of CO 2 capture in post combustion process using active carbon modified by amino acids Davoud Houshmand, Fariborz Rashidi, Sepideh Amjad-Iranagh, Meysam hajilari This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8252812/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 30 Mar, 2026 Read the published version in Scientific Reports → Version 1 posted 10 You are reading this latest preprint version Abstract In this study, surface modification of a general purpose activated carbon is achieved through impregnation of three common amino acids namely, glycine, serine and lysine for CO₂ adsorption in post-combustion processes at temperature range 298–358 K. Glycine, due to its smaller molecular size, higher nitrogen content, and enhanced pore accessibility caused the adsorption to increase by 25% compared to original activated carbon uptake. Also, N₂ adsorption was evaluated to assess selectivity and competitive behavior. The modified adsorbents with glycine exhibited similar N₂ uptake to initial activated carbon at 358K. All adsorbents exhibited physisorption behavior, with ΔH st values ranging from − 11 to − 33.9 kJ/mol for CO₂ and − 15.6 to − 17.8 kJ/mol for N 2 . Overall, the results demonstrate that functionalizing by glycine, significantly enhances CO 2 adsorption capacity of activated carbon. These findings provide valuable insights for designing tailored adsorbents for post combustion CO 2 capture. Physical sciences/Chemistry Physical sciences/Energy science and technology Earth and environmental sciences/Environmental sciences Post combustion CO2 capture activated carbon amino-acids functionalized Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 Figure 13 1. Introduction With the advancement and development of human societies, the need for energy resources is continuously increasing. Currently, fossil fuels are used as the main source of energy production in the world. The use of fossil fuels leads to the emission of carbon dioxide, a greenhouse gas, which is one of the main factors contributing to global warming. Nowadays, there is no doubt that carbon dioxide emissions into the atmosphere must be prevented due to their greenhouse effect, which causes global warming. The NASA Earth temperature measurements show that the atmospheric temperature has been rising over the last century. The main cause of this phenomenon is the CO 2 emission into the atmosphere. Statistics show that the cumulative net CO 2 emissions from 1850 to 2019 were about 2400 gigatons of CO 2 . Approximately 17% of the cumulative net CO2 emissions since 1850 were emitted into the atmosphere between 2010 and 2019 1–3 . Therefore, numerous efforts are being made to reduce and eliminate this gas. Carbon capture and storage, or carbon capture sequestration, was introduced to address this problem, where CO 2 captured by various techniques is stored underground, converted to fuels, or used in various applications. The methods of CO 2 removal and recycling/reusing are divided into three categories: pre-combustion capture, post-combustion capture, and oxy-combustion methods. Post-combustion capture methods are more popular due to their applicability to many active units in the industry, such as thermal power plants and boilers in industrial and non-industrial units. The process of capturing CO 2 after combustion of a hydrocarbon fuel is called post-combustion, which has many advantages that make this approach applicable to many existing power plants and industrial plants that utilize hydrocarbon combustion energy 4 , 5 . The post-combustion CO 2 capture process removes CO 2 at low partial pressures from the stream of combustion gases. This carbon capture technology is widely used as a suitable option for retrofitting existing power plants, and research has shown that it can recover up to 800 tons of CO 2 daily. A significant advantage of post-combustion carbon capture technology is its technological maturity compared to other existing carbon capture methods 6 , 7 . Various separation techniques, such as chemical absorption, adsorption, membrane, cryogenic, and biotechnology, have been introduced in the last decades. Every separation process has its own merits 8 – 11 . While conventional methods rely on chemical absorption, post-combustion adsorption shows great potential for CO 2 capture at low partial pressure of CO 2 in flue gas, where a solid adsorbent will adsorb the CO 2 in its pore in an adsorption step and release it in a regeneration step by changing the pressure or temperature of the solid bed. For adsorbents used in Post-Combustion Carbon Capture (PCC), a high adsorption capacity at low partial pressures of CO 2 (typically less than 0.2 bar) is crucial 12 . Other desired properties of adsorbents include high porosity, high thermal stability, and low technical complexity. From a process design perspective, resistance to changes in the concentration of accompanying impurities plays a crucial role in extending the lifespan of the adsorbent for repeated cycles 13 . Combustion flue gases from power plants typically contain between 3% and 15% by volume CO 2 , with the remainder consisting of gases such as nitrogen, oxygen, NOx, and SOx 14 . Combustion gases are typically at atmospheric pressure and have a wide range of temperatures. In post-combustion applications, the temperature ranges from 40 to 200°C. However, in some industries, flue gas streams may reach temperatures up to around 400°C 15 . Such temperature conditions can be challenging for carbon-based adsorbents that operate at a temperature range of 50°C and a pressure of 1 bar, as well as for zeolites that operate up to 100°C and MOFs that operate at ambient temperature 16 . Other important features for adsorbents in cyclic adsorption/desorption processes include resistance to abrasion (mechanical resistance), low manufacturing costs, high recyclability, and a small temperature difference between the adsorption and regeneration processes. Regeneration ability and reversibility of the adsorption process are key features for selecting adsorbent material and regeneration process 17 , 18 . In recent years, numerous solid adsorbents have been proposed for CO 2 capture, including activated carbons, zeolites, metal-organic frameworks, porous polymers, carbon nanosheets, metal oxides, nitrogen-doped TiO 2 , activated carbon-TiO 2 composites, amine-modified TiO 2 , TiO 2 -titanate composite nanorods, and carbon nanotubes 19 . Among the materials mentioned above, activated carbons have been shown to be a promising adsorbent due to their low production cost, high porosity, large surface area, easy structure control, good thermal and chemical stability, and high efficiency. High CO 2 adsorption is considered the most relevant parameter in selecting an adsorbent. Activated carbons are primarily microporous materials and typically exhibit high CO 2 adsorption. Additionally, CO 2 adsorption can be enhanced by altering its chemical or structural properties using appropriate techniques. Another important parameter in using activated carbon adsorbents in CO 2 capture processes is the selectivity of the adsorbent for carbon dioxide gas. In post-combustion capture processes, due to the presence of large amounts of nitrogen in the flue gas stream, high CO 2 /N 2 selectivity is desired 20 – 23 . Furthermore, if there are high percentages of oxygen in the output stream, CO 2 /O 2 selectivity is also important. Activated carbon is activated by physical and chemical methods. In physical activation methods, the structure of the carbon precursor is partially preserved, and the surface area can reach up to 2000 square meters per gram 24 . Carbon dioxide, water vapor, and oxygen are commonly used compounds for physical activation. In contrast, chemical activation methods allow for control over the adsorbent's surface, pores, and porosity. Many activated carbons are chemically activated. Various studies on the structural modification of activated carbon adsorbents have been conducted in reputable scientific journals and research publications. Among these, the use of materials or compounds with high nitrogen content, such as urea, ammonia, amine solvents, nitric acid, alkaline metal-based bases like KOH, and phosphoric acid, has been the most commonly used in research 25 – 28 . The main parameters that significantly affect or control adsorption using activated carbon adsorbents are the chemical nature of CO 2 , the surface morphology of the adsorbent, the activation method, the preparation pressure and temperature, and the binding energies between surface functional groups and CO 2 . The use of amine-modified activated carbon adsorbents results in a significant increase in adsorption capacity, even though the specific surface area is reduced due to the blockage of pore openings by amine molecules. This is because the amine groups on the surface act as active sites for adsorption. Consequently, adsorption on this category of structurally modified adsorbents will be a chemical adsorption process. Amination structure modification, due to the blockage of pore openings, significantly decreases the adsorption capacity at temperatures close to ambient, but increases it with rising temperatures due to the chemical nature of this type of adsorbent. Therefore, to utilize these adsorbents for CO 2 adsorption in both ambient and high-temperature conditions, physical adsorption must also be maintained at a high level. Thus, it is necessary to prevent the blockage of pore openings by amine molecules 29 . The use of mesoporous carbons for amine loading is a more favorable option, as amine functional groups are more uniformly distributed on the surface, and the pore size is such that it prevents the blockage of pore openings by amine molecules 30 , 31 . The presence of nitrogen-containing functional groups and the pore size are important factors in CO 2 adsorption 32 . Among the materials used to add heteroatoms to the surface of activated carbon in the literature, various amine compounds, ammonia, air, nitric acid, and alkali metals have been employed. The researchers' findings, along with the advantages and disadvantages of each, are presented in detail, demonstrating that the use of new compounds and innovative methods is necessary. In these studies, new compounds will be used to add nitrogen heteroatoms to the surface of activated carbon. The compounds this research intends to use for adding nitrogen-containing functional groups to the adsorbent surface are amino acid salts. Amino acids are nitrogen-based substances and are widely present in the structure of plants and proteins. A peptide is a molecule composed of several amino acids, and a polypeptide is a chain of many amino acids. The main elements of amino acids are carbon, hydrogen, oxygen, and nitrogen. Amino acids are a group of amines, like ammonia and alkanolamines. Recently, aqueous amino acid salt solutions have been proposed as an alternative to alkanolamines. The advantages of amino acids include resistance to structural degradation, low volatility, and lower amounts of oxidative degradation products 33 . These compounds have high nitrogen content and also include salts containing sodium, potassium, and calcium, which can be added to the surface as well. Thus, for the first time, this work intends to use these compounds as nitrogen heteroatom enhancers on the surface of activated carbon adsorbents. The idea behind this approach is that, given the presence of metals such as sodium and potassium in the salt solutions of these compounds, it is possible to simultaneously add oxygen and nitrogen groups to the adsorbent surface, thereby achieving greater capacity and selectivity. By performing the chemical functionalization method, the structure of the adsorbent surface will be modified. Adsorption isotherms of CO 2 and N 2 at near-stack flue gas temperatures, experimentally measured, and the heat of adsorption was calculated for both components on the adsorbents. 1. Materials and methods Three amino acids, including Glycine, Lysine, and Serine, as well as potassium hydroxide and sodium hydroxide, were purchased from Merck Co. Activated carbon samples were obtained from Jacobi and Samchun Co. Deionized water was provided locally in Iran. All materials were analytical grade and used without further purification. 1.1. Experimental apparatus and procedure for adsorbent modification For activated carbon modification, a reflux apparatus was assembled as follows. A 3-head flask was used: the first head was attached to a condenser with a valve, the second to a water condenser, and the third held a thermometer. The assembled flask and condensers were placed in an oil bath, positioned on a hot plate with a magnetic stirrer. The schematic of the apparatus is presented in Fig. 1 . In a typical experiment, 100 mL of solvent containing a modification agent (amino acid or amino acid plus alkaline metal hydroxide at various molar concentrations) was placed in a flask, and approximately 2g of unmodified adsorbent was added to the flask using a magnet. The setup was assembled, and the experiment commenced with the apparatus at its desired temperature and stirring settings. When it reaches the desired temperature, the timing of the modification reaction begins. After the desired modification time, the heater stopped. The solution, which contained activated carbon in the flask, was filtered and then washed several times with water. It was then dried at 120°C in an oven overnight and stored for further use. 1.2. Adsorption measurements For the adsorption isotherm measurement, a schematic setup, as shown in Fig. 2 , was employed. The basis of this apparatus is to volumetrically measure the adsorption of a gas on a solid adsorbent. The setup consists of three vessels of different sizes. The first vessel is used to store enough gas as feed to the adsorption cell measurement. The second and third vessels were used to measure the gas volume during the adsorption process. These two vessels were placed in a temperature-controlled bath to ensure isothermal conditions during the experiment. The first vessel is 2L, the second vessel is 235 ml, and the third vessel has a total volume of 200 ml, including the connection lines. In each isotherm measurement, approximately 0.5g of adsorbent was placed in a stainless-steel container under high vacuum at approximately 90°C for 8 hours to remove impurities. Afterward, it was quickly placed inside the adsorption cell, and the vessel was sealed. All lines and vessels were vacuumed for at least 15 minutes. Then, the feed was injected from a gas cylinder (CO 2 , air, nitrogen) into the first vessel to the desired pressure. Pressure and temperature of various parts of the setup were automatically logged. The V2 valve was opened to equalize pressure in the first and second vessels, then closed. After a few minutes, once the temperature of the second vessel reached equilibrium with the bath, the V3 valve was rapidly opened and closed to inject a small amount of gas into the adsorption cell. The pressure of the second vessel was measured before and after opening V3. Using thermodynamic calculations, the amount of injected gas was determined. After sufficient contact time between gas and adsorbent in the third vessel, the equilibrium pressure was recorded. From this, the remaining gas in the adsorption vessel was calculated. The difference between the injected and final gas gave the amount adsorbed at each equilibrium pressure. This procedure was repeated for each isotherm temperature. For this work, isotherms were measured over the temperature range of 25 to 80°C. The accuracy of the setup has been tested many times, with results presented in the literature 34 , 35 . The adsorbent volume was subtracted from the vessel volume in calculations. The adsorbent bulk density was measured three times, and the uncertainty is reported in the results section. 2. Experimental design As the modification of activated carbon with amino acids is a new process, there is insufficient information on their reaction activity. Temperature, amino acid concentration, and modification times were used as effective parameters in the modification of activated carbon. The temperature range of 60–100°C was used. Higher temperature was avoided due to the stability of these compounds. As for amino acid concentration, a range of 0.3 to 2 mol/L was selected. The reaction time was varied from 3 to 8 hours. A Box-Behnken experimental design method was used to obtain the design matrix. Based on the experimental design, 16 tests were suggested. The primary response was chosen as the amount of CO 2 adsorption. The design matrix is presented in Table 1 . Table 1 Design matrix for adsorbent modification StdOrder RunOrder Temperature Time Amino acid loading 13 1 60 3 0.3 9 2 60 1 0.1 4 3 80 5 0.3 5 4 40 3 0.1 7 5 40 3 0.5 12 6 60 5 0.5 3 7 40 5 0.3 1 8 40 1 0.3 15 9 60 3 0.3 11 10 60 1 0.5 2 11 80 1 0.3 14 12 60 3 0.3 6 13 80 3 0.1 8 14 80 3 0.5 10 15 60 5 0.1 2.1. Adsorbent Characterization The characterization technique was employed to investigate the adsorption phenomena on the microscopic level. In this work, FT-IR spectroscopy is used to determine the surface functional groups on the adsorbent surface. Scanning electron microscopy is used to find the surface morphology and particle shape of the adsorbent. The BET-BJH test was conducted to measure the N2 adsorption/desorption of activated carbon before and after modification to find the surface area, pore volume, and pore size distribution of the adsorbent. The TGA analysis was used to measure the thermal stability of the adsorbent at various temperatures. 3. Results and discussion 3.1. Adsorbent characterization 3.1.1. SEM characterization Scanning electron microscopy was used to analyze the shape and morphology of the adsorbent particles. The SEM image of both unmodified and modified activated carbon at optimal conditions is presented in Fig. 3 . As can be seen from the figure, the modification of the adsorbent surface resulted in increased texture, which can be attributed to the reaction of the amino acid with the functional groups on the adsorbent surface. The SEM micrographs of the pristine Jacobi activated carbon and the glycine-modified sample (AC-g9) reveal clear morphological changes after functionalization. The unmodified Jacobi AC exhibits irregular, angular particles with relatively rough surfaces and exposed pore entrances. After glycine treatment, surfaces appear covered by a more uniform, fine-textured layer, and small particulate deposits are visible on the particle faces (Figure X). The coating observed for AC-g9 is consistent with surface decoration or partial filling of pore walls by organic moieties rather than gross pore blocking. Taken together with the BET results (Section 3.2), the SEM observations suggest that glycine anchors on pore surfaces and external areas while preserving the overall porous architecture. The more homogeneous surface texture of AC-g9 plausibly provides an increased density of surface functional groups accessible to gas molecules, which can enhance chemical affinity toward CO₂ without sacrificing the textural area. 3.1.2. BET-BJH characterization The nitrogen adsorption/desorption at 77K was measured to obtain the adsorbent characteristic, and the results are presented in Fig. 4. As can be seen from the figure above, the adsorbent exhibits a Type I IUPAC isotherm, which represents a microporous material with a sharp jump at near-zero pressure. The adsorption hysteresis is presented in partial pressure of 0.4, indicating that the adsorbent possesses some mesopore structure. The BET analysis is used to evaluate the surface area of both adsorbents. The results are presented in Table 2 . Table 2 BET characteristic measurement of adsorbents sample a BET (m 2 /g) Total pore volume (cm 3 /g) Average pore diameter (nm) Jacobi activated carbon 1018.9 0.7256 2.8487 AC-g9 1015.8 0.7241 2.8516 As can be seen from the table above, the active surface area of both adsorbents is nearly identical. This means that the modification did not plug pores or reduce the surface area, which indicates great success. The negligible loss in BET area and pore volume (< 0.3% change in surface area) indicates that glycine functionalization does not induce significant pore blocking at the scale resolved by the BET measurement. This preservation of porosity is crucial because it suggests that any observed enhancement in adsorption performance is more likely to originate from chemical (functional group) effects than from an increase in accessible surface area. As for pore distribution, the BJH plot was used to compare both adsorbents before and after modification, and the results are presented in Fig. 5 . As shown in Fig. 5 , the pore distribution on the adsorbents is nearly identical. This implies that the modification did not change the pore structure or pore opening. BJH-derived pore size distributions for Jacobi AC and AC-g9 are essentially overlapping. Both show a dominant contribution in the micropore–small mesopore range (≈ 1–3 nm) and a smooth decay toward larger mesopores. The near-identical profiles confirm that the overall pore architecture remains intact after modification. Small differences at the low end of the distribution—a slight reduction in the smallest pore contribution for AC-g9—are consistent with a thin, conformal glycine coating on pore walls but not with wholesale pore filling. 3.1.3. XRD Characterization X-ray diffraction (XRD) analysis was conducted to evaluate the crystalline structure of the activated carbon before and after surface modification. Figure 6 displays the XRD patterns of the pristine Jacobi activated carbon and the modified sample (AC-g9). The above results indicate that both adsorbents exhibit similar spectra, except at low angles, where the modified adsorbent shows some peaks with higher counts. However, the activated carbon does not have crystalline phases in its structure. Both samples exhibit broad diffraction peaks at 2θ angles of approximately 10°, 25°, and 45°, which are characteristic of amorphous or turbostratic carbon structures. The modified sample exhibits a slight reduction in peak intensity and minor shifts in peak positions, indicating a decrease in crystallinity and the introduction of structural disorder resulting from the incorporation of nitrogen- and oxygen-containing functional groups. No new crystalline phases were observed, suggesting that the modification process preserved the overall carbon framework while altering its surface characteristics 3.1.4. FT-IR characterization Fourier transform infrared spectroscopy shows the surface functional groups and a great indication of the effect of modification on the adsorbent surface. This characterization was employed to analyze the surface function group of the adsorbent. The results are presented in Fig. 7. The results presented in the figure show that the Jacobi activated carbon demonstrates 9 peaks in the entire range of 400 ~ 4000 wavelength. In comparison, the modified adsorbent with glycine amino acid represents only 7 peaks. The peaks are listed in Table 3 . Table 3 Peaks properties presented in FT-IR Spectra of adsorbents AC-g9 Jacobi AC Peak No. X (cm-1) Y (%T) Peak No. X (cm − 1 ) Y (%T) 1 3952.17 97.97 1 3878.96 97.47 2 3876.36 97.98 2 3766.41 97.18 3 3840.74 97.87 3 3427.26 82.07 4 3422.76 85.55 4 2351.31 98.04 5 2900.76 95.29 5 1383.55 92.01 6 1038.69 80.86 6 1054.11 90.36 7 466.34 93.93 7 616.33 96.73 8 504.12 97.35 9 465.01 95.29 As can be seen, the modification removes the peaks at 504 and 616 wavelengths. These peaks are attributed to the activated carbon structure. This indicates that the amino-acid modification removes these functional groups from the adsorbent surface. Peaks in the range of 1000–1100 cm⁻¹ correspond to C–N and C–O stretching vibrations, further confirming successful functionalization. Also, Additional peaks observed around 1650–1550 cm⁻¹ are attributed to C = O stretching and N–H bending, suggesting the incorporation of carboxyl and amide functionalities. The peak at 2400 on the Jacobi activated carbon was eliminated during the modification. The modified sample exhibits enhanced absorption bands in the region of 3200–3400 cm⁻¹, corresponding to O–H and N–H stretching vibrations, indicating the presence of hydroxyl and amine groups. The peaks in the range of 3400 ~ 3900 show the hydroxyl group on the surface of the adsorbent and N-H stretching vibrations 36 , 37 which can be an indication of nitrogen functional groups. These spectral changes demonstrate the effective introduction of nitrogen- and oxygen-containing groups onto the activated carbon surface, which are expected to enhance CO 2 adsorption performance. 3.1.5. Thermogravimetric analysis TGA is used to analyze the mass change of the adsorbent during heating in neutral atmosphere of N 2 . The results are presented in Fig. 8 . This analysis is used to study the stability of the adsorbent and its decomposition conditions. The comparative TGA analysis conclusively demonstrates the successful functionalization of the activated carbon with glycine. The pristine sample (AMERICA) exhibited high thermal inertia, with minimal weight loss across the 25–600°C temperature range. In stark contrast, the functionalized sample (AC-G9) showed a significant, multi-stage decomposition event between 270°C and 340°C, resulting in a substantial 31.23% mass loss. This event is a direct fingerprint of the thermal decomposition of the grafted glycine molecules and their derived functional groups, confirming their presence on the carbon surface. Regarding stability, the glycine-functionalized carbon is structurally stable for routine applications at low temperatures but exhibits lower thermal stability overall. The grafted organic layer begins to decompose sharply around 270°C, making the material unsuitable for high-temperature processes above this point. However, for typical uses like adsorption in post combustion processes (temperature range up to 150°C), the functionalized structure is perfectly stable, as the decomposition temperature provides a wide safety margin. Thus, the functionalization successfully introduced valuable surface groups while maintaining sufficient stability for its intended purpose. 3.2. Adsorption isotherm measurement of unmodified adsorbent As it was stated above, two activated carbons were selected for CO 2 adsorption in this work. At the first step, their isotherm was measured at 298K to see their adsorption performance. The isotherm of both adsorbents is presented in Fig. 9. The Jacobi adsorbent shows stronger CO 2 adsorption than the Samchun adsorbent. The product catalogue lists the Jacobi as a coal-derived activated carbon, and the Samchun as a char-derived adsorbent. BET surface area measurements reveal that both have nearly identical active surface areas. In physisorption-dominated systems, adsorption capacity increases with pressure and decreases with temperature. At 298 K, maximum uptake reaches about 4.5 mmol/g; at 358 K, it drops to around 2.5 mmol/g, a significant reduction of nearly 45%. This temperature-dependent behavior confirms the physical nature of CO 2 adsorption on unmodified activated carbon, highlighting the impact of operating conditions on adsorption performance. 3.3. Adsorption isotherm of modified activated carbon with various agents In the next step, the adsorption of Jacobi activated carbon with various modification agents, including glycine, serine, and lysine, with and without NaOH/KOH, was measured. In this step, the modification was carried out at 100°C with a 2M solution concentration and a reaction time of 8 hours. After each modification, the CO 2 adsorption isotherm at various temperatures was measured. The results are presented in Fig. 10. The results show that the modification of active carbon with Glycine and Serine increased the adsorption capacity of the adsorbent, while the lysine-modified adsorbent shows a lower capacity than the original adsorbent. Regarding the addition of KOH/NaOH to the amino acid modification solvent, it is evident that samples with lysine modified by KOH/NaOH exhibited lower capacity, with the KOH-modified sample showing the lowest adsorption capacity. As for serine and glycine, the behavior is different. The serine shows higher adsorption capacity with the addition of NaOH and KOH, which later shows greater adsorption capacity improvement. As for glycine, it can be observed that the addition of NaOH or KOH did not result in a significant increase in adsorption. The modification of activated carbon with an amino acid improved the adsorption capacity of the adsorbent by up to 25% for glycine. The amino acids used in this study were from various sources. Glycine is from non-polar aliphatic R-group amino acids. Lysine is from a group of amino acids with a positively charged R group, and serine is from polar non-charged R group amino acids. The common characteristic of serine and glycine is that they both have a non-charged R group in their structure, while lysine has a negatively charged R group. It seems that the negative/positive charged R group is responsible for the reduction in adsorption capacity of the sample modified with this agent. The addition of KOH/NaOH may cause the precipitation of alkaline salts on the adsorbent surface or block the adsorbent pores. Additionally, it is possible that the reaction of the functional groups on the surface of the adsorbent with R groups or even free hydroxyl groups in the solution is responsible for the reduction in adsorption capacity in the samples modified with lysine. 3.4. Optimizing the modification conditions Based on the results obtained for adsorbent modification with three amino acids, the optimization of the modification condition was performed with glycine, which was selected as the best modification agent, and the experimental design optimization was carried out with this agent. The experimental table is presented in Table 4 . Table 4 Experimental parameters for adsorbent modification Sample code Run order Temperature (°C) Time (h) Amino acid loading (mol/L) AC-G-1 1 60 3 0.3 AC-G-2 2 60 1 0.1 AC-G-3 3 80 5 0.3 AC-G-4 4 40 3 0.1 AC-G-5 5 40 3 0.5 AC-G-6 6 60 5 0.5 AC-G-7 7 40 5 0.3 AC-G-8 8 40 1 0.3 AC-G-9 9 60 3 0.3 AC-G-10 10 60 1 0.5 AC-G-11 11 80 1 0.3 AC-G-12 12 60 3 0.3 AC-G-13 13 80 3 0.1 AC-G-14 14 80 3 0.5 AC-G-15 15 60 5 0.1 The CO 2 adsorption capacity was selected as the response variable. After each modification, the CO 2 adsorption isotherm was carried out at 298 K and the results obtained were compared in each run. The isotherm obtained is presented in Fig. 11 . The adsorption uptake increases with pressure across all samples, consistent with physisorption behavior. Notably, sample AC-G-9 exhibits the highest CO 2 uptake, confirming the optimal modification conditions of 60°C, 3 h, and 0.3 mol/L glycine concentration. In contrast, samples such as AC-G-2 and AC-G-4 show significantly lower adsorption capacities, likely due to insufficient functionalization at lower temperatures or concentrations. These results underscore the importance of precise control over modification parameters to maximize adsorbent performance. In the next step, the adsorption isotherm of AC-G-9, as the best candidate, was examined at temperatures of 298, 318, 338, and 358 K, and the results are presented in Fig. 12 . As can be seen from Fig. 12 , the adsorption capacity decreases when the temperature is increased from 298 K to 358 K (3.4 mmol/g to 1.8 mmol/g). Therefore, in real practice, considering the high temperature of stack flue gas one should seek methods to decrease flue gas temperature in order to increase the process efficiency. One way to reduce the stack temperature is to preheat the combustion air by recuperation technique, which is a common practice. A recuperator is a heat exchanger that transfers heat from hot flue gases to the combustion air. Various recuperators are adapted in the industry 38 . In this technique, flame temperature increases, while the flue gas temperature decreases due to heat transfer. This method increases the combustion efficiency and saves energy. 3.3.1. N 2 Adsorption capacity The N 2 adsorption on unmodified and modified adsorbents is measured experimentally, and the results are provided in Fig. 13 . For all samples, N₂ uptake increases with pressure, consistent with physisorption behavior. The modified sample AC-g9 at 298 K exhibits the highest N₂ adsorption capacity, followed by Jacobi AC at the same temperature. At 358 K, both samples show reduced uptake, highlighting the temperature sensitivity of N₂ adsorption. The enhanced N₂ uptake in AC-g9 may be attributed to the introduction of polar functional groups and changes in pore structure due to surface modification. While beneficial for overall adsorption capacity, this increase in N₂ uptake could potentially affect CO₂/N₂ selectivity, which is a critical parameter in post-combustion capture applications. 3.3.2. Isosteric heat of adsorption The isosteric heat of adsorption, also referred to as the differential enthalpy of adsorption, is an important thermodynamic property for designing and optimizing adsorption processes. It provides insights into interactions between adsorbate molecules and between adsorbate and adsorbent. For instance, in homogeneous adsorbents, the isosteric heat of adsorption remains unchanged at low adsorbate loadings, indicating that interactions among adsorbed molecules do not impact the adsorption energy. However, at higher adsorbate loadings, significant lateral interactions between adsorbate and adsorbent emerge, leading to an increase in the isosteric heat of adsorption. Conversely, in energetically heterogeneous adsorbents, the isosteric heat of adsorption decreases with increasing adsorbate loading because molecules preferentially adsorb on high-energy sites even at low pressures 38 , 39 . The isosteric heat of adsorption for pure components can be determined through calorimetric measurements or from adsorption isotherms obtained at various temperatures. This method employs a Clapeyron-type relationship to calculate the isosteric heat of adsorption, denoted as ΔH st . $$\:{\varDelta\:H}_{st}=R{T}^{2}{\left(\frac{\partial\:\text{ln}P}{\partial\:T}\right)}_{\stackrel{´}{n}}$$ Where n’ is the amount of adsorbed, P and T are pressure and temperature, respectively, R is the universal gas constant, and ΔHst is the isosteric heat of adsorption. It must be noted that this equation is applicable to low-pressure and pure gas isotherms, and its derivation can be found elsewhere 40 . The isosteric heat of adsorption for various adsorbents is calculated from related isotherms, and the results are shown in Table 4 . Table 5 Isosteric heat of adsorption Adsorbent ΔH st . (kJ/mol) (CO 2 ) ΔH st . (kJ/mol) (N 2 ) Jacobi AC -11 -15.6 AC-G-9 -33.9 -17.8 All values are negative, confirming the exothermic nature of the adsorption process and supporting the physisorption mechanism for both gases. The ΔHₛₜ value for pristine activated carbon aligns well with literature reports (10–28.5 kJ/mol for CO 2 , 9.5–16 kJ/mol for N₂) 41 . The significantly higher ΔHₛₜ for the modified adsorbent (AC-G-9) suggests stronger adsorbate–adsorbent interactions, likely due to the presence of nitrogen- and oxygen-containing functional groups introduced during modification. This enhanced interaction leads to a more pronounced temperature sensitivity, as evidenced by the 60% reduction in CO 2 adsorption capacity when the temperature increased from 298 K to 358 K. Therefore, the exothermic nature of the modified adsorbent is the primary factor contributing to the observed decline in adsorption performance at elevated temperatures. 4. Conclusion This study systematically examined the adsorption behavior of activated carbon modified with various amino acids for carbon dioxide (CO 2 ) capture in post-combustion processes. Amino acid functionalization substantially increased CO 2 adsorption capacity, while it didn’t alter the N 2 adsorption capacity. Glycine-functionalized activated carbon demonstrated the highest CO 2 uptake, surpassing both unmodified and other modified activated carbon. This enhanced performance is attributed to the small molecular size of glycine, which enables uniform surface coverage and maximizes the availability of nitrogen-containing functional groups. The isosteric heats of adsorption (ΔH st ) confirmed the physisorption mechanism. Temperature-dependent isotherms indicated a reduction in adsorption capacity as temperature increased, especially for modified adsorbents with higher ΔH st values. These findings emphasize the exothermic character of the adsorption process and the necessity of cooling the flue gas in order to increase the efficiency of CO 2 capture process. This optimization may include cooling of the exhaust gas from stacks by means heat exchange with combustion air. Although, this approach may complicate the process but it has the advantages of higher efficiency and energy saving. Declarations Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Declaration of funding The authors declare that this research did not receive any funding sources. 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4","display":"","copyAsset":false,"role":"figure","size":27700,"visible":true,"origin":"","legend":"\u003cp\u003eThe N\u003csub\u003e2\u003c/sub\u003e adsorption/desorption isotherm at 77 K, a) pristine Jacobi activated carbon, b) modified AC-g9\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-8252812/v1/a2a41e259fe6daf9258cc9fe.png"},{"id":98390549,"identity":"cc8e6ddf-404e-4f06-8e1c-e532ca5e42b2","added_by":"auto","created_at":"2025-12-17 09:25:14","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":49478,"visible":true,"origin":"","legend":"\u003cp\u003eThe BJH analysis for both adsorbents\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-8252812/v1/16a9c0efbf524e6aef898ae1.png"},{"id":98390532,"identity":"70168814-cdc8-481b-8209-04e6e842a5f8","added_by":"auto","created_at":"2025-12-17 09:25:12","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":97584,"visible":true,"origin":"","legend":"\u003cp\u003eXRD spectra of activated carbon before and after modification\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-8252812/v1/30e7cb3b845a6cf855d3af46.png"},{"id":98390565,"identity":"95dadf11-929f-4273-8230-2ebdea778e2c","added_by":"auto","created_at":"2025-12-17 09:25:16","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":25526,"visible":true,"origin":"","legend":"\u003cp\u003eThe FT-IR analysis of the a) Jacobi activated carbon, b) AC-g9\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-8252812/v1/8ec45e8474c2c7c818f05503.png"},{"id":98390548,"identity":"ed45e220-a6d9-4ca3-8eae-1df331a13957","added_by":"auto","created_at":"2025-12-17 09:25:14","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":67681,"visible":true,"origin":"","legend":"\u003cp\u003eThermogravimetric analysis of a) Jacobi AC, b) AC-g9\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-8252812/v1/b5f5c142f73fa7cc1dcbe246.png"},{"id":98390540,"identity":"9da9e2bb-226f-4f0f-831e-760ee717ac0a","added_by":"auto","created_at":"2025-12-17 09:25:13","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":28555,"visible":true,"origin":"","legend":"\u003cp\u003eAdsorption isotherm of (a) Jacobi activated carbon sample, (b) Samchun activated carbon.\u003c/p\u003e","description":"","filename":"9.png","url":"https://assets-eu.researchsquare.com/files/rs-8252812/v1/657f04bb057d83665a0a0418.png"},{"id":98390567,"identity":"3041d4f0-87c7-492f-aebb-73e6b5651c65","added_by":"auto","created_at":"2025-12-17 09:25:16","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":277323,"visible":true,"origin":"","legend":"\u003cp\u003eCO\u003csub\u003e2\u003c/sub\u003e adsorption isotherm of activated carbon modification with various agents at temperatures of a) 298K,\u0026nbsp;\u0026nbsp; b) 318 K,\u0026nbsp;\u0026nbsp; c)338 K,\u0026nbsp;\u0026nbsp; d)358 K\u003c/p\u003e","description":"","filename":"10.png","url":"https://assets-eu.researchsquare.com/files/rs-8252812/v1/68b4cb0ccde82a7c3fcff2eb.png"},{"id":98390509,"identity":"3ebc04ba-ce43-4bf8-97fd-1da78742def5","added_by":"auto","created_at":"2025-12-17 09:25:06","extension":"png","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":86642,"visible":true,"origin":"","legend":"\u003cp\u003eAdsorption isotherm at 298K of activated carbon modified with glycine at various experimental conditions\u003c/p\u003e","description":"","filename":"11.png","url":"https://assets-eu.researchsquare.com/files/rs-8252812/v1/0c34a93a230dfad284a65c15.png"},{"id":98390566,"identity":"0932d3e9-faf6-415e-8a66-f8e81a15a92f","added_by":"auto","created_at":"2025-12-17 09:25:16","extension":"png","order_by":12,"title":"Figure 12","display":"","copyAsset":false,"role":"figure","size":29966,"visible":true,"origin":"","legend":"\u003cp\u003eCO\u003csub\u003e2\u003c/sub\u003e adsorption isotherm of modified activated carbon with glycine at various temperatures\u003c/p\u003e","description":"","filename":"12.png","url":"https://assets-eu.researchsquare.com/files/rs-8252812/v1/fdfc4c2b25df947b2672267f.png"},{"id":98390553,"identity":"5f7b30ec-ddbb-4c67-9940-62fcaa2e874a","added_by":"auto","created_at":"2025-12-17 09:25:14","extension":"png","order_by":13,"title":"Figure 13","display":"","copyAsset":false,"role":"figure","size":82318,"visible":true,"origin":"","legend":"\u003cp\u003eN2 Adsorption uptake of the unmodified activated carbon sample and the chemically modified adsorbent\u003c/p\u003e","description":"","filename":"13.png","url":"https://assets-eu.researchsquare.com/files/rs-8252812/v1/9fdb02a9b5ed1ad43e56d71b.png"},{"id":106343731,"identity":"96ea806e-2951-41ff-848c-284749f4c360","added_by":"auto","created_at":"2026-04-07 16:08:17","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2939204,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8252812/v1/f3e1c149-d234-49a7-9847-31f78e0dd56a.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"\u003cp\u003eEnhancement of CO\u003csub\u003e2\u003c/sub\u003e capture in post combustion process using active carbon modified by amino acids\u003c/p\u003e","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eWith the advancement and development of human societies, the need for energy resources is continuously increasing. Currently, fossil fuels are used as the main source of energy production in the world. The use of fossil fuels leads to the emission of carbon dioxide, a greenhouse gas, which is one of the main factors contributing to global warming. Nowadays, there is no doubt that carbon dioxide emissions into the atmosphere must be prevented due to their greenhouse effect, which causes global warming. The NASA Earth temperature measurements show that the atmospheric temperature has been rising over the last century. The main cause of this phenomenon is the CO\u003csub\u003e2\u003c/sub\u003e emission into the atmosphere. Statistics show that the cumulative net CO\u003csub\u003e2\u003c/sub\u003e emissions from 1850 to 2019 were about 2400 gigatons of CO\u003csub\u003e2\u003c/sub\u003e. Approximately 17% of the cumulative net CO2 emissions since 1850 were emitted into the atmosphere between 2010 and 2019 \u003csup\u003e1\u0026ndash;3\u003c/sup\u003e. Therefore, numerous efforts are being made to reduce and eliminate this gas. Carbon capture and storage, or carbon capture sequestration, was introduced to address this problem, where CO\u003csub\u003e2\u003c/sub\u003e captured by various techniques is stored underground, converted to fuels, or used in various applications. The methods of CO\u003csub\u003e2\u003c/sub\u003e removal and recycling/reusing are divided into three categories: pre-combustion capture, post-combustion capture, and oxy-combustion methods. Post-combustion capture methods are more popular due to their applicability to many active units in the industry, such as thermal power plants and boilers in industrial and non-industrial units. The process of capturing CO\u003csub\u003e2\u003c/sub\u003e after combustion of a hydrocarbon fuel is called post-combustion, which has many advantages that make this approach applicable to many existing power plants and industrial plants that utilize hydrocarbon combustion energy \u003csup\u003e\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e,\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u003c/sup\u003e. The post-combustion CO\u003csub\u003e2\u003c/sub\u003e capture process removes CO\u003csub\u003e2\u003c/sub\u003e at low partial pressures from the stream of combustion gases. This carbon capture technology is widely used as a suitable option for retrofitting existing power plants, and research has shown that it can recover up to 800 tons of CO\u003csub\u003e2\u003c/sub\u003e daily. A significant advantage of post-combustion carbon capture technology is its technological maturity compared to other existing carbon capture methods \u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e,\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u003c/sup\u003e. Various separation techniques, such as chemical absorption, adsorption, membrane, cryogenic, and biotechnology, have been introduced in the last decades. Every separation process has its own merits \u003csup\u003e\u003cspan additionalcitationids=\"CR9 CR10\" citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u003c/sup\u003e. While conventional methods rely on chemical absorption, post-combustion adsorption shows great potential for CO\u003csub\u003e2\u003c/sub\u003e capture at low partial pressure of CO\u003csub\u003e2\u003c/sub\u003e in flue gas, where a solid adsorbent will adsorb the CO\u003csub\u003e2\u003c/sub\u003e in its pore in an adsorption step and release it in a regeneration step by changing the pressure or temperature of the solid bed.\u003c/p\u003e \u003cp\u003eFor adsorbents used in Post-Combustion Carbon Capture (PCC), a high adsorption capacity at low partial pressures of CO\u003csub\u003e2\u003c/sub\u003e (typically less than 0.2 bar) is crucial \u003csup\u003e\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u003c/sup\u003e. Other desired properties of adsorbents include high porosity, high thermal stability, and low technical complexity. From a process design perspective, resistance to changes in the concentration of accompanying impurities plays a crucial role in extending the lifespan of the adsorbent for repeated cycles \u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003e. Combustion flue gases from power plants typically contain between 3% and 15% by volume CO\u003csub\u003e2\u003c/sub\u003e, with the remainder consisting of gases such as nitrogen, oxygen, NOx, and SOx \u003csup\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003e. Combustion gases are typically at atmospheric pressure and have a wide range of temperatures. In post-combustion applications, the temperature ranges from 40 to 200\u0026deg;C. However, in some industries, flue gas streams may reach temperatures up to around 400\u0026deg;C \u003csup\u003e15\u003c/sup\u003e. Such temperature conditions can be challenging for carbon-based adsorbents that operate at a temperature range of 50\u0026deg;C and a pressure of 1 bar, as well as for zeolites that operate up to 100\u0026deg;C and MOFs that operate at ambient temperature \u003csup\u003e\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eOther important features for adsorbents in cyclic adsorption/desorption processes include resistance to abrasion (mechanical resistance), low manufacturing costs, high recyclability, and a small temperature difference between the adsorption and regeneration processes. Regeneration ability and reversibility of the adsorption process are key features for selecting adsorbent material and regeneration process \u003csup\u003e\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e,\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eIn recent years, numerous solid adsorbents have been proposed for CO\u003csub\u003e2\u003c/sub\u003e capture, including activated carbons, zeolites, metal-organic frameworks, porous polymers, carbon nanosheets, metal oxides, nitrogen-doped TiO\u003csub\u003e2\u003c/sub\u003e, activated carbon-TiO\u003csub\u003e2\u003c/sub\u003e composites, amine-modified TiO\u003csub\u003e2\u003c/sub\u003e, TiO\u003csub\u003e2\u003c/sub\u003e-titanate composite nanorods, and carbon nanotubes \u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eAmong the materials mentioned above, activated carbons have been shown to be a promising adsorbent due to their low production cost, high porosity, large surface area, easy structure control, good thermal and chemical stability, and high efficiency. High CO\u003csub\u003e2\u003c/sub\u003e adsorption is considered the most relevant parameter in selecting an adsorbent. Activated carbons are primarily microporous materials and typically exhibit high CO\u003csub\u003e2\u003c/sub\u003e adsorption. Additionally, CO\u003csub\u003e2\u003c/sub\u003e adsorption can be enhanced by altering its chemical or structural properties using appropriate techniques. Another important parameter in using activated carbon adsorbents in CO\u003csub\u003e2\u003c/sub\u003e capture processes is the selectivity of the adsorbent for carbon dioxide gas. In post-combustion capture processes, due to the presence of large amounts of nitrogen in the flue gas stream, high CO\u003csub\u003e2\u003c/sub\u003e/N\u003csub\u003e2\u003c/sub\u003e selectivity is desired \u003csup\u003e\u003cspan additionalcitationids=\"CR21 CR22\" citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e. Furthermore, if there are high percentages of oxygen in the output stream, CO\u003csub\u003e2\u003c/sub\u003e/O\u003csub\u003e2\u003c/sub\u003e selectivity is also important. Activated carbon is activated by physical and chemical methods. In physical activation methods, the structure of the carbon precursor is partially preserved, and the surface area can reach up to 2000 square meters per gram \u003csup\u003e\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u003c/sup\u003e. Carbon dioxide, water vapor, and oxygen are commonly used compounds for physical activation. In contrast, chemical activation methods allow for control over the adsorbent's surface, pores, and porosity. Many activated carbons are chemically activated. Various studies on the structural modification of activated carbon adsorbents have been conducted in reputable scientific journals and research publications. Among these, the use of materials or compounds with high nitrogen content, such as urea, ammonia, amine solvents, nitric acid, alkaline metal-based bases like KOH, and phosphoric acid, has been the most commonly used in research \u003csup\u003e\u003cspan additionalcitationids=\"CR26 CR27\" citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e. The main parameters that significantly affect or control adsorption using activated carbon adsorbents are the chemical nature of CO\u003csub\u003e2\u003c/sub\u003e, the surface morphology of the adsorbent, the activation method, the preparation pressure and temperature, and the binding energies between surface functional groups and CO\u003csub\u003e2\u003c/sub\u003e. The use of amine-modified activated carbon adsorbents results in a significant increase in adsorption capacity, even though the specific surface area is reduced due to the blockage of pore openings by amine molecules. This is because the amine groups on the surface act as active sites for adsorption. Consequently, adsorption on this category of structurally modified adsorbents will be a chemical adsorption process. Amination structure modification, due to the blockage of pore openings, significantly decreases the adsorption capacity at temperatures close to ambient, but increases it with rising temperatures due to the chemical nature of this type of adsorbent. Therefore, to utilize these adsorbents for CO\u003csub\u003e2\u003c/sub\u003e adsorption in both ambient and high-temperature conditions, physical adsorption must also be maintained at a high level. Thus, it is necessary to prevent the blockage of pore openings by amine molecules \u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e. The use of mesoporous carbons for amine loading is a more favorable option, as amine functional groups are more uniformly distributed on the surface, and the pore size is such that it prevents the blockage of pore openings by amine molecules \u003csup\u003e\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e,\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u003c/sup\u003e. The presence of nitrogen-containing functional groups and the pore size are important factors in CO\u003csub\u003e2\u003c/sub\u003e adsorption \u003csup\u003e\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eAmong the materials used to add heteroatoms to the surface of activated carbon in the literature, various amine compounds, ammonia, air, nitric acid, and alkali metals have been employed. The researchers' findings, along with the advantages and disadvantages of each, are presented in detail, demonstrating that the use of new compounds and innovative methods is necessary. In these studies, new compounds will be used to add nitrogen heteroatoms to the surface of activated carbon. The compounds this research intends to use for adding nitrogen-containing functional groups to the adsorbent surface are amino acid salts. Amino acids are nitrogen-based substances and are widely present in the structure of plants and proteins. A peptide is a molecule composed of several amino acids, and a polypeptide is a chain of many amino acids. The main elements of amino acids are carbon, hydrogen, oxygen, and nitrogen. Amino acids are a group of amines, like ammonia and alkanolamines. Recently, aqueous amino acid salt solutions have been proposed as an alternative to alkanolamines. The advantages of amino acids include resistance to structural degradation, low volatility, and lower amounts of oxidative degradation products \u003csup\u003e\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u003c/sup\u003e. These compounds have high nitrogen content and also include salts containing sodium, potassium, and calcium, which can be added to the surface as well. Thus, for the first time, this work intends to use these compounds as nitrogen heteroatom enhancers on the surface of activated carbon adsorbents.\u003c/p\u003e \u003cp\u003eThe idea behind this approach is that, given the presence of metals such as sodium and potassium in the salt solutions of these compounds, it is possible to simultaneously add oxygen and nitrogen groups to the adsorbent surface, thereby achieving greater capacity and selectivity. By performing the chemical functionalization method, the structure of the adsorbent surface will be modified. Adsorption isotherms of CO\u003csub\u003e2\u003c/sub\u003e and N\u003csub\u003e2\u003c/sub\u003e at near-stack flue gas temperatures, experimentally measured, and the heat of adsorption was calculated for both components on the adsorbents.\u003c/p\u003e"},{"header":"1. Materials and methods","content":"\u003cp\u003eThree amino acids, including Glycine, Lysine, and Serine, as well as potassium hydroxide and sodium hydroxide, were purchased from Merck Co. Activated carbon samples were obtained from Jacobi and Samchun Co. Deionized water was provided locally in Iran. All materials were analytical grade and used without further purification.\u003c/p\u003e \u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e1.1. Experimental apparatus and procedure for adsorbent modification\u003c/h2\u003e \u003cp\u003eFor activated carbon modification, a reflux apparatus was assembled as follows. A 3-head flask was used: the first head was attached to a condenser with a valve, the second to a water condenser, and the third held a thermometer. The assembled flask and condensers were placed in an oil bath, positioned on a hot plate with a magnetic stirrer. The schematic of the apparatus is presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eIn a typical experiment, 100 mL of solvent containing a modification agent (amino acid or amino acid plus alkaline metal hydroxide at various molar concentrations) was placed in a flask, and approximately 2g of unmodified adsorbent was added to the flask using a magnet. The setup was assembled, and the experiment commenced with the apparatus at its desired temperature and stirring settings. When it reaches the desired temperature, the timing of the modification reaction begins. After the desired modification time, the heater stopped. The solution, which contained activated carbon in the flask, was filtered and then washed several times with water. It was then dried at 120\u0026deg;C in an oven overnight and stored for further use.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e1.2. Adsorption measurements\u003c/h2\u003e \u003cp\u003eFor the adsorption isotherm measurement, a schematic setup, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, was employed.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe basis of this apparatus is to volumetrically measure the adsorption of a gas on a solid adsorbent. The setup consists of three vessels of different sizes. The first vessel is used to store enough gas as feed to the adsorption cell measurement. The second and third vessels were used to measure the gas volume during the adsorption process. These two vessels were placed in a temperature-controlled bath to ensure isothermal conditions during the experiment. The first vessel is 2L, the second vessel is 235 ml, and the third vessel has a total volume of 200 ml, including the connection lines.\u003c/p\u003e \u003cp\u003eIn each isotherm measurement, approximately 0.5g of adsorbent was placed in a stainless-steel container under high vacuum at approximately 90\u0026deg;C for 8 hours to remove impurities. Afterward, it was quickly placed inside the adsorption cell, and the vessel was sealed. All lines and vessels were vacuumed for at least 15 minutes. Then, the feed was injected from a gas cylinder (CO\u003csub\u003e2\u003c/sub\u003e, air, nitrogen) into the first vessel to the desired pressure. Pressure and temperature of various parts of the setup were automatically logged. The V2 valve was opened to equalize pressure in the first and second vessels, then closed. After a few minutes, once the temperature of the second vessel reached equilibrium with the bath, the V3 valve was rapidly opened and closed to inject a small amount of gas into the adsorption cell. The pressure of the second vessel was measured before and after opening V3. Using thermodynamic calculations, the amount of injected gas was determined. After sufficient contact time between gas and adsorbent in the third vessel, the equilibrium pressure was recorded. From this, the remaining gas in the adsorption vessel was calculated. The difference between the injected and final gas gave the amount adsorbed at each equilibrium pressure. This procedure was repeated for each isotherm temperature. For this work, isotherms were measured over the temperature range of 25 to 80\u0026deg;C. The accuracy of the setup has been tested many times, with results presented in the literature \u003csup\u003e\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e,\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u003c/sup\u003e. The adsorbent volume was subtracted from the vessel volume in calculations. The adsorbent bulk density was measured three times, and the uncertainty is reported in the results section.\u003c/p\u003e \u003c/div\u003e"},{"header":"2. Experimental design","content":"\u003cp\u003eAs the modification of activated carbon with amino acids is a new process, there is insufficient information on their reaction activity. Temperature, amino acid concentration, and modification times were used as effective parameters in the modification of activated carbon. The temperature range of 60\u0026ndash;100\u0026deg;C was used. Higher temperature was avoided due to the stability of these compounds. As for amino acid concentration, a range of 0.3 to 2 mol/L was selected. The reaction time was varied from 3 to 8 hours. A Box-Behnken experimental design method was used to obtain the design matrix. Based on the experimental design, 16 tests were suggested. The primary response was chosen as the amount of CO\u003csub\u003e2\u003c/sub\u003e adsorption. The design matrix is presented in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eDesign matrix for adsorbent modification\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eStdOrder\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eRunOrder\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eTemperature\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eTime\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eAmino acid loading\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e13\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e60\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.3\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e60\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.1\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e80\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.3\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e40\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.1\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e40\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.5\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e60\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.5\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e40\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.3\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e40\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.3\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e60\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.3\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e11\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e60\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.5\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e11\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e80\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.3\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e14\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e60\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.3\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e13\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e80\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.1\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e14\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e80\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.5\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e60\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.1\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e2.1. Adsorbent Characterization\u003c/h2\u003e \u003cp\u003e \u003cdiv class=\"BlockQuote\"\u003e \u003cp\u003eThe characterization technique was employed to investigate the adsorption phenomena on the microscopic level. In this work, FT-IR spectroscopy is used to determine the surface functional groups on the adsorbent surface. Scanning electron microscopy is used to find the surface morphology and particle shape of the adsorbent. The BET-BJH test was conducted to measure the N2 adsorption/desorption of activated carbon before and after modification to find the surface area, pore volume, and pore size distribution of the adsorbent. The TGA analysis was used to measure the thermal stability of the adsorbent at various temperatures.\u003c/p\u003e \u003c/div\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"3. Results and discussion","content":"\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\n\u003ch2\u003e3.1. Adsorbent characterization\u003c/h2\u003e\n\u003cdiv id=\"Sec9\" class=\"Section3\"\u003e\n\u003ch2\u003e3.1.1. SEM characterization\u003c/h2\u003e\n\u003cdiv class=\"BlockQuote\"\u003e\n\u003cp\u003eScanning electron microscopy was used to analyze the shape and morphology of the adsorbent particles. The SEM image of both unmodified and modified activated carbon at optimal conditions is presented in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv class=\"BlockQuote\"\u003e\n\u003cp\u003eAs can be seen from the figure, the modification of the adsorbent surface resulted in increased texture, which can be attributed to the reaction of the amino acid with the functional groups on the adsorbent surface. The SEM micrographs of the pristine Jacobi activated carbon and the glycine-modified sample (AC-g9) reveal clear morphological changes after functionalization. The unmodified Jacobi AC exhibits irregular, angular particles with relatively rough surfaces and exposed pore entrances. After glycine treatment, surfaces appear covered by a more uniform, fine-textured layer, and small particulate deposits are visible on the particle faces (Figure X). The coating observed for AC-g9 is consistent with surface decoration or partial filling of pore walls by organic moieties rather than gross pore blocking. Taken together with the BET results (Section 3.2), the SEM observations suggest that glycine anchors on pore surfaces and external areas while preserving the overall porous architecture. The more homogeneous surface texture of AC-g9 plausibly provides an increased density of surface functional groups accessible to gas molecules, which can enhance chemical affinity toward CO₂ without sacrificing the textural area.\u003c/p\u003e\n\u003c/div\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec10\" class=\"Section3\"\u003e\n\u003ch2\u003e3.1.2. BET-BJH characterization\u003c/h2\u003e\n\u003cdiv class=\"BlockQuote\"\u003e\n\u003cp\u003eThe nitrogen adsorption/desorption at 77K was measured to obtain the adsorbent characteristic, and the results are presented in Fig.\u0026nbsp;4.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv class=\"BlockQuote\"\u003e\u0026nbsp;\u003c/div\u003e\n\u003cdiv class=\"BlockQuote\"\u003e\n\u003cp\u003eAs can be seen from the figure above, the adsorbent exhibits a Type I IUPAC isotherm, which represents a microporous material with a sharp jump at near-zero pressure. The adsorption hysteresis is presented in partial pressure of 0.4, indicating that the adsorbent possesses some mesopore structure. The BET analysis is used to evaluate the surface area of both adsorbents. The results are presented in Table\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv class=\"gridtable\"\u003e\n\u003cdiv class=\"colspec\" align=\"left\"\u003e\u0026nbsp;\u003c/div\u003e\n\u003ctable id=\"Tab2\" border=\"1\"\u003e\u003ccaption\u003e\n\u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e\n\u003cdiv class=\"CaptionContent\"\u003e\n\u003cp\u003eBET characteristic measurement of adsorbents\u003c/p\u003e\n\u003c/div\u003e\n\u003c/caption\u003e\n\u003cthead\u003e\n\u003ctr\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003esample\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003ea\u003csub\u003eBET\u003c/sub\u003e (m\u003csup\u003e2\u003c/sup\u003e/g)\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eTotal pore volume (cm\u003csup\u003e3\u003c/sup\u003e/g)\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eAverage pore diameter (nm)\u003c/p\u003e\n\u003c/th\u003e\n\u003c/tr\u003e\n\u003c/thead\u003e\n\u003ctbody\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eJacobi activated carbon\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e1018.9\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e0.7256\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e2.8487\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eAC-g9\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e1015.8\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e0.7241\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e2.8516\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003c/tbody\u003e\n\u003c/table\u003e\n\u003c/div\u003e\n\u003cdiv class=\"BlockQuote\"\u003e\n\u003cp\u003eAs can be seen from the table above, the active surface area of both adsorbents is nearly identical. This means that the modification did not plug pores or reduce the surface area, which indicates great success. The negligible loss in BET area and pore volume (\u0026lt;\u0026thinsp;0.3% change in surface area) indicates that glycine functionalization does not induce significant pore blocking at the scale resolved by the BET measurement. This preservation of porosity is crucial because it suggests that any observed enhancement in adsorption performance is more likely to originate from chemical (functional group) effects than from an increase in accessible surface area. As for pore distribution, the BJH plot was used to compare both adsorbents before and after modification, and the results are presented in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003e.\u003c/p\u003e\n\u003c/div\u003e\n\u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003e, the pore distribution on the adsorbents is nearly identical. This implies that the modification did not change the pore structure or pore opening. BJH-derived pore size distributions for Jacobi AC and AC-g9 are essentially overlapping. Both show a dominant contribution in the micropore\u0026ndash;small mesopore range (\u0026asymp;\u0026thinsp;1\u0026ndash;3 nm) and a smooth decay toward larger mesopores. The near-identical profiles confirm that the overall pore architecture remains intact after modification. Small differences at the low end of the distribution\u0026mdash;a slight reduction in the smallest pore contribution for AC-g9\u0026mdash;are consistent with a thin, conformal glycine coating on pore walls but not with wholesale pore filling.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec11\" class=\"Section3\"\u003e\n\u003ch2\u003e3.1.3. XRD Characterization\u003c/h2\u003e\n\u003cdiv class=\"BlockQuote\"\u003e\n\u003cp\u003eX-ray diffraction (XRD) analysis was conducted to evaluate the crystalline structure of the activated carbon before and after surface modification. Figure\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003e displays the XRD patterns of the pristine Jacobi activated carbon and the modified sample (AC-g9).\u003c/p\u003e\n\u003c/div\u003e\n\u003cp\u003eThe above results indicate that both adsorbents exhibit similar spectra, except at low angles, where the modified adsorbent shows some peaks with higher counts. However, the activated carbon does not have crystalline phases in its structure. Both samples exhibit broad diffraction peaks at 2\u0026theta; angles of approximately 10\u0026deg;, 25\u0026deg;, and 45\u0026deg;, which are characteristic of amorphous or turbostratic carbon structures. The modified sample exhibits a slight reduction in peak intensity and minor shifts in peak positions, indicating a decrease in crystallinity and the introduction of structural disorder resulting from the incorporation of nitrogen- and oxygen-containing functional groups. No new crystalline phases were observed, suggesting that the modification process preserved the overall carbon framework while altering its surface characteristics\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec12\" class=\"Section3\"\u003e\n\u003ch2\u003e3.1.4. FT-IR characterization\u003c/h2\u003e\n\u003cdiv class=\"BlockQuote\"\u003e\n\u003cp\u003eFourier transform infrared spectroscopy shows the surface functional groups and a great indication of the effect of modification on the adsorbent surface. This characterization was employed to analyze the surface function group of the adsorbent. The results are presented in Fig.\u0026nbsp;7.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv class=\"BlockQuote\"\u003e\n\u003cp\u003eThe results presented in the figure show that the Jacobi activated carbon demonstrates 9 peaks in the entire range of 400\u0026thinsp;~\u0026thinsp;4000 wavelength. In comparison, the modified adsorbent with glycine amino acid represents only 7 peaks. The peaks are listed in Table\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv class=\"BlockQuote\"\u003e\u0026nbsp;\u003c/div\u003e\n\u003cdiv class=\"gridtable\"\u003e\n\u003cdiv class=\"colspec\" align=\"left\"\u003e\u0026nbsp;\u003c/div\u003e\n\u003ctable id=\"Tab3\" border=\"1\"\u003e\u003ccaption\u003e\n\u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e\n\u003cdiv class=\"CaptionContent\"\u003e\n\u003cp\u003ePeaks properties presented in FT-IR Spectra of adsorbents\u003c/p\u003e\n\u003c/div\u003e\n\u003c/caption\u003e\n\u003cthead\u003e\n\u003ctr\u003e\n\u003cth colspan=\"3\" align=\"left\"\u003e\n\u003cp\u003eAC-g9\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\u0026nbsp;\u003c/th\u003e\n\u003cth colspan=\"3\" align=\"left\"\u003e\n\u003cp\u003eJacobi AC\u003c/p\u003e\n\u003c/th\u003e\n\u003c/tr\u003e\n\u003c/thead\u003e\n\u003ctbody\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003ePeak No.\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003eX (cm-1)\u003c/strong\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003eY (%T)\u003c/strong\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003ePeak No.\u003c/strong\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003eX (cm\u003c/strong\u003e\u003csup\u003e\u003cstrong\u003e\u0026minus;\u0026thinsp;1\u003c/strong\u003e\u003c/sup\u003e\u003cstrong\u003e)\u003c/strong\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003eY (%T)\u003c/strong\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e1\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e3952.17\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e97.97\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003e1\u003c/strong\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e3878.96\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e97.47\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e2\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e3876.36\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e97.98\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e3766.41\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e97.18\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e3\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e3840.74\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e97.87\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003e3\u003c/strong\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e3427.26\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e82.07\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e4\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e3422.76\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e85.55\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003e4\u003c/strong\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e2351.31\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e98.04\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e5\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e2900.76\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e95.29\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003e5\u003c/strong\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e1383.55\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e92.01\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e6\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e1038.69\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e80.86\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003e6\u003c/strong\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e1054.11\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e90.36\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e7\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e466.34\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e93.93\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003e7\u003c/strong\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e616.33\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e96.73\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003e8\u003c/strong\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e504.12\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e97.35\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e\u003cstrong\u003e9\u003c/strong\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e465.01\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e95.29\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003c/tbody\u003e\n\u003c/table\u003e\n\u003c/div\u003e\n\u003cp\u003eAs can be seen, the modification removes the peaks at 504 and 616 wavelengths. These peaks are attributed to the activated carbon structure. This indicates that the amino-acid modification removes these functional groups from the adsorbent surface. Peaks in the range of 1000\u0026ndash;1100 cm⁻\u0026sup1; correspond to C\u0026ndash;N and C\u0026ndash;O stretching vibrations, further confirming successful functionalization. Also, Additional peaks observed around 1650\u0026ndash;1550 cm⁻\u0026sup1; are attributed to C\u0026thinsp;=\u0026thinsp;O stretching and N\u0026ndash;H bending, suggesting the incorporation of carboxyl and amide functionalities. The peak at 2400 on the Jacobi activated carbon was eliminated during the modification. The modified sample exhibits enhanced absorption bands in the region of 3200\u0026ndash;3400 cm⁻\u0026sup1;, corresponding to O\u0026ndash;H and N\u0026ndash;H stretching vibrations, indicating the presence of hydroxyl and amine groups. The peaks in the range of 3400\u0026thinsp;~\u0026thinsp;3900 show the hydroxyl group on the surface of the adsorbent and N-H stretching vibrations \u003csup\u003e\u003cspan class=\"CitationRef\"\u003e36\u003c/span\u003e,\u003cspan class=\"CitationRef\"\u003e37\u003c/span\u003e\u003c/sup\u003e which can be an indication of nitrogen functional groups. These spectral changes demonstrate the effective introduction of nitrogen- and oxygen-containing groups onto the activated carbon surface, which are expected to enhance CO\u003csub\u003e2\u003c/sub\u003e adsorption performance.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec13\" class=\"Section3\"\u003e\n\u003ch2\u003e3.1.5. Thermogravimetric analysis\u003c/h2\u003e\n\u003cdiv class=\"BlockQuote\"\u003e\n\u003cp\u003eTGA is used to analyze the mass change of the adsorbent during heating in neutral atmosphere of N\u003csub\u003e2\u003c/sub\u003e. The results are presented in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e8\u003c/span\u003e. This analysis is used to study the stability of the adsorbent and its decomposition conditions.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv class=\"BlockQuote\"\u003e\n\u003cp\u003eThe comparative TGA analysis conclusively demonstrates the successful functionalization of the activated carbon with glycine. The pristine sample (AMERICA) exhibited high thermal inertia, with minimal weight loss across the 25\u0026ndash;600\u0026deg;C temperature range. In stark contrast, the functionalized sample (AC-G9) showed a significant, multi-stage decomposition event between 270\u0026deg;C and 340\u0026deg;C, resulting in a substantial 31.23% mass loss. This event is a direct fingerprint of the thermal decomposition of the grafted glycine molecules and their derived functional groups, confirming their presence on the carbon surface.\u003c/p\u003e\n\u003cp\u003eRegarding stability, the glycine-functionalized carbon is structurally stable for routine applications at low temperatures but exhibits lower thermal stability overall. The grafted organic layer begins to decompose sharply around 270\u0026deg;C, making the material unsuitable for high-temperature processes above this point. However, for typical uses like adsorption in post combustion processes (temperature range up to 150\u0026deg;C), the functionalized structure is perfectly stable, as the decomposition temperature provides a wide safety margin. Thus, the functionalization successfully introduced valuable surface groups while maintaining sufficient stability for its intended purpose.\u003c/p\u003e\n\u003c/div\u003e\n\u003c/div\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec14\" class=\"Section2\"\u003e\n\u003ch2\u003e3.2. Adsorption isotherm measurement of unmodified adsorbent\u003c/h2\u003e\n\u003cp\u003eAs it was stated above, two activated carbons were selected for CO\u003csub\u003e2\u003c/sub\u003e adsorption in this work. At the first step, their isotherm was measured at 298K to see their adsorption performance. The isotherm of both adsorbents is presented in Fig.\u0026nbsp;9.\u003c/p\u003e\n\u003cdiv class=\"BlockQuote\"\u003e\n\u003cp\u003eThe Jacobi adsorbent shows stronger CO\u003csub\u003e2\u003c/sub\u003e adsorption than the Samchun adsorbent. The product catalogue lists the Jacobi as a coal-derived activated carbon, and the Samchun as a char-derived adsorbent. BET surface area measurements reveal that both have nearly identical active surface areas. In physisorption-dominated systems, adsorption capacity increases with pressure and decreases with temperature. At 298 K, maximum uptake reaches about 4.5 mmol/g; at 358 K, it drops to around 2.5 mmol/g, a significant reduction of nearly 45%. This temperature-dependent behavior confirms the physical nature of CO\u003csub\u003e2\u003c/sub\u003e adsorption on unmodified activated carbon, highlighting the impact of operating conditions on adsorption performance.\u003c/p\u003e\n\u003c/div\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec15\" class=\"Section2\"\u003e\n\u003ch2\u003e3.3. Adsorption isotherm of modified activated carbon with various agents\u003c/h2\u003e\n\u003cp\u003eIn the next step, the adsorption of Jacobi activated carbon with various modification agents, including glycine, serine, and lysine, with and without NaOH/KOH, was measured. In this step, the modification was carried out at 100\u0026deg;C with a 2M solution concentration and a reaction time of 8 hours. After each modification, the CO\u003csub\u003e2\u003c/sub\u003e adsorption isotherm at various temperatures was measured. The results are presented in Fig.\u0026nbsp;10.\u003c/p\u003e\n\u003cp\u003eThe results show that the modification of active carbon with Glycine and Serine increased the adsorption capacity of the adsorbent, while the lysine-modified adsorbent shows a lower capacity than the original adsorbent. Regarding the addition of KOH/NaOH to the amino acid modification solvent, it is evident that samples with lysine modified by KOH/NaOH exhibited lower capacity, with the KOH-modified sample showing the lowest adsorption capacity. As for serine and glycine, the behavior is different. The serine shows higher adsorption capacity with the addition of NaOH and KOH, which later shows greater adsorption capacity improvement. As for glycine, it can be observed that the addition of NaOH or KOH did not result in a significant increase in adsorption. The modification of activated carbon with an amino acid improved the adsorption capacity of the adsorbent by up to 25% for glycine. The amino acids used in this study were from various sources. Glycine is from non-polar aliphatic R-group amino acids. Lysine is from a group of amino acids with a positively charged R group, and serine is from polar non-charged R group amino acids. The common characteristic of serine and glycine is that they both have a non-charged R group in their structure, while lysine has a negatively charged R group. It seems that the negative/positive charged R group is responsible for the reduction in adsorption capacity of the sample modified with this agent. The addition of KOH/NaOH may cause the precipitation of alkaline salts on the adsorbent surface or block the adsorbent pores. Additionally, it is possible that the reaction of the functional groups on the surface of the adsorbent with R groups or even free hydroxyl groups in the solution is responsible for the reduction in adsorption capacity in the samples modified with lysine.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec16\" class=\"Section2\"\u003e\n\u003ch2\u003e3.4. Optimizing the modification conditions\u003c/h2\u003e\n\u003cp\u003eBased on the results obtained for adsorbent modification with three amino acids, the optimization of the modification condition was performed with glycine, which was selected as the best modification agent, and the experimental design optimization was carried out with this agent. The experimental table is presented in Table\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e.\u003c/p\u003e\n\u003cdiv class=\"gridtable\"\u003e\n\u003cdiv class=\"colspec\" align=\"left\"\u003e\u0026nbsp;\u003c/div\u003e\n\u003cdiv class=\"colspec\" align=\"char\"\u003e\u0026nbsp;\u003c/div\u003e\n\u003cdiv class=\"colspec\" align=\"char\"\u003e\u0026nbsp;\u003c/div\u003e\n\u003cdiv class=\"colspec\" align=\"char\"\u003e\u0026nbsp;\u003c/div\u003e\n\u003cdiv class=\"colspec\" align=\"char\"\u003e\u0026nbsp;\u003c/div\u003e\n\u003ctable id=\"Tab4\" border=\"1\"\u003e\u003ccaption\u003e\n\u003cdiv class=\"CaptionNumber\"\u003eTable 4\u003c/div\u003e\n\u003cdiv class=\"CaptionContent\"\u003e\n\u003cp\u003eExperimental parameters for adsorbent modification\u003c/p\u003e\n\u003c/div\u003e\n\u003c/caption\u003e\n\u003cthead\u003e\n\u003ctr\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eSample code\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eRun order\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eTemperature (\u0026deg;C)\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eTime (h)\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eAmino acid loading (mol/L)\u003c/p\u003e\n\u003c/th\u003e\n\u003c/tr\u003e\n\u003c/thead\u003e\n\u003ctbody\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eAC-G-1\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e1\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e60\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e3\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e0.3\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eAC-G-2\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e2\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e60\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e1\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e0.1\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eAC-G-3\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e3\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e80\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e5\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e0.3\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eAC-G-4\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e4\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e40\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e3\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e0.1\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eAC-G-5\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e5\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e40\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e3\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e0.5\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eAC-G-6\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e6\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e60\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e5\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e0.5\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eAC-G-7\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e7\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e40\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e5\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e0.3\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eAC-G-8\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e8\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e40\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e1\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e0.3\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eAC-G-9\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e9\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e60\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e3\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e0.3\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eAC-G-10\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e10\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e60\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e1\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e0.5\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eAC-G-11\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e11\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e80\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e1\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e0.3\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eAC-G-12\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e12\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e60\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e3\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e0.3\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eAC-G-13\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e13\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e80\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e3\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e0.1\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eAC-G-14\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e14\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e80\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e3\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e0.5\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eAC-G-15\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e15\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e60\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e5\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e0.1\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003c/tbody\u003e\n\u003c/table\u003e\n\u003c/div\u003e\n\u003cp\u003eThe CO\u003csub\u003e2\u003c/sub\u003e adsorption capacity was selected as the response variable. After each modification, the CO\u003csub\u003e2\u003c/sub\u003e adsorption isotherm was carried out at 298 K and the results obtained were compared in each run. The isotherm obtained is presented in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e11\u003c/span\u003e.\u003c/p\u003e\n\u003cp\u003eThe adsorption uptake increases with pressure across all samples, consistent with physisorption behavior. Notably, sample AC-G-9 exhibits the highest CO\u003csub\u003e2\u003c/sub\u003e uptake, confirming the optimal modification conditions of 60\u0026deg;C, 3 h, and 0.3 mol/L glycine concentration. In contrast, samples such as AC-G-2 and AC-G-4 show significantly lower adsorption capacities, likely due to insufficient functionalization at lower temperatures or concentrations. These results underscore the importance of precise control over modification parameters to maximize adsorbent performance.\u003c/p\u003e\n\u003cp\u003eIn the next step, the adsorption isotherm of AC-G-9, as the best candidate, was examined at temperatures of 298, 318, 338, and 358 K, and the results are presented in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e12\u003c/span\u003e.\u003c/p\u003e\n\u003cp\u003eAs can be seen from Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e12\u003c/span\u003e, the adsorption capacity decreases when the temperature is increased from 298 K to 358 K (3.4 mmol/g to 1.8 mmol/g). Therefore, in real practice, considering the high temperature of stack flue gas one should seek methods to decrease flue gas temperature in order to increase the process efficiency. One way to reduce the stack temperature is to preheat the combustion air by recuperation technique, which is a common practice. A recuperator is a heat exchanger that transfers heat from hot flue gases to the combustion air. Various recuperators are adapted in the industry \u003csup\u003e\u003cspan class=\"CitationRef\"\u003e38\u003c/span\u003e\u003c/sup\u003e. In this technique, flame temperature increases, while the flue gas temperature decreases due to heat transfer. This method increases the combustion efficiency and saves energy.\u003c/p\u003e\n\u003cdiv id=\"Sec17\" class=\"Section3\"\u003e\n\u003ch2\u003e3.3.1. N\u003csub\u003e2\u003c/sub\u003e Adsorption capacity\u003c/h2\u003e\n\u003cp\u003eThe N\u003csub\u003e2\u003c/sub\u003e adsorption on unmodified and modified adsorbents is measured experimentally, and the results are provided in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e13\u003c/span\u003e.\u003c/p\u003e\n\u003cp\u003eFor all samples, N₂ uptake increases with pressure, consistent with physisorption behavior. The modified sample AC-g9 at 298 K exhibits the highest N₂ adsorption capacity, followed by Jacobi AC at the same temperature. At 358 K, both samples show reduced uptake, highlighting the temperature sensitivity of N₂ adsorption. The enhanced N₂ uptake in AC-g9 may be attributed to the introduction of polar functional groups and changes in pore structure due to surface modification. While beneficial for overall adsorption capacity, this increase in N₂ uptake could potentially affect CO₂/N₂ selectivity, which is a critical parameter in post-combustion capture applications.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec18\" class=\"Section3\"\u003e\n\u003ch2\u003e3.3.2. Isosteric heat of adsorption\u003c/h2\u003e\n\u003cp\u003eThe isosteric heat of adsorption, also referred to as the differential enthalpy of adsorption, is an important thermodynamic property for designing and optimizing adsorption processes. It provides insights into interactions between adsorbate molecules and between adsorbate and adsorbent. For instance, in homogeneous adsorbents, the isosteric heat of adsorption remains unchanged at low adsorbate loadings, indicating that interactions among adsorbed molecules do not impact the adsorption energy. However, at higher adsorbate loadings, significant lateral interactions between adsorbate and adsorbent emerge, leading to an increase in the isosteric heat of adsorption. Conversely, in energetically heterogeneous adsorbents, the isosteric heat of adsorption decreases with increasing adsorbate loading because molecules preferentially adsorb on high-energy sites even at low pressures \u003csup\u003e\u003cspan class=\"CitationRef\"\u003e38\u003c/span\u003e,\u003cspan class=\"CitationRef\"\u003e39\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eThe isosteric heat of adsorption for pure components can be determined through calorimetric measurements or from adsorption isotherms obtained at various temperatures. This method employs a Clapeyron-type relationship to calculate the isosteric heat of adsorption, denoted as \u0026Delta;H\u003csub\u003est\u003c/sub\u003e.\u003c/p\u003e\n\u003cdiv id=\"Equa\" class=\"Equation\"\u003e\n\u003cdiv id=\"FileID_Equa\" class=\"mathdisplay\"\u003e$$\\:{\\varDelta\\:H}_{st}=R{T}^{2}{\\left(\\frac{\\partial\\:\\text{ln}P}{\\partial\\:T}\\right)}_{\\stackrel{\u0026acute;}{n}}$$\u003c/div\u003e\n\u003c/div\u003e\n\u003cp\u003eWhere n\u0026rsquo; is the amount of adsorbed, P and T are pressure and temperature, respectively, R is the universal gas constant, and \u0026Delta;Hst is the isosteric heat of adsorption. It must be noted that this equation is applicable to low-pressure and pure gas isotherms, and its derivation can be found elsewhere \u003csup\u003e\u003cspan class=\"CitationRef\"\u003e40\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eThe isosteric heat of adsorption for various adsorbents is calculated from related isotherms, and the results are shown in Table\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e.\u0026nbsp;\u003c/p\u003e\n\u003cdiv class=\"gridtable\"\u003e\n\u003ctable id=\"Tab5\" border=\"1\"\u003e\u003ccaption\u003e\n\u003cdiv class=\"CaptionNumber\"\u003eTable 5\u003c/div\u003e\n\u003cdiv class=\"CaptionContent\"\u003e\n\u003cp\u003eIsosteric heat of adsorption\u003c/p\u003e\n\u003c/div\u003e\n\u003c/caption\u003e\n\u003cthead\u003e\n\u003ctr\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eAdsorbent\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003e\u0026Delta;H\u003csub\u003est\u003c/sub\u003e. (kJ/mol) (CO\u003csub\u003e2\u003c/sub\u003e)\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003e\u0026Delta;H\u003csub\u003est\u003c/sub\u003e. (kJ/mol) (N\u003csub\u003e2\u003c/sub\u003e)\u003c/p\u003e\n\u003c/th\u003e\n\u003c/tr\u003e\n\u003c/thead\u003e\n\u003ctbody\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eJacobi AC\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e-11\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e-15.6\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eAC-G-9\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e-33.9\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e-17.8\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003c/tbody\u003e\n\u003c/table\u003e\n\u003c/div\u003e\n\u003cp\u003eAll values are negative, confirming the exothermic nature of the adsorption process and supporting the physisorption mechanism for both gases. The \u0026Delta;Hₛₜ value for pristine activated carbon aligns well with literature reports (10\u0026ndash;28.5 kJ/mol for CO\u003csub\u003e2\u003c/sub\u003e, 9.5\u0026ndash;16 kJ/mol for N₂) \u003csup\u003e41\u003c/sup\u003e. The significantly higher \u0026Delta;Hₛₜ for the modified adsorbent (AC-G-9) suggests stronger adsorbate\u0026ndash;adsorbent interactions, likely due to the presence of nitrogen- and oxygen-containing functional groups introduced during modification.\u003c/p\u003e\n\u003cp\u003eThis enhanced interaction leads to a more pronounced temperature sensitivity, as evidenced by the 60% reduction in CO\u003csub\u003e2\u003c/sub\u003e adsorption capacity when the temperature increased from 298 K to 358 K. Therefore, the exothermic nature of the modified adsorbent is the primary factor contributing to the observed decline in adsorption performance at elevated temperatures.\u003c/p\u003e\n\u003c/div\u003e\n\u003c/div\u003e"},{"header":"4. Conclusion","content":"\u003cp\u003eThis study systematically examined the adsorption behavior of activated carbon modified with various amino acids for carbon dioxide (CO\u003csub\u003e2\u003c/sub\u003e) capture in post-combustion processes. Amino acid functionalization substantially increased CO\u003csub\u003e2\u003c/sub\u003e adsorption capacity, while it didn\u0026rsquo;t alter the N\u003csub\u003e2\u003c/sub\u003e adsorption capacity. Glycine-functionalized activated carbon demonstrated the highest CO\u003csub\u003e2\u003c/sub\u003e uptake, surpassing both unmodified and other modified activated carbon. This enhanced performance is attributed to the small molecular size of glycine, which enables uniform surface coverage and maximizes the availability of nitrogen-containing functional groups. The isosteric heats of adsorption (ΔH\u003csub\u003est\u003c/sub\u003e) confirmed the physisorption mechanism. Temperature-dependent isotherms indicated a reduction in adsorption capacity as temperature increased, especially for modified adsorbents with higher ΔH\u003csub\u003est\u003c/sub\u003e values. These findings emphasize the exothermic character of the adsorption process and the necessity of cooling the flue gas in order to increase the efficiency of CO\u003csub\u003e2\u003c/sub\u003e capture process. This optimization may include cooling of the exhaust gas from stacks by means heat exchange with combustion air. Although, this approach may complicate the process but it has the advantages of higher efficiency and energy saving.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eDeclaration of competing interest\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eDeclaration of funding\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that this research did not receive any funding sources.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eDeclaration of authors contribution\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eDavoud Houshmand conducted the experiments and wrote the main manuscript text. Fariborz Rashidi edit the text and supervised this research work.\u0026nbsp;Meysam Hajilari designed the experiments and helped in experimental section and preparing the manuscript. Sepide Amjad Iranagh helped to prepare the discussion section and analyze the results. All authors reviewed the manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData availability statement\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll data generated or analysed during this study are included in this published article\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eGISTEMP Team. GISS Surface Temperature Analysis (GISTEMP v4). NASA Goddard Institute for Space Studies. (2023).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eCouncil, N. 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Gas Chem.\u003c/em\u003e \u003cb\u003e15\u003c/b\u003e, 223\u0026ndash;229 (2006).\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"Post combustion CO2 capture, activated carbon, amino-acids functionalized","lastPublishedDoi":"10.21203/rs.3.rs-8252812/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8252812/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eIn this study, surface modification of a general purpose activated carbon is achieved through impregnation of three common amino acids namely, glycine, serine and lysine for CO₂ adsorption in post-combustion processes at temperature range 298\u0026ndash;358 K. Glycine, due to its smaller molecular size, higher nitrogen content, and enhanced pore accessibility caused the adsorption to increase by 25% compared to original activated carbon uptake. Also, N₂ adsorption was evaluated to assess selectivity and competitive behavior. The modified adsorbents with glycine exhibited similar N₂ uptake to initial activated carbon at 358K. All adsorbents exhibited physisorption behavior, with ΔH\u003csub\u003est\u003c/sub\u003e values ranging from \u0026minus;\u0026thinsp;11 to \u0026minus;\u0026thinsp;33.9 kJ/mol for CO₂ and \u0026minus;\u0026thinsp;15.6 to \u0026minus;\u0026thinsp;17.8 kJ/mol for N\u003csub\u003e2\u003c/sub\u003e.\u003c/p\u003e \u003cp\u003eOverall, the results demonstrate that functionalizing by glycine, significantly enhances CO\u003csub\u003e2\u003c/sub\u003e adsorption capacity of activated carbon. These findings provide valuable insights for designing tailored adsorbents for post combustion CO\u003csub\u003e2\u003c/sub\u003e capture.\u003c/p\u003e","manuscriptTitle":"Enhancement of CO2 capture in post combustion process using active carbon modified by amino acids","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-12-17 09:24:35","doi":"10.21203/rs.3.rs-8252812/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2025-12-29T11:39:42+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-12-27T11:19:29+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-12-26T06:38:45+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"137458651066721839856521572427491192950","date":"2025-12-16T04:14:19+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"176315177597708075581628743718648958542","date":"2025-12-15T08:29:32+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-12-12T11:17:40+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"","date":"2025-12-12T10:09:45+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-12-03T12:14:37+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-12-03T12:12:07+00:00","index":"","fulltext":""},{"type":"submitted","content":"Scientific Reports","date":"2025-12-01T17:00:11+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"2c426cd8-14f3-4d8e-8af5-f0d25f504724","owner":[],"postedDate":"December 17th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":59796443,"name":"Physical sciences/Chemistry"},{"id":59796444,"name":"Physical sciences/Energy science and technology"},{"id":59796445,"name":"Earth and environmental sciences/Environmental sciences"}],"tags":[],"updatedAt":"2026-04-07T16:04:45+00:00","versionOfRecord":{"articleIdentity":"rs-8252812","link":"https://doi.org/10.1038/s41598-026-44400-z","journal":{"identity":"scientific-reports","isVorOnly":false,"title":"Scientific Reports"},"publishedOn":"2026-03-30 15:59:34","publishedOnDateReadable":"March 30th, 2026"},"versionCreatedAt":"2025-12-17 09:24:35","video":"","vorDoi":"10.1038/s41598-026-44400-z","vorDoiUrl":"https://doi.org/10.1038/s41598-026-44400-z","workflowStages":[]},"version":"v1","identity":"rs-8252812","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8252812","identity":"rs-8252812","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}