New Porous Amine Functionalized Biochar based Desiccated Coconut Waste as Efficient CO2 Adsorbents

doi:10.21203/rs.3.rs-3343072/v1

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Research Article

New Porous Amine Functionalized Biochar based Desiccated Coconut Waste as Efficient CO2 Adsorbents

https://doi.org/10.21203/rs.3.rs-3343072/v1

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published 05 Feb, 2024

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Environmental problems such as climate change and global warming caused by greenhouse gases, mainly CO_2, have become a worldwide topic of concern. Adsorption is a promising method for CO₂ capture. In this research, a series of amine functionalized on biochar based desiccated coconut waste (amine-biochar@DCW) namely ethylenediamine functionalized biochar@DCW (EDA-biochar@DCW), diethylenetriamine functionalized biochar@DCW (DETA-biochar@DCW), triethylenetetramine functionalized biochar@DCW (TETA-biochar@DCW), tetraethylenepentamine functionalized biochar@DCW (TEPA-biochar@DCW), and pentaethylenehexamine functionalized biochar@DCW (PEHA-biochar@DCW) adsorbents were synthesized and characterized. From the characterization analyses, series of amine-biochar@DCW adsorbents had better developed pore structure and larger specific surface area than that of pristine desiccated coconut waste (DCW). Furthermore, the results showed that the increase percentage of elemental of C and N as well the presence peaks NH stretching, NH bending, CN stretching, and CN bending, revealing the presence of amine on the surface of biochar@DCW. From the CO₂ adsorption experiment, among amine modified biochar adsorbents, TETA-biochar@DCW had higher CO₂ adsorption capacity (61.78 mg/g) with mass ratio (m:m) of biochar@DCW:TETA (1:2). The adsorption kinetics on the TETA-biochar@DCW was best fitted by the pseudo-second model, suggesting the adsorption process occurs through chemisorption. Additionally, TETA-biochar@DCW depicts high selectivity towards CO₂ gas and good reusability after five CO₂ adsorption–desorption cycles.

Desiccated coconut waste

Biochar

Amine functionalization

CO2 adsorption

The temperature of the earth gradually increased, beginning with the Industrial Revolution in the early 20th century. Recent weather conditions have intensified the debate about rising global temperatures. Temperatures are explained by high manufacturing and economic activity levels including major emissions (Mikhaylov et al. 2020). The main source of greenhouse gas emissions is from human activities, for instance, the combustion of fossil fuels for electricity, food, and transport. If uncontrolled emissions are not reduced, the current CO₂ level of 394.5 parts per million volume (ppmv) is predicted to increase to 500 ppmv by 2050 (Thakur et al. 2022). Several techniques have been introduced to minimize the concentration of CO₂ in the environment, such as absorption (Shi et al. 2022), membrane separation (Lei et al. 2022), and cryogenic separation (Zainab et al 2020). However, all these methods are costly and not environmentally friendly (Pardakhti et al. 2020; Zubbri et al. 2020). The solid adsorption method is widely used for its advantages of low cost, simple operation, high stability, low renewable energy consumption, and low equipment corrosion (Ismail et al. 2022). Common adsorbents have been synthesized for CO₂ adsorption, including polymer materials (Kunalan et al. 2022), metal-organic framework (Li et al. 2020), porous metal oxide (Lu et al. 2023), and mesoporous silica (Hanif et al. 2020). However, these adsorbents have several drawbacks such as low CO₂ uptake, expensive and poor recyclability performance (Liu et al. 2019; Ali et al. 2020).

Recently, more attention has been focused on utilizing emerging carbonaceous adsorbent, biochar as CO₂ adsorbents (Shafawi et al. 2021). Biochar is much cheaper than other common CO₂ capture materials such as metal-organic frameworks (MOFs) because it is usually produced from various biowaste residues through one-step slow pyrolysis without sophisticated equipment (He at al. 2021). Meanwhile, the research and utilization of biomass waste materials could help discover new materials for long-term energy storage (Li et al. 2021), reduce costs (Dissanayake et al. 2020) and improve competitiveness in energy supply markets, thus having the potential solution to problems faced by the other adsorbents (Lin et al. 2020: Manasa et al. 2022). Agricultural wastes (AWs) represent a promising alternative as raw materials for producing cost-effective for biochar. High availability, locally available material and minimal treatment requirements before utilization makes AWs an attractive alternative (Lim et al. 2020; Yana et al. 2022). For biochar, various agricultural wastes can be used as feedstock, such as rambutan peel (Zubbri et al. 2021), walnut shell (Serafin et al. 2023), and etc. Desiccated coconut waste (DCW) is one of the promising raw materials for biochar production as it has higher cellulose, hemicellulose, and lignin contents (Ajien et al. 2023). DCW is an attractive choice because of its high availability, cheap and easily regenerable, and the fact that DCW requires minor preparation before it is utilized (Rahim et al. 2020). In addition, using DCW promotes the management of biomass wastes for biochar production. As the utilization of DCW based biochar has been rarely reported for CO₂ adsorption application, it is a good idea to explore biomass from DCW as raw materials to develop biochar as an adsorbent for CO₂ capture.

Properties of biochar such as surface area, porosity and surface functional groups need to be further improved to enhance CO₂ capture (Liu et al. 2022) Thereby, surface modification and grafting of functional groups onto the surface of the material may be an effective approach to improve the adsorption performance of biochar toward CO₂ adsorption. Adio et al. (2020) used nitrogen modification to alter the surface porous carbon derived from sugarcane bagasse for CO₂ capture. They reported that nitrogen functionalization enhances the adsorption of CO₂, as their modified carbon demonstrates the highest CO₂ uptake at ambient pressure and temperature with 3.34 mmol/g. Wei et al. (2018) mentioned that nitrogen can be added to a porous carbon framework using nitrogen containing precursors such as amine, melamine and urea.

Although the role of nitrogen in enhancing CO₂ adsorption is established, a significant knowledge gap exists concerning the influence of gradual increases in nitrogen content, derived from amine precursors, influence the adsorption process. Therefore, in this study, a series of amine functionalized on biochar based desiccated coconut waste (amine-biochar@DCW) namely ethylenediamine (EDA), diethylenetriamine (DETA), triethylenetetramine (TETA), tetraethylenepentamine (TEPA), and pentaethylenehexamine (PEHA) were chosen to functionalize the surface of biochar as adsorbents for CO₂ capture. The synthesized adsorbents were further characterized using Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscope (SEM), Energy Dispersive X-Ray (EDX) spectroscopy, Brunauer-Emmett-Teller (BET) and Thermogravimetric Analysis (TGA) analyses. In this study, we investigate the effect of amine modifications and amine loading performance for CO₂ adsorption experiment. Furthermore, kinetic study, selectivity and regeneration of prepared adsorbents were also studied.

2.1 Materials

Desiccated coconut waste (DCW) was obtained from the local seller in Perlis. Analytical grade of amines; ethylenediamine (EDA), diethylenetriamine (DETA), triethylenetetramine (TETA), tetraethylenepentaamine (TEPA), pentaethylenehexamine (PEHA), ethanol, and toluene were purchased from Sigma-Aldrich (Missouri Mo, USA). CO₂ (99.9% purity) was supplied by Air Product Malaysia. These chemicals were used directly without any further purification. Table S1 shows the structure of the series of amines used for the functionalization procedure.

2.2 Characterizations

The functional group analysis of the prepared adsorbents was performed by Fourier Transform Infrared Spectroscopy (FTIR) using (PerkinElmer, USA) using ATR technique in adsorption mode with 4 scans at a resolution of ± 4 cm^-1, and wavenumber in the range of 4000 − 400 cm^-1 with a diamond as a detector. The morphology and elemental analyses of the prepared adsorbents were investigated by Scanning Electron Microscopy (SEM) using Hitachi Tabletop Microscope (TM300, Japan) equipped with Energy Dispersive X-ray (EDX) spectroscopy (Bruker, Quatax70). The surface area and pore size of the synthesized adsorbents were measured using Brunauer-Emmett-Teller (BET) by nitrogen adsorption-desorption isotherms at 423.15 K in Micromeritics (ASAP2020, Georgia, and USA). Thermal behaviour of the synthesized adsorbents was study by using thermogravimetric analyzer (PerkinElmer, TGA 4000, Waltham, MA, USA) from 20 to 800°C with a heating rate at 10°C/min under nitrogen flow. A Thermogravimetric Analyzer (TGA, SDTQ-600) was used to analyze CO₂ adsorption of adsorbents.

2.3 Preparation of adsorbent

2.3.1 Preparation of biochar@DCW

Desiccated coconut waste (DCW) samples were washed multiple times and dried for 48 hours in oven at 50°C. Once dried, the pristine DCW were ground into fine particles (Rahim et al. 2020). The preparation of biochar@DCW was adapted and modified from published methods by Adebisi et al. (2017) and Ma et al. (2018). Briefly, the pristine DCW samples were pyrolyzed under 30 mL/min nitrogen flow to vaporize the DCW, leaving a char behind. The furnace was heated until the desired temperature of 500°C reached. Pyrolysis activity was conducted for 2 hours, and the obtained biochar’s powders were left for cooling down to room temperature before taking out the biochar@DCW from the tube furnace.

2.3.2 Synthesis a series of amine-biochar@DCW

The functionalization of amine towards the surface of biochar was adapted and modified from the published methods by Rozi et al. (2017) and Zulkurnai et al. (2017). In this synthesis, biochar@DCW was mixed with the amine, ethylenediamine (EDA) using 1:2 mass ratio (m:m) and dispersed in 20 mL toluene solution. The mixed solution was stirred for 4 hours at 70°C. After the solution was cooled at room temperature, the resulting product were washed several times with ethanol and deionized water before oven-dried at 110°C overnight. This procedure was then repeated by other amine modifying agents which were diethylenetriamine (DETA), triethylenetetramine (TETA), tetraethylenepentamine (TEPA), and pentaethylenehexamine (PEHA). The series of amine-biochar@DCW adsorbents that were synthesized were ethylenediamine functionalized biochar@DCW (EDA-biochar@DCW), diethylenetriamine functionalized biochar@DCW (DETA-biochar@DCW), triethylenetetramine functionazlied biochar@DCW (TETA-biochar@DCW), tetraethylenepentamine functionalzed biochar@DCW (TEPA-biochar@DCW), and pentaethylenehexamine functionalized biochar@DCW (PEHA-biochar@DCW). The schematic diagram of synthesis series amine-biochar@DCW adsorbents is represented in Fig. 1.

2.4 CO₂ adsorption experiments

CO₂ adsorption measurement was adapted from the previous method by Foo et al. (2020). The adsorption tests were conducted under isothermal conditions using a thermogravimetric analyzer (TGA, SDTQ-600). In each gasification experiment, about 8–12 mg of each sample were loaded in an alumina pan and heated at a rate of 20°C /min to 110°C/min in an N₂ atmosphere (75 mL/min) to remove contaminants in the sample. The sample was kept at the gasification temperature under N₂ for 15 min. Then, N₂ was switched to CO₂ (75 mL/min) to initiate isothermal gasification for 70 min. The continuous weight losses of the prepared adsorbents were recorded at an isothermal of 30°C. The CO₂ adsorption capacity was the amount of CO₂ gas adsorbed starting at 30 min until the end of the experiment. The performance of pristine DCW, biochar@DCW, EDA-biochar@DCW, DETA-biochar@DCW, TETA-biochar@DCW, TEPA-biochar@DCW and PEHA-biochar@DCW was screened towards CO₂ adsorption. From the CO₂ adsorption experiment, the amine-biochar@DCW adsorbent that obtained the highest adsorption capacity of CO₂ was chosen and its performance toward CO₂ adsorption was further studied using different mass ratio (m:m) of biochar@DCW and selected amine at 1:1, 1:2, 1:3 and 1:4 to get an optimum mass ratio (m:m) of biochar@DCW and amine content for the synthesis of selected adsorbent. Then, the chosen adsorbent with an optimum mass ratio of biochar@DCW and amine was further evaluated in terms of kinetic, selectivity, and regeneration studies. The CO₂ adsorption capacity of the adsorbent was calculated using Eq. 1.

$${\text{C}\text{O}}_{2} \text{a}\text{d}\text{s}\text{o}\text{r}\text{p}\text{t}\text{i}\text{o}\text{n} \text{c}\text{a}\text{p}\text{a}\text{c}\text{i}\text{t}\text{y} \left(\text{m}\text{g}/\text{g}\right)= \frac{\text{I}\text{n}\text{s}\text{t}\text{a}\text{t}\text{e}\text{n}\text{e}\text{o}\text{u}\text{s} \text{w}\text{e}\text{i}\text{g}\text{h}\text{t} \left(\text{m}\text{g}\right)-\text{I}\text{n}\text{i}\text{t}\text{i}\text{a}\text{l} \text{w}\text{e}\text{i}\text{g}\text{h}\text{t} \left(\text{m}\text{g}\right)}{\text{I}\text{n}\text{i}\text{t}\text{i}\text{a}\text{l} \text{w}\text{e}\text{i}\text{g}\text{h}\text{t} \left(\text{m}\text{g}\right)} \text{x} 1000$$

3.1. Characterization of adsorbents

3.1.1 Functional group analysis

The functional group of the synthesized adsorbents was analyzed using Fourier Transform Infrared Spectroscopy (FTIR) analysis and the results are shown in Fig. 2 ((A) and (B)). The peaks at 3600 cm^− 1, 2922 cm^− 1, 2853 cm^− 1, 1743 cm^− 1, and 1155 cm^− 1 in pristine DCW Fig. 2 (A) indicate the strong stretchings of -OH, asymmetric -CH, symmetric -CH, -C = O, and CN respectively (Zhang et al. 2022). These peaks are commonly found in coconut materials due to cellulose, lignin, and hemicellulose (Ramkumar et al. 2022). The spectrum of biochar@DCW showed the intensity of the CH group at the peak of 2922 cm^− 1 and 2853 cm^− 1 were reduced after the pyrolysis activity, suggesting the breakage of a weak bond between C and H in the alkyl group at high temperatures (Zhao et al. 2022). The peaks at 3600 cm^− 1 and 1743 cm^− 1 that belong to hydroxyl and carbonyl bonds which appeared broadly initially in pristine DCW, become less intense for biochar@DCW adsorbent due to moisture loss caused by the high temperatures reached during the pyrolysis process (Figueredo et al. 2021). New bands for CH, -CH₂, and -CH₃ symmetric or asymmetric stretching at 2330 cm^− 1 and 1980 cm^− 1 increased peak intensity for biochar@DCW. Similar findings were found in published work byMa et al (2018).

After amine functionalization on the surface of biochar@DCW (Fig. 2 (B)), the peaks at 3593 cm^− 1, 1590 cm^− 1, 2250 cm^− 1, and 1055 cm^− 1, which corresponds to the NH stretching, NH bending, CN stretching, and CN bending, confirming the presence of amine (EDA) group on the surface of biochar@DCW, respectively (Ariyanta et al. 2021; Figueiredo et al. 2021). Similar peaks were observed for the spectrum of DETA-biochar@DCW, TETA-biochar@DCW, TEPA-biochar@DCW, and PEHA-biochar@DCW adsorbents. Similar results were also obtained from the study by Zhu et al. (2023). They found the asymmetric bending of the primary amine at peaks at 1570 and 1479 cm^− 1, respectively in the acid-modified sepiolite with pentaethylenehexamine (PEHA) adsorbent. The analyses of IR spectra in the amine-biochar@DCW confirmed the presence of amine functional groups on biochar@DCW surfaces. These results proved the series of amines including EDA, DETA, TETA, TEPA, and PEHA, were successfully functionalized on the surface of biochar@DCW adsorbents. Henceforth, these amine modifications generate more active sites for CO₂, thus increasing the adsorption efficiency and selectivity of adsorbent towards CO₂ gases.

3.1.2 Morphological analysis

The morphology of the prepared adsorbent was studied using Scanning Electron Microscopy (SEM). As seen in Fig. 3 (A), the surface of pristine DCW shows that the surface of the adsorbent has a smooth surface with no visible pores (Rahim et al. 2020). The SEM image of biochar@DCW shows the formation of irregularly shaped pores on the surface of adsorbents, as circled in Fig. 3 (B). The presence of pores structure in the biochar@DCW is due to the release of volatile gases from the components (Cheng et al. 2021). Furthermore, carbonization at higher temperatures (500°C) gave the surface of the adsorbent to collapse and fragment, creating more pores on the surface (Thakur et al. 2022). This finding aligns with the published study by Jia et al. (2018).

After amine functionalization on the surface of biochar@DCW, no clear difference was observed in surface morphologies between EDA-biochar@DCW (Fig. 3 (C)), DETA-biochar@DCW (Fig. 3 (D)), and TETA-biochar@DCW (Fig. 3 (E)). However, SEM images of TEPA-biochar@DCW (Fig. 3 (F)) and PEHA-biochar@DCW (Fig. 3 (G)) which have longer amine chains, showed the pore structures of these adsorbents were clogged due to bulky amine filling on these pores during functionalization process.Faisal et al. (2021) and Gunawardene et al. (2022) mentioned that the functionalization of bulky amine content would block the pores of the adsorbents. Therefore, it would be expected PEHA-biochar@DCW and TEPA-biochar@DCW have low performances in terms of CO₂ adsorption capacity due to the limitation of pores for CO₂ adsorption.

3.1.3 Elemental analysis

Elemental analysis was performed to confirm the presence of elements in the prepared adsorbents and the results are shown in Fig. 4. The EDX result of pristine DCW (Fig. 4 (A)) contains 37.6% of C, 56.8% of O and 5.6% of N. A similar result was recorded from research by Rahim et al. (2020). The spectrum of biochar@DCW (Fig. 4 (B)) depicts that the percentages of C increased to 67.1% while the percentage of O eventually declined to 16.1% after carbonization of pristine DCW at 500°C. The increased % of C is due to the release of volatile matter from the materials and the formation stable C structure. Meanwhile, the decrement in the O compositions was related to the dehydration reaction of hydrophilic compounds from the adsorbent material during the carbonization process (He et al. 2018). These results align with the studies reported by Rahim et al. (2020) and Reza et al. (2020).

After amine modification on the surface of biochar@DCW, EDX results revealed an increase % of C and N elements in the synthesized amine-biochar@DCW adsorbents. The composition of the C and N atoms had increased significantly from EDA-biochar@DCW to PEHA-biochar@DCW. EDA-biochar@DCW exhibited an additional 69.9% C and 15.4% N (Fig. 4 (C)), while DETA-biochar@DCW (Fig. 4 (D)) showed an additional 70.2% C and 16.9 N. The TETA-biochar@DCW, on the other hand, had shown about an additional 70.5% C and 17.7% N (Fig. 4 (E)), while TEPA-biochar@DCW had shown an additional 72.1% C and 18.1% N (Fig. 4 (F)). PEHA-biochar@DCW had the highest compositions of 72.9% C and N at 19.8% (Fig. 4 (G)), as PEHA-biochar@DCW possesses the longest alkyl amine chain compared to the other amine-biochar@DCW adsorbents. The obtained results proved the successful functionalization of the series of amines on the surface of biochar@DCW. This result agrees with the reported work by Liu et al. (2015). Liu and coworkers found that when the number of amine chains of an adsorbent increased, the elements of C and N in EDX analysis also increased. Generally, an adsorbent with high nitrogen content provides more active sites for selective CO₂ adsorption (Xiao et al. 2018; Qiao et al. 2022).

3.1.4 Thermal stability

Thermogravimetric analysis was employed to assess the thermal stability over the surface of the synthesized adsorbents and the thermogram of the synthesized adsorbent is presented in Fig. 5 ((A) and (B)). As seen in figure, the weight loss for all the adsorbents started below 100°C. Moisture and light volatile components are evaporated around temperatures below 120°C (Escalante et al. 2022). At 230 to 800°C, cellulose, hemicellulose, and lignin decomposition mainly occurred in pristine DCW. Hemicellulose starts to degrade at temperatures 220°C to 325°C, while cellulose degrades at temperatures 315°C to 400°C (Escalante et al. 2022). Lignin degrades around 350°C to 800°C since it has a complex degradation mechanism due to its highly aromatic structure (Escalante et al. 2022; Mukherjee et al. 2022). For biochar@DCW, it shows a weight loss of low molecular weight hydrocarbons of about 4.29% occurred between 50–180°C. A similar statement was mentioned by Tomczyk et al. (2020) in their review of biochar in terms of different pyrolysis temperatures and types of feedstocks.

The thermogram in Fig. 5 (B) shows after functionalization of biochar@DCW with a series of amine, EDA biochar@DCW, DETA-biochar@DCW, TETA-biochar@DCW, TEPA-biochar@DCW, and PEHA-biochar@DCW adsorbents experienced a weight loss about 32.2%, 33.7%, 37.8% 39.8% and 40.9%, respectively due to the breakdown of amines, including EDA, DETA, TETA, TEPA and PEHA at 110–670°C.Zhu et al. (2023) also reported a similar finding with 45% mass loss due to the decomposition of PEHA at temperature of 130°C − 600°C. As seen in result, PEHA-biochar@DCW has a higher weight loss of amine than other amine-biochar@DCW adsorbents as it has the longest amine chain, as was corroborated with the EDX result.

3.1.4 Surface area analysis

BET analysis was performed to investigate the porous structure of synthesized adsorbents and the result is shown in Figure S1 and Table 1. Pristine DCW has a poor surface area (1.34 m²/g) and total pore volume (0.0009 cm³/g), while biochar@DCW has improvement in specific surface area (15.2 m²/g) and total pore volume (0.0017 cm³/g) after undergoing pyrolysis process, suggesting this procedure enhance the porosity of the biochar@DCW. After functionalization with a series of amines, the modified adsorbent improved its specific surface area and total pore size. The increment of surface area and total pore volume can be seen from EDA-biochar@DCW, DETA-biochar@DCW and TETA-biochar@DCW where the surface area is 16.3, 17.8, and 37.1 m²/g with a total pore volume of 0.0018, 0.0021 and 0.0029 cm³/g, respectively.

Table 1

BET results of prepared adsorbents.
Adsorbents	Specific Surface Area (m²/g)	Pore Volume (cm³/g)	Average Pore Diameter (nm)
Pristine DCW	1.34	0.0009	2.67
biochar@DCW	15.2	0.0017	4.50
EDA-biochar@DCW	16.3	0.0018	5.62
DETA-biochar@DCW	17.8	0.0021	6.48
TETA-biochar@DCW	37.1	0.0029	2.23
TEPA-biochar@DCW	26.6	0.0024	6.62
PEHA-biochar@DCW	16.2	0.0013	3.29

The findings showed that TETA-biochar@DCW had the highest surface area and total pore volume in this analysis which is in line with the study by Faisal et al. (2021), where they reported that TETA had minimum pore blockages, thus resulting in a higher surface area of its porous structure. However, the surface area and total pore volume of TEPA-biochar@DCW and PEHA-biochar@DCW were decreased, suggesting that increasing the amine chain reduces their surface area and total pore volume due to the bulky amine loading that clogged their porous structure. This result supports the explanation mentioned above for the results of their morphological analysis. Since the amine-biochar@DCW was applied to adsorb small molecules (CO₂), microporosity plays a major role. Based on the results of average pore diameter, it showed that TETA-biochar@DCW was closest to micropores diameter (diameter < 2 nm), which suggested it has a suitable pore size diameter for the adsorption of CO₂ (Chen et al. 2018). This finding demonstrates that TETA-biochar@DCW is a good adsorbent for potential application in CO₂ adsorption as it has a higher surface area and suitable pore size for the adsorption of CO₂ compared to the other synthesized adsorbents.

3.2. CO₂ adsorption results

3.2.1 Screening of CO₂ uptake of synthesized adsorbents

CO₂ adsorption experiment was conducted to investigate the adsorption capacities of the synthesized series of amine-biochar@DCW (EDA-biochar@DCW, DETA-biochar@DCW, TETA-biochar@DCW, TEPA-biochar@DCW and PEHA-biochar@DCW) adsorbents in this research. The adsorption experiment was also conducted on pristine DCW and biochar@DCW. Figure 6 demonstrates the CO₂ adsorption capacity obtained by the synthesized adsorbent in this experiment. As seen from the result, pristine DCW demonstrated a minimal CO₂ adsorption capacity of 9.98 mg/g as it does not have any porous structure on its surface according to morphological analysis. However, in the case of biochar@DCW adsorbent, it recorded CO₂ adsorption capacity two times higher than pristine DCW. This result is due to the presence of a porous structure after the pyrolysis process of pristine DCW. These kinds of porous structures will act as the main pathways connecting the inner surface of the biochar and offer additional space for CO₂ adsorption (Li et al. 2020). On the other hand, the series of amine-biochar@DCW adsorbents had shown greater removal percentages than the pristine DCW and [email protected] et al. (2019) suggested that the high content of amine groups on the surface of the series amine-biochar@DCW enhances the interaction system and the selectivity of the adsorbents towards CO₂. The elemental N in amine sites on the surface of amine-biochar@DCW increased the number of active sites for CO₂ adsorption (Guo et al. 2022).

Among the series of amine-biochar@DCW, TETA-biochar@DCW is a superior adsorbent with an adsorption capacity of 61.78 mg/g. This result can be explained by its surface consisting of a large porous structure and high surface area obtained from SEM and BET analyses. A study by Yan et al. (2023) stated that TETA is easily dispersed in the pores of the solid support which not only effectively prevents the agglomeration effect and improves the accessibility of the active sites, but also facilitates the inhibition of the amine leaching. On the other hand, TETA also has the least steric hindrance and higher affinity for CO₂, thus reducing the mass transfer resistance and improving the adsorption performance. The explanation from this research support well the good performance achieved for TETA-biochar@DCW in this study. EDA-biochar@DCW and DETA-biochar@DCW exhibit lower removal percentages as compared to TETA-biochar@DCW due to the available binding sites of EDA-biochar@DCW and DETA-biochar@DCW that have shorter amine chains and lower numbers of nitrogen atoms, therefore, limit the adsorption of CO₂ to occur which decreased their adsorption efficiencies (Zhang et al. 2019).

However,Liu et al. (2015) further stated that certain CO₂ adsorption capacities of longer amine chains and rise in nitrogen could decline due to the increase in organic steric hindrance, which explains the lower CO₂ uptake of the TEPA-biochar@DCW and PEHA-biochar@DCW as compared to TETA-biochar@DCW. The steric hindrance occurs because the long organic chains tend to shroud some nitrogen atoms, inhibiting access to CO₂. Furthermore, morphological and surface area analyses also proved that TEPA-biochar@DCW and PEHA-biochar@DCW consist of blocked pore structures and small surface area due to bulky amine filling during the functionalization process, which can be an additional reason for their low CO₂ performance. Therefore, it is evident that TETA-biochar@DCW displayed the highest CO₂ uptake compared to the rest of the prepared adsorbents. This justifies using TETA-biochar@DCW as the adsorbent for subsequent adsorption studies. Figure 7 depicts the interaction of TETA-biochar@DCW with CO₂ during the adsorption process. From the interaction, the lone pair of electrons on the amine attack CO₂ molecules to form the CO₂-amine zwitterions (Hu et al. 2020).

3.2.2 Amine loading

The amount of amine loading can greatly influence the efficiency of an adsorbent for CO₂ adsorption [63]. Since TETA-biochar@DCW had shown the best performance in the CO₂ adsorption experiment, therefore, the mass ratio (m:m) of biochar@DCW and amine (TETA) was studied in the range of 1:1, 1:2, 1:3 and 1:4. Figure 8 shows the comparison of CO₂ adsorption capacities of TETA-biochar@DCW with different mass ratio (m:m) of biochar@DCW and amine (TETA). From the graph, TETA-biochar@DCW with a mass ratio of 1:2 shows the highest adsorption capacity with 61.78 mg/g. A similar finding has been found by Kamarudin et al. (2018). They also investigated the effect of amine loading towards CO₂ adsorption and found that the optimum mass ratio (adsorbent:amine) for CO₂ adsorption was 1:2.

Increasing the amount of TETA (mass ratio of 1:3 and 1:4) towards functionalizing biochar@DCW would decrease CO₂ adsorption capacity due to the amine loading clogged on the adsorbent pores, as revealed from SEM images from Fig. 9. Additionally, after the porous structure of the adsorbent is filled, the excess amine starts to coat the outer surface, indirectly limiting CO₂ and amine interaction inside the pores because of mass transfer resistance (Azmi et al. 2019). Thus, biochar@DCW and TETA with mass ratio of 1:2 was selected for the synthesis of TETA-biochar@DCW for CO₂ adsorption.

3.2.3 Kinetic study

It is important to determine the equilibrium and kinetic behaviour of the adsorbents to evaluate their performance and understand the adsorption mechanism and overall mass transfer in the process (Najafabadi et al. 2021). This study investigated the kinetics of CO₂ adsorption using the pseudo-first order (PFO) and pseudo-second order (PSO) models. These adsorption rate models are often utilised for gas–solid systems with adsorption reaction-controlled kinetics. Pseudo first order kinetic model (Eq. 2) mainly studies the adsorption process controlled by surface diffusion.

$${q}_{t}={q}_{e}( 1- {e}^{-{k}_{1}t})$$

In the equation, ${q}_{t}$ is amount of adsorbate adsorbed at a time, t in mg/g, ${q}_{e}$ is the equilibrium of CO₂ uptake (mg/g), $t$ is adsorption time, min and ${k}_{1}$ is the PFO adsorption rate constant, 1/min.

Pseudo-second order kinetic model establishes a hypothesis that the chemical reaction is the rate-controlling step of the adsorption process on the gas–solid interface. It mainly describes the chemical adsorption process and chemical bond formation ( Guo et al. 2019). The equation for the Pseudo second order kinetic model is shown in Eq. 3.

$${q}_{t}=\frac{{q}_{e}^{2} {k}_{2}t}{1+ {q}_{e}^{2} {k}_{2}t}$$

In the equation, ${q}_{e}$ and ${q}_{t}$ stand for the amount of adsorbate adsorbed at equilibrium and at a time (mg/g), ${k}_{2}$ is the pseudo-second order rate constant mg (g/min).

As seen in Table 2 and Figure S2 and, the pseudo second order kinetic model was the best-fitted kinetic model in this present study as it has the highest correlation coefficient (R²) of 0.9998 and good agreement between theoretical (68.4932 mg/g) and experiment q_e (61.78 mg/g) values compared to the pseudo first order kinetic model. Thus, it could be concluded that the adsorption of the CO₂ by TETA-biochar@DCW follows the pseudo second order kinetic model which developed from the assumption of a limiting step which may be a chemisorption process involving valency forces through sharing or exchange of electrons between adsorbent and adsorbate (Lahuri et al. 2023). It could be attributed to surface modification with amine (TETA) that could interact with CO₂ forming CO₂ - amine zwitterion (Lahajini et al. 2018; Lahuri et al. 2023). Huang et al. (2019) found a similar result using biochar-based sewage sludge and leucaena wood for CO₂ capture. Huang and coworkers found that the pseudo second order kinetic model best-fitted CO₂ adsorption on their synthesized biochar.

Table 2

Adsorption kinetic parameters of CO₂
Kinetic Model	Parameter	Value
	q_e	47.1154
Pseudo-first order	k₁	0.0578
	R²	0.9866
Pseudo-second order	q_e	68.4932
	k₁	-0.0293
	R²	0.9998

3.2.4 Selectivity

The gas adsorption capacities of TETA-biochar@DCW were measured for each gas including CO₂, N₂, CH₄, and air, to study the gas selectivity properties of TETA-biochar@DCW towards several gases (Fig. 10). From the findings, TETA-biochar@DCW exhibited excellent selectivity towards CO₂ (61.78 mg/g) over the air (0.36 mg/g), CH₄ (0.48 mg/g) and N₂ (0.42 mg/g). Shafawi et al. (2021) have mentioned that the presence of N-functional groups will increase the surface basicity and polarity of the amine-functionalized biochar, thus strengthening the adsorption of CO₂ molecules. This statement also agrees well with research work by Wang et al. (2019), which also stated that nitrogen amount is frequently cited as the primary variable affecting the selectivity of CO₂ adsorption in carbon sorbents. Thus, this result proved that the synthesized adsorbent, TETA-biochar@DCW was highly selective towards CO₂, suggesting CO₂ can be easily separated from the gas mixture using this adsorbent.

3.2.5 Regeneration Study

Regeneration also plays a vital role in selecting the optimal biochar for CO₂ capture, in addition to the adsorption capacities, since it is closely related to the economic benefits of the involved industry. To further investigate the regeneration performance of TETA-biochar@DCW, five cycles of CO₂ adsorption-desorption experiments were conducted in this study. As shown in Fig. 11, this adsorbent exhibited excellent CO₂ adsorption capacities even after five cycles of CO₂ adsorption-desorption, suggesting it has a high regeneration ability for CO₂ and beneficial properties for its economical and practical applications as CO₂ adsorbent. A similar finding was found by Cao et al. (2022), that used straw and wood-based biochar to adsorb CO₂.

3.2.6 Comparison with other reported adsorbents in the literature

The current adsorbent (TETA-biochar@DCW) was evaluated by comparing the obtained results with finding from published adsorbents for CO₂ adsorption. The performance of the TETA-biochar@DCW in terms of BET surface area, total pore volume and adsorption capacity as shown in Table 3. As seen in the table, the surface area and adsorption capacity of TETA-biochar@DCW towards CO₂ adsorption are comparable to other reported works. This result demonstrates that TETA-biochar@DCW as a potential tool for CO₂ adsorption.

Table 3

A summary of biochar adsorbents for CO₂ adsorption
Feedstock	Pyrolysis system	Adsorption system	BET surface area (m²/g)	Total pore volume (cm³/g)	Adsorption capacity (mg/g)	Reference
Rambutan peel	Pyrolysis at 500°C	TGA	7.80	0.011	27.83	(Zubbri et al. 2021)
Rambutan peel	Pyrolysis at 700°C	TGA	175.84	0.111	56.64	(Zubbri et al. 2021)
Coffee ground	Pyrolysis at 600°C	Fixed bed column	9.8	-	69.10	(Guo et al. 2022)
Hickory chip	Pyrolysis at 600°C	TGA	548	0.356	37.40	(Xu et al. 2019)
Chicken manure	Pyrolysis at 400°C	TGA	22.22	0.052	76.80	(Yildiz et al. (2019)
Digestate from anaerobic digestion	Pyrolysis at 600°C	Surface area analyzer	6.89	0.038	53.59	(Qiao et al. 2020)
Sugarcane baggase	Pyrolysis at 700°C	TGA	-	0.18	95.37	(Chatterjee et al. 2020)
Sargassum seaweed	Pyrolysis at 800°C	TGA	292	-	46.2	(Ding et al. 2020)
TETA-biochar@DCW	Pyrolysis at 500°C	TGA	37.1	0.0029	61.78	This present study

A series of amine-functionalized biochar based desiccated coconut waste (amine-biochar@DCW) were successfully synthesized and characterized as new adsorbents for CO₂ capture. There is significant effect in nitrogen content derived from amine precursors that influence the adsorption process. The functionalization of amine enhanced the performance of modified biochar based desiccated coconut waste (biochar@DCW) with high CO₂ adsorption capacity (61.78 mg/g). Furthermore, this adsorbent showed high selectivity, stability and regenerabiliy towards CO₂ capture. Taking into account the principles of the circular economy, including the utilization of a low-cost, locally available, and environmentally benign feedstock, coupled with the efficient performance of the prepared adsorbent, the synthesis and application of the material presented in this study represent a significant stride toward sustainable CO₂ capture. This work exemplifies circular economy principles by converting potential waste into a valuable resource for carbon dioxide mitigation.

Ethical Approval

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Consent to Publish

The publisher has the author's permission to publish research findings.

Consent to participate

Not applicable

Author contributions

Dina Sofiea Zakaria: methodology, formal analysis, investigation, resources, data curation, writing—original draft, writing—review and editing, and visualization. Siti Khalijah Mahmad Rozi: conceptualization, methodology, formal analysis, investigation, resources, data curation, writing—original draft, writing—review and editing, visualization, supervision, and project administration. Hairul Nazirah Abdul Halim: validation, investigation, writing—review and editing, and visualization. Sharifah Mohamad: investigation and visualization. Ghee Kang Zheng: investigation and visualization.

Funding

This work was supported by FRGS RACER grant (RACER/1/2019/STG01/UNIMAP//1) funded by the Ministry of Higher Education Malaysia

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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New Porous Amine Functionalized Biochar based Desiccated Coconut Waste as Efficient CO2 Adsorbents

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Abstract

Figures

1. Introduction

2. Experiment section

2.1 Materials

2.2 Characterizations

2.3 Preparation of adsorbent

2.3.1 Preparation of biochar@DCW

2.3.2 Synthesis a series of amine-biochar@DCW

2.4 CO₂ adsorption experiments

3. Results and discussion

3.1. Characterization of adsorbents

3.1.1 Functional group analysis

3.1.2 Morphological analysis

3.1.3 Elemental analysis

3.1.4 Thermal stability

3.1.4 Surface area analysis

3.2. CO₂ adsorption results

3.2.1 Screening of CO₂ uptake of synthesized adsorbents

3.2.2 Amine loading

3.2.3 Kinetic study

3.2.4 Selectivity

3.2.5 Regeneration Study

3.2.6 Comparison with other reported adsorbents in the literature

4. Conclusions

Declarations

References

Supplementary Files

Status:

Journal Publication

Version 1

New Porous Amine Functionalized Biochar based Desiccated Coconut Waste as Efficient CO2 Adsorbents

Status:

Journal Publication

Version 1

Abstract

Figures

1. Introduction

2. Experiment section

2.1 Materials

2.2 Characterizations

2.3 Preparation of adsorbent

2.3.1 Preparation of biochar@DCW

2.3.2 Synthesis a series of amine-biochar@DCW

2.4 CO2 adsorption experiments

3. Results and discussion

3.1. Characterization of adsorbents

3.1.1 Functional group analysis

3.1.2 Morphological analysis

3.1.3 Elemental analysis

3.1.4 Thermal stability

3.1.4 Surface area analysis

3.2. CO2 adsorption results

3.2.1 Screening of CO2 uptake of synthesized adsorbents

3.2.2 Amine loading

3.2.3 Kinetic study

3.2.4 Selectivity

3.2.5 Regeneration Study

3.2.6 Comparison with other reported adsorbents in the literature

4. Conclusions

Declarations

References

Supplementary Files

Status:

Journal Publication

Version 1

2.4 CO₂ adsorption experiments

3.2. CO₂ adsorption results

3.2.1 Screening of CO₂ uptake of synthesized adsorbents