What makes biochar an interesting CO₂ adsorbent?

Abstract

Biochar belongs to the category of low-cost, stable, and environmentally benign carbon-based materials. In this article, the reasons that highlight the advantages of biochar materials to be used in carbon dioxide (CO₂) adsorption are briefly viewed with recent examples. Also, the issues to be solved for recommending biochar materials in the practical applications are listed.

Carbon dioxide (CO₂), an anthropogenic greenhouse gas, contributes to global warming and climate change with its rising concentrations in the atmosphere as a result of fossil fuel combustion. CO₂ capture has thus received special attention in the field of environmental science, which can be done by capturing CO₂ from point sources or the air. The captured CO₂ will then be either utilized or stored. The efficient adsorbent materials are necessary for the effective CO₂ capture. So far, a variety of CO₂ adsorbents are available, including carbon-based materials, metal oxides, covalent organic frameworks, metal-organic frameworks, silica, and polymers. A special attention is paid to carbon-based adsorbents due to their cost-effectiveness, stability, and environmental friendliness. Especially, biochar is a fascinating CO₂ adsorbent because its manufacture could cut CO₂ emissions by avoiding the open space combustion of agricultural and domestic wastes, i.e., the sources of biochar. Besides, this leads to the conversion of waste into valuable material (circular economy through sustainable waste management). In line with the life cycle analysis, preparing biochar from waste materials promotes environmental sustainability with a reduced environmental impact. According to the United States Environmental Protection Agency, improper food waste management emits 170 million MT of CO₂-equivalent greenhouse gases each year, which is equivalent to the annual CO₂ emissions of 42 coal-fired power plants [1]. Therefore, preparing biochar from carbonaceous waste and using it as CO₂ adsorbent is advantageous, i.e., getting two mangoes with one stone (carbon abatement and sustainable management).

A wide range of carbon resources (e.g., biomass wastes, municipal wastes, plastic wastes, food wastes, municipal wastes) provides the platform to tune the characteristics and CO₂ adsorption capacity of biochar simply by varying its carbon source. Also, many synthetic methods (hydrothermal carbonization, gasification, pyrolysis, and torrefaction) are available for preparing biochar, which can also have impacts on the properties of biochar [2]. This variety of sources and preparation methods highlights an excellent opportunity to develop a highly efficient CO₂ adsorbent in the coming years.

Biochar has inherent microporosity and specific functional groups (e.g., C=O and –OH), which are required for the physisorption and chemisorption of CO₂, respectively. Biochar’s macropores and mesopores facilitate CO₂ diffusion and transportation but not adsorption (Fig.1). Unlike other adsorbents, biochars can be easily activated by increasing microporosity, surface area, and functional groups to enrich the active sites for enhanced CO₂ adsorption. Biochars are activated either physically (using steam or CO₂) or chemically (using alkali or acid). Zhang et al. [3] made biochar from wood pellets and activated it separately using steam (physical) and KOH (chemical). Chemical treatment improves microporosity in a better way; hence biochar activated by the chemical approach has a higher CO₂ absorption (50.75 mg·g⁻¹) than biochar activated by the physical method (38.30 mg·g⁻¹) under the same conditions. Although yielding biochar with a higher adsorption capacity, the chemical treatment may release secondary contaminants [3]. Zhang et al. [4] used a unique activation approach that involved impregnating bamboo-derived biochar with lignin and then treating it with the microwave. As a result, activated biochar has a better porosity and a CO₂ uptake of 134.46 mg·g⁻¹. Luo et al. [5] also used the microwave to install micropores in biochar made from marine biomass.

Fig.1 Porosity in biochar and mechanism of CO2 adsorption over biochar.

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To achieve maximum CO₂ adsorption, the synergistic interplay of physisorption and chemisorption is essential, underlining the combined presence of active sites for physisorption (microporosity) and chemisorption (functional groups) in biochar. As the functional groups located in the pore walls may block the micropores [6], it is important to have functional groups without interrupting microporosity. Chemisorption dominates at both low and high-pressure conditions whereas physisorption dominates usually at the enhanced pressure. On the contrary, the higher temperature does not favor physisorption, attributing to the desorption of physisorbed CO₂ molecules due to their weak interaction with active sites. Therefore, optimizing adsorption conditions and porosity of biochar is essential to establish a better synergism between physisorption and chemisorption. Like enriching microporosity simply by physical and chemical activation methods, the functional groups, in addition to the inherent groups, can be introduced into biochar by facile methods. Especially, enriching the nitrogen-based functional groups is attractive due to their basicity that facilitates the Lewis acid-base interaction with CO₂ and promotes the adsorption of CO₂ (Fig.1). With proper biomass source selection (e.g., microalgae), the intrinsic nitrogen groups in biochar can be obtained without extra treatment [7]. For example, biochar made from Enteromorpha, a marine biomass, contains basic nitrogen groups (e.g., pyridinic-N, pyrrolic-N, and graphitic-N) that aid in CO₂ adsorption through chemical interactions [5]. Also, nitrogen groups can be added through the co-carbonization of carbon and nitrogen compounds (e.g., urea). Special emphasis is given to amine-modified biochar adsorbents because the amine groups involve strong and selective interaction with CO₂ molecules, forming carbamates [7]. Ji et al. [8] created a biochar-silica hybrid material with amine groups connected to silica rather than biochar, utilizing siloxane linkers with numerous amine groups. A linker molecule comprises three amine sites, which enables the increased CO₂ adsorption.

The compatibility of biochar with both inorganic and organic materials enables the formation of a synergistic hybrid composite for optimal CO₂ adsorption efficiency. Because of their basicity, alkaline earth metal oxides (e.g., magnesium) are frequently added to biochar, allowing for both physisorption and chemisorption of CO₂. Pu et al. [9] added MgO into walnut shell-derived biochar to improve dynamic CO₂ uptake. Fortunately, many biomass sources contain alkali earth metals in their structure. For example, tamarind bark contains calcium oxalate, which can be used to make biochar with a high calcium content for good CO₂ adsorption [10]. Similarly, organic molecules (e.g., ionic liquids) capable of CO₂ chemisorption are mixed into biochar, resulting in effective composite materials. Arjona-Jaime et al. [11] created a biochar-ionic liquid composite that shows 90% better efficiency and faster kinetics than the bulk ionic liquid. As stated above, physisorption dominates at high pressure.

As known, biochar materials exhibit different CO₂ adsorption capacities depending upon the nature of their biomass sources. Interestingly, the same biochar could also display different CO₂ capacity values according to the conditions like CO₂ concentration, temperature, and competitive adsorbates. The CO₂ concentration usually reveals positive impacts on the CO₂ adsorption capacity of biochar, i.e., the CO₂ adsorption capacity increases with the concentration of CO₂. For instance, Ding and Liu [12] studied the CO₂ adsorption capacity of SCK-800-1 (a biochar prepared using KOH and sargassum with 1:1 weight ratio at 800 °C) under various CO₂ concentrations (3%, 8%, 12%, and 18%) without changing the feed flow rate of total flue gas. As shown in Fig.2(a), the breakthrough times decrease from 6.6 to 5.1 min while increasing CO₂ concentration from 3% to 18%. This is because the higher CO₂ concentration allows more CO₂ molecules to pass via the reactor per unit time, enabling the faster saturation of active sites. Notably, the CO₂ adsorption capacity of SCK-800-1 enhances from 0.23 to 1.08 mmol·g⁻¹, with the increase of CO₂ concentration from 3% to 18%, which is attributed to the increase of CO₂ concentration gradient and the resultant CO₂ mass transfer rate. The authors also studied the variation of CO₂ adsorption capacity with temperature. The adsorption capacity of SCK-800-1 is maximum (1.05 mmol·g⁻¹) at 25 °C. With the increase in temperature, the CO₂ adsorption capacity gradually decreases and reduces to 0.45 mmol·g⁻¹, at 100 °C. This is because the rise in temperature increases the surface energy and the molecular diffusion of CO₂ on the biochar’s surface. As a result, the adsorption force of biochar is reduced for CO₂, increasing the chance of CO₂ desorption at elevated temperatures [12]. Though this seems a common trend in the literature, exceptions cannot be excluded based on the modification or activation carried out on biochar materials [5]. Like CO₂ concentration and temperature, humidity can also have a significant impact on the CO₂ adsorption capacity. Especially, the CO₂ adsorption capacity of hydrophilic biochar (i.e., activated with oxygen-related polar functional groups) gradually loses its CO₂ adsorption capacity with the increase of humidity. As illustrated in Fig.2(b), the increase in humidity levels does not have a significant impact on the CO₂ adsorption capacity of wood-derived raw biochar (BC-Raw), as biochar adsorbs 1.3 and 0.85 mg·g⁻¹ CO₂ at 8.8% and 87.9% relative humidity (RH) levels, respectively. However, for the KOH-activated biochar (BC-KOH1:1) containing more polar functional groups, the impact of humidity is apparent, i.e., the CO₂ adsorption capacity of BC-KOH1:1 decreases from 3.7 to 1.3 mg·g⁻¹ when increasing RH from 8.8% to 87.9%, respectively [3]. This might be due to the strong interaction between the hydrophilic polar groups of biochar and water. Hence, preparing biochar materials with hydrophobic and non-polar properties is important to limit the competitive adsorption of water molecules and realize the practical applications of biochar.

Fig.2 (a) CO2 breakthrough curves of SCK-800-1 at various CO2 inlet concentrations [12]. Reprinted with permission from ref. [12], copyright 2020, Elsevier. (b) 15% CO2 adsorption ability of wood-derived biochar (BC) samples at different relative humidity (RH) levels [3]. Reprinted with permission from ref. [3], copyright 2022, Elsevier. (BC-Raw: wood-derived raw biochar; BC-Steam: steam activated biochar; BC-KOH1:1: KOH activated biochar; BC-KOH-lmp: biochar actirated with KOH by impregnation) (c) Multicomponent breakthrough curves of CO2, NO, and SO2 through LBB20 (a kind of biochar) at 25 °C [4]. Reprinted with permission from ref. [4], copyright 2024, Springer Nature. (d) Unique advantages of biochar materials for CO2 adsorption.

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Both flue gas and open air are a mixture of multiple gases, including CO₂. Therefore, to realize the practical applications, biochar materials should achieve the selective adsorption of CO₂. In the literature, the CO₂ adsorption selectivity of biochar is studied in the presence of various gases like N₂, NO_x, SO_x, etc. In general, the typical flue gas from coal-fired power plants possesses 10%–16% CO₂, and small traces of NO_x, and SO_x. So, Zhang et al. [4] prepared a gas mixture (CO₂-12 vol %, NO-0.4 vol %, and SO₂-0.4 vol %) for a multicomponent competitive adsorption experiment using a biochar (LBB₂₀). During the individual gas adsorption experiments, LBB₂₀ adsorbs 28.50 mg·g⁻¹ CO₂, 0.24 mg·g⁻¹ NO, and 50.10 mg·g⁻¹ SO₂ under identical conditions. While using the prepared multicomponent gaseous mixture, the adsorption capacity of LBB₂₀ is invariably declined for CO₂ (23.70 mg·g⁻¹), NO (0.14 mg·g⁻¹), and SO₂ (45.06 mg·g⁻¹). Owing to its small size and dynamic diameter, NO quickly passes the column. Both CO₂ and SO₂ slowly reach the active site and replace weakly adsorbed NO molecules, as supported by the increasing outlet concentration of NO. Unlike NO, SO₂ demonstrates an adverse effect on CO₂ adsorption because both SO₂ and CO₂ display an acidic nature and involve competitive adsorption. However, the faster movement of CO₂ could allow the reduction of CO₂ adsorption capacity only by 16.8% even in the presence of SO₂ (Fig.2(c)) [4]. This might be assigned to the lignin impregnation of biochar at a suitable quantity which selectively enhances the mobility of CO₂ inside the pore channels. Therefore, a higher selectivity for CO₂ adsorption is possible by modifying raw biochar using suitable methods. The selective adsorption of CO₂ from N₂ is equally important for the post-combustion CO₂ capture and direct air capture. At low pressures, the biochar materials achieve a higher CO₂ adsorption selectivity due to the dominant role of their functional groups and non-reactive adsorption of N₂. Differently, while increasing the pressure, the CO₂ adsorption selectivity is gradually decreased owing to the dominant role of pore filling and eventually reaches the stable selectivity [13]. During the competitive adsorption between CO₂ and CO over biochar materials, achieving a maximum CO₂ adsorption selectivity is not difficult due to the easy and complete removal of weakly adsorbed CO molecules by CO₂ molecules even with 50% CO concentration [14]. These reports suggest that biochar selection, biochar modification, and adsorption conditions can control the CO₂ adsorption selectivity. However, attaining the maximum CO₂ adsorption selectivity using biochars remains a big challenge because it demands many optimization experiments, especially, in real conditions. To help with this problem, Yuan et al. [15] attempted an active learning strategy for preparing a high-performance engineered biochar by recommending optimal synthesis parameters and feedstocks.

In summary, the following unique characteristics (Fig.2(d)) make biochar an interesting CO₂ adsorbent than other adsorbents: (1) low cost, thermal stability, and environmental friendliness; (2) preparation promoting carbon abatement and sustainable management; (3) availability of various precursors; (4) availability of simple preparation methods; (5) ease of activation to enrich microporosity and functionalization; (6) physisorption and chemisorption in synergy; (7) compatibility with organic and inorganic compounds, resulting in highly efficient hybrid adsorbents. Despite these advantages, a few issues must be solved before proposing biochar as a commercial CO₂ adsorbent. Biochar regeneration, or CO₂ desorption, must take place at a lower temperature, even for nitrogen-rich biochar materials. There have been very few CO₂ adsorption experiments with biochar under practical conditions. The real environment comprises complicated gas combinations. Therefore, acidic gases such as SO_x and NO_x can occupy the basic nitrogen groups of biochar, preventing CO₂ adsorption. Humidity, temperature, and pressure are the variables in the environment; hence the CO₂ adsorption behavior of biochar materials must be studied under fluctuating humidity, pressure, and temperature conditions, but such studies are rare. Hope these issues will be addressed by rationally designing biochar materials and studying their CO₂ adsorption capacity under practical conditions.

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Competing interests

The authors declare that they have no competing interests.

Acknowledgements

The authons gratefully acknowledge financial suppport from the National Key Research and Development Program of China (2022YFE0208400); National Natural Science Foundation of China (Grant No. 22038008).

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