Amine Infused Fly Ash Grafted Acrylic Acid/Acrylamide Hydrogel for Carbon Dioxide (CO2) Adsorption and Its Kinetic Analysis

In most carbon dioxide (CO2) capture processes, chemical absorption using an amine solvent is widely used technology; however, the solvent is prone to solvent degradation and solvent loss which leads to the formation of corrosion. This paper investigates the adsorption performance of amine-infused hydrogels (AIFHs) to increase carbon dioxide (CO2) capture by leveraging the potency of amine absorption and adsorption properties of class F fly ash (FA). The solution polymerization method was used to synthesize the FA-grafted acrylic acid/acrylamide hydrogel (FA-AAc/AAm), which was then immersed in monoethanolamine (MEA) to form amine infused hydrogels (AIHs). The prepared FA-AAc/AAm showed dense matrices morphology with no obvious pore at the dry state but capable of capturing up to 0.71 mol/g CO2 at 0.5 wt% FA content, 2 bar pressure, 30 °C reaction temperature, 60 L/min flow rate, and 30 wt% MEA contents. Cumulative adsorption capacity was calculated and Pseudo-first order kinetic model was used to investigate the CO2 adsorption kinetic at different parameters. Remarkably, this FA-AAc/AAm hydrogel is also capable of absorbing liquid activator that was 1000% more than its original weight. FA-AAc/AAm can be used as an alternative AIHs that employ FA waste to capture CO2 and minimize the GHG impact on the environment.


Introduction
One of the world's key focus is to minimize greenhouse gas (GHG) emissions impact on climate resilience and environmental sustainability. GHG such as CO 2 accumulate in the atmosphere and cause climate change by trapping heat. The main GHG emission contributors are the transport, manufacturing industries, industrial processes, waste, agriculture sector and energy industries. As of July 2022, statistics from Global Energy Monitor shows there are more than 2215 power plants that actively generate electricity from the burning of coal [1]. A conventional post-combustion capture technology (PCC) used in plants normally utilize amine as the absorbent solvent that binds with CO 2 . Amine as a solvent is generally used for the removal of CO 2 as it has a faster rate of reaction with CO 2 . However, the main problem related to the use of amine as a solvent for the CO 2 absorption is its high cost [2]. The high cost is associated with its regeneration cost which results in only 50% of the solvent at the end. Another problem with amine solvent is it affects operating problems such as solvent degradation, solvent loss and corrosion to the equipment [3].
Adsorption is an appealing method because it requires little energy to regenerate and is easy to process. However, it has limitations when used in high moisture environments and at low pressure conditions, which can affect its capacity and selectivity [4,5]. To improve its performance, amine-functionalized adsorbents were introduced. Amine-functionalized adsorbents are prepared by attaching amines to a porous support, either through in situ Gels 2023, 9,229 2 of 16 formed polymeric amines or impregnation route. Impregnated route adsorbent is created by physically adsorbing amines onto the support, while amine-grafted/covalently bonded systems are made through chemical reaction during synthesis or post-synthesis modification [4].
For instance, Chen et al. (2017) employed a post-functionalization technique to introduce amines into silica and observed that the CO 2 adsorption capacity remained stable even after 10 cycles of adsorption [6]. Liu et al. (2019) developed a thermosensitive solid amine CO 2 adsorbent by grafted polyethyleneimine (PEI) on the poly(N-isopropylacrylamide) via inverse suspension polymerization and it recorded 3.15 mmol/g of CO 2 adsorption at 10 • C under wet condition [5]. Similarly, Choi et al. (2022) employed the in-situ method to bond hyperbranched poly(amidoamine)s on the ethylenediamine via oil-water-oil suspension polymerization and successfully formed microporous hydrogel particles that exhibited a CO 2 adsorption capacity of 104.4 mg/g from flue gas in its dry state. The average pore size of the particles also varies from 2.9 µm to 1.3 µm depending on the agitation speed [7]. In another study, Kim et al. (2021) physically impregnated MEA into the dried hydrogel, resulting in an amine infused hydrogel (AIHs) with increased CO 2 selectivity in the methane/CO 2 mixture gas system compared to bulk MEA solution [8]. White et al. (2021) also utilized the same method to infused the amine into the hydrogel and investigated the use of water and ethylene glycol (EG) as the amine solvent in order to increase the uptake kinetic and minimize solvent degradation after adsorbent recycling. The study found that a combination of EG-diethanolamine resulted in a CO 2 capture capacity of 0.75 mol CO 2 per mol amine and that the adsorption capacity of the AIHs was highly dependent on the type of hydrogel used, with poly(acrylamide) found to be more effective in capturing CO 2 compared to poly(N-2-hydroxythyleneacrylamide) and sodium polyacrylate [9].
Hydrogels, as a solid support, possess unique characteristics such as the ability to retain high amounts of liquid while maintaining their structural integrity through chemical or physical crosslinking of the polymer chains [10]. In order to exploit its potential as a CO 2 adsorbent, the properties of hydrogels can be manipulated by infusing amine solvents into the hydrogel matrix. This AIHs can be prepared through the impregnation route, which is a simple and efficient process that reduces both operation time and capital costs [11]. The AIHs act as microcontainers when CO 2 molecules diffuse through the surface of the hydrogel and react with the amine solution in-situ. This interaction with CO 2 results in the formation of carbamates or bicarbonates. The AIHs exhibit a combination of physical adsorption inherited from the parent adsorbent and chemical adsorption from the loaded amine groups, resulting in a much higher CO 2 adsorption capacity when compared to separate microcontainers and bulk amines [12]. Additionally, the use of AIHs technology eliminates the need for extra packing materials, as the hydrogel itself serves as the "amine storage and packing" [11].
The utilization of solid support mechanisms in the development of gas adsorbents to minimize the drawback of amine solvent has been a topic of ongoing research for an extended period of time [13][14][15][16][17][18]. However, the use of power plant generated waste, specifically fly ash (FA) as an additive in polymer adsorbents to enhance gas adsorption properties, has not been widely explored in the research literature. Previous FA adsorbent studies primarily focused on the surface treatment of the FA itself and it was proven that chemically treated FA did improve the surface area, morphology, porosity, and adsorbent affinity of fly ash [19]. The enhancement of CO 2 adsorption by FA can been done via various methods such as alkaline activator treatment [20,21], conversion of FA into zeolite [22,23] and impregnation of raw FA using amine such as MEA [24]. For instance, this solid waste was transformed into mesoporous adsorbent called silica-alumina composite aerogel with 400 m 2 /g of active surface area and 1.9 cm 3 /g of specific pores volume via alkaline fusion and acid leached method of FA. It able to adsorb 2.02 mmol/g of CO 2 [25] compared to the pristine FA which only can capture CO 2 up to 870.1 µmol/g FA [20]. FA is the coal power plant byproduct that trapped at the end of the process using precipitator or baghouse [26]. It contains high carbon content, silica dioxide (SiO 2 ) and aluminum oxide (Al 2 O 3 ) that can Gels 2023, 9, 229 3 of 16 facilitate the CO 2 adsorption [20]. Within the last 20 years, an average quantity of 64.8 Mts FA was produced every year by the coal power plant in United State (US) and the highest utilization of FA was only at 60% which is in 2017 [27].
In this work, newly explored FA grafted AIHs was fabricated via direct impregnation of MEA in FA grafted AAc/AAm hydrogel. The impact of FA content, type of hydrogel activator, and operational temperature on CO 2 capture was systematically evaluated in order to maximize the CO 2 adsorption capacity. To gain a deeper understanding of the CO 2 adsorption mechanism of MEA-infused FA grafted AAc/AAm hydrogels, adsorption kinetic model analysis was performed using five different kinetic models. The results of this work suggest that the utilization of FA to enhance the adsorption properties of amine-infused hydrogels is a promising area for future investigation.  Table 1 presents the chemical composition of FA. The composition of FA varies depending on the source of the FA. In this study, the coal FA that obtained from local power station recorded the chemical composition of silicon oxide (SiO 2 ), aluminum oxide (Al 2 O 3 ) and iron oxide (Fe 2 O 3 ) were 78.58 wt%, which indicated that the currently used FA was classified as a Class F FA as described in ASTM C618. The Class F FA contains more than 70% of Al 2 O 3 , SiO 2 and less than 10% of CaO. While some analysis reported that the composition of SiO 2 and Al 2 O 3 can be up to 55% and 35% respectively [28,29]. The properties of the FA are highly dependent on its composition especially silicon (Si). High amount of silicon is able to contribute to high mechanical strength and adsorption capacity of the synthesized material [30,31]. On the other hand, the FA will be categorized as Class C if the total of Al 2 O 3 , SiO 2 and ferrite is less or equal to 50% of the total FA composition and the composition of CaO is more than 10% [29].  Figure 1 shows the particle size distribution of raw FA. The particle size distribution of the FA was measured by the laser diffraction particle size analyzer. The particle size of FA ranged from 0.55 to 440 µm with median, D 50 of 15.37 µm, D 90 90% of the particles size was 122.95 µm and span of 7.82. According to Nath et al., the size of filler is one of the most important factors that might affect the mechanical properties of the composite materials. Frequently, nanofiller with a size of 2 nm to 50 nm was used in synthesizing the composite hydrogel [32][33][34]. Large-sized filler might create a weak point in the material [35]. Several reports found to utilize raw FA in its pristine state without grinding and sieving [35,36] [37,38].  [37,38].

Results and Discussion
Referring to Figure 2b, the additional FA in 0.5FA-AAc/AAm can be validated by observing the disappearance of C=C bending at 1100 and 936 cm −1 . This can be used to presume that FA was successfully grafted into the AAc/AAm hydrogel. The IR spectra of 0.5FA-AAc/AAm+CO2 also displayed a few key differences at 1540 and 1320 cm −1 which correspond to the symmetric and asymmetric COO-stretching vibration. The presence of COO-stretching indicates the carbamate formation as an outcome from the reaction between MEA and CO2. Thus, this means that CO2 molecules could readily penetrate hydrogel and react with the infused MEA. Similar IR spectra with similar bands were observed in previous study by Xu et al. using DEA Infused Hydrogels (AIH) for CO2 capture [11]. Several common peaks were also observed between the AAc/AAm and 0.5FA-AAc/AAm+CO2 resulted from the use of copolymer of acrylic acid and acrylamide. Broad transmittance bands between 3600 cm −1 and 2750 cm −1 were assigned to -OH stretching which were originated by the hydroxyl group from carboxylic acid of acrylic acid. At 1650 cm −1 , the common stretching of C=O groups were observed for both spectra. These align peaks were assigned to C=O group connected to the amide group of acrylamides.  octahedral Al and bending of Si-O, respectively. The spectra of raw FA is comparable to the previous studies [37,38].
Referring to Figure 2b, the additional FA in 0.5FA-AAc/AAm can be validated by observing the disappearance of C=C bending at 1100 and 936 cm −1 . This can be used to presume that FA was successfully grafted into the AAc/AAm hydrogel. The IR spectra of 0.5FA-AAc/AAm+CO2 also displayed a few key differences at 1540 and 1320 cm −1 which correspond to the symmetric and asymmetric COO-stretching vibration. The presence of COO-stretching indicates the carbamate formation as an outcome from the reaction between MEA and CO2. Thus, this means that CO2 molecules could readily penetrate hydrogel and react with the infused MEA. Similar IR spectra with similar bands were observed in previous study by Xu et al. using DEA Infused Hydrogels (AIH) for CO2 capture [11]. Several common peaks were also observed between the AAc/AAm and 0.5FA-AAc/AAm+CO2 resulted from the use of copolymer of acrylic acid and acrylamide. Broad transmittance bands between 3600 cm −1 and 2750 cm −1 were assigned to -OH stretching which were originated by the hydroxyl group from carboxylic acid of acrylic acid. At 1650 cm −1 , the common stretching of C=O groups were observed for both spectra. These align peaks were assigned to C=O group connected to the amide group of acrylamides.  Referring to Figure 2b, the additional FA in 0.5FA-AAc/AAm can be validated by observing the disappearance of C=C bending at 1100 and 936 cm −1 . This can be used to presume that FA was successfully grafted into the AAc/AAm hydrogel. The IR spectra of 0.5FA-AAc/AAm+CO 2 also displayed a few key differences at 1540 and 1320 cm −1 which correspond to the symmetric and asymmetric COO-stretching vibration. The presence of COO-stretching indicates the carbamate formation as an outcome from the reaction between MEA and CO 2 . Thus, this means that CO 2 molecules could readily penetrate hydrogel and react with the infused MEA. Similar IR spectra with similar bands were observed in previous study by Xu et al. using DEA Infused Hydrogels (AIH) for CO 2 capture [11]. Several common peaks were also observed between the AAc/AAm and 0.5FA-AAc/AAm+CO 2 resulted from the use of copolymer of acrylic acid and acrylamide. Broad transmittance bands between 3600 cm −1 and 2750 cm −1 were assigned to -OH stretching which were originated by the hydroxyl group from carboxylic acid of acrylic acid. At 1650 cm −1 , the common stretching of C=O groups were observed for both spectra. These align peaks were assigned to C=O group connected to the amide group of acrylamides.

Scanning Electron Microscope (SEM)
The microstructure of AAc/AAm hydrogel and 0.5FA-AAc/AAm composite hydrogels are shown in Figure 3(a(i,ii),b(i,ii)). The morphology of both AAm/AAc and 0.5FA-AAc/AAm for Figure 3(ai,bi) were taken at 64× magnification while Figure 3(aii,bii) were taken at 5KX magnification. Based on the surface morphology of Figure 3(bi), it can be Gels 2023, 9, 229 5 of 16 observed that there was no crack between FA and polymer matrix, which might signal the compatibility of the FA and AAc/AAm hydrogel. These morphologies were seen during the collapse state. As a result, the interlinkage matrices were obscured because of the shrinkage during the drying process and when the liquid is absorbed, the hydrogel can revive to its original form [39]. Anyhow, some pores still can be observed in Figure 3(aii,bii). With the addition of FA, as pointed by the arrows in Figure 3(bii), it is examined that the surface porosity volume of 0.5FA-AAc/AAm hydrogel was more than AAm/AAc hydrogel in Figure 3(aii). The surface of 0.5FA-AAc/AAm hydrogel exhibits tiny and denser pores in comparison to AAc/AAm hydrogel, which has wider diameter but only a few pores. FA presents a wide range of benefits due to its silica, alumina, and unburned carbon content which is useful as a precursor for the synthesis of porous materials.
The microstructure of AAc/AAm hydrogel and 0.5FA-AAc/AAm composite hydro are shown in Figure 3(ai,ii,bi,ii). The morphology of both AAm/AAc and 0.5FA-AAc/A for Figure 3(ai,bi) were taken at 64X magnification while Figure 3(aii,bii) were taken at magnification. Based on the surface morphology of Figure 3(bi), it can be observed that was no crack between FA and polymer matrix, which might signal the compatibility o FA and AAc/AAm hydrogel. These morphologies were seen during the collapse state. result, the interlinkage matrices were obscured because of the shrinkage during the dr process and when the liquid is absorbed, the hydrogel can revive to its original form Anyhow, some pores still can be observed in Figure 3(aii,bii). With the addition of FA pointed by the arrows in Figure 3(b(ii)), it is examined that the surface porosity volum 0.5FA-AAc/AAm hydrogel was more than AAm/AAc hydrogel in Figure 3(aii). The su of 0.5FA-AAc/AAm hydrogel exhibits tiny and denser pores in comparison to AAc/A hydrogel, which has wider diameter but only a few pores. FA presents a wide range of efits due to its silica, alumina, and unburned carbon content which is useful as a precu for the synthesis of porous materials.

Thermogravimetric Analysis (TGA)
TGA was used to determine the effect of temperature on the desorption of CO 2 and the evaporation of the MEA. The TGA analysis was carried out by heating up the samples from 25 to 600 • C. All samples were loaded with same amount of 30 wt% MEA and 0.5FA-AAc/AAm pre-saturated with CO 2 to access the evaporation and desorption conditions. The TGA and DTG curves of AAc/AAm hydrogel, 0.5FA-AAc/AAm composite hydrogel and CO 2 pre-saturated 0.5FA-AAc/AAm composite hydrogel are shown in Figure 4a-c. Combination evaporation of 30 wt% MEA which contain 70% of water was observed at the temperature <250 • C for all samples. From the TGA curves, more weight loss was recorded by 0.5FA-AAc/AAm composite hydrogel at the first stage of decomposition (temperature <250 • C) which shows that the incorporation of FA in the hydrogel can Gels 2023, 9, 229 6 of 16 promote the absorption of amine. In Figure 4c, clear DTG peaks correspond to loss of water and MEA can be seen at 120 • C and 220 • C. While the desorption of CO 2 occurred at 160 • C. A similar curve was also reported by Xu et al. [11]. The DTG curves show that the degradation of the polymer backbone or referred as hydrogel composite occurred at 380 to 420 • C, suggesting the hydrogels were thermally stable up until 380 • C. Base on the results, the hydrogel adsorbent can be regenerated until certain temperature and the water, MEA, and CO 2 can be regenerated. Gels 2023, 9, x FOR PEER REVIEW 6 of 16 the absorption of amine. In Figure 4c, clear DTG peaks correspond to loss of water and MEA can be seen at 120 °C and 220 °C. While the desorption of CO2 occurred at 160 °C. A similar curve was also reported by Xu et al. [11]. The DTG curves show that the degradation of the polymer backbone or referred as hydrogel composite occurred at 380 to 420 °C, suggesting the hydrogels were thermally stable up until 380 °C. Base on the results, the hydrogel adsorbent can be regenerated until certain temperature and the water, MEA, and CO2 can be regenerated.  Figure 5 presents the kinetics data and the fitting plots for pseudo-first order, pseudosecond order, Weber-Morris, Alovich and Avrami fractional adsorption kinetic model and Table 2 tabulates the obtained kinetics form the models. The fitting calculation was carried out on the cumulative adsorption of the sample 30 wt% MEA infused 0.5FA-AAc/AAm which was performed at reaction temperature of 30 °C, CO2 gauge pressure of 2 bar and gas flow rate of 60 mL/min. The experimental and fitting results are shown in Figure 5.

Carbon Dioxide Adsorption Kinetics
It can be seen that from Table 2, the correlation coefficients, R 2 of the pseudo-second order and Elovich kinetic model are 0.9905 and 0.9644, respectively. Meanwhile, the R 2 of the Weber-Morris kinetic model is 0.8687. These kinetic models show relatively low R 2 compared to pseudo-first order and Avrami kinetic model, so it is not fit to explain the kinetic behavior of 30 wt% MEA infused 0.5FA-AAc/AAm hydrogel. The Avrami kinetic model demonstrated a very close degree of correlation with the pseudo-first order kinetic model, as evidenced by R 2 values of 0.9996 and 0.9999, respectively. However, on top of R 2 analysis, other statistical indicators that represent residual error like SSE must be analyzed to validate the modelling [40]. Thus, upon analysis of the sum of squared estimate errors (SSE), it was determined that the pseudo-first order model was the most appropriate for describing the adsorption kinetics of CO2 by a 30 wt% MEA-infused 0.5FA-AAc/AAm hydrogel. From this kinetic model, it can be said that the adsorption of CO2 is a reversible process. Assuming there was no dissociation of CO2 happened on the surface of 30 wt% MEA-  Figure 5 presents the kinetics data and the fitting plots for pseudo-first order, pseudosecond order, Weber-Morris, Alovich and Avrami fractional adsorption kinetic model and Table 2 tabulates the obtained kinetics form the models. The fitting calculation was carried out on the cumulative adsorption of the sample 30 wt% MEA infused 0.5FA-AAc/AAm which was performed at reaction temperature of 30 • C, CO 2 gauge pressure of 2 bar and gas flow rate of 60 mL/min. The experimental and fitting results are shown in Figure 5.

Carbon Dioxide Adsorption Kinetics
It can be seen that from Table 2, the correlation coefficients, R 2 of the pseudo-second order and Elovich kinetic model are 0.9905 and 0.9644, respectively. Meanwhile, the R 2 of the Weber-Morris kinetic model is 0.8687. These kinetic models show relatively low R 2 compared to pseudo-first order and Avrami kinetic model, so it is not fit to explain the kinetic behavior of 30 wt% MEA infused 0.5FA-AAc/AAm hydrogel. The Avrami kinetic model demonstrated a very close degree of correlation with the pseudo-first order kinetic model, as evidenced by R 2 values of 0.9996 and 0.9999, respectively. However, on top of R 2 analysis, other statistical indicators that represent residual error like SSE must be analyzed to validate the modelling [40]. Thus, upon analysis of the sum of squared estimate errors (SSE), it was determined that the pseudo-first order model was the most appropriate for describing the adsorption kinetics of CO 2 by a 30 wt% MEA-infused 0.5FA-AAc/AAm hydrogel. fusion-controlled process. Besides, if there are no adsorbate initially present on the bent, the rate of change of CO2 uptake by the 30 wt% MEA-infused 0.5FA-AAc/A drogel with time is directly proportional to the difference in saturation concentrat the amount of CO2 uptake with time which is generally applicable over the initial an adsorption process. The pseudo-first order was used to further investigate the tion kinetic under different conditions.    Alekseeva et al. (2020) also demon a similar trend where the adsorption kinetic rate increased as the FA content inc despite a decline in adsorption capacity [41].
FA is an aluminosilicate mineral, which means it consists primarily of silica, num, iron and calcium oxide [42] and has the capability to adsorb CO2 [20]. In g  From this kinetic model, it can be said that the adsorption of CO 2 is a reversible process. Assuming there was no dissociation of CO 2 happened on the surface of 30 wt% MEA-infused 0.5FA-AAc/AAm hydrogel, the sorption phenomenon can be described as the diffusion-controlled process. Besides, if there are no adsorbate initially present on the adsorbent, the rate of change of CO 2 uptake by the 30 wt% MEA-infused 0.5FA-AAc/AAm hydrogel with time is directly proportional to the difference in saturation concentration and the amount of CO 2 uptake with time which is generally applicable over the initial stage of an adsorption process. The pseudo-first order was used to further investigate the adsorption kinetic under different conditions.

Effect of FA Content
In order to study the influence of FA content on the CO 2 adsorption kinetics, the adsorption reaction experiment was carried out under the condition of reaction temperature of 30 • C, CO 2 flow rate of 60 mL/min and FA content of 0 to 2.0 wt%. The experimental and pseudo-first order fitting results are shown in Figure 6a trend where the adsorption kinetic rate increased as the FA content increased, despite a decline in adsorption capacity [41].
incorporating a nanocomposite or filler into a polymer adsorbent increases porosity c pared to the neat polymer. But, in this instance, only 0.5 wt% concentration of FA proved the adsorption capacity in this situation. A higher FA concentration led to duction in CO2 capture. According to Zhu et al. (2019), once the hydrogel has been vated and swollen, the cavities and active sites produced in the polymer network by solidified FA particle would increase the number of adsorption points [43]. The SiO2 in promoting the hydrogel to form a hydrogen bond with other monomers [35,44]. Ex sive FA induces more crosslinked points, thus limiting the hydrogel's porosity.

Effect of AAc/AAm Hydrogel Activator
Hydrogel is a versatile material that exhibits unique properties such as shrink swelling, and the ability to retain large amounts of liquids. The activation of the hydro through the infusion of a liquid solution, causes the material to swell and expand. swelling results in the creation of additional void spaces within the hydrogel matrix imbuing it with additional properties based on the properties of infused liquid [43]. swelling properties of the hydrogel are highly affected by the ionic strength, pH and perature. It is important to investigate the most effective hydrogel activator in orde maximize the adsorption of CO2. 0.5FA-AAc/AAm hydrogel exhibited 12% swelling centage in 100 wt% MEA. Contrarily, a significant amount of water (800%) and 30 MEA solution (1150%) were able to be infused in the 0.5FA-AAc/AAm hydrogel mat as presented in Figure 7. This implies that a smaller amount of hydrogel is require support the same amount of pure and 30 wt% amine solution and the utilization of FA is an aluminosilicate mineral, which means it consists primarily of silica, aluminum, iron and calcium oxide [42] and has the capability to adsorb CO 2 [20]. In general, incorporating a nanocomposite or filler into a polymer adsorbent increases porosity compared to the neat polymer. But, in this instance, only 0.5 wt% concentration of FA improved the adsorption capacity in this situation. A higher FA concentration led to a reduction in CO 2 capture. According to Zhu et al. (2019), once the hydrogel has been activated and swollen, the cavities and active sites produced in the polymer network by the solidified FA particle would increase the number of adsorption points [43]. The SiO 2 aids in promoting the hydrogel to form a hydrogen bond with other monomers [35,44]. Excessive FA induces more crosslinked points, thus limiting the hydrogel's porosity.

Effect of AAc/AAm Hydrogel Activator
Hydrogel is a versatile material that exhibits unique properties such as shrinkage, swelling, and the ability to retain large amounts of liquids. The activation of the hydrogel, through the infusion of a liquid solution, causes the material to swell and expand. This swelling results in the creation of additional void spaces within the hydrogel matrix and imbuing it with additional properties based on the properties of infused liquid [43]. The swelling properties of the hydrogel are highly affected by the ionic strength, pH and temperature. It is important to investigate the most effective hydrogel activator in order to maximize the adsorption of CO 2 . 0.5FA-AAc/AAm hydrogel exhibited 12% swelling percentage in 100 wt% MEA. Contrarily, a significant amount of water (800%) and 30 wt% MEA solution (1150%) were able to be infused in the 0.5FA-AAc/AAm hydrogel matrices as presented in Figure 7. This implies that a smaller amount of hydrogel is required to support the same amount of pure and 30 wt% amine solution and the utilization of dry hydrogel also can be minimized during adsorption process. The water and MEA solution, which had been infused into the hydrogel matrix, served as the activating agent for the CO 2 capture process, effectively adsorbing CO 2 from the surrounding environment. hydrogel also can be minimized during adsorption process. The water and MEA solution, which had been infused into the hydrogel matrix, served as the activating agent for the CO2 capture process, effectively adsorbing CO2 from the surrounding environment. As shown in Figure 8a,b, at temperature of 30 °C, pressure of 2 bar and flowrate of 60 mL/min, the CO2 adsorption capacity of water infused 0.5FA-AAc/AAm hydrogel was 0.51 mol/g. A further analysis of the kinetic model revealed that the pseudo-first order kinetic rate increases in the following order: 30 wt% MEA < water < 100 wt% MEA, with values of 0.76 min −1 , 0.96 min −1 , and 0.98 min −1 , respectively, as demonstrated in Figure 8c and summarized in Table 3.
Water exhibited a comparable adsorption capacity to the 100 wt% MEA infused hydrogel. 1 mol of diffused CO2 can react with 1 mol of hydroxyl groups to form carbonic acid (shown in Equation (1)), and 1 mol of amine (C2H7NO) can only react with 0.5 mol of CO2 (shown in Equation (2)), but the nitrogen atom in amine molecules is more electronegative and attractive towards CO2. Despite the high adsorption of water creating more void within the hydrogel adsorbent, the use of amine as the activator was found to be more effective in adsorbing CO2, supported by the slightly higher kinetic rate of the 100 wt% MEA-infused hydrogel. Thus, it is reasonable to assume that pure amine infused hydrogel would be more efficient in capturing CO2 than one infused with water.
Meanwhile, the adsorption capacity shows a significant increment with the use of 30 wt% of MEA solution. It also can be inferred that an increase in the concentration of MEA leads to an increase in the kinetic rate, indicating a faster reaction with CO2. However, with the presence of water in amine solution, 1 mol of MEA now can react with 1 mol of CO2 and form bicarbonate, as shown in Equation (3)  As shown in Figure 8a,b, at temperature of 30 • C, pressure of 2 bar and flowrate of 60 mL/min, the CO 2 adsorption capacity of water infused 0.5FA-AAc/AAm hydrogel was 0.51 mol/g. A further analysis of the kinetic model revealed that the pseudo-first order kinetic rate increases in the following order: 30 wt% MEA < water < 100 wt% MEA, with values of 0.76 min −1 , 0.96 min −1 , and 0.98 min −1 , respectively, as demonstrated in Figure 8c and summarized in Table 3.

Effect of Operational Temperature
A series of adsorption experiments were investigated in fixed bed column at 30, 45 and 60 °C. Figure 9 shows the breakthrough curve and adsorption capacity of 0.5FA-AAc/AAm at three different operational temperature. It shows that the higher adsorption   (1)), and 1 mol of amine (C 2 H 7 NO) can only react with 0.5 mol of CO 2 (shown in Equation (2)), but the nitrogen atom in amine molecules is more electronegative and attractive towards CO 2 . Despite the high adsorption of water creating more void within the hydrogel adsorbent, the use of amine as the activator was found to be more effective in adsorbing CO 2 , supported by the slightly higher kinetic rate of the 100 wt% MEA-infused hydrogel. Thus, it is reasonable to assume that pure amine infused hydrogel would be more efficient in capturing CO 2 than one infused with water.
Meanwhile, the adsorption capacity shows a significant increment with the use of 30 wt% of MEA solution. It also can be inferred that an increase in the concentration of MEA leads to an increase in the kinetic rate, indicating a faster reaction with CO 2 . However, with the presence of water in amine solution, 1 mol of MEA now can react with 1 mol of CO 2 and form bicarbonate, as shown in Equation (3)

Effect of Operational Temperature
A series of adsorption experiments were investigated in fixed bed column at 30, 45 and 60 • C. Figure 9 shows the breakthrough curve and adsorption capacity of 0.5FA-AAc/AAm at three different operational temperature. It shows that the higher adsorption temperature, the shorter the break curve and saturation time. At adsorption temperature of 30 • C, the CO 2 adsorption amount of 0.5FA-AAc/AAm was 0.71 mol/g. However, it decreased to 0.53 and 0.49 mol/g at 45 and 60 • C, respectively. From the Table 3, it can see that the kinetic constant, k 1 increases from 0.76 min −1 to 1.14 min −1 as the adsorption temperature increase from 30 • C to 60 • C, indicating that the increase in temperature promotes the reaction rate. In general, as temperature increases, the kinetic energy of gas particles also increases, leading to more frequent collisions between gas and adsorbent molecules and resulting in a higher adsorption capacity. Conversely, the reaction rate and CO 2 equilibrium adsorption capacity were inversely proportional to each other. At temperatures above 60 • C, thermodynamic effect become more significant in controlling the reaction compared to kinetic effect, and the carbamate molecules begin to reverse back to CO 2 as shows in Equation (14), resulted the decrease in equilibrium adsorption capacity [5,25]. the reaction rate. In general, as temperature increases, the kinetic energy of gas particles also increases, leading to more frequent collisions between gas and adsorbent molecules and resulting in a higher adsorption capacity. Conversely, the reaction rate and CO2 equilibrium adsorption capacity were inversely proportional to each other. At temperatures above 60 °C, thermodynamic effect become more significant in controlling the reaction compared to kinetic effect, and the carbamate molecules begin to reverse back to CO2 as shows in Equation (14), resulted the decrease in equilibrium adsorption capacity [5,25].  The CO 2 adsorption of FA-AAc/AAm hydrogel in this study varied from 0.39 to 0.71 mol/g depending on the FA content, type of hydrogel activator and operational temperature. It is noted that the use of different amine, starting monomer, filler or polymerization technique will give different properties to the adsorbent. For instance, MEA-hydrogel with the dodecyltrimethylammonium bromide (DTAB) surfactant showed an adsorption of 2.96 mmol/g [8]. Meanwhile, silica that functionalize with 3-aminopropyltriethoxysilane (APTES) via wet grafting method able to capture CO 2 up to 1.67 mmol/g [45]. Another example of amine infused adsorbent is APTES functionalized aluminosilicate; which came from metakaolin and silica fume. The study successfully absorbed 1.17 mmol/g of CO 2 and the adsorbent also exhibited low adsorption and desorption energy of 14.47 and 51.05 kJ/mol [46]. The concept of incorporating amine in solid adsorbent has been established for some time, but the search for the best adsorbent in term of sorption capacity, industrial pliability and sustainable for a long run should be continue with more material to be explored.

Conclusions
The FA-AAc/AAm hydrogel was synthesized via in-situ solution polymerization and hydrogel activator was infused by direct impregnation method in which the hydrogel was immersed in the water or MEA solution for 24 h. The morphology of the hydrogel featured less visible pores but demonstrated a high capacity for hydrogel activator absorption. It is suggested that the amine absorption properties of the hydrogel are also influenced by the functional groups present. The incorporation of fly ash into AIHs was found to effectively improve CO 2 adsorption. At a temperature of 30 • C and pressure of 2 bar, the use of 0.5 wt% of FA increased CO 2 adsorption by 54% compared to neat AIHs. Data from this study were best simulated by the pseudo-first order > Avrami fractional > pseudo-second order > Elovich > Weber-Morris kinetic models, as supported by the SSE value. It can be inferred that the rate of change in CO 2 uptake with respect to time is directly proportional to the difference in saturation concentration and the amount of solid uptake. However, the Avrami fractional kinetic model cannot be completely disregarded due to only 0.99% difference in SSE between both kinetic models. Although the hydrogel material is stable at high temperature up to 380 • C, the breakthroughs analysis suggest that the adsorption of CO 2 is not favorable at high temperatures. As a result, to optimize the adsorption capacity of the amine infused FA-AAc/AAm hydrogel, it is recommended to lower the flue gas temperature prior to the adsorption. Further optimization studies, such as varying the FA composition, FA particle size, type of amine solution, operational pressure, and increasing Gels 2023, 9, 229 12 of 16 temperature, are also recommended in order to maximize CO 2 capture. With further exploration of these remaining factors, this easily synthesized AIHs has the potential to become a commercially viable adsorbent for CO 2 capture.

Materials
Acrylamide (analytical reagent grade) and acrylic acid (99% purity) were supplied by Merck. Ammonium peroxodisulphate (analytical reagent grade) was supplied by R&M Chemicals. N,N'-methylenebis(acrylamide) (99% purity), was purchased from Sigma-Aldrich. A synthesis-grade of monomers and cross-linker were used as-received without further purification. Distilled water was used throughout this research work as the polymerization medium. FA was obtained from a coal power plant located in Northern Malaysia.

Preparation of MEA Infused FA-AAc/AAm Hydrogel Adsorbent
The amine infused hydrogel was synthesized by solution polymerization using common monomers, acrylic acid (AAc) and acrylamide (AAm). The schematic illustration of synthesis process and CO 2 adsorption is shown in Figure 10. The synthesis process started by diluting 5 g of AAm into 100 mL distilled water. The solution was then combined with sieved FA ranging from 0.5 wt% to 2 wt%. The stirring continued for another 20 min with temperature maintained at 60 • C. Then, 0.5 g of AAc and 0.05g of N,N"-methylene bisacrylamide (NMBA) as cross-linking agent were introduced to the solution while the mixture was continuously stirred. The initiator, 0.25 g of ammonium peroxodisulphate (APS) was added after 30 min. The FA-AAc/AAm hydrogel was dried at 55 • C for 8 h in the vacuum oven before it was infused with activator namely water, 100 wt% monoethanolamine (MEA) and 30 wt% MEA solution.

Materials
Acrylamide (analytical reagent grade) and acrylic acid (99% purity) were supplied by Merck. Ammonium peroxodisulphate (analytical reagent grade) was supplied by R&M Chemicals. N,N'-methylenebis(acrylamide) (99% purity), was purchased from Sigma-Aldrich. A synthesis-grade of monomers and cross-linker were used as-received without further purification. Distilled water was used throughout this research work as the polymerization medium. FA was obtained from a coal power plant located in Northern Malaysia.

Preparation of MEA Infused FA-AAc/AAm Hydrogel Adsorbent
The amine infused hydrogel was synthesized by solution polymerization using common monomers, acrylic acid (AAc) and acrylamide (AAm). The schematic illustration of synthesis process and CO2 adsorption is shown in Figure 10. The synthesis process started by diluting 5 g of AAm into 100 mL distilled water. The solution was then combined with sieved FA ranging from 0.5 wt% to 2 wt%. The stirring continued for another 20 min with temperature maintained at 60 °C. Then, 0.5 g of AAc and 0.05g of N,N"-methylene bisacrylamide (NMBA) as cross-linking agent were introduced to the solution while the mixture was continuously stirred. The initiator, 0.25 g of ammonium peroxodisulphate (APS) was added after 30 min. The FA-AAc/AAm hydrogel was dried at 55 °C for 8 h in the vacuum oven before it was infused with activator namely water, 100 wt% monoethanolamine (MEA) and 30 wt% MEA solution.

Characterization
The elemental composition of raw FA was analyzed by energy dispersive XRF (X-ray fluorescence spectrometry, Shimadzu XRF-1700, Tokyo, Japan) according to BS EN ISO 12677:2011 method. The sampling of FA was conducted by taking the FA from five different sampling points. The FA was ground using a planetary ball mill to obtain the particle size of less than 100 µ m prior to mounting into the sample holder.
Malvern Instrument Mastersizer 3000 was used to determine the particle size distribution (PSD) of FA before and after the ball-mill grinding. FA samples were analyzed in triplicates and an average result recorded. The particle refractive index and adsorption index was set at 1.6 and 1.0, respectively.
The functional groups of raw FA and the effect of incorporation of FA on the hydrogel matrices was determined using FTIR spectra model Perkin-Elmer Spectrum One, FTIR-frontier. The FTIR spectra was determined at wavenumber ranges, scan number per sample and wave resolution of 4000-500 cm −1 , 16 and 4 cm −1 , respectively. The hydrogel samples were prepared using vacuum oven-dried for 5 h at 50 °C. The pellets for FTIR analysis were prepared from a uniform mixture of hydrogel and potassium bromide (KBR) using a steel die. KBr was used as the sample carrier because its optically transparent properties would not impose any absorbance interference to the IR ray.

Characterization
The elemental composition of raw FA was analyzed by energy dispersive XRF (X-ray fluorescence spectrometry, Shimadzu XRF-1700, Tokyo, Japan) according to BS EN ISO 12677:2011 method. The sampling of FA was conducted by taking the FA from five different sampling points. The FA was ground using a planetary ball mill to obtain the particle size of less than 100 µm prior to mounting into the sample holder.
Malvern Instrument Mastersizer 3000 was used to determine the particle size distribution (PSD) of FA before and after the ball-mill grinding. FA samples were analyzed in triplicates and an average result recorded. The particle refractive index and adsorption index was set at 1.6 and 1.0, respectively.
The functional groups of raw FA and the effect of incorporation of FA on the hydrogel matrices was determined using FTIR spectra model Perkin-Elmer Spectrum One, FTIRfrontier. The FTIR spectra was determined at wavenumber ranges, scan number per sample and wave resolution of 4000-500 cm −1 , 16 and 4 cm −1 , respectively. The hydrogel samples were prepared using vacuum oven-dried for 5 h at 50 • C. The pellets for FTIR analysis were prepared from a uniform mixture of hydrogel and potassium bromide (KBR) using a steel die. KBr was used as the sample carrier because its optically transparent properties would not impose any absorbance interference to the IR ray. The microstructure of hydrogel samples was determined using Scanning electron microscopy (SEM, Zeiss Evo LS15, Munich, Germany) with an energy of 10 keV at various magnification from ×60 to ×5000. The hydrogels samples were dried using a freeze dryer for 48 h to preserve the hydrogel structure. The samples were coated with a thin layer of gold to add the conductivity into the insulted hydrogel surface before analysis.
The AAc/AAm, 0.5FA-AAc/AAm and CO 2 loaded 0.5FA-AAc/AAm samples were put into the sample pan of a Perkin Ekmer STA6000 thermal gravimetric analyzer. Waltham, MA USA The chamber was then purged with nitrogen gas before it was heated from 20 to 600 • C at a heating rate of 10 • C/min. The system temperature, sample mass and mass derivative rate as a function of time were recorded throughout the heating process.

CO 2 Adsorption Analysis
To study the influencing factors of FA-AAc/AAm hydrogel on the CO 2 adsorption performance, the experiments were conducted using 10 mm inner diameter and 100 mm high adsorption column. The setup of the self-assembled CO 2 adsorption column is shown in Figure 11. Firstly, 1 g of MEA infused FA-AAc/AAm was packed into the stainlesssteel column supported with glass wool on both ends. Then, the operational pressure was regulated to 2 bar and the temperature set to 30, 45 and 60 • C, depending on the predetermined experimental parameter. After that, the CO 2 was passed into the adsorption column at a flow rate of 60 mL/min and the outlet CO 2 concentration was automatically analyzed and recorded by the computer. The adsorption capacity of CO 2 was calculated by the follow equation: where, Q is the adsorption capacity (mol/g), F is inlet molar flow rate (mol/min), C in and C eff represent the inlet and outlet concentration and m stands for the mass of adsorbent (g). The microstructure of hydrogel samples was determined using Scanning electron microscopy (SEM, Zeiss Evo LS15, Munich, Germany) with an energy of 10keV at various magnification from X60 to X5000. The hydrogels samples were dried using a freeze dryer for 48 h to preserve the hydrogel structure. The samples were coated with a thin layer of gold to add the conductivity into the insulted hydrogel surface before analysis.
The AAc/AAm, 0.5FA-AAc/AAm and CO2 loaded 0.5FA-AAc/AAm samples were put into the sample pan of a Perkin Ekmer STA6000 thermal gravimetric analyzer. Waltham, MA USA The chamber was then purged with nitrogen gas before it was heated from 20 to 600 °C at a heating rate of 10 °C/min. The system temperature, sample mass and mass derivative rate as a function of time were recorded throughout the heating process.

CO2 Adsorption Analysis
To study the influencing factors of FA-AAc/AAm hydrogel on the CO2 adsorption performance, the experiments were conducted using 10 mm inner diameter and 100 mm high adsorption column. The setup of the self-assembled CO2 adsorption column is shown in Figure 11. Firstly, 1 g of MEA infused FA-AAc/AAm was packed into the stainless-steel column supported with glass wool on both ends. Then, the operational pressure was regulated to 2 bar and the temperature set to 30, 45 and 60 °C, depending on the predetermined experimental parameter. After that, the CO2 was passed into the adsorption column at a flow rate of 60 mL/min and the outlet CO2 concentration was automatically analyzed and recorded by the computer. The adsorption capacity of CO2 was calculated by the follow equation: where, Q is the adsorption capacity (mol/g), F is inlet molar flow rate (mol/min), Cin and Ceff represent the inlet and outlet concentration and m stands for the mass of adsorbent (g).

Adsorption Kinetic
The adsorption kinetic models were used to analyze the performance of the adsorbent and to predict the adsorption mass transfer mechanism. Several models are available to describe the adsorption of adsorbate on the adsorbent. Table 4 lists five these models and their corresponding linear expressions. One commonly used model is the pseudofirst order kinetic model, which is often employed to study the gas-solid adsorption processes that are controlled by surface diffusion [25]. The pseudo-second order kinetic model assumes that the rate of adsorption is limited by chemical sorption or chemisorption. The k1 in Equations (5) and (6) represents the rate constant, meanwhile k2 in Equations (7) and (8) denotes the pseudo-second order rate constant.
The Weber-Morris kinetic model or also known as intra-particle diffusion model suggest that the rate of adsorption is governed by the availability of active sites on the

Adsorption Kinetic
The adsorption kinetic models were used to analyze the performance of the adsorbent and to predict the adsorption mass transfer mechanism. Several models are available to describe the adsorption of adsorbate on the adsorbent. Table 4 lists five these models and their corresponding linear expressions. One commonly used model is the pseudo-first order kinetic model, which is often employed to study the gas-solid adsorption processes that are controlled by surface diffusion [25]. The pseudo-second order kinetic model assumes that the rate of adsorption is limited by chemical sorption or chemisorption. The k 1 in Equations (5) and (6) represents the rate constant, meanwhile k 2 in Equations (7) and (8) denotes the pseudo-second order rate constant. Table 4. Adsorption kinetic models in linear and non-linear expressions.
The Weber-Morris kinetic model or also known as intra-particle diffusion model suggest that the rate of adsorption is governed by the availability of active sites on the adsorbent surface rather than concentration of adsorbate. Referring to Equations (9) and (10), k id represents the kinetic rate constant. The Elovich kinetic model was initially applied in gaseous systems to predict the mass and surface diffusion, activation and deactivation energy of the system. Referring to Equations (11) and (12), α is initial adsorption rate and β is desorption constant. The Avrami fractional kinetic model is a fractional kinetic with order one for particle nucleation and applicable for describing the adsorption of multicomponent gases by solid adsorbent [25]. In Equations (13) and (14), k a is the Avrami rate constant, and n is the Avrami exponent which use to reflect the dimensionality of crystal growth [47]. The CO 2 adsorption capacity data of 30 wt% MEA infused 0.5FA-AAc/AAm hydrogel was used to fit into all these models and determine the most suitable kinetic model to explain the kinetic behavior of CO 2 adsorption of MEA infused hydrogel adsorbent.