The effect of the active carbonyl groups and residual acid on the ammonia adsorption over the acid-modified activated carbon

1 Introduction

Ammonia (NH₃) is a highly toxic and extremely corrosive pollutant, which may bring undesired odors, lead to severe burning of respiratory systems, and contribute to fine particulate matter (PM2.5) concentration in the atmospheric environment (Stokstad 2014; Han et al., 2021; Jiang et al., 2022). Many different sources can release NH₃, including home/public toilets, farm manure, liquid NH₃ refrigerants, construction waste antifreeze, expansion agents, industrial desulfurization/denitrification units (Dai et al., 2019; Zhong et al., 2019; Liang et al., 2020). In recent years, ammonia pollution has caused increasing concerns about the environment and human health around the world, and rigid standards (e.g., National Emission Ceilings Directive 2016/2284/EU, GB/T 34340-2017) have been put out to restrict the ammonia emission (Wang et al., 2017a; Giannakis et al., 2019). Generally, NH₃ removal technologies include catalytic oxidation, biological processes, membrane technology, and scrubbing processes (Vikrant et al., 2017). Although the high removal efficiency of NH₃ can be achieved, they may still suffer possible problems such as temperature/pressure requirements, high processing costs, and the generation of harmful byproducts. By comparison, the adsorption of NH₃ over porous materials is one of the most simple and efficient methods to remove the odorous gas (Zheng et al., 2016; Choi et al., 2020; Mirzaie et al., 2021), and carbon materials always draw a lot of attention owing to their high surface area, rich pores, adjustable structure and low cost (Kamran et al., 2020; Kamran and Park 2020; Kamran and Park 2021b). The physical and chemical activization of carbon materials may further increase their adsorption capacity through the enriched pore structure and active functional groups (Kamran and Park 2020; Kamran and Park 2021a; Kamran and Park 2021b; Kamran et al., 2022). In particular, acid modification of activated carbon were reported to have more hydroxyl and the acidic carboxyl groups, which significantly elevated the adsorption capacity of NH₃ (Huang et al., 2008; Kamran and Park 2021a). However, the removal ability of NH₃ by acid-modified activated carbon may decrease obviously as the operating temperature above 40°C, which severely constrains its application for purification of hot industrial gas with NH₃ (Rodrigues et al., 2007). Therefore, the deep insight into the adsorption mechanism of NH₃ over the acid-modified activated carbon will be of great significance to develop more efficient carbon materials for NH₃ removal.

The interactions between activated carbon and adsorbate are very complex owing to the various functional groups in activated carbon. Both the physical and chemical mechanisms may occur during the adsorption of NH₃ over activated carbon (Guo et al., 2005). The physical adsorption of NH₃ over the unmodified-activated carbon is so weak that the NH₃ adsorption capacity even has no obvious relationship with the surface area and pore structure (Rodrigues et al., 2007). On the other hand, the oxygen-containing functional groups (especially for the hydroxyl and carboxyl groups) on the surface of modified activated carbon may provide the main active sites for NH₃ adsorption through the chemical action (Jiang et al., 2022). The activated carbon modified by acid always exhibits excellent NH₃ adsorption capacity. Huang et al. (2008) found that the breakthrough capacity of NH₃ is linearly proportional to the amount of acidic functional groups of the acid-modified activated carbon. The further study by Qajar et al. (2015) indicates that high density of carboxylic acid groups may be produced with enhanced NH₃ adsorption capacity after modifying activated carbon by HNO₃. The hydrogen bond between the hydrogen atoms of NH₃ and oxygen atoms of the hydroxyl/carboxyl groups is regarded as the main chemical interaction during the adsorption of NH₃ over the acid-modified activated carbon (Guo et al., 2005; Tamai et al., 2006). Despite these progress on NH₃ adsorption mechanism, the previous reports ignored the influence of the residual acid on the NH₃ removal performance, and it is still unclear that how the active functional groups and residual acid functions for NH₃ removal. Moreover, the molecular level insight into the adsorption mechanism of NH₃ is still necessary to uncover the hydrogen-bond interaction between NH₃ and acid-modified activated carbon.

In this work, the modified activated carbon samples by three common inorganic acids (HNO₃, HCl and H₂SO₄) were prepared to investigate the effect of the active functional groups and residual acid on the NH₃ adsorption. The NH₃ adsorption efficiency over different modified activated carbon samples was evaluated under different conditions, and the detailed structural features of the typical activated carbon samples were investigated by SEM, BET, FT-IR, TG and NH₃-TPD. Moreover, the density functional theory (DFT) calculations were further carried out to obtain the molecular-level insight into NH₃ adsorption mechanism over the acid-modified activated carbon.

2 Experimental method

2.1 Materials and reagents

The activated carbon used in this study was purchased from Green-source Activated Carbon Company Limited, China. Prior to modification, the activated carbon was crushed and sieved into .3–.6 mm. The inorganic acids (HNO₃, HCl and H₂SO₄) and H₂O₂ were purchased from Aladdin. Deionized water was used in all the experimental processes.

2.2 Preparation of modified activated carbon

The activated carbon was modified by HNO₃, HCl, and H₂SO₄ under different conditions, respectively. Typically, 6 g of activated carbon was impregnated in 30 mL solution with different acids at the stirring speed of 200 rpm/min for 6 h. After treatment, the modified activated carbon samples were thoroughly washed by distilled water at least three times until the pH value of mother solution was close to neutral, and then dried at 100°C for 3 h. The H⁺ concentration of the mother solution was kept same (10 M) when investigating the effect of acid types. Four concentrations (1 M, 5 M, 10 M and 15 M) of HNO₃ were tried, and a reflux condenser was used when the treatment temperature was 90°C. By comparison, the activated carbon samples modified by H₂O₂ were also prepared to assess the influence of surface hydroxyl groups on NH₃ removal. The modified activated carbon samples are named as “concentration-reagent-temperature” such as “5M-HNO₃-90”, which means the activated carbon was modified by 5 M HNO₃ at 90°C.

2.3 Characterizations

The microstructure of activated carbon was analyzed by scanning electron microscopy (SEM, JSM-7800P, JEOL, Japan). The specific surface area, pore size and pore volume of typical activated carbon samples were measured by the automatic surface area and porosity analyzer (BET, iPore400, PhysiChem Instruments, China). The fourier transform spectroscopy (FTIR, Nicolet iS50, Thermoscientific, America) was used to characterize the surface functional groups of samples. The weight loss analysis was achieved by thermogravimetric analyzer (TG, HCT-1, BHEE, China). Temperature programmed desorption of NH₃ was performed on a chemisorption analyzer (NH₃-TPD, Autochem1 II 2920, micromeritics, America).

2.4 Adsorption performance tests

Adsorption performance of NH₃ over modified activated carbon was tested in a fixed-bed quartz reactor with 1,000 ppm NH₃ with N₂ balance. The total flow of the experiment was maintained at 200 mL/min. The adsorption temperature was controlled by a temperature controller, and the outlet concentrations of NH₃ and NO_x were continuously monitored using a Gasmet portable FTIR analyzer (Gasmet DX4000, Finland). The removal efficiency of NH₃ was determined by inlet and outlet concentration of NH₃.

${\mathrm{N}\mathrm{H}}_3\mathrm{r}\mathrm{e}\mathrm{m}\mathrm{o}\mathrm{v}\mathrm{a}\mathrm{l}\mathrm{e}\mathrm{f}\mathrm{f}\mathrm{i}\mathrm{c}\mathrm{i}\mathrm{e}\mathrm{n}\mathrm{c}\mathrm{y}=\frac{{\left[{\mathrm{N}\mathrm{H}}_3\right]}_{\mathrm{i}\mathrm{n}}-{\left[{\mathrm{N}\mathrm{H}}_3\right]}_{\mathrm{o}\mathrm{u}\mathrm{t}}}{{\left[{\mathrm{N}\mathrm{H}}_3\right]}_{\mathrm{i}\mathrm{n}}}\times 100\%$

2.5 Theoretical calculation models and methods

The models of NH₃ and HNO₃ are shown in Figures 1A, B, respectively. The original activated carbon model is derived from the graphite model (Hassel and Mark 1924). A periodic four-layer graphene structural model (5 × 5 × 4) was constructed to represent the activated carbon with different functional groups (Figures 1C–E and Supplementary Figure S1). The defected carbon was modeled by removing one of the surface central carbon atoms (Figure 1C). The carbonyl or carboxyl modified carbon models were directly built on the defected sites (Figures 1D–E). The Dmol³ was used to optimize configuration models and find optimal adsorption configurations through DFT. The Perdew-Burke-Ernzerhof (PBE) in generalized gradient approximation (GGA) was used as the exchange-correlation functional (Payne et al., 1992; Perdew et al., 1996). The Tkatchenko Scheffler method was used for the DFT dispersion correction to deal with the hydrogen bond and van der Waals interactions (Tkatchenko and Scheffler 2009). The core treatment of all electron was selected with the basis set of DNP 4.4 (Li et al., 2020). The calculation parameters are with the energy tolerance of 1 × 10⁻⁵ Ha per atom, a maximum force tolerance of .002 Ha/Å, and a maximum displacement tolerance of .005 Å. A smearing of .005 Ha and the global orbital cutoff of 5.2 Å were used. Before geometry optimization, a periodic cube cell with the side length of 20 Å was built. The adsorption energy (E_ad) is defined as Eq. 1. E_{adsorbate/carbon} is the total energy of the adsorbate/carbon system after the gas molecule being adsorbed on model carbon. E_adsorbate is the energy of gas molecule and E_carbon is the energy of the model carbon after geometry optimization.

$E_{ad}=E_{adsorbate/carbon}-E_{adsorbate}-E_{carbon}(1)$

FIGURE 1

FIGURE 1. Geometrically optimized configurations of NH₃ (A), HNO₃ (B), defected carbon (C), defected carbon with carbonyl (D) and defected carbon with carboxyl (E) from the top view. (Only the top layer is displayed. Atomic legend: -C, -H, -O, -N).

3 Result and analysis

3.1 Adsorption performance of NH₃ over modified activated carbon

The removal performance of NH₃ over modified activated carbon samples under different conditions is shown in Figure 2. Compared with the fresh activated carbon, all the acid-modified activated carbon samples show significantly improved NH₃ adsorption capacity. Specially, the HNO₃-modified activated carbon samples have the best NH₃ adsorption capacity (Figure 2A). Huang et al. (2008) also reported that the active carbon modified by HNO₃ had much better NH₃ removal performance than that modified by other acids, which is the same with our results. Both the increased acid concentration and modification temperature are beneficial to the NH₃ adsorption over the HNO₃-modified activated carbon. But the high concentration of HNO₃ (15 M) can not further increase the NH₃ adsorption, and the best NH₃ removal performance can be achieved over the 10M-HNO₃-90 sample, whose adsorption capacity can reach 40 mg/g at room temperature. Compared with the previous reports about NH₃ adsorption over different carbon materials (Table 1), the NH₃ adsorption capacity of 10M-HNO₃-90 sample is in relatively high level. Moreover, the increased application temperature can significantly decrease the NH₃ removal performance, and the adsorption capacity can decrease to 15 mg/g over the 10M-HNO₃-90 sample at 100°C. By contrast, H₂O₂-modified activated carbon samples show only a little increase of NH₃ adsorption capacity even if treated at 90°C (Figures 2A, C). According to the previous reports (Takaoka et al., 2007; Ye et al., 2022), H₂O₂-modified activated carbon possesses mainly hydroxyl groups, and HNO₃-modified activated carbon may have much more carboxyl groups. The indistinctively increased NH₃ adsorption capacity of H₂O₂-modified activated carbon indicates that hydroxyl groups are not the active sites for NH₃ adsorption. HNO₃-modified activated carbon possess abundant carboxyl functional groups owing to the oxidation of carbon by HNO₃, and these acidic carboxyl groups may play a key role in the NH₃ adsorption (Huang et al., 2008). In addition, the dramatically weakened adsorption of NH₃ at elevated temperature implies the weak interaction between the adsorbed NH₃ and active oxygen-containing functional groups in acid-modified activated carbon.

FIGURE 2

FIGURE 2. Removal efficiency of NH₃ over modified activated carbon samples under different conditions: (A) modification by different acids, (B) modification by HNO₃ of different concentrations, (C) modification at 90°C, and (D) the removal of NH₃ over 10M-HNO₃-90 at different temperatures.

TABLE 1

TABLE 1. The NH₃ adsorption capacity over different carbon materials in the literatures.

3.2 Regeneration performance of HNO₃-modified activated carbon

The removal performance of NH₃ over the used HNO₃-modified activated carbon regenerated at different temperatures is shown in Figure 3A. After the thermal regeneration of the used HNO₃-modified activated carbon, the adsorption capacity of NH₃ dramatically decreases compared with its corresponding fresh one (results in Figure 2A). And the NH₃ adsorption capacity of sample regenerated at 200°C is almost the same with the fresh activated carbon. Moreover, the abundant NO_x (mainly NO and NO₂) as well as NH₃ were detected during the thermal regeneration process of the used HNO₃-modified activated carbon (Figure 3B). According to the previous reports, HNO₃-modified activated carbon possess abundant hydroxyl and carboxyl groups (Guo et al., 2015; Demiral et al., 2021). The NH₃ removal performance over H₂O₂-modified activated carbon (Figure 2) implies the hydroxyl groups may have little influence on the NH₃ removal capacity. Thus, the acidic carboxyl groups can be the main active sites for NH₃ adsorption over the HNO₃-modified activated carbon. On the other hand, the release of NO_x verifys the that HNO₃ molecule was left on the HNO₃-modified activated carbon even after thorough washing. Considering the strong acidity of HNO₃, the residual HNO₃ on the modified activated carbon may also dramatically affect the adsorption capacity of NH₃, which was always ignored by previous reports.

FIGURE 3

FIGURE 3. Removal efficiency of NH₃ over the used HNO₃-modified activated carbon regenerated at different temperatures (A) and the gas products released during the thermal regeneration process (B).

3.3 Structural characteristics of the modified activated carbon

The morphology of the modified activated carbon samples was observed by the SEM as shown in Figure 4. The surface of the fresh activated carbon is smooth (Figure 4A). The modification of activated carbon by H₂O₂ does not obviously change its morphology (Figure 4B). But the surface of the activated carbon modified by HNO₃ becomes very rough, especially for the sample of 10M-HNO₃-90 (Figures 4C, D). HNO₃ possesses the strong oxidizing/etching properties, and its oxidizing/etching capacity will be enhanced at elevated temperature (Vinke et al., 1994; Zhang et al., 2008). Thus, the treatment of activated carbon by HNO₃ at 90°C will oxidize partial surface carbon atoms and etch some mineral substance in activated carbon, which results in the rough surface of HNO₃-modified activated carbon.

FIGURE 4

FIGURE 4. Morphology features of the typical activated carbon samples: (A) Fresh, (B) 5M-H₂O₂-90, (C) 10M-HNO₃-25, (D) 10M-HNO₃-90.

To have deep insight into the change of surface area and pore structure after modification, the BET tests were performed, and the surface area and pore features of the typical activated carbon are shown in Figure 5 and Table 2. As it can be seen, these carbon samples exhibit the typical IV isotherms with the H3 hysteresis loops (Figure 5A), and more pores are produced after modification by H₂O₂ or HNO₃ (Figure 5B). It can been seen from Table 2 that the modified activated carbon samples by both H₂O₂ and HNO₃ can increase the specific surface, pore volume and pore size. The modified activated carbon by HNO₃ at 90°C exhibits the largest specific surface (581.27 m²/g), pore volume (.34 cm³/g) and pore size (2.10 nm). According to a previous reports, the oxidation reaction will occur between HNO₃ and activated carbon during the acid-modification process, which can enlarge the original pores or form new pore structure (Wang et al., 2017b; Giraldo et al., 2020). The BET results are also in accordance with the SEM observation, further implying that the oxidation and corrosion of activated carbon by HNO₃ enrich the pore structure of activated carbon.

FIGURE 5

FIGURE 5. N₂ adsorption-desorption isotherms (A) and corresponding pore size distributions (B) of the typical activated carbon samples: (A) Fresh, (B) 5M-H₂O₂-90, (C) 10M-HNO₃-25, (D) 10M-HNO₃-90.

TABLE 2

TABLE 2. Surface area and pore features of the typical activated carbon samples.

Surface functional groups on the modified activated carbon samples were also detected by FTIR, and the spectra are shown in Figure 6. The signal at 1,570 cm⁻¹ is the C=C stretch, and the signal at about 1725 cm⁻¹ is the stretching of carboxyl (Laszlo et al., 2004; Guo et al., 2005). The infrared signal at about 1,240 cm⁻¹ can be assigned to the stretching of saturated oxygen-containing functional groups such as ethers (Moreno-Castilla et al., 1998). Bands in the 900–1,200 cm⁻¹ region are difficult to assign because there is a superposition of a number of broad overlapping bands, which thus cannot be described in terms of simple motion of specific functional groups or chemical bonds (Moreno-Castilla et al., 1998). Compared with the fresh activated carbon, the oxidation of active carbon by both H₂O₂ and HNO₃ can increase the amount of oxygen-containing functional groups. But the oxidation of activated carbon by HNO₃ can produce more oxygen-containing functional groups than H₂O₂, especially for carboxyl groups. Considering the observation of abundant NO_x (mainly NO and NO₂) during the thermal regeneration of the used HNO₃-modified activated carbon (Figure 3B), the contribution of the strong signal intensity of oxygen-containing functional groups may also result from the residual HNO₃ molecule adsorbed on activated carbon. For the used 10M-HNO₃-90 sample, a new shoulder peak appears at 1,273 cm⁻¹, which may be some kind of adsorbed NH₃ complex. It is generally recognized that hydroxyl and carboxyl groups in activated carbon provide the main active sites for NH₃ adsorption through the chemical action (Jiang et al., 2022). Although the H₂O₂-modified activated carbon may possesses much more hydroxyl groups (Wang et al., 2012; Liu et al., 2018), its weak NH₃ adsorption capacity implys the hydroxyl groups in activated carbon are not so acitve for NH₃ adsorption. Only the acidic oxygen-containing groups such as carboxyl and adsorbed HNO₃ may play a dominant role in its enhanced NH₃ adsorption capacity. After the NH₃ molecule diffuses into the pore of AC, the adsorbed NH₃ may strongly interact with the neighbor acidic groups (carboxyl and adsorbed HNO₃) with increased NH₃ adsorption capacity.

FIGURE 6

FIGURE 6. FTIR results of the typical activated carbon samples.

For the TG curves of the typical activated carbon samples (Figure 7), the weight loss of all the samples occurs within 200°C, implying the easy elimination of these acquired functional groups including carboxyl and residual HNO₃ after modification (Doğan et al., 2020). The more weight loss for 10M-HNO₃-90 than 5M-H₂O₂-90 indicates the more oxygen-containing functional groups in the sample of 10M-HNO₃-90, which is consistent with the FTIR results in Figure 6. For the used 10M-HNO₃-90 sample, much more weight loss about 4 wt% can be observed than its corresponding fresh 10M-HNO₃-90 sample. The result is also well coincidence with the calculated adsorption capacity of 40 mg/g according to the NH₃ removal performance in Figure 2A. Moreover, the easy elimination of the active functional groups in activated carbon even at low thermal-treatment temperature below 200°C accounts for the bad regeneration performance of the acid-modified activated carbon as shown in Figure 3A.

FIGURE 7

FIGURE 7. TG results of the typical activated carbon samples.

To further make clear the NH₃ adsorption mechanism, the NH₃-TPD tests were carried out for the typical activated carbon samples as shown in Figure 8. Similar with the TG results in Figure 7, most NH₃ adsorbed on all the three samples can be easily desorbed below 200°C, which indicates the weak binding force between NH₃ and active functional groups in activated carbon. The amount of the adsorbed NH₃ over both the fresh and H₂O₂-modified activated carbon is far below that over the 10M-HNO₃-90 sample. Ye et al. (2022) also reported the weak NH₃ adsorption on the activated carbon modified by different acids, which is nearly the same with Figure 8. These results demonstrate the modification of activated carbon by HNO₃ can significantly increase the number of active sites for NH₃ adsorption, but cannot increase the binding force between NH₃ and active functional groups in activated carbon.

FIGURE 8

FIGURE 8. NH₃-TPD profiles of the typical activated carbon samples.

3.4 Theoretical insight into the NH₃ adsorption over the acid-modified activated carbon

According to the above performance and structure data, the oxidation of activated carbon by HNO₃ will produce more oxygen-containing functional groups including carboxyl and residual HNO₃, and these acid sites may strongly interact with NH₃ and facilitate its adsorption. Thus, the activated carbon models with defected, carbonyl and carboxyl groups were established to investigate the detailed adsorption mechanism of NH₃ on the activated carbon with different functional groups (See the structural models in Figure 1 and Supplementary Figure S1).

Compared with the activated carbon with defect or carbonyl groups, the acidic carboxyl group indeed has strong binding force with NH₃ with the lowest adsorption energy (−.66 eV) and the shortest N···H hydrogen bond length (1.66 Å) (Figures 9A1–A3). For the adsorption of the residual HNO₃, the binding force between the carboxyl functional group and HNO₃ molecule are also the strongest with the lowest adsorption energy (−.79 eV) (Figures 9B1–B3). A circular hydrogen-bond network is formed between HNO₃ and carboxyl group, in which the O···H hydrogen bond lengths are 1.98 Å and 1.58 Å, respectively (Figure 9B3). For the co-adsorption of NH₃ and HNO₃, the adsorption energies over all the three carbon models obviously decrease, and the co-adsorption energy of NH₃ and HNO₃ over activated carbon with carboxyl group is also the lowest (−1.63 eV) (Figures 9C1–C3). In particular, H atom in HNO₃ is dissociated to form NH₄⁺ ions, and a large circular hydrogen-bond network among NO₃⁻, NH₄⁺ and carbonyl in activated carbon is formed. The hydrogen bond lengths of the three O···H bonds in the network are 1.53 Å, 1.58 Å and 1.67 Å, respectively, whose average value is the lowest of all (Figure 9C3). As is well-known, the lower adsorption energy and the shorter length of hydrogen bond mean the stronger adsorption with bigger binding force (Pan and Xing 2008). The carboxyl in carbon can strong interact with both NH₃ and HNO₃ through the hydrogen bond, and the formation of the circular hydrogen-bond network among NO₃⁻, NH₄⁺ and carbonyl further decreases the adsorption energy with enhanced adsorption capacity of NH₃.

FIGURE 9

FIGURE 9. Geometrically optimized configurations of adsorbed NH₃ (A1–A3), adsorbed HNO₃ (B1–B3) and co-adsorbed NH₃/HNO₃ (C1–C3) on defected carbon, defected carbon with carbonyl and defected carbon with carboxyl from the top view with their adsorption energy and hydrogen-bond length labeled, respectively. (Only the top layer is displayed. Atomic legend: -C, -H, -O, -N).

Taking all the perforemance, structure and calculation results together, the oxidation reaction occurs between HNO₃ and activated carbon during the acid-modification process, and more pores and active surface carboxyl groups are formed in HNO₃-modified activated carbon. The thermal treatment of the used HNO₃-modified activated carbon confirms the existence of the residual HNO₃ in modified activated carbon. The acidic carboxyl groups and residual HNO₃ play a dominant role in its enhanced NH₃ adsorption capacity. On the other hand, these acidic species in activated carbon can be easily eliminated even at low thermal-treatment temperature below 200°C, which accounts for its bad regeneration performance of the acid-modified activated carbon. DFT calculations further display that the carboxyl functional group can strongly interplay with NH₃ or HNO₃ with the lowest adsorption energy (−.66 eV and −.79 eV, respectively), and the co-adsorption of NH₃ and HNO₃ around the carboxyl functional group forms a strong circular hydrogen-bond network among NO₃⁻, NH₄⁺ and carbonyl and further decreases the adsorption energy (−1.63 eV), which provides a molecular level insight into the adsorption mechanism of NH₃ over the acid-modified activated carbon.

4 Conclusion

In summary, the effect of the active functional groups and residual acid on the NH₃ adsorption over the acid-modified activated carbon was investigated to make clear its enhanced NH₃ adsorption mechanism by both experimental and theoretical methods. The activated carbon modified by HNO₃ possesses the best NH₃ removal performance with the maximum NH₃ adsorption amounts of 40 mg/g over the 10M-HNO₃-90 sample. The multiple structural characterizations (SEM, BET, FTIR, TG, NH₃-TPD) of the typical activated carbon samples further reveal that the activated carbon can be oxidized and etched by HNO₃ with enriched pore structure, carbonyl groups and residual HNO₃. These acidic species including carbonyl and residual HNO₃ play a key role in the improved NH₃ adsorption capacity. DFT calculations further reveal the carboxyl functional group can strongly interplay with both NH₃ and HNO₃ with the lowest adsorption energies (−.66 eV and −.79 eV, respectively), and the co-adsorption of NH₃ and HNO₃ on the carboxyl functional group forms a strong circular hydrogen-bond network among NO₃⁻, NH₄⁺ and carbonyl and further decreases the adsorption energy (−1.63 eV). The experimental and theoretical results together demonstrate that the oxidation and etching of activated carbon by acid enrich the active surface carboxyl functional groups, and the co-adsorption of NH₃ with the residual HNO₃ around the carboxyl functional group through the strong circular hydrogen-bond network accounts for the enhanced NH₃ adsorption capacity of the acid-modified activated carbon. The demonstrated adsorption behaviours and mechanism suggests that the modified activated carbon with more acidic oxygen-containing functional groups will significantly elevate the NH₃ adsorption capacity for its further commercial applications.

Data availability statement

The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding authors.

Author contributions

Academic idea, DFT calculation, and original draft preparation by CL; Sample preparation, characterizations and performance tests by SZ, ML, and YL; Calculation methods by S-MX; Suggestions by CZ and JL; Funding, modification and supervision by ZY and JY. All authors read and approved the final manuscript.

Funding

This work was supported by Natural Science Foundation of Beijing Municipality (No. 8222042), Zhongye Changtian Basic Research Foundation (No. 2021JCYJ10) and National Natural Science Foundation of China (No. 21601192).

Conflict of interest

CZ was employed by Everbright Environmental Technology Co., Ltd. JL was employed by Zhongye Changtian International Engineering Co., Ltd.

The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fenvs.2023.976113/full#supplementary-material

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