Introduction

Carbon dioxide (CO2) is currently recognized as the most prominent contributor to global warming1,2. The main sectors of CO2 emission are energy production (~ 40%), industry (~ 23%), buildings (~ 10%), transport (~ 23%), and others (agriculture, forestry, and other land uses) (~ 5%)3,4. Most of the CO2 emissions from energy production are derived from the burning of fossil fuels, like coal (~ 72.5%) and oil and natural gas (~ 27.5%)4. The technology currently used to produce energy in power plants is the Integrated Gasification Combined Cycle (IGCC). With this technology, the energy carriers, such as coals, are gasified with oxygen and steam to form syngas, a mixture of carbon monoxide (CO) and hydrogen (H2). The obtained syngas is then further processed through the water–gas shift reaction to convert CO to CO2 by the reaction with H2O, yielding a CO2/H2 mixed gas5,6,7. To meet the target of the Paris Agreement (2015), which urged to reduce the greenhouse gas emissions by 45% by 2030 compared to 2010 and then to zero emission by 20508, various strategies have been developed and applied to separate CO2 from a mixed gas stream. These include membrane separation, physical and chemical absorptions, cryogenic separation, and adsorption. Among these developed CO2 separation technologies, adsorption is the most promising and versatile technology due to its low energy consumption and operating cost, high separation efficiency, and high possibility of adsorbent regeneration9,10,11. Based on the literature, activated carbons (ACs) are one of the most appropriate adsorbents for CO2 capture based on their high performance and stability12,13.

Typically, a high CO2 adsorption capacity is achieved at a low temperature and high pressure14,15,16. Besides, the properties of ACs also play an important role on their CO2 uptake. The volume of micropores in the range of 0.33–0.82 nm of bamboo-derived ACs was the main factor responsible for the CO2 adsorption at 273 K and 1 bar, while neither the surface area nor the total pore volume were significant factors13. In contrast, both the surface area and micropore volume played a crucial role on the CO2 uptake by coconut shell-derived ACs17, local coal-derived AC activated by KOH18 and corncob-derived AC activated by KOH19. A high CO2 uptake of Mesua ferrea seed cake-derived AC was obtained when the AC had a high micropore quantity and surface functionality20. However, different results were observed with corn stalk-derived ACs, in which the mesopore volume played the key role in the CO2 adsorption capacity at a low BET surface area (< 500 m2/g), while the micropore area played the main role at a high BET surface area (> 500 m2/g)11. The CO2 uptake of water caltrop shell-derived nitrogen-doped porous carbons was enforced by the synergetic effect of N content and narrow microporous volume21, similar to the hazelnut shell derived N and S co-doped porous carbons, in which its CO2 uptake was dictated by the joint effect of narrow microporosity and N and S content22. The amine-impregnated AC exhibited considerably low BET surface area but importantly high CO2 uptake compared with the virgin AC23. This is because the impregnated amines acted as the active sites to adsorb the CO2 molecules through the chemical adsorption mechanism. The BaO-impregnated AC exhibited a considerably higher CO2 uptake than the unimpregnated one due to its high surface basicity24. The MgO-impregnated AC nanofiber can promote the CO2 uptake of the virgin material due to the generation of chemical bindings between the acidic CO2 molecules and existing basic functional groups25. According to above results, it seems to be that both the textural property and surface chemistry affect the quantity of the CO2 uptake. Nevertheless, it is still controversial to conclude which textural properties of ACs affect the CO2 adsorption capacity, probably due to the differences in the utilized raw materials and conditions used to prepare the ACs as well as the condition used to test the CO2 uptake. Nevertheless, the preparation of ACs with good textural properties and high surface might benefit for the CO2 adsorption.

Typically, there are two sequential steps that are involved in the production process of ACs, including carbonization and activation26. Carbonization (or pyrolysis) is the thermal decomposition of the raw material at high temperatures (400–1200 °C) in an inert atmosphere, such as nitrogen (N2) or argon (Ar), in order to eliminate volatile compounds, getting the carbonized carbonaceous material with high fixed carbon (or biochar)12,27. For activation, there are two established processes including physical and chemical activations. Physical activation involves thermal elimination of carbon oxides from the carbon surface using activating gases, such as CO2, steam, ammonia, or a combination of them20,28,29, while the chemical activation involves the impregnation of dehydrating agents or oxidants, such as potassium hydroxide (KOH), sodium hydroxide (NaOH), potassium carbonate (K2CO3), or zinc chloride (ZnCl2), and heating the mixture in an inert atmosphere18,20,30. Compared with physical activation, chemical activation can be achieved at a lower temperature (< 600 °C)20 with higher yields26 and higher surface areas28. However, it is energy consuming process due to the required severe condition to proceed the reaction20 and requires chemical reagents that can contaminate the obtained ACs as well as the environment29. Thus, physical activation is more preferable than chemical activation when considered in terms of environmental safety. The frequently used gases for physical activation are CO2 and steam because both gases provided ACs with a comparable BET surface area12. The CO2 activation usually facilitates the development of new micropores that are responsible for the CO2 adsorption31,32. However, it exhibits a four-fold slower reaction rate than that of steam activation31, leading to a long production time and high energy consumption. To conform to the need of economic feasibility, the steam activation seems more favorable than the CO2 activation. Nevertheless, the steam activation still faces the weakness of that the excessively high steam activation temperatures and/or times promote the creation of new micro-pores and/or widen the existing micropores, which consequently decrease the surface area and total pore volume29,33. Moreover, they induce a high burn-off, resulting in a low yield of ACs. Thus, knowing which preparation parameters (temperature and time) or their interaction affect the textural property of ACs and yield might help the sustainable production of ACs.

Previously, the typically precursors used to produce ACs were coal, peat, lignite, and petroleum residues34. However, the production of ACs from these finite resources is expensive, requires intensive regeneration, and cannot serve a high and increasing demand for global AC consumption12 Thus, a plethora of research have focused on the synthesis of ACs from sustainable resources, such as biomass/agricultural wastes14,16,24,35,36,37,38, municipal wastes30,32,39, and industrial wastes40,41,42. The production of AC from wastes is not only a sustainable process but also an environmentally friendly and a cost-effective strategy based on the reduction of waste disposal and the low production cost of AC43.

In this work, spent disposable wooden chopsticks were used as a raw material to prepare ACs by steam activation and then used to capture CO2 from a mixed CO2/H2 gas. A 2k factorial design was carried out to investigate the effect of the activation temperature and time as well as their interactions on the yield and CO2 adsorption capacity of ACs. The benefit of this work is the utilization of spent disposable wooden chopsticks, one of the large scales generated municipal wastes coming from the sharp growth of food delivery services in Thailand as a sustainable carbon source to prepare ACs. This can reduce the environmental and economic burden of waste management by the government and related agencies as well as achieve the cost-effective production of ACs.

Methods

Spent disposable wooden chopsticks were collected from an urban area in Thailand and employed as the raw material to prepare ACs. Prior to utilizing, they were cleaned, naturally dried, crushed in a knife mill and sieved to get a particle size in the range of 0.21–4.76 mm. The dry-basis proximate analysis displayed the presence of volatile matter, fixed carbon, and ash of 80.15 ± 0.38, 18.74 ± 0.37, and 1.12 ± 0.01 wt%, respectively. The ultimate analysis showed the existence of C, H, N, O, and S contents of around 54.05 ± 7.40, 6.86 ± 0.92, 0.21 ± 0.04, 38.75 ± 8.47, and 0.14 ± 0.11 wt%, respectively, and also trace minerals, such as potassium, magnesium, silicon, calcium, or iron.

Preparation of disposable wooden chopstick-derived AC

A two-step process (carbonization and physical activation) was performed to prepare the disposable wooden chopstick-derived ACs. The carbonization was carried out at 500 °C for 15 min in a N2 atmosphere. In each experiment, the raw material was pre-dried at 105 °C for 3 h to eliminate the free moisture. Then, approximately 100 g of dried raw material was placed in a cylindrical stainless-steel reactor. Gaseous N2 (99.999%, Alternative Chem) was continuously supplied throughout the reactor at a constant flow rate of 1,000 mL/min for 30 min to build up an inert environment. Next, the reactor was slowly heated at a constant heating rate of 10 °C/min from room temperature (~ 30 °C) to 500 °C and maintained at this final temperature for 15 min. Afterwards, the reactor was left to cool down slowly to below 105 °C and the carbonized wooden material or biochar was withdrawn. For the steam activation, a 2k factorial design was performed to explore the effect of the activation temperature (A: 700–900 °C) and activation time (B: 1–2 h) on the production yield and CO2 adsorption capacity of the obtained ACs. In each batch, approximately 40 g of the obtaining biochar was physically activated by steam in a horizontal fixed bed reactor. The steam generated from the deionized water was continuously supplied at a flow rate of 8 mL/min, while N2 (protecting gas) was supplied into the reactor at a rate of 1000 mL/min. After completion of the processing time, the reactor was left to cool down overnight and the resulting ACs were kept in desiccator for further characterization and utilization. Samples were coded as ACx–y, where x represents the activation temperature (in 100 °C units) and y represents the time (h). For example, AC7-1 indicates the AC which was activated at 700 °C for 1 h. The yield of ACs was computed from the weight ratio between the obtained AC and the utilized raw material as Eq. (1).

$$Y = \frac{{w_{AC} }}{{w_{SC} }} \times 100.$$
(1)

Characterization

The micromorphological characteristics of the biochar and ACs were determined by scanning electron microscopy and energy dispersive X-ray spectrometry (SEM–EDX; IT-500HR JEOL) and high-resolution transmission electron microscopy (HRTEM; JEOL-JEM-3100F) with an accelerating voltage of 300 kV. The qualitative functional groups presented on the surface of all ACs were characterized by Fourier-transform infrared spectroscopy (FTIR; FT/IT-6800 JASCO). The textural properties of the ACs, including the specific surface area and pore size distribution, were computed by N2 adsorption/desorption isotherms at 77 K using a Multipoint Surface Area Analyzer (Micromeritics, Tristar II3020) coupled with the classical adsorption theories of Brunauer–Emmett–Teller (BET) methods.

Adsorption capacity test

The adsorption capacity of all adsorbents was tested via the CO2 adsorption from a CO2/H2 mixed gas in a horizontal glass tube reactor having an inside diameter of 8 mm ID and 600 mm length at constant temperature of 25 °C and 1 atm. Prior to conducting the experiment, the AC was dried at 105 °C for 5 h to eliminate free moisture and then approximately 2 g of AC was carefully packed in the glass column, providing an effective adsorption length of around 230–250 mm. Afterwards, a CO2/H2 mixed gas was supplied at a constant flow rate of 100 mL/min into the reactor. The concentration of CO2 in the mixed gas stream was varied over the range of 10 to 50 mol%, controlled by mass flow controller (S48-2-HMT, Horiba). As the adsorption proceeded, the outlet gas stream was sampled to analyze the gas concentration using gas chromatography (GC; Shimadzu GC-8A) with a thermal conductivity detector (TCD) and an INJ/DET temperature of 120 °C, column temperature of 100 °C, and current of 100 mA. The amount of CO2 adsorption (mg CO2 per gram of bulk adsorbent) was obtained from integration of the transient CO2 concentration from the breakthrough curves using Eq. (2). The average value of at least three experimental data was reported to reduce the relative errors (3%).

$$q = \frac{1}{{wM_{W} }}\int\limits_{0}^{t} {\left( {C_{in} - C_{out} } \right)dt} .$$
(2)

Modelling of adsorption isotherms

Three adsorption isotherm models were used to fit the experimental adsorption data, including Langmuir, Freundlich, and Dubinin–Radushkevich (D–R) models. The Langmuir model describes a monolayer adsorption of adsorbates onto a homogeneous surface with a constant adsorption energy in the absence of interaction between the adsorbates and neighboring sites44,45. The nonlinear- and linearized equations of the Langmuir model are shown in Table 1. A plot of Pe against Pe/qe provides the slope and y-intercept, which can be used to estimate qm and kL, respectively. The Freundlich model explains a multilayer adsorption of adsorbates on the heterogeneous surface of adsorbents46. The adsorption energy is initially high and exponentially decreases as the degree of occupied sites increases47,48. Both nonlinear- and linearized forms of the Freundlich model are given in Table 1. A plot of log Pe versus log qe allows the estimation of n and kF from the slope and y-intercept, respectively. Lastly, the D–R model is appropriate to describe the equilibrium adsorption of gases and vapor on the heterogeneous surface of carbonaceous materials with a wide distribution range of pore sizes44. The adsorption occurs via pore volume filling rather than film formation on the perforated walls44,49. The original nonlinear form of the D–R model together with its linear form are also tabulated in Table 1. A plot of (lnB)2 versus lnqD gives the slope and intercept, which can be used to compute the energy parameter (E) and qD, respectively.

Table 1 Adsorption isotherm models employed in this study48,49.
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The goodness of fit between each isotherm model and the experimental data was determined via the determination coefficient (R2) and the normalized standard deviation (S), as expressed by Eqs. (3) and (4), respectively:

$$R^{2} = 1 - \frac{{\sum\nolimits_{i = 1}^{n} {\left( {q_{{\text{e}}} - q_{{{\text{e}},\bmod }} } \right)^{2} } }}{{\sum\nolimits_{i = 1}^{n} {\left( {q_{{\text{e}}} - \overline{q}_{{\text{e}}} } \right)^{2} } }},$$
(3)
$$S = \sqrt {\frac{{\left( {q_{{\text{e}}} - q_{{{\text{e}},\bmod }} } \right)/q_{{\text{e}}} }}{N - 1}} \times 100.$$
(4)

Results and discussion

Effect of the activation temperature and activation time

Representative SEM and HRTEM images showing the microstructures of AC7-2 and AC9-2 together with the original biochar are illustrated in Fig. 1. The biochar showed the development of some large pores in a longitudinal direction due to the opening of vascular bundles of the wooden material during the carbonization (Fig. 1a). The HRTEM images revealed a concentric arrangement of small packets of carbon layers. Compared with the original biochar, the steam activation induced the generation of well-developed pores as well as a surface roughness due to the formation of more gasified components during the activation process. This is because the reactions between carbon and steam are endothermic, and so well-developed carbons form efficiently under elevated temperatures33. That is, a high temperature can effectively remove the disordered carbon coming from the deposition and decomposition of the generated tar and then facilitate the development of new pores51. Analysis of the AC structures by HRTEM revealed the defective graphene-like layers (dark area) of different sizes and shapes, which were bonded with the neighboring layers to create the spaces or porosity (grey area) on the surface of ACs (Fig. 1b,c).

Figure 1
figure 1

Representative SEM images (left) and HRTEM images (right) of the (a) biochar50, (b) AC7-2 and (c) AC9-2 samples.

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Figure 2 shows the FTIR spectra of the parental biochar and all ACs prepared by steam activation at 700–900 °C for 1–2 h. The FTIR spectrum of the biochar that appeared at a wavenumber lower than 920 cm−1 indicated the presence of aromatic C–H out-of-plane bending52. Bands of intensity between wavenumbers of 920–1300 cm−1 are the overlapping C–O stretchings of various surface groups, including the C–O vibration of ethers (942–1300 cm−1), esters (1100–1250 cm−1), cyclic ethers (1140 cm−1), lactonic groups (1160–1370 cm−1), phenolic groups (1180–1220 cm−1), and also carboxylic acids and cyclic anhydrides (1180–1300 cm−1)53. The bands at 1480–1650 cm−1 indicated the presence of polyaromatic C=C stretching vibration of sp2 hybridized carbons53. The peaks found at a wavenumber of 1650–1800 cm−1 were attributed to the presence of the C=O stretching vibration of carboxylic and lactones. Intense spectra appeared a wavenumber of 2300–2400 cm−1 due to atmospheric CO254,55. After steam activation, qualitative changes were observed in all six ACs, from which some bands were diminished. That is, the peak intensities of the aromatic C–H out-of-plane bending mode, C–O stretching vibration of different surface groups, C=C vibration of sp2 hybridized carbon, and C=O stretching vibration were all reduced. This was attributed to the loss of more volatile compounds that were released due to the gasification and the reaction between biochar and steam during the steam activation.

Figure 2
figure 2

Representative FTIR spectra of the parental biochar and the six ACs prepared by steam activation.

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The physical adsorption/desorption isotherms and pore size distribution of the ACs are shown in Fig. 3. The isotherm of AC7-1 and AC7-2 (Fig. 3a) conformed to the Type I isotherm according to the IUPAC classification56, indicating the presence of a predominately microporous structure with a narrow pore size distribution and a well-developed mesoporous structure29. The isotherms of the four ACs prepared at higher activation temperatures and times displayed a hysteresis loop; the usual characteristics of some mesopore-dominant porous materials associated from the capillary condensation in their mesopores29. This suggested the emergence of a mesoporous structure in the different distributions. As also displayed as inset of Fig. 3b, the generation of medium-size mesopores was initially observed for AC8-1 and was more pronounced for AC8-2, AC9-1, and AC9-2. This is attributed to the widening of the original micropores to mesopores in the presence of a high activation temperature and long activation time as well as the generation of new mesopores.

Figure 3
figure 3

Representative (a) N2 adsorption/desorption isotherms and (b) pore size distribution of the parental biochar and six ACs prepared by steam activation at different activation temperatures and activation times.

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The quantitative values of the textural properties of all six ACs are tabulated in Table 2. It can be seen that, at an activation time of 1 h, all the monitored textural properties, including the SBET, St-plot, Vmic and Vmes, increased as the activation temperature increased from 700 to 900 °C. The AC8-1 exhibited the highest micropore volume ratio (Vmic/Vmic + Vmes) of 81.57%. At an activation time of 2 h, the SBET, St-plot, and Vmic also increased as the activation temperature increased from 700 to 800 °C but then slightly decreased at 900 °C. This was not the case for Vmes, in which it continuously increased over the whole range of activation temperatures. The micropore volume ratio decreased slightly from 79.83 to 77.18%, indicating the relatively low existence of a microporous structure at a high activation temperature and long activation time.

Table 2 Textural property and CO2 adsorption capacity at 25 °C and 1 atm of the six ACs and the parental biochar.
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The CO2 adsorption capacity of all six ACs prepared by steam activation from a CO2/H2 mixed gas is also summarized in Table 2. The steam activation significantly improved the adsorption capacity of the biochar from around 19.20 mg/g to greater than 74.46 mg/g, a greater than 3.88-fold improvement. Increasing the activation time from 1 to 2 h increased the CO2 adsorption capacity of the ACs prepared at 700 °C, but a longer activation time at 800 °C was not significant. However, it negatively affected the CO2 adsorption capacity of ACs prepared at 900 °C. The AC9-1 exhibited the maximum adsorption capacity (around 89.85 mg/g), while AC9-2 displayed a remarkably lower CO2 adsorption capacity (81.13 mg/g).

The relationship between the CO2 adsorption capacity and textural properties of ACs prepared at different activation temperatures and activation times is plotted in Fig. 4. The CO2 adsorption exhibited a direct relationship to the SBET, St-plot, and Vmic of the ACs and a relatively fluctuating trend with respect to the Vmes. This suggested that both the surface area and micropore volume were the dominant factors promoting the CO2 adsorption, in accord with previous studies that mentioned that micropores play a crucial rule in CO2 adsorption11,57. This is because a large quantity of CO2 molecules can diffuse throughout a high surface area of ACs and strongly adsorb at their micropores via van der Waal’s forces24,58.

Figure 4
figure 4

Effect of the (a) surface area and (b) pore volume of the obtained ACs on the CO2 adsorption capacity at 25 °C and 1 atm.

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In comparison, the CO2 uptake of the ACs prepared from various types of biomass-waste by physical activation reported in the literature are displayed in Table 3. The adsorption capacity of the chopstick-derived AC (AC7-2) was on par with those reported in the literature. The difference in adsorption capacity might be due to the differences in biomass-waste type and properties, the condition used to prepare the AC and to test the adsorption capacity (ex. gas composition and gas flow rate), as well as the equipment or reactors used to test the adsorption capacity.

Table 3 Comparison of the CO2 uptake by biomass-waste derived ACs prepared by physical activation.
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Optimization of the AC preparation condition

To further understand the impact of the AC preparation condition, in terms of the activation temperature (A) and activation time (B), on the production yield (Y) and CO2 adsorption capacity (q) of ACs, a collection of statistical models and associated estimation procedures known as analysis of variance (ANOVA) was performed. Table 4 tabulates each experimental condition in terms of coded variables and response values. It can be seen that case 1 exhibited the highest production yield, but the lowest CO2 adsorption, whilst case 5 showed the maximum adsorption capacity with an extremely low production yield but still in the acceptable range of the dry biomass-derived ACs, of 5–40 wt%53, while the lowest production yield was obtained at case 6. Table 5 illustrates the ANOVA analysis of these two response variables. The manipulated variables that had a p-value of less than 0.05 were considered as a statistically significant effect at the 95% confidential interval level. Conspicuously, the activation temperature played an important role on the production yield of ACs, while the interaction between the activation temperature and the activation time exhibited an important effect on the CO2 adsorption capacity.

Table 4 Experimental condition and response values.
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Table 5 ANOVA analysis of the response variables.
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Plots of the main and interaction effects of the manipulated variables on the production yield of ACs are shown in Fig. 5. Both a high activation temperature and long activation time exhibited a negative effect on the AC production yield. The activation temperature exhibited a much steeper plot than that of the activation time, indicating its greater impact on the AC production yield than the activation time (Fig. 5a), while the interaction effect of both manipulated variables was not pronounced in this study range, as can be seen by the parallel graph lines (Fig. 5b). This is because the steam activation induced the decomposition of cellulose and hemicellulose leading to the formation of a high porosity in the structure of AC, which allowed the diffusion of the oxidizing agent into the carbon structures and consequently reacted with the lignin62,63. Upon increasing the temperature and time, more volatiles were released due to the gasification and the reaction between biochar (Cf) and steam, according to reactions (R1) and (R2)64, thus resulting in the decreasing yield of ACs65.

$${\text{C}}_{{\text{f}}} + {\text{H}}_{{2}} {\text{O}} \to {\text{CO}}_{{2}} + {\text{2H}}_{{2}} ,$$
(R1)
$${\text{C}}_{{\text{f}}} + {\text{H}}_{{2}} {\text{O}} \to {\text{C}}({\text{O}}) \, + {\text{H}}_{{2}} .$$
(R2)
Figure 5
figure 5

Plots of the (a) main effect and (b) interaction effect on the AC production yield.

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Figure 6 depicts the plots of the main and interaction effects for both manipulated variables on the CO2 adsorption capacity. Both the activation temperature and time exhibited a slight impact on the CO2 adsorption capacity. The maximum adsorption was observed at a suitable activation temperature (Fig. 6a), indicating a non-linear relationship between the activation temperature and CO2 adsorption capacity. According to the interaction effect plot (Fig. 6b), the intersect of the two linear-curves was observed, indicting a significant interaction effect between the activation temperature and activation time on the CO2 adsorption capacity. A high CO2 adsorption was achieved for the ACs prepared at a lower activation temperature and a longer activation time (700 °C, 2 h), or those prepared at a high activation temperature and short activation time (900 °C, 1 h). The AC prepared at an elevated activation temperature and long activation time exhibited a markedly low CO2 adsorption capacity (ex. case 6), because a long activation time at an elevated temperature can induce a high degree of widening of the existing pores instead of pore-deepening and/or new pore generation33,66,67. This pore-widening effect was experimentally confirmed by the decreased micropore volume ratio from 79.20 to 77.18% as the reaction time increased from 1 to 2 h at 900 °C (Table 2). Based on the statistical analysis, the regression models used to predict the production yield and adsorption capacity can be written as Eqs. (5) and (6), respectively.

$$Y = 13.446039 - 9.852355A - 2.505472B,$$
(5)
$$q = 84.168498 + 2.8292162A - 4.862424AB,$$
(6)

where A and B are the coded activation temperature and time, respectively.

Figure 6
figure 6

Plots of the (a) main and (b) interaction effects on the CO2 adsorption.

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Figure 7 shows the contour plots of both manipulated variables against both response variables. A high AC production yield was obtained at a low activation temperature and short activation time (Fig. 7a), while a high CO2 adsorption was achieved at a high activation temperature and short activation time (Fig. 7b). From an economical point of view and adsorption performance, the optimal activation temperature and activation time for the preparation of AC was found to be at 700 °C for 2 h, respectively. At this predicted condition, the approximated values of production yield and CO2 adsorption capacity were around 20.79 and 86.20 mg/g, respectively, which were closed to those obtained from the bench-scale experiment.

Figure 7
figure 7

Contour plots of the (a) AC yield and (b) CO2 adsorption as a function of the activation temperatures and activation times.

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Figure 8a–e shows the breakthrough curve of an equimolar CO2/H2 mixture at 25 °C and 1 atm and the concentration profile of the exit gas stream at different inlet CO2 concentrations of all the adsorbents. The breakthrough curve exhibited a huge roll-up of H2 (C/C0 > 1) at the early adsorption period (< 2 min). This indicted a fast exit of H2 in the exhaust stream or its lower adsorption compared with CO268,69. In other words, the as-prepared AC7-2 exhibited a strong CO2 adsorption and a weak H2 adsorption. This roll-up behavior was observed over the entire investigated range of CO2 concentrations (10–50 mol%; data not shown), supporting that the separation of CO2 from H2 was due to thermodynamic separation70,71. This is because the weaker adsorbed H2 exhibited a fast diffusion in the porous structure of AC compared with a stronger adsorbed CO2, leading to a transient H2 rich adsorbed phase71. Besides, as CO2 is heavier than H2 it exhibited a higher adsorption affinity towards the AC adsorbents9,72, which has previously been ranked in the order of CO2 >  > CH4 > CO >  > H272. Due to the low adsorption affinity of H2 from the CO2/H2 mixed gas via the AC7-2, the calculation of either the CO2 or H2 selectivity using the thermodynamic analysis by Ideal adsorbed solution theory (IAST) is not applicable71,73. Thus, the composition of the outlet gas stream (M) or the exit gas concentration profile was calculated according to Eqs. (7) and (8) and depicted in Fig. 8f.

$$M_{{{\text{H}}_{2} }} = \frac{{m_{{{\text{H}}_{2} }} }}{{m_{{{\text{H}}_{2} }} + m_{{{\text{CO}}_{2} }} }} \times 100,$$
(7)
$$M_{{{\text{CO}}_{2} }} = 100 - M_{{{\text{H}}_{2} }} .$$
(8)
Figure 8
figure 8

(a–e) Breakthrough curve of an equimolar CO2/H2 mixture at 25 °C and 1 atm and (f) the exit gas concentration profile at different inlet CO2 concentrations over the range of 10–50 mol% of the AC7-2.

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According to the plot, the obtained profiles can be categorized into two distinct regions; (i) the breakthrough of H2 at the early adsorption period and the production of a H2-rich gas stream, and (ii) the breakthrough of CO2 and transient to reach the feed composition. The gas stream with the high CO2 concentration exhibited a breakthrough faster than that with the low CO2 concentration. The mixed gas stream with a low CO2 concentration (< 20 mol%) gave an exit stream with a high H2 purity (> 96.8 mol%) during the first 3 min of adsorption and then lessened afterwards to reach the feed composition, while the mixed gas stream with a CO2 concentration of more than 30 mol% provided an exit gas stream with a H2 purity greater than 90% during the first 2 min of adsorption time. This information will help engineers to design an industrial scale CO2 capture system using the adsorption-based separation process from a CO2/H2 mixed gas over a wide range of CO2 concentrations.

Adsorption isotherm

Figure 9 depicts the fitting curves between the experimental data (marker point) and isotherm results (dashed line) for the CO2 adsorption by three ACs at 25 °C and 1 atm in the presence of different CO2 concentrations. The obtained coefficients and fitting quality were considered in terms of the determination coefficient (R2) and the normalized standard deviation (S), with the results summarized in Table 6.

Figure 9
figure 9

Adsorption isotherms of CO2 on three ACs fitted by the (a) Langmuir, (b) Freundlich, and (c) D–R models.

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Table 6 Obtained constants for different adsorption isotherm models.
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The adsorption capacity increased as the increased CO2 concentration, which was attributed to the high driving force of the CO2 concentration between the bulk phase and the surface of AC that can promote a high mass transfer rate74,75. For all the explored ACs, the Freundlich model provided a better fit with the experimental results over the entire range of CO2 concentrations than the Langmuir model, considered in terms of the higher R2 and S values. This suggests that the CO2 adsorption on the spent chopstick-derived AC occurred predominantly via a multilayer adsorption with a heterogeneous surface binding44,49. The value of n was higher than 1 (n > 1), confirming a favorable adsorption75 as well as its high degree of heterogeneity and good adsorption intensity49,76. The adsorption energy parameters (E) obtained from the D-R isotherm model varied in the range of 11.3 to 11.7 kJ/mol, which were between 8 and 20 kJ/mol, and so were neither purely physical adsorption (< 8 kJ/mol) nor chemical adsorption (> 20 kJ/mol)77. Nevertheless, a deviation of energy parameters from a value of 8 kJ/mol of around 25% indicated a predominately physical adsorption. In other words, the CO2 molecules were dominantly adsorbed via the intermolecular cohesion forces at the pore surface and small part of them were adsorbed via the surface functionalities that originated from pyrolysis as well as inorganic matters20.

The recyclability of an adsorbent plays an essential role in the economics of a commercial scale operation, where a high cyclic stability of any employed adsorbent is required. In this work, the cyclic stability of the AC7-2 sample was tested by repetitive adsorption of CO2 from a CO2/H2 mixed gas (50 mol% CO2) at 25 °C and 1 atm. After each particular adsorption, the adsorbed CO2 on the surface of the AC7-2 was simply desorbed in air at 105 °C at ambient pressure and then subjected to re-adsorb CO2 from the mixed gas stream at the same CO2 concentration. As shown in Fig. 10, the fresh AC exhibited a CO2 adsorption capacity of 85.19 mg/g and this dropped slightly with increasing regenerative cycles. This suggested that the regeneration of AC via the heating process is effective to remove the weak adsorbed CO2 at the outer layer of a multi-layer adsorption. At the even after six adsorption/desorption cycles, the employed adsorbent depicted a minimal loss of CO2 adsorption capacity of around 18% of its initial capacity without any significant loss of ACs. Based on the regeneration results, the as-prepared chopstick-derived AC7-2 is recommended as the candidate adsorbent for CO2 adsorption in the temperature swing adsorption process in air at low temperature (ex. 105 °C) and ambient pressure. Otherwise, to enhance a higher recyclability, other regeneration procedures such as depressurization or chemical regeneration should be investigated78.

Figure 10
figure 10

The CO2 adsorption capacity at 25 °C and 1 atm of the AC7-2 over six successive adsorption/desorption cycles.

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Energy consumption analysis

The analysis of required energy to produce the spent disposable wooden chopstick-derived AC by steam activation was adopted from the energy model of Maski et al.79 and Chen et al.80. The energy demand in the production process was separated into two sub-production processes, including the carbonization for biochar production and the steam activation. To perform the energy balance, the heat capacitis of chopstick (Cp,sc) and biochar (Cp,char) were estimated from Eqs. (9) and (10), respectively81.

$$C_{p,sc} = 1500 + T,$$
(9)
$$C_{p,char} = 420 + 2.09T - 6.85 \times 10^{ - 4} T^{2} .$$
(10)

For the basis of 1 kg of chopstick with 7.73 wt% moisture content, the energy consumption used to produce the chopstick-derived AC was determined as shown below.

For the carbonization, there are two types of energy required to prepare biochar; (i) energy required to dry the moisture-bearing chopstick and (ii) energy required to heat chopstick (ECS) from room temperature (30 °C) to 500 °C and holding at this temperature for 15 min.

  1. (i)

    Energy required to dry the moisture-bearing chopstick was the summation of energy required to raise the temperature of moisture from 30 to 100 °C (Ew) and energy required to vaporize the moisture at 100 °C (Evap) as expressed by Eqs. (11) and (12), respectively.

    $$E_{w} = w_{m} C_{p,w} \Delta T = 0.0228\,{\text{MJ}}.$$
    (11)
    $$E_{vap} = w_{m} \Delta H_{vap} = 0.1745\,{\text{MJ}}$$
    (12)
  2. (ii)

    Energy required to heat chopstick was composed of the energy required to heat chopstick from 30 to 105 °C (Ecs,heat-1), energy required for holding chopstick at 105 °C for 3 h (Ecs,hold-1), energy required to heat chopstick from 105 to 500 °C (ECS,heat-2) and energy required for holding chopstick at 500 °C for 15 min (Esc,hold-2) The values of Ecs,heat-1 and Ecs,heat-2 were estimated by Eqs. (13) and (14), respectively. Due to the complexity of reaction pathways during drying and carbonization, the energy consumptions during both periods were computed from the voltage–current–time profile of the oven and furnace, which were 0.0009 and 0.0011 MW, respectively. Based on the utilized processing time, the values of Ecs,hold-1 and Ecs,hold-2 were 9.979 and 0.993 MJ, respectively. Therefore, the total energy required to produce biochar per kg of chopstick was 11.7977 MJ.

    $$E_{sc,heat - 1} = w_{sc} C_{{p,{\text{sc}}}} \Delta T = 0.1274\,{\text{MJ}},$$
    (13)
    $$E_{sc,heat - 2} = w_{sc} C_{{p,{\text{sc}}}} \Delta T = 0.4990\,{\text{MJ}}.$$
    (14)

For the steam activation, the energy requirements are mainly for (i) heating the biochar from 500 to 700 °C (Echar,heat), (ii) holding the biochar at 700 °C for 2 h (Echar,hold) and (iii) producing steam (Eboiler).

  1. (i)

    Energy required to heat biochar was estimated from the sensible heat according to Eq. (15).

    $$E_{{{\text{char}},heat}} = w_{char} C_{{p,{\text{char}}}} \Delta T = 0.1013\,{\text{MJ}}.$$
    (15)
  2. (ii)

    Energy required for holding the biochar at 700 °C for 2 h was computed from the voltage–current–time profile of the furnace, which was equal to 40.3200 MW.

  3. (iii)

    Energy required in boiler was estimated from the property of generated steam, which was at 1.2 barg. Based on the quantity of required steam, the amount of required energy for boiler was around 0.0016 MJ.

In summary, the total energy required to produce AC from spent disposable wooden chopsticks was the summation of the energy required for carbonization (11.7977 MJ/kg chopstick) and activation (40.4229 MJ/kg chopstick) with a total of 52.2206 MJ/kg chopstick. Therefore, the total energies required to produce AC and to adsorb CO2 based on the production yield of AC (23.18%) were 225.28 MJ/kg AC and 116.4 MJ/g-mol CO2, respectively. The estimated energy consumption was seemed to be high but was still in the range of biomass-derived AC, of 43.4 – 277 MJ/kg AC81.

Conclusion

In this work, a series of ACs was prepared from spent disposable wooden chopsticks by steam activation for CO2 separation from a CO2/H2 mixed gas. From ANOVA analysis, it was found that a high activation temperature and long activation time (900 °C, 2 h) negatively affected the production yield and properties of ACs. From an economical point of view and adsorption performance, the optimal activation temperature and activation time for the preparation of AC was found to be 700 °C for 2 h, providing an experimental production yield of 23.18% and CO2 adsorption of 85.19 mg/g at 25 °C and 1 atm, respectively with the total energies required to produce AC carbon and to adsorb CO2 of 225.28 MJ/kg AC and 116.4 MJ/g-mol CO2, respectively. A fast breakthrough of H2 was observed via the as-prepared ACs over an inlet CO2 concentration in a mixed gas of 10–50 mol%, with the release of an almost pure H2 gas stream during the first 2 min of adsorption. The experimental data of CO2 adsorption was adequately described by the Freundlich isotherm model, where the physical adsorption played a predominate role on the interaction between the AC-CO2 molecules. The optimal AC exhibited a 18% loss of CO2 adsorption after six adsorption/desorption cycles. To further increase the CO2 absorption capacity, future research should be carried out to develop a large number of basic sites on the surface of AC such as adding basic metal oxides or alkali metals.