Amine-Modiﬁed Biochar for the Efﬁcient Adsorption of Carbon Dioxide in Flue Gas

: Biochar-based carbonaceous adsorbents are gaining interest due to their high availability, ease of modiﬁcation, and low cost; however, they show limited adsorption of CO 2 in ﬂue gas due to common textural properties. In this study, TEPA-modiﬁed biochar was used to prepare a solid amine adsorbent for the efﬁcient capture of CO 2 in ﬂue gas. First, the porous biochar was prepared with FeCl 3 , Mg(NO 3 ) 2 , and H 2 O (g) as activators and walnut shells as carbon sources. Next, the biochar was modiﬁed with TEPA to obtain a solid amine adsorbent. Porous texture properties and sample surface functional groups were characterized, and we measured the adsorption CO 2 of the amine-modiﬁed biochar in a breakthrough adsorption device. Results showed that biochar has a large speciﬁc surface area (744.38 m 2 g − 1 ), a total pore volume of 1.41 cm 3 g − 1 , and a high mesoporous volume ratio (82.7%). The high pore volume provided a more efﬁcient support space for loading tetraethylenepentamine (TEPA). The adsorbent had an excellent CO 2 adsorption capacity, corresponding to 2.82 mmol g − 1 , which increased to 3.31 mmol g − 1 and kept water resistance at 10% H 2 O (g) simulated ﬂue gas (SFG). The FTIR analysis showed that H 2 O (g) inhibited urea production after cyclic adsorption. Therefore, solid amine adsorbent created by amine-modiﬁed biochar has potential advantages in its application for capturing CO 2 in SFG.


Introduction
CO 2 emissions caused by fossil energy are important causes of global warming and the greenhouse effect [1]. Adsorption is the most promising method for reducing CO 2 emissions from SFG because of its low corrosiveness and low energy consumption. At the same time, it does not require significant modifications to the considerable existing equipment [2][3][4]. The excellent adsorbent is the core of efficient CO 2 adsorption. Solid amine adsorbents remove CO 2 from SFG because of their multiple active sites, high adsorption capacity, broad application temperature range, and eco-friendliness [5][6][7][8]. The carrier is the core and essential for synthesizing the solid amine adsorbent. Commonly, carriers can be divided into noncarbon and carbon materials [7]. Noncarbon carriers include alkali-metal materials, metal oxides, metal-organic frameworks (MOFs), and molecular sieves [5][6][7]. Carbon material carriers include graphene oxide (GO), activated carbon fibers, carbon nanotubes, and ordered mesoporous carbon. Although these materials have excellent pore structure parameters, complex preparation methods and high production cost limit their large-scale production.
Compared with the abovementioned carriers, biochar has been extensively used in various fields because of its broad availability, low cost, well-strung pores, and ample surface Note: a indicates the use of differential subtraction; Ad: air-dried basis. M: moisture; A: ash; V: volatile; FC: fixed-carbon.

Synthesis of the Amine-Functionalized Sorbent 2.2.1. Preparation of Biochar
First, walnut shells were rinsed with water and dried at 100 • C. Then, they were crushed and sieved to form 50-100 mesh-sized particles. FeCl 3 ·6H 2 O (3 g) and Mg(NO 3 ) 2 ·6H 2 O (6 g) were placed in 500 mL of deionized water and stirred until the solution was dispersed; 10 g of biomass as the carbon source was then added to the metal salts solution for impregnation, and the solution was then evaporated to dryness by stirring at 150 • C, and 400 r min −1 . In a tube furnace, one-step pyrolysis and carbonization were performed in an inert atmosphere (H 2 O 0.1 mL min −1 , N 2 flow 200 mL min −1 ) at 900 • C for 100 min. After carbonization, H 2 O (g) was switched to pure N 2 and cooled to a low temperature. The carbonization biochar was washed with 0.3 M HCl for 6 h to remove the metal oxides and a small amount of ash and then repeatedly washed using distilled water to neutralize it. Finally, the biochar was obtained by drying. Samples were named RBC (without activation), HBC (H 2 O (g) activated), FMBC (FeCl 3 , and Mg(NO 3 ) 2 coactivated) and CBC (FeCl 3 , Mg(NO 3 ) 2 , and H 2 O (g) coactivated).

Preparation of Amine-Functionalized Biochar
TEPA was uniformly dispersed in anhydrous ethanol after the addition of the CBC. The solution was stirred with a heating plate at 35 • C and 200 r min −1 until it was completely evaporated. Finally, samples were dried (80 • C dry for 10 h under vacuum) to obtain the TEPA-loaded adsorbent. Samples were named CBC-X-TEPA, where X (X = 20, 30, 40, 50, and 60) refers to the mass fraction of TEPA.

Characterization
The ultimate analysis of samples was characterized by the elemental analyzer (EA 3000, Euro Vector, Pavia, Italy). The textural properties of the samples were measured using the method of low-temperature N 2 adsorption-desorption with a rapid surface area and porosity analyzer (Autosorb-iQ, Quantachrome Instruments, Boynton Beach, FL, USA). The morphology features of samples were observed using scanning electron microscopy (SEM Quanta 400, FEI company, Hillsboro, OR, USA). The crystalline phase of samples was tested by X-ray diffractometer (XRD, Bruker, Germany) at Cu radiation, 40 kV, 40 mA, and 2θ range of 3 • -85 • . The samples graphitization was analyzed by Raman spectra (DXR2xi Raman, Thermo Fisher Scientific, Waltham, Massachusetts, U.S. wavelength range from 500 to 3500 cm −1 ). The functional groups on the surface of the samples were analyzed by using FTIR (TENSOR II, Bruker, Germany). Thermogravimetric analysis (TGA STA449 F3, NETZSCH-Gerätebau GmbH, Selb, Germany) was employed to analyze the thermal stability of samples.

CO 2 Adsorption
CO 2 sorption on adsorbent was performed in a breakthrough adsorption setup that is shown in Figure 1. The adsorption column (with length 40 cm and inner diameter 1 cm) was filled with adsorbent 1 g. In SFG (15% CO 2 /85% N 2 ), the sorption CO 2 capacity of adsorbent with different TEPA loaded was tested to select the best. Subsequently, CO 2 sorption of the best adsorbent was performed in SFG (15% CO 2 /75% N 2 /10% H 2 O). The Atmosphere 2022, 13, 579 4 of 16 specific steps of the adsorption experiment were as follows: N 2 (30 mL/min) was aerated in the adsorption bed, and the temperature was slowly heated to 105 • C and kept for 1 h. Then, the temperature of the adsorption bed was lowered to a constant temperature of 60 • C; the adsorption experiment was started by introducing SFG into the adsorption bed. When the outlet and inlet gases had the same CO 2 concentration, the experiment was stopped. For the recycle experiment, the absorbent was exposed to SFG (30 mL min −1 ) at 60 • C to adsorb CO 2 . When the adsorption reaction ended, the high-purity N 2 (30 mL min −1 ) was aerated in the adsorption bed, and the temperature was heated to 105 • C and maintained for 1 h. The test was repeated ten times. The adsorption capacity of the adsorbent was calculated using the following equation: where q represents the adsorption capacity of CO 2 , mmol g −1 ; M is the quality of the CBC-X-TEPA, g; t is adsorption time, min. Q is the total SFG flow, cm 3 min −1 ; C 0 and C are the input and output CO 2 concentrations; Vol.%; t is the adsorption time, min; T 0 represents absolute zero, 273 K; T is adsorption temperature, K; and V m is the standard molar volume, 22.4 L mol −1 .
CO2 sorption on adsorbent was performed in a breakthrough adsorption setup that is shown in Figure 1. The adsorption column (with length 40 cm and inner diameter 1 cm) was filled with adsorbent 1 g. In SFG (15% CO2/85% N2), the sorption CO2 capacity of adsorbent with different TEPA loaded was tested to select the best. Subsequently, CO2 sorption of the best adsorbent was performed in SFG (15% CO2/75% N2/10% H2O). The specific steps of the adsorption experiment were as follows: N2 (30 mL/min) was aerated in the adsorption bed, and the temperature was slowly heated to 105 °C and kept for 1 h. Then, the temperature of the adsorption bed was lowered to a constant temperature of 60 °C; the adsorption experiment was started by introducing SFG into the adsorption bed. When the outlet and inlet gases had the same CO2 concentration, the experiment was stopped. For the recycle experiment, the absorbent was exposed to SFG (30 mL min −1 ) at 60 °C to adsorb CO2. When the adsorption reaction ended, the high-purity N2 (30 mL min −1 ) was aerated in the adsorption bed, and the temperature was heated to 105 °C and maintained for 1 h. The test was repeated ten times. The adsorption capacity of the adsorbent was calculated using the following equation: where q represents the adsorption capacity of CO2, mmol g -1 ; M is the quality of the CBC-X-TEPA, g; t is adsorption time, min. Q is the total SFG flow, cm 3 min -1 ; C0 and C are the input and output CO2 concentrations; Vol.%; t is the adsorption time, min; T0 represents absolute zero, 273 K; T is adsorption temperature, K; and Vm is the standard molar volume, 22.4 L mol -1. Amine (TEPA) efficiency was employed to investigate the utilization efficiency of organic amine loaded into biochar and the adsorption capacity of the adsorbent for CO2. The calculation formula of amine efficiency was as follows: where nCO2 is the saturated adsorption capacity of the adsorbent, mmol g −1 ; mamine refers to the quality of amine, g. Amine (TEPA) efficiency was employed to investigate the utilization efficiency of organic amine loaded into biochar and the adsorption capacity of the adsorbent for CO 2 . The calculation formula of amine efficiency was as follows: where nCO 2 is the saturated adsorption capacity of the adsorbent, mmol g −1 ; m amine refers to the quality of amine, g.  (7) and (8) [24]. MgO and Fe 3 O 4 formed the MgO and FeO solid solution during pyrolysis. H 2 O (g) reacted with the surface carbon of biochar generated after biomass pyrolysis and then gradually diffused into the internal pores of biochar as the reaction proceeded, and the reactions formulae of H 2 O (g) and C are shown in Equations (9)-(11) [23].

Biochar Preparation Mechanism
The XRD patterns were employed to investigate the metal-conversion process during the preparation of biochar. The specific reaction is shown in Figure S1. During the initial 10 (12) and (13) [19,25], then they sintered and agglomerated as MgO  (14) and (15) [27]. In addition, the sintered metal oxides separated from each other. The reaction mechanism is shown in Figure 2. Since the metal oxides were distributed on the biochar surface after pyrolysis, they could prevent the H 2 O (g) from entering the pores and hence prevent it from overreacting and collapsing the internal pore. In addition, the carbon peak in the biochar gradually weakened, which is attributed to high temperatures forming a more disordered structure [22].

Sample Characterization
The SEM characterization of samples was carried out to investigate the effect of the preparation method on the structure and morphology. Figure 3a shows the RBC without metal salts, and H 2 O (g) activation exhibited macropores, with an undeveloped pore structure and no lamellar structure. Figure 3b shows results after being coactivated with metal salts and H 2 O (g); the surface of CBC possessed distributed metal particles, which were produced by FeCl 3 and Mg(NO 3 ) 2 . Figure 3c shows results after the removal of these metal particles. The CBC surface showed a stacked lamellar structure with multiple large pores, which was different from what was roughly observed with irregular crakes, plates, and particles reported by Guo et al. [25]. The large pores on the surface were partly produced by the spillage of the volatile fraction during thermal decomposition and partly formed by the removal of large-sized metal particles sintered on the biochar surface. Compared with RBC, the surface structure and morphology of the CBC had undergone tremendous changes, which showed that metal salts and H 2 O (g) had a great influence on the formation of biochar. This could be more conducive to TEPA loading and CO 2 adsorption on the surface of biochar. Figure 3d is an SEM image of the adsorbent that shows the stacked lamellar structure and surface pores on the CBC did not change significantly after the TEPA

Sample Characterization
The SEM characterization of samples was carried out to investigate the effect of the preparation method on the structure and morphology. Figure 3a shows the RBC without metal salts, and H2O (g) activation exhibited macropores, with an undeveloped pore structure and no lamellar structure. Figure 3b shows results after being coactivated with metal salts and H2O (g); the surface of CBC possessed distributed metal particles, which were produced by FeCl3 and Mg(NO3)2. Figure 3c shows results after the removal of these metal particles. The CBC surface showed a stacked lamellar structure with multiple large pores, which was different from what was roughly observed with irregular crakes, plates, and particles reported by Guo et al. [25]. The large pores on the surface were partly produced by the spillage of the volatile fraction during thermal decomposition and partly formed by the removal of large-sized metal particles sintered on the biochar surface. Compared with RBC, the surface structure and morphology of the CBC had undergone tremendous changes, which showed that metal salts and H2O (g) had a great influence on the formation of biochar. This could be more conducive to TEPA loading and CO2 adsorption on the surface of biochar. Figure 3d is an SEM image of the adsorbent that shows the stacked lamellar structure and surface pores on the CBC did not change significantly after the TEPA loading. The presence of pores of different sizes on the adsorbent surface could provide large passages, accelerating CO2 diffusion into the internal part of the adsorbent.  XRD is one of the best techniques to characterize the effect of the preparation method on the crystalline biochar phase. The XRD patterns of samples are shown in Figure 4a. The RBC and HBC had broad and weak diffraction peaks at 2θ = 25.8° and 42.8°, corresponding to the graphite (002) and (100) crystal face diffraction of amorphous carbon [28][29][30][31]. The intensity of the carbon peak gradually strengthened for FMBC (compared with RBC) after being added to FeCl3 and Mg(NO3)2. This confirmed that the addition of metal salts XRD is one of the best techniques to characterize the effect of the preparation method on the crystalline biochar phase. The XRD patterns of samples are shown in Figure 4a. The RBC and HBC had broad and weak diffraction peaks at 2θ = 25.8 • and 42.8 • , corresponding to the graphite (002) and (100) crystal face diffraction of amorphous carbon [28][29][30][31]. The intensity of the carbon peak gradually strengthened for FMBC (compared with RBC) after being added to FeCl 3 and Mg(NO 3 ) 2 . This confirmed that the addition of metal salts formed a large and orderly crystal structure. CBC had a more intense carbon peak than FMBC, which was attributed to FeO being continuously oxidized to Fe 3 O 4 by H 2 O (g), making the reaction between carbon and Fe 3 O 4 continue. At the same time, it increased the degree of graphitization and the thermal conductivity of biochar and sped up the CO 2 adsorption and desorption processes of biochar. The XRD peak intensity of carbon weakened for CBC-50-TEPA (compared with CBC). This indicated TEPA was impregnated into the pores of CBC. In addition, TEPA impregnation induced a structure in which the edges or sides of a stacked, layered, and connected structure were destroyed so that the XRD pattern peak intensity weakened [30]. After TEPA incorporation, there was a bulging peak in front of the (002) crystal face peak, and the half peak width widened. This confirmed that the amine impregnation prevented the restacking of a part of the CBC's lamellar structure [31].
Atmosphere 2022, 13, x FOR PEER REVIEW 8 of 17 The double resonance transition of two phonons of carbon atoms with opposite momentum produced 2D peaks, reflecting the number of layers of carbon [33]. The Raman spectra of CBC-50-TEPA were similar to that of the CBC, indicating that the structure was unchanged after TEPA modification, which agreed with SEM results. The ratio of D to G (ID: IG) is an essential parameter for investigating carbon graphitization.   Figure 5a shows adsorption-desorption isotherms of samples. According to the IU-PAC classification, hysteresis loops appeared in the adsorption-desorption isotherm so that mesopores were present in all samples. Figure 5b shows that the pores of the CBC were mainly mesopores, primarily distributed between 4 and 20 nm, whereas few were micropores. There were primarily micropores and some mesopores regarding the RBC, HBC, and FMBC. The TEPA blocked micropores in the adsorbent w, but some large mesopores were retained.   Figure 4b shows the Raman spectra of the samples. The RBC had a weak G peak at 1570 cm −1 , a D peak at 1340 cm −1 , and no 2D peaks at 2670 cm −1 . The HBC had intensity G and D peaks. However, the 2D peak weakened. FMBC had high-intensity D and G peaks and a 2D peak. CBC spectra had both high-intensity G and D peaks and weak 2D peaks. The symmetrical stretching vibration of the sp 2 carbon atom in the aromatic ring produced the D peak, which represents the purity and defects of the graphite structure [32]. The tensile vibration between the carbon atoms of sp 2 caused the appearance of the G peak. The double resonance transition of two phonons of carbon atoms with opposite momentum produced 2D peaks, reflecting the number of layers of carbon [33]. The Raman spectra of CBC-50-TEPA were similar to that of the CBC, indicating that the structure was unchanged after TEPA modification, which agreed with SEM results. The ratio of D to G (I D : I G ) is an essential parameter for investigating carbon graphitization. The I D : I G of the RBC, HBC, FMBC, CBC, and CBC-50-TEPA were calculated as 2.62, 2.42, 1.86, 1.65, and 1.46, respectively. Figure 5a shows adsorption-desorption isotherms of samples. According to the IUPAC classification, hysteresis loops appeared in the adsorption-desorption isotherm so that mesopores were present in all samples. Figure 5b shows that the pores of the CBC were mainly mesopores, primarily distributed between 4 and 20 nm, whereas few were micropores. There were primarily micropores and some mesopores regarding the RBC, HBC, and FMBC. The TEPA blocked micropores in the adsorbent w, but some large mesopores were retained.  Figure 5a shows adsorption-desorption isotherms of samples. According to the IU-PAC classification, hysteresis loops appeared in the adsorption-desorption isotherm so that mesopores were present in all samples. Figure 5b shows that the pores of the CBC were mainly mesopores, primarily distributed between 4 and 20 nm, whereas few were micropores. There were primarily micropores and some mesopores regarding the RBC, HBC, and FMBC. The TEPA blocked micropores in the adsorbent w, but some large mesopores were retained.  Table 2 lists the specific pore properties of the RBC, HBC, FMBC, CBC, and CBC-50-TEPA. Comparing CBC with other biochars, SBET slightly increased. However, VT, Smeso, and Vmeso significantly increased, which indicated the high efficiency of combining H2O  Table 2 lists the specific pore properties of the RBC, HBC, FMBC, CBC, and CBC-50-TEPA. Comparing CBC with other biochars, S BET slightly increased. However, V T , S meso , and V meso significantly increased, which indicated the high efficiency of combining H 2 O (g) with bimetal salts to change the pore structure. According to Sun et al. [34], the pore channels of biochar collapsed at high temperatures, thereby increasing both the V T and P d of biochar, whereas the corresponding S BET and the number of pores decreased. However, in this study, while effectively increasing the V T and P d of CBC, the S BET was also increased. This was due to the new pores created by metal salts and H 2 O (g), and it removed the Fe 3 O 4 , while MgO particles formed the stacked lamellar structure. The S BET and V T of CBC had a positive effect on the CO 2 adsorption capacity of the solid amine adsorbent, and the S meso and V meso are more vital than other pores [35]. In addition, compared to the reported results [11][12][13][14][15][16][17], when biochar had a large specific surface area, it also had a small pore volume and average pore size. In the process of amine modification, the micropores are easily blocked by organic amine molecules. Therefore, microporous biochar is not suitable for organic amine modification.
Carbon content in CBC was extremely high, whereas the N, H, and O contents were low (Table 3). During pyrolysis, a dehydration reaction reduced the H: C and O: C ratios, and the breakdown of macromolecular organic nutrients into smaller molecules occurred, which spilled over along with volatile fractions and decreased the N content in the CBC at 900 • C [36]. In addition, the N content of CBC-50-TEPA (7.34%) was approximately 28.6 times that of CBC (0.47%), which indicated that a mass of amine was attached to the CBC during functionalization. This further confirmed the effectiveness of TEPA as an ample origin of N (37%) with high amino density. According to the N content of elemental analysis, the actual loading of TEPA was calculated as 35.0% less than 50.0%, which was because, during TEPA immersion, some pores of the CBC were blocked, preventing the amine from continuing to diffuse deeply into the interior. In addition, the O content in the adsorbent increased because of the adsorption of CO 2 and H 2 O by the samples.  The FTIR spectra of CBC and CBC-50-TEPA are shown in Figure S2. Regarding CBC, the peak at 1585 cm −1 was the C=C vibration peak [37,38]. There were no other oxygenand nitrogen-containing groups, indicating low N and O contents, which agreed with the elemental analysis. After TEPA modification, the stretching vibration peak of primary amine (−NH 2 ) appeared at 3292 cm −1 . Peaks at 2821 and 2930 cm −1 were CH 2 stretching vibrations, whereas C-N stretching vibrations were observed at 1308 cm −1 [39,40]. The newly added N groups in the CBC showed that TEPA was successfully loaded. Figure 6 shows the CBC and CBC-50-TEPA TG analyses. Figure 6a shows that the CBC had almost no weight loss in the range of 30-500 • C, which is attributed to the organic matter in the biochar having been volatilized and no organic matter left for decomposition at high temperatures. So, the CBC has excellent thermal stability at high temperatures. There are two weight-loss stages in the curve of the CBC-50-TEPA. The results showed that CBC-50-TEPA only lost 4.7% of its weight within 150 • C. Therefore, it is suitable for CO 2 capture in SFG (50-100 • C). The DTG curves of CBC-50-TEPA and CBC are shown in Figure 6b,c. CBC-50-TEPA has two weight-loss peaks through 38-500 • C and has a wide temperature range of weight loss. CBC-50-TEPA weight loss in the range of 38-105 • C is mainly due to CO 2 and H 2 O heat volatilization, and weight loss within 105-500 • C is caused by pyrolysis of TEPA. CBC DTG curve has one weight loss peak at 95-103 • C. The excellent heat resistance of the biochar and aminated biochar makes it potentially applicable to CO 2 adsorption in SFG. Figure 6b,c. CBC-50-TEPA has two weight-loss peaks through 38-500 °C and has a wide temperature range of weight loss. CBC-50-TEPA weight loss in the range of 38-105 °C is mainly due to CO2 and H2O heat volatilization, and weight loss within 105-500 °C is caused by pyrolysis of TEPA. CBC DTG curve has one weight loss peak at 95-103 °C. The excellent heat resistance of the biochar and aminated biochar makes it potentially applicable to CO2 adsorption in SFG.

CO 2 Adsorption-Desorption
The sorption of CO 2 was performed on a breakthrough adsorption setup. In dry SFG containing 15% CO 2 /N 2 85%, the adsorption capacity of CO 2 was tested for CBC-X-TEPA adsorbent (at 101 kPa, 60 • C). Figure 7a,b shows breakthrough adsorption curves and CO 2 adsorption capacities of the adsorbent, which gradually increased as the TEPA loading increased from 20% to 50%; the adsorption time extended, as shown in Figure 7a, and reached a maximum value of 2.82 mmol g −1 at 50% TEPA, as shown in Figure 7b. The large pore volume of the CBC provided a large space for the loading of TEPA, which could allow a sufficient amount of amine to penetrate the pore and provide more adsorption sites for CO 2 adsorption.

CO2 Adsorption-Desorption
The sorption of CO2 was performed on a breakthrough adsorption setup. In dry SFG containing 15% CO2/N2 85%, the adsorption capacity of CO2 was tested for CBC-X-TEPA adsorbent (at 101 kPa, 60 °C). Figure 7a,b shows breakthrough adsorption curves and CO2 adsorption capacities of the adsorbent, which gradually increased as the TEPA loading increased from 20% to 50%; the adsorption time extended, as shown in Figure 7a, and reached a maximum value of 2.82 mmol g −1 at 50% TEPA, as shown in Figure 7b. The large pore volume of the CBC provided a large space for the loading of TEPA, which could allow a sufficient amount of amine to penetrate the pore and provide more adsorption sites for CO2 adsorption. The adsorption capacity started to decrease at 60% TEPA loading, which was because the pores of the adsorbent were blocked so that CO2 only reacted with the amine-based active sites loaded on the surface of the CBC. In contrast, the organic amines immersed in the pores could not function and reduced CO2 adsorption capacity. When the amine efficiency was 8.05 mmol g −1 , the adsorption capacity of the adsorbent began to decrease, and the adsorbent reached the optimal load of the amine group.
The adsorption capacity of biochar for CO2 at 0 °C and 60 °C was also tested. At low temperatures (0 °C, pure CO2), biochar with high microporosity has a high physical adsorption capacity for CO2, 4.17 mmol g −1 . At 60 °C, the capacity of biochar with high mi- The adsorption capacity started to decrease at 60% TEPA loading, which was because the pores of the adsorbent were blocked so that CO 2 only reacted with the amine-based active sites loaded on the surface of the CBC. In contrast, the organic amines immersed in the pores could not function and reduced CO 2 adsorption capacity. When the amine efficiency was 8.05 mmol g −1 , the adsorption capacity of the adsorbent began to decrease, and the adsorbent reached the optimal load of the amine group.
The adsorption capacity of biochar for CO 2 at 0 • C and 60 • C was also tested. At low temperatures (0 • C, pure CO 2 ), biochar with high microporosity has a high physical adsorption capacity for CO 2, 4.17 mmol g −1 . At 60 • C, the capacity of biochar with high microporosity to adsorb CO 2 is low, only 0.76 mmol g −1 , and micropores are no longer the dominant factor for CO 2 adsorption. The results of pore-size analysis showed that the micropores in the biochar after TEPA modification were greatly reduced, and more mesopores and macropores were retained. At 60 • C, the CO 2 adsorption experiment results show that the adsorbent has a high CO 2 adsorption capacity, mainly due to the chemical reaction between the amine group and CO 2 , and the adsorption at this time is mainly chemical adsorption. It can be seen that the effect of micropores on chemisorption is not significant. In general, in the adsorption of CO 2 at 60 • C, the adsorbent CBC-50-TEPA was mainly used for chemical adsorption. At the same time, there was also some physical adsorption caused by biochar CBC.
In addition, the adsorption capacity of CBC-50-TEPA was tested using 10% H 2 O (g) SFG [15% CO 2 /75% N 2 /10% H 2 O (g)]. The results are shown in Figure 8. Remarkably, it reached 3.31 mmol g −1 , 0.49 mmol g −1 higher than in the dry SFG, as shown in Figure 8a. TEPA efficiency increased from 8.05 to 9.4 mmol g −1 , as shown in Figure 8b, since the presence of H 2 O (g) changed the reaction mechanism of the amine with CO 2 . The reactions of the amino group with CO 2 in the dry SFG are shown in Equations (16)- (18). When one CO 2 molecule was consumed, two amino functional groups were also consumed [41,42]. In the 10% H 2 O (g) SFG, the reactions of the amino group with CO 2 are shown in Equations (19) and (20) [42,43]. One CO 2 molecule was absorbed, and one amino functional group was expended. Therefore, H 2 O (g) increased the adsorption efficiency of the amino-functional group and the adsorption capacity of CO 2 .
Atmosphere 2022, 13, x FOR PEER REVIEW 12 of 17 the amino group with CO2 in the dry SFG are shown in Equations (16)- (18). When one CO2 molecule was consumed, two amino functional groups were also consumed [41,42]. In the 10% H2O (g) SFG, the reactions of the amino group with CO2 are shown in Equations (19) and (20) [42,43]. One CO2 molecule was absorbed, and one amino functional group was expended. Therefore, H2O (g) increased the adsorption efficiency of the amino-functional group and the adsorption capacity of CO2. 2R NH + CO → (R NH )(R NHCOO ) (16) 2R R NH + CO → (R R NH )(R R NHCOO ) (17) R NH + R R NH + CO → (R NH )(R R NHCOO ) The CO 2 adsorption capacity of CBC-50-TEPA was compared with those of reported solid amine adsorbents in Table 4. CBC-50-TEPA exhibited an adsorption capacity (3.31 mmol g −1 ) equivalent to GO (graphene oxide), MCM-41, CNT, and MOFS functionalized by TEPA. This proved that the solid amine adsorbent with biochar as the carrier is also a kind of adsorbent, with potential advantages for removing CO 2 from FG.

Cyclic Adsorption Experiment
The cycle stability determines the service life of the adsorbent. Figure 9 shows the cycle experiments (adsorption temperature: 60 • C, desorption temperature: 100 • C) of CBC-50-TEPA in the dry and 10% H 2 O (g) SFG. The experimental results show that the adsorption capacity of adsorbents decreased from the initial 2.82 to 0.86 mmol g −1 after ten cycles in dry SFG. It was not easy to regenerate the adsorption of the product just by changing the temperature. Under dry conditions, amine-group adsorption of CO 2 generated extremely stable adsorption products of urea functional groups [43,45]. Whenever H 2 O (g) was present, the adsorption capacity of the adsorbent decreased by only 0.08 mmol g −1 from the initial 3.31 after ten cycles, which was almost unchanged. H 2 O (g) caused the reaction products between amino groups and CO 2 to be transformed into carbamates [44,46]. It could be effectively regenerated by increasing the temperature, thus keeping the capacity of the adsorbent stable.

Cyclic Adsorption Experiment
The cycle stability determines the service life of the adsorbent. Figure 9 shows the cycle experiments (adsorption temperature: 60 °C, desorption temperature: 100 °C) of CBC-50-TEPA in the dry and 10% H2O (g) SFG. The experimental results show that the adsorption capacity of adsorbents decreased from the initial 2.82 to 0.86 mmol g −1 after ten cycles in dry SFG. It was not easy to regenerate the adsorption of the product just by changing the temperature. Under dry conditions, amine-group adsorption of CO2 generated extremely stable adsorption products of urea functional groups [43,45]. Whenever H2O (g) was present, the adsorption capacity of the adsorbent decreased by only 0.08 mmol g −1 from the initial 3.31 after ten cycles, which was almost unchanged. H2O (g) caused the reaction products between amino groups and CO2 to be transformed into carbamates [44,46]. It could be effectively regenerated by increasing the temperature, thus keeping the capacity of the adsorbent stable.  Figure 10 shows the FTIR spectra of the fresh adsorbent and the adsorbent circulating in the 10% H2O (g) SFG and dry SFG. A new absorption peak appeared at 1658 cm −1 after circulating in 10% H2O (g) SFG, which was the carbonyl peak in urea. Meanwhile, the N-H and C-H vibration peaks were located at 1570 cm −1 and 1460 cm −1 , whose intensity weakened compared with the fresh adsorbent [47][48][49]. However, the adsorbent recycled in dry SFG had a lower peak at 1570 cm −1 ; the C-H vibration peak at 1460 cm −1 almost disappeared. The carbonyl vibration peak at 1658 cm −1 was further strengthened. FTIR spectra show that in dry SFG, CBC-50-TEPA formed many urea groups after ten cycles.  N-H and C-H vibration peaks were located at 1570 cm −1 and 1460 cm −1 , whose intensity weakened compared with the fresh adsorbent [47][48][49]. However, the adsorbent recycled in dry SFG had a lower peak at 1570 cm −1 ; the C-H vibration peak at 1460 cm −1 almost disappeared. The carbonyl vibration peak at 1658 cm −1 was further strengthened. FTIR spectra show that in dry SFG, CBC-50-TEPA formed many urea groups after ten cycles. There are two forms of urea. The open-chain urea is attributed to the dehydration condensation of CO2 with the different amines in two molecules. The cyclic urea is attributed to the reaction of CO2 with two amines in the molecules [46,50]. The results indicated that 10% H2O (g) SFG could delay the CBC-50-TEPA formatted urea, thereby maintaining the stability of the adsorbent.

Conclusions
We developed an eco-friendly and cost-effective method to prepare biochar (CBC). At the same time, FeCl3, Mg(NO3)2, and H2O (g) activated and increased the ratio of biochar mesopores through a continuous redox reaction. The large VT (1.41 cm 3 g -1 ) and SBET (744.38 m 2 g -1 ) of biochar provided immersion space and channels for the loading of TEPA. The solid amine adsorbent (CBC-50-TEPA) prepared with biochar as a carrier exhibited good thermal stability and a high CO2 adsorption capacity (3.31 mmol g -1 ) in 10% H2O (g) SFG. The H2O (g) could delay the generation of urea groups, thus allowing the adsorbent to maintain high stability for ten cycles.
Supplementary Materials: The following supporting information can be downloaded at: www.mdpi.com/xxx/s1, Figure S1 XRD spectra of Metal oxide during the reaction process, Figure  S2   There are two forms of urea. The open-chain urea is attributed to the dehydration condensation of CO 2 with the different amines in two molecules. The cyclic urea is attributed to the reaction of CO 2 with two amines in the molecules [46,50]. The results indicated that 10% H 2 O (g) SFG could delay the CBC-50-TEPA formatted urea, thereby maintaining the stability of the adsorbent.