Modification of biomass-derived biochar: A practical approach towards development of sustainable CO₂ adsorbent

Abstract

The persistent increase in the atmospheric concentration of carbon dioxide (CO₂), the primary anthropogenic greenhouse gas contributing to global warming, makes research directed towards carbon capture and storage (CCS) imperative. In the past few years, among the available adsorbents, biochar has drawn significant interest as a promising carbon-based material for low-temperature CO₂ capture from flue/fuel gas (such as biogas or gasification-derived syngas) owing to its environmentally friendly nature, cost-effective and facile preparation method, and sustainable adsorption performance. This work provides a review of recent studies on the development of biochar from biomass feedstocks and its subsequent modification through various approaches, including physical, chemical and physicochemical activations for post-combustion CO₂ capture. An overview of the factors, including pyrolysis temperature, heating rate and time, and different modification methods, affecting the physicochemical attributes of biochar such as surface area, microporosity, surface properties and functional groups is presented. Biochar with a large micropore volume, a narrow microporosity (0.3–0.8 nm) and basic surface characteristics would be effective in adsorbing CO₂ molecules. In this regard, physical modification of biochar is closely related to pore formation, whereas chemical modification emphasizes the creation of oxygen and nitrogen-containing functional groups; hence, they contribute to the enhanced CO₂ capture through porosity development and surface chemistry alteration, respectively. Biochar has presented a strong selectivity towards CO₂ compared to other gasses and has revealed a sustainable performance in multi-cycles of CO₂ adsorption–desorption; these are crucial features to ensure the large-scale application of biochar for CO₂ capture.

1 Introduction

Nowadays, the issues of global climate change have attracted worldwide attention. The pollutant gasses such as nitrogen oxides (NO_x), carbon monoxide (CO), carbon dioxide (CO₂)_, and sulphur dioxide (SO₂) are generated from energy sectors such as industrial plants, thermoelectric power plants, and combustion of fossil fuels [1]. In between, CO₂ emission from fossil fuel combustion is considered the main contributor to greenhouse gas (GHG) emissions. In addressing these environmental concerns, there is a continuous effort by the scientific community in proposing the priority actions and sectors that require a detailed look to fulfil net-zero emissions in 2050 [2]. The United Nations has taken the responsibility to manage a series of discussions at the international level to develop appropriate guidelines for accommodating the climate change impacts. As a result, the Kyoto Protocol was adopted on 11 December 1997 and enforced on 16 February 2005, focusing on the industrial countries to reduce GHG emissions. They have been recognized as the main contributor to the current high levels of GHG emissions in the atmosphere [3]. A decade after Kyoto Protocol, the Paris Agreement was adopted in Paris on 12 December 2015. The involved countries aimed to achieve a neutral climate by limiting global warming increase to 2 ℃ above the pre-industrial levels [4]. However, predictions on worldwide energy-related CO₂ emissions propose the CO₂ release from the energy sector will increase by 6%, from 33 Gt in 2015 to 35 Gt in 2050 [5]. Hence, to ensure the reduction of CO₂ concentration in the atmosphere, implementing efficient Carbon Capture and Storage (CCS) technologies, especially for large scale applications, are of great interest.

CO₂ capture technologies can be classified into three groups: pre-combustion, oxy-fuel combustion and post-combustion CO₂ capture. Interestingly, post-combustion CO₂ capture technologies are more favourable due to better compatibility with the existing gas emission control systems and low technological risk [6]. Solvent absorption, adsorption with solid adsorbents, cryogenic separation, and membrane separation are commonly well-known methods for post-combustion CO₂ capture [7]. Among these methods, adsorption with solid sorbents is preferred because of its ability to comply with a broad range of temperatures, low energy consumption, and ease of adsorbent regeneration [8]. Over the past few years, many types of adsorbents have been studied for CO₂ adsorption, including metal–organic frameworks (MOFs), zeolites, metal oxides, ion-exchange resins, layered double hydroxide, activated carbons, mesoporous carbon, and carbon nanomaterials [9,10,11,12,13]. Even though these materials exhibit excellent CO₂ adsorption performance, their use at a large scale has some drawbacks, such as high operational cost and adsorption competition issues [14]. In striving to find sustainable and cost-effective adsorbents, biochar has attracted considerable attention and has become a research hotspot as a valuable material to combat the global climate change problem. Biochar is a carbon-based solid product obtained from the thermal processing of biomass through various methods, including pyrolysis, gasification, torrefaction and hydrothermal carbonization [15, 16]. It has many multifunctional properties that are affected by the type of feedstock and production condition. Biochar can be generated from various biomass feedstocks such as wood and woody biomass [17,18,19], crop residues [20, 21], animal manure [22, 23], food waste [24, 25] and sewage sludge [26,27,28], as presented in Fig. 1.

Fig. 1

Different biomass feedstocks for the production of biochar

Biochar has found numerous applications in the fields of environmental remediation (adsorption of different contaminants, heavy metals, nitrogen and phosphorous) [29,30,31], agriculture (improvement of soil fertility, stabilizing soil nutrients, and reduction of soil GHGs emission) [32, 33], climate change (adsorption of pollutant gases such as NO_x, SO_x, H₂S and GHGs) [34, 35], and material science (development of catalyst, building materials and batteries) [36, 37]. Figure 2 visualizes the word cloud of the most frequently used keywords in journal articles in the field of biochar application in 2021(bibliographic data from Scopus).

Fig. 2

Visualized word cloud of the most frequently used keywords in journal articles in the field of biochar applications. Bibliographic data were extracted from the Scopus database, with a total of 1970 articles only in the year 2021

Recently biochar has emerged as a material of interest for carbon capture. To reflect this interest in research studies, the Scopus database was used to retrieve the number of articles per year from 1991 to March 13, 2022, with the keywords “char” AND “CO₂ adsorption” OR “CO₂ capture” OR “carbon capture”. The results of this analysis are reflected in Fig. 3, which shows the growing scientific interest in using biochar-based adsorbents for CO₂ capture in recent years.

Fig. 3

Annual scientific growth in the number of published articles containing the keywords “char” AND “CO₂ adsorption” OR “CO₂ capture” OR “carbon capture”. The bibliographic data were obtained from Scopus from 1991 to March 13, 2022

The type of sorbent used in CO₂ capture significantly impacts the CO₂ adsorption capacity [38]. In this regard, sorbents can be categorized into zeolites, MOFs, metal-oxide-based adsorbents and carbonaceous materials. Amongst all, activated carbons are promising carbonaceous materials that present good thermal, chemical and mechanical stability [39]. Despite their advantages, they have relatively low selectivity towards CO₂ over other gases such as N₂ and CH₄ [40]. Zeolites demonstrate an excellent selectivity to CO₂, yet they show a poor adsorption uptake and stability in the presence of moisture and impurities (such as NO_x, SO_x) [41]. MOFs present high CO₂ adsorption at elevated pressures; nevertheless, they have a lower CO₂ uptake than other adsorbents at low CO₂ partial pressures [42]. Even though MOFs possess high CO₂ adsorption capacity, their large-scale manufacturing cost should be considered due to the use of expensive precursors [43]. Metal-oxides, such as calcium oxide (CaO), are extensively used to capture CO₂ at a large scale. However, a major problem of CaO-based adsorbents is the sintering of adsorbent particles during the regeneration stage, which drastically reduces their adsorption capacity [44]. In striving to develop sustainable, low-cost CO₂ adsorbents, biochar has become a research hotspot. Biochar can be easily produced from various abundant and low-cost materials, such as woody and crop residues which also addresses the waste disposal problem in many agro and forestry-based industries. In comparison with activated carbon, the break-even price of biochar is about one-sixth of activated carbon [45]. Moreover, the average energy consumption for the production of biochar (6.1 MJ/kg) is significantly lower than that of activated carbon (97 MJ/kg) [46]. With all the facts from the economic points of view, biochar is one of the potential candidates for producing low-cost adsorbents for CO₂ adsorption with the potential to be used in large-scale operations.

Biochar has shown promising potential as a CO₂ adsorbent, yet the adsorption uptake of pristine biochar is not very high as it does not have a well-developed porous structure and has poor surface chemistry. Therefore, physical and/or chemical modifications are usually implemented to enhance the CO₂ capture capacity of biochar [47,48,49]. In this context, this review demonstrates the potential of pristine and modified biochar derived from various biomass feedstocks for CO₂ capture based on the literature data. An outline of the parameters influencing the microstructure and surface chemistry of biochar, including pyrolysis conditions and the type of modification approaches, is provided. The performance of pristine and modified biochar in CO₂ adsorption is compared, and the mechanisms through which the CO₂ uptake capacity of modified biochar is enhanced are extensively discussed. Apart from that, the selectivity and reusability of the modified biochar are also elucidated. To the best of our knowledge, reviews covering such aspects of biomass-derived biochar for CO₂ capture are only a few. This review provides advanced access to emerging ideas on the current trends for the development and implementation of biochar to control CO₂ emissions from various emission sources. A complete overview starting from CO₂ capture technologies and ending with the challenges of using biochar as CO₂ adsorbent would provide insightful information that will be beneficial for the scientific community and those working on air pollution control and related biochar applications.

2 CO₂ capture technologies

The CCS technology captures and stores CO₂ before entering the atmosphere. CCS can be applied at large-scale emission sources, including natural gas processing, coal and gas-fired power generation, and manufacturing industries such as pulp, paper, cement, iron, and steel [50,51,52]. Figure 4 depicts a scheme of CCS technologies, including pre-combustion, oxy-fuel and post-combustion CO₂ capture processes.

Fig. 4

Diagram of CO₂ capture technologies, including pre-combustion, oxy-fuel combustion and post-combustion capture. Adapted with permission from [292]

The principle of pre-combustion technology is to capture CO₂ from the syngas after converting CO into CO₂ [53]. Initially, a fuel is reacted with air to produce a gas that is rich in CO and hydrogen (H₂). Then, the reaction of CO with the steam forms CO₂ and H₂ via water–gas shift (WGS) reaction, where CO₂ is then separated using chemical absorption processes such as those applied in Purisol, Fluor, Rectisol and Selexol, as presented in Table 1. Meanwhile, H₂ can be directly consumed as fuel. It is convenient to adsorb CO₂ since the CO₂ concentration is relatively high. The advantages of the pre-combustion technology include: the gas volume needed in the pre-combustion capture is significantly reduced because the processing takes place before the syngas is diluted with the combusted air [54], and the CO₂ is produced under pressure; therefore, less compression is required for CO₂ storage and transportation [55]. Even though this technology fulfils the industrial scale specification, retrofitting the existing plants is complicated and costly. Moreover, the syngas must be dried before the CO₂ separation can be performed [56].

Table 1 Capturing solvent and disadvantages of different pre-combustion technologies

In the second approach, oxy-fuel combustion, the fuel is burned in nearly pure oxygen instead of air, which induces a flue gas stream consisting of CO₂, water, and other trace impurities. Pure oxygen is obtained from an air separation unit that separates oxygen from nitrogen. The advantage of using this method is that it can be employed in the existing or new power plants along with the utilization of various biomass feedstocks [59]. In the oxy-fuel CO₂ capture process, cryogenic distillation is found to be the most suitable process for producing high purity oxygen for a large-scale operation [60]. However, the major drawback of this technology is that the supply of expensive pure oxygen and the high energy consumption for oxygen separation from the air would prevent the applicability of this method for CO₂ capture [59, 60].

As the third approach, CO₂ is directly captured from flue gas streams after combustion using wet or dry adsorbents in post-combustion technology. Generally, the fuel is combusted with air in a boiler to produce steam in a coal-fired power generation system. Then, electricity will be generated using a turbine [61]. The flue gas produced is mainly composed of CO₂ and N₂. At present, solvent scrubbing using amine solution is a promising method to react with CO₂ in the flue gas and produce purified CO₂ that can be compressed for storage [62]. The post-combustion technologies can be divided into (i) absorption-based, (ii) adsorption-based and (iii) membrane-based post-combustion processes. Table 2 represents the advantages and disadvantages of different methods for post-combustion technologies.

Table 2 Advantages, disadvantages and CO₂ capture performance of various post-combustion technologies

Chemical absorption is a favourable approach for absorbing CO₂ from the flue gas streams consisting of low to moderate CO₂ partial pressures in the range of 3–20% [56]. Absorption can be explained by the use of a liquid to separate the gaseous component from the flue gas, and this liquid is known as an absorbent or solvent for CO₂ capture. In this process, the gas phase is turned into a liquid phase as the gaseous components contact the absorbent. Various chemical absorption processes, including amine absorption, aqua ammonia absorption, dual alkali approach and sodium carbonate slurry, have been widely used for carbon capture and storage [63,64,65,66,67,68].

CO₂ adsorption using solid adsorbents is one of the well-known methods to reduce the CO₂ concentration in the atmosphere. The solid material is known as adsorbent, while the adsorbed CO₂ gas is referred to as adsorbate. During the adsorption process, the gaseous constituent comes into contact with a solid adsorbent, where CO₂ molecules are adsorbed onto the solid surface. Most adsorbents experience a severe reduction in sorption capacity at high adsorption temperatures [56]. The interaction of CO₂ with the biochar surface could be through weak physical adsorption (physisorption) or strong chemical reaction (chemisorption), or a combination of the both, depending on the structural features and surface chemistry of biochar, as well as the implemented adsorption condition (such as temperature and pressure) [47, 69, 70]. Physisorption is often associated with a lower heat of reaction compared to chemisorption [56]. After the adsorption process is completed, the desorption stage is conducted, where CO₂ is removed from the adsorbent, and the adsorbent is consequently regenerated. A number of techniques related to regeneration of the adsorbent can be enumerated [71, 72]: (1) PSA: pressure-swing adsorption [73], (2) TSA: temperature-swing adsorption [74], (3) PTSA: pressure and temperature-swing adsorption [69], (4) VPSA: vacuum pressure-swing adsorption [75], (5) ESA: electric-swing adsorption [76], (6) RPSA: rapid pressure-swing adsorption, and (7) URPSA: ultra-rapid pressure-swing adsorption [72]. Among the listed adsorption methods, TSA and PSA are the two most commonly applied techniques in the adsorption–desorption of biochar. In the TSA system, CO₂ is desorbed from the adsorbent as the temperature of the system is increased. Whereas in the PSA system, the adsorption is performed at elevated pressures; consequently, reducing pressure within the system releases CO₂ from the solid material. The benefit of a PSA system is that the regeneration can be accomplished in a few seconds compared to hours in the TSA system [77]. However, in the TSA system, the solid adsorbent can still be regenerated while preserving a high CO₂ partial pressure [56].

Recently, membrane separation processes have been commercially used to remove CO₂ from the natural gas streams [78], consisting of CH₄ and CO₂, where the CO₂ concentration and the overall pressure are relatively high. The membrane separation strongly relies on selectivity and permeability. Here, CO₂ is selectively separated from the other gas components and transported to the other side of the membrane by the use of a permeable or semi-permeable membrane [56]. For efficient separation, it is suggested that the flue gas must be pre-treated to avoid any impurities such as NO_x and SO_x, which can cause an adverse effect during the separation process [56].

From the economic perspective, pre-combustion technology could offer a lower cost than oxy-fuel and post-combustion technologies by approximately 21–24 and 38–45%, respectively [59]. However, the additional cost and the complexity of setting up the process due to the retrofitting of current equipment may limit its commercialization. Among the CCS technologies, post-combustion CO₂ capture is a widely used technique to tackle escalating CO₂ concentrations [59]. Most power plants favour the adsorption of CO₂ after a complete occurrence of the reaction [79]. Ideally, an efficient adsorbent for post-combustion CO₂ capture must present a high CO₂ capture capacity and stability, high selectivity and low manufacturing cost for large-scale operations. In addition, a detailed design of the process in the adsorption/desorption cycles is essential to minimize the energy consumption in the post-combustion operating conditions [80].

3 Biochar production

Biochar could be developed through conventional pyrolysis, flash carbonization, gasification, torrefaction and hydrothermal carbonization, as illustrated in Fig. 5.

Fig. 5

Production of biochar through various thermochemical processes. Adapted with permission from [101]

Conventional pyrolysis under continuous inert gas flow can be categorized into slow, fast, and flash pyrolysis. This thermal process produces three main products; biochar, bio-oil and non-condensable gaseous such as carbon monoxide, hydrogen, methane and carbon dioxide [95]. In fast pyrolysis, the primary product is bio-oil, which constitutes 50–75% of the feedstock mass. The reaction normally occurs at 450–600 ℃ for less than 10 min at a heating rate of 16–150 ℃/min [96,97,98]. An improved form of fast pyrolysis is flash pyrolysis which operates at a temperature ranging from 600–1300 ℃, which can be attained within 3 min. Conversely, lower pyrolysis temperature and slow heating rates contribute to higher char yield, as represented by slow pyrolysis. The process is performed at the temperature range of 300–900 ℃ for about 1.5 h, depending on the process condition. The primary product is biochar, which relatively accounts for 25–35% of the feedstock mass [99]. Apart from conventional pyrolysis, an advanced pyrolysis technique known as microwave-assisted pyrolysis is applied, which is a rapid, efficient, selective and controllable technique to obtain solid, liquid and gaseous products from biomass. Microwave-assisted pyrolysis has manifold benefits compared to conventional pyrolysis, such as volumetric heating, energy transfer rather than heat transfer, non-contacting heating and heating from the inside material body [100]. In flash carbonization, biochar is produced from biomass feedstock at the following conditions: (1) temperature: 300–600 ℃, pressure: 1–2 MPa, and residence time: 30 min [101, 102]. In this process, biomass is efficiently converted into biochar with 70–80% fixed carbon content, and biochar yield is approximately 40–50% [103].

Gasification is performed at high temperatures in the range of 600–1000 ℃ for 2–3 h using a gasifying agent such as steam, air and oxygen. This process involves two steps: (1) production of biochar and volatile matter through pyrolysis and (2) syngas production by gasification of biochar and secondary cracking of volatile matters [104]. The main product generated is a non-condensable gas rich in carbon dioxide and hydrogen. However, the biochar yield is relatively low (5–10% of the feedstock mass) as most organic compounds are gasified into gas [105].

Torrefaction is a thermochemical treatment process conducted at a lower temperature, around 200–300 ºC for 15–120 min [106], where the biomass is subjected to slow heating in an inert condition. It is also referred to as mild pyrolysis, as the heating condition is similar to pyrolysis, generally performed at a temperature of 350 to 650 ℃ [107]. During torrefaction, the biomass decomposes slowly and emits H₂O and CO₂. With increasing torrefaction temperature, the elemental compositions (carbon, hydrogen, nitrogen, oxygen), the biochar’s yield and volatile matter decrease, while higher heating value (HHV), fixed carbon and ash content increase [108].

The hydrothermal carbonization (HTC) process, also known as wet torrefaction, is carried out in subcritical water [109] under autogenous pressure (0.3–4.0 MPa) [110], where the raw material is heated in the hydrothermal reactor at a temperature between 170 and 260 ℃ for 15–90 min. This process generates three main products, namely: solid products (hydrochar), aqueous compounds and small fractions of gases (major gas: CO₂) [111]. Interestingly, a carbonization reaction is performed in water at a temperature lower than that of pyrolysis. Moreover, during the HTC process, the ash content could be reduced as the inorganic compound can be washed away into the liquid phase [112]. Among all the biochar production methods, slow pyrolysis has a higher production yield (25–35%). Although the hydrothermal process operates at temperatures less than 300 ℃, which is lower than that of pyrolysis temperature, the hydrochar needs to undergo the drying process for 24 h before being subjected to any modification techniques [113, 114]. Additionally, no “high-end equipment” is required to synthesise the biochar in slow pyrolysis. Table 3 summarizes the thermochemical processes for biochar production.

Table 3 Thermochemical conversion techniques for biochar production and their process conditions

4 Physicochemical characteristics of biochar for CO₂ capture

4.1 Surface area and porosity

The physicochemical characteristics of biochar are crucial for CO₂ uptake and depend on various factors. These parameters include feedstock properties, pyrolysis temperature, residence time and heating rate, and the implemented modification technique (physical, chemical or physicochemical treatment) [124,125,126,127]. In the case of gas adsorption, the development of highly microporous biochar with a large specific surface area is desired [47]. The porous structure of biochar is created during the pyrolysis of feedstock due to the volatilization of organic matters [128]. According to the International Union of Pure and Applied Chemistry (IUPAC), the distribution of pore size is as follows: micropores (< 2 nm), mesopores (2–50 nm) and macropores (> 50 nm) [129]. Figure 6 shows the porosity type and the possible functional groups on the carbon structure of biochar.

Fig. 6

Morphology and the presence of surface functional groups on biochar. Adapted with permission from [37]

Studies have indicated that for efficient CO₂ capture at 1 bar, it is necessary to generate a high volume of micropores with pore size in the range of 0.5–0.7 nm [130]. For example, Dang et al. [131] who obtained biochar from pine nut shell modified by KOH that had a pore size between 0.33 and 0.63 nm, reported an excellent CO₂ capture of 220 mg/g at 25 ℃ and 1 bar. Studies have also reported that the development of micropores has a greater impact on CO₂ capture compared to total pore volume and surface area development [131, 132]. Notably, pores less than 0.8 nm significantly contribute to CO₂ uptake at 1 bar, while pores with a diameter smaller than 0.5 nm capture CO₂ molecules at low partial pressure (0.1 bar) [130]. This is consistent with the previous finding that the maximum CO₂ adsorption (145.20 mg/g) at 0 ℃ and 0.15 bar was observed for the biochar with micropores in the range of 0.33–0.50 nm [131]. The kinetic diameter of CO₂ (0.33 nm) is relatively smaller than methane (0.38 nm) and nitrogen (0.364 nm) [133]. Therefore, the CO₂ adsorption will be facilitated if the adsorbent has a pore size close to the CO₂ diameter.

4.1.1 Effects of pyrolysis temperature

The structural properties and surface chemistry of biochar determine its performance in CO₂ adsorption. Apart from the properties of biomass feedstock, pyrolysis conditions, especially pyrolysis temperatures, have considerable impacts on biochar characteristics [134, 135]. Previous studies demonstrated that pyrolysis temperature plays a crucial role in producing biochar which presents high surface area and micropore volume with superior CO₂ adsorption capacity [20, 117, 136].

When biomass undergoes pyrolysis, pore development occurs due to the loss of water molecules in the dehydration process and the release of volatile matters from the carbon structure of biochar. At low pyrolysis temperature (< 400 ℃), this condition is not adequate to complete the devolatilization of volatile matters; thus, the creation of new pores is hindered [137, 138]. As the temperature goes up to 500 ℃, more volatiles are released, creating sparse regions, leading to cracks in the material and, consequently, developing more pores [139, 140]. At high pyrolysis temperatures (500–900 ℃), the generated energy could be used to develop microporosity and boost the evolution of pore structure [49]. Table 4 displays the effects of different pyrolysis temperatures on the pristine biochar properties derived from various biomass feedstocks. The generally observed trend is that surface area and micropores volume/total pore volume increase as the pyrolysis temperature is increased. It should be noted that at high pyrolysis temperatures (> 900 ℃), softening and sintering of the high molecular weight volatiles may occur, resulting in the shrinkage of the total pore volume of biochar. As such, the extreme pyrolysis temperature reduces the micropore volume and surface area [141]. Hence, the pyrolysis temperature should be carefully controlled to obtain a suitable microporosity and surface area for a high CO₂ uptake. A literature survey suggests that the pyrolysis temperature of 400–900 ℃ is suitable for converting biomass feedstocks to biochar [20, 142, 143].

Table 4 Effect of pyrolysis temperature on pristine biochar properties derived from various biomass feedstocks

In general, biochar yield decreased over the temperature of 300–900 ℃ [144,145,146]. At higher temperatures, the rapid decomposition of lignocellulosic components reduces biochar yield [144]. Moreover, more volatile matters are released as the biomass is heated up [145]. Lahijani et al. [117] pyrolyzed walnut shells at three different temperatures (500, 700 and 900 ℃) under N₂ gas for 90 min. The obtained char yields were 31.7, 28.4 and 23.8% with the respective temperatures. Even though the biochar yield decreased at 900 ℃, the highest micropore volume of 0.159 cm³/g was obtained for this sample. In this context, the development of micropores should be taken into account when considering the optimum pyrolysis temperature in producing biochar as CO₂ capturing medium. Pyrolysis has been carried out under different gas environments such as nitrogen (N₂), carbon dioxide (CO₂), helium (He), argon (Ar), and steam (H₂O) [147,148,149,150,151]. Among all, nitrogen is the most popular and extensively used gas due to its availability, cost-effectiveness and inert behaviour [54]. Guizani et al. [152] reported that the char yield produced under N₂ gas (13.10%) was higher than that obtained in the CO₂ atmosphere (11.32%). According to them, the additional mass decay in the char pyrolyzed under CO₂ could be explained by CO₂ gasification of char, which occurred concurrently with biomass pyrolysis. Gas flow rate is also an important parameter during the pyrolysis, which may affect the char yield. A high gas flow rate removes volatile matters faster from the hot zone, reducing secondary exothermic reactions such as thermal cracking, partial oxidation, repolymerization and recondensation, leading to the reduction of char formation [153]. In a study by Liu et al. [153], peanut shells were carbonized at 500 ℃ for 60 min. N₂ gas at various flow rates (20, 50, 100, and 200 ml/min) was used for carbonization. They found that the obtained biochar yield reduced approximately from 35 to 28%, as the gas flow increased from 20 to 200 ml/min. Similar results were reported in the production of laurel residue-derived biochar; when the nitrogen flow rate was increased from 50 to 400 ml/min, the biochar yield reduced from 28.48 to 27.2% [154]. Therefore, the selection of the appropriate carbonization gas and its flow rate is important to obtain a high yield of biochar.

4.1.2 Effect of pyrolysis holding time and heating rate

Apart from pyrolysis temperature, holding time and heating rate are other two factors that influence the development of micropores and surface area. Increasing the pyrolysis time boosts the rudimentary pore generation as the carbon surface releases the volatile matter [49]. However, prolonged pyrolysis time at high temperatures may lead to an intermediate melt formation due to progressively softening and sintering of the low molecular weight volatiles [141]. Here, the intermediate melt could partially block the pores, thus reducing the surface area. Lua et al. [141] observed that a maximum surface area of 519 m²/g and micropore volume of 0.215 cm³/g was achieved at 120 min of residence time for oil palm shells. While prolonging the residence time to 180 min reduced the surface area and micropores volume to 380 m²/g and 0.155 cm³/g, respectively. On the other hand, insufficient holding time to release the volatiles would result in the accumulation of these matters between and within the particles, and thus the deposition of these matters causes pore entrance blocking. In terms of biochar yield and fixed carbon content, Yang et al. [158] found that a holding time of 120 min produced a high yield and fixed carbon content of 32.67% and 79.38%, respectively, using pruned apple tree branches. Generally, the literature survey shows that a holding time between 60 and 120 min is suitable to improve the surface area and porosity of biochar for CO₂ uptake [141, 158, 159].

The heating rate is strongly associated with heat and mass transfer inside the particles. At a low heating rate, the reaction is relatively slow, while, at a high heating rate, the reaction rate is more pronounced due to progressive heat and mass transfer [49]. For example, increasing the heating rate from 1 to 20 ℃/min increased the surface area of rapeseed stem-derived biochar from 259.9 to 384.1 m²/g and micropores volume from 0.097 to 0.116 cm³/g [159]. However, an excessive heating rate also melts the biochar particles and likely smooths the biochar surface [126]. In a study carried out by Angin et al. [160], a reduction in the surface area and micropores volume from 4.23 to 3.64 m²/g and 0.0067 to 0.0057 cm³/g, respectively, was experienced with an increase of the heating from 10 to 50 ℃/min. Chen et al. [127] reported that increasing the heating rate from 5 to 30 ℃/min increased the surface area and micropores volume of biochar from ~ 400 to 411.06 m²/g and ~ 0.120 to ~ 0.125 cm³/g, respectively. Further increasing the heating rate to 50 ℃/min reduced the surface area and micropores volume to 385.38 m²/g and ~ 0.10 cm³/g, respectively. In conclusion, the available literature suggests that a heating rate in the range of 5 to 30 ℃ is preferable for promoting the evolution of biochar porosity and its surface area development for CO₂ adsorption application [127, 159].

4.2 Biochar pH

Generally, biochar is alkaline, and its pH is around 8.0 to 11.0 (Table 4). Studies have indicated that pyrolysis temperature influences the biochar pH [161]. The relevance of pyrolysis temperature and pH of several biochar samples derived from various biomass feedstocks, such as animal manure, woody biomass, and agricultural residues, is shown in Table 4. The data demonstrate a positive correlation, where the biochar pH increases as the pyrolysis temperature is increased. It should be highlighted that increasing the pyrolysis temperature results in higher pH of biochar due to the disappearance of the acidic group at higher temperatures [162]. Conversely, at low temperature (< 300℃), the acidic value could be attributed to the remaining organic acids and phenolic constituents resulting from the decomposition of cellulose and hemicellulose on biochar surface [163]. For example, Al-wabel et al. [164] showed that with the increase of the temperature from 200 to 800 ℃, the pH of the biochar derived from Conocarpus wastes increased from 7.37 to 12.38, corresponding to the decrease of acidic surface groups from 4.17 to 0.22 mmol/g biochar, which was determined by Boehm’s titration. Yuan et al. [165] reported that pyrolysis temperature above 300 ℃ for canola straw, corn straw, peanut straw and soybean biochar might lead to the formation of carbonates (i.e. MgCO₃, CaCO₃), thus resulting in pH increment up to 10.76, 11.32, 11.15 and 11.10, respectively. However, at lower pyrolysis temperature (200 ℃), Zhang et al. [163] showed that the biochar derived from wheat straw and lignosulfonate had acidic pH ranging from 4.87–6.11. Similar to this study, vegetable waste and pine cone-derived biochar also exhibited acidic pH of 5.95 and 4.15, at the same pyrolysis temperature of 200 ℃ [155]. The acidic condition is due to the decomposition of cellulose and hemicelluloses at temperatures around 180–250 ℃, which produce organic acid and phenolic compounds that remain on the biochar surface and lower the pH of the biochar.

Ash content also has a significant effect on the pH of biochar. An increase in the pyrolysis temperature results in higher ash content of biochar, thus affecting its pH. In a study by Ghaffar et al. [166], as the pyrolysis temperature increased from 350 to 500℃, the higher ash content and the removal of acid functional groups (such as carboxylic (-COOH), phenolic (-C₆H₅) and carbonyl (-C = O) groups) from the Brazilian pepper-derived biochar surface contributed to the increment of pH from 7.72 to 9.65. This is a generally observed trend, but the observed results are not the same in some cases, probably depending on biomass feedstock. Therefore, pyrolysis temperatures above 400 ℃ are appropriate to develop biochar with basic characteristics. Due to the acidic character of CO₂ gas, the interaction with basic biochar would be feasible for CO₂ adsorption. According to the literature, it can be concluded that higher pyrolysis temperatures lead to increased biochar surface area and microporosity with a higher concentration of basic functional groups, which are beneficial attributes for CO₂ capture [134, 156, 163, 166, 167].

4.3 Surface functional groups

Besides the surface area and microporosity of biochar, surface functional groups on biochar also play a significant role in determining the surface chemistry of biochar and thus its CO₂ adsorption performance [168, 169]. At low adsorption pressures, the surface functional groups contribute to the enhanced CO₂ capture performance, regardless of the porosity of biochar. While, at high pressures, for the pores larger than 1.0 nm, the surface functional groups play an important role in giving a higher CO₂ adsorption capacity [168]. Here, basic surface characteristics and high aromaticity are desirable to ensure the high CO₂ capture capacity of the biochar, which are discussed in the following.

4.3.1 Surface basicity

Basically, the CO₂ adsorption can be improved by increasing the biochar surface alkalinity [117]. Here, the oxygen and nitrogen surface functional groups have been recognized as the main contributors to carbon’s surface acidity and alkalinity [170, 171]. According to Boehm et al. [170], surface functional groups can be categorized into acidic, basic, and neutral types. Oxygen-containing functional groups are mainly acidic, as shown in Fig. 7. Specifically, lactol, phenols, lactones and carboxylic acid have been postulated as the sources of surface acidity [172]. However, not all the oxygen-containing groups tend to show acidic characteristics. For instance, chromene, ketone and pyrone are more likely to be basic groups and contribute to surface basicity [125].

Fig. 7

Acidic and basic oxygen functionalities on biochar surface. Adapted with permission from [171]

In the case of CO₂ gas adsorption with an acidic character, biochar with basic surface functional groups is much favoured. In this regard, acidic oxygen functionalities are not beneficial for CO₂ adsorption. The relationship between thermal desorption temperature and related desorption products during temperature-programmed desorption (TPD), as illustrated in Fig. 8, indicates that acidic oxygen functional groups such as phenol, lactone and carboxyl dissociate to CO and CO₂ at the temperature range of 100–700℃ (373-973 K). Most basic functional groups decompose to CO₂ and CO above 600 ℃ (873 K) except anhydride, which starts to decompose at 350 ℃ (623 K). It can be concluded that most oxygen-containing acidic groups could be removed from the biochar surface at high temperatures, thus producing biochar favourable for adsorption of CO₂ acidic gas.

Fig. 8

Surface oxygen-containing groups and their decomposition by TPD. Adapted with permission from [293]

On the other hand, the presence of nitrogen-containing groups such as pyrrolic, pyridinic, lactam, imide, and amide enhance biochar’s surface basicity [48]. These functional groups can be incorporated into the biochar surface using different nitrogen-containing reagents such as ammonia, amines, nitric acid, and other nitrogen-containing precursors (i.e. melamine, polyacrylonitrile) through biochar activation [173, 174]. In an attempt to increase the surface basicity of biochar for enhancing the CO₂ uptake, Yaumi et al. [175] impregnated rice husk with melamine, and by introducing the N-containing group onto the biochar surface, the concentration of basic surface groups increased from 1.43 to 4.10 mmol/g, corresponding to improved surface alkalinity.

4.3.2 Aromaticity

Aromaticity is a chemical property that facilitates CO₂ adsorption on the biochar surface. Fixed carbon fraction is strongly related to biochar’s aromaticity produced from the pyrolysis and gasification [176]. High aromaticity can indicate the carbon stability and its resistance to biodegradation [177], and van Krevelen diagram is used to determine the degree of aromaticity and maturation of char based on atomic H/C and O/C ratios. A low ratio of H/C and O/C (< 0.2) indicates that the biochar is chemically stable [178, 179]. Aromaticity and hydrophobicity are interrelated properties; when the aromaticity of biochar increases, consequently its hydrophobicity enhances [47]. Biochar with non-polar and hydrophobic characteristics may favour the sorption of CO₂ molecules by limiting the accessibility of H₂O molecules on the biochar surface [48]. It was reported that biochar derived from white oak possessed an extremely low O/C ratio of 0.051, implying low polarity and high hydrophobicity [180, 181], which both factors contribute to the enhancement of CO₂ sequestration.

4.4 Elemental composition of biochar

In general, the elemental composition of biochar is highly affected by pyrolysis temperature. The carbon content increases as a function of pyrolysis temperature. By increasing the pyrolysis temperature, the heat treatment-driven loss of the OH functional group from the lignocellulosic biomass occurs due to dehydration, resulting in the disappearance of H and O atoms [182]. Additionally, the elimination of water, CO₂, CO, hydrocarbons, and tarry vapours during the carbonization contributes to the decrement of H, O and N contents as the biomass is heated up [183]. Moreover, the losses of H and C at the elevated temperature may result from the breakage and cleavage of the weak bonds in the carbon structure [184, 185]. Accordingly and as the data in Table 4 show, the H, O and N contents reduce when the pyrolysis temperature increases.

The elemental contents data obtained from CHNS analysis provides insightful information about the chemistry of biochar. For example, the O/C, H/C and (O + N)/C values are known as hydrophobicity, aromaticity and polarity indexes, respectively [186,187,188]. A high H/C ratio suggests a low degree of aromaticity and carbonization, while high O/C and (O + N)/C ratios indicate low hydrophobicity and high polarity, respectively [186]. In the case of CO₂ adsorption, biochar with lower H/C and O/C presents better efficiency in the adsorption. Zubbri et al. [87] investigated the effect of various thermochemical treatments on the CO₂ adsorption capacity of the biomass-derived adsorbents. The implemented treatments included hydrothermal carbonization (at 170 °C) to obtain hydrochar, pyrolysis (850 °C) to obtain biochar, and KOH impregnation of hydrochar followed by activation (850 °C); the samples were designated as HC _170–90, biochar _850–120 and HC-2KOH _850–120, respectively. They observed that the value of O/C reduced along with increasing the severity of thermal treatment in the following order HC _170–90 (1.51) > HC-2KOH _850–120 (1.18) > biochar _850–120 (0.86). At high temperatures, the reduction of the hydrophilic sites may be attributed to the dehydration process (loss of O- and H- functional groups), making the char surface more hydrophobic. Similarly, the H/C ratio reduced with the thermal treatment processes in the following order HC _170–90 (0.38) > HC-2KOH _850–120 (0.34) > biochar _850–120 (0.18), indicating higher aromaticity. Other than that, the polarity index, represented by the ratio of (O + N)/ C decreased with heat treatment. Among all samples, the KOH activated hydrochar (HC-2KOH _850–120) possessed the highest adsorption capacity of 122.37 mg/g at 30 ℃ and 1 bar. In another study by Gargiulo et al. [86], the authors obtained cellulose fibres-derived biochar by employing steam-assisted slow pyrolysis at various temperatures (600, 650 and 700 ℃). The biochar pyrolyzed at 700 ℃ exhibited the lowest H/C ratio of 0.09 among the prepared biochars. In this case, high aromaticity was closely related to the stability of biochar which enhanced CO₂ sorption capacity to 102.52 mg/g at 25 ℃ and 1 bar. In another attempt to investigate the performance of different biomass feedstocks towards CO₂ adsorption, Bamdad et al. [189] pyrolyzed softwood bark, softwood sawdust, hardwood and a mixture of softwood bark and sawdust at a temperature between 400 and 500 ℃. They found softwood sawdust pyrolyzed at 500 ℃ with the lowest H/C ratio of 0.03 showed the maximum CO₂ adsorption capacity of 105.60 mg/g at 20 ℃ and 1 bar compared to the other resultant biochars.

4.5 Analytical techniques to determine the physicochemical properties of biochar

After preparing pristine and modified biochar, it is important to carry out some characterization analyses on the biochar to gain some insights into the structural features of biochar and its surface chemistry. The common methods used to characterize the physicochemical properties of biochar include Raman spectroscopy, surface area and porosity analysis using Brunauer–Emmett–Teller (BET), Fourier transform infrared spectrometry (FTIR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), solid-state ¹³C nuclear magnetic resonance (NMR), and scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM–EDX), as shown in Fig. 9.

Fig. 9

Characterization methods used to analyse the physicochemical properties of biochar. Adapted with permission from [47]

Raman spectroscopy is normally used to determine the carbon structural characteristic of the biochar, where the two prominent peaks, which respectively represent the amorphous ($D$-band) and crystalline ($G$-band) region formed during the pyrolysis of biochar [190]. The surface area and porosity of biochar are measured via nitrogen adsorption at 77 K. The assessment of the micropore volume is usually accomplished by the Dubinin–Radushkevich (DR) method or the t-plot method [191]. Mesopore volume can be calculated by the difference between the total pore volume and micropore volume [49]. For pore size distribution, the Barrett-Joyner-Halenda (BJH) method is only suitable for calculating mesopores [192], while the density functional theory (DFT) is applicable for micropores and mesopores determination [49]. FTIR can be used to analyse the presence of the related functional groups within the wavelength between 4000 and 400 cm⁻¹, while XPS is used to determine the chemical state and the concentration of elements on the adsorbent surface. Specifically, the FTIR absorption peak and XPS binding energy for fresh and spent (after adsorption) adsorbent, which provide insightful information regarding the various surface functionalities, such as O and N-containing functional groups are crucial to interpret the CO₂ adsorption mechanism, which is highly influenced by the basic or acidic characteristic of the biochar surface [34, 190]. To examine the carbon structure and the mineralogical analysis of biochar, XRD could be performed [117]. NMR is commonly used to investigate the quantitative aromaticity and non-protonated aromatic fraction of the biochar [193]. SEM analysis is conducted to observe the changes in the surface morphology of biochar. Here, porosity development, pore widening and pore-clogging could be observed after the implementation of various modification techniques. Additionally, EDX is used to determine the elemental composition of the biochar surface [20].

5 CO₂ capture mechanisms by biochar

Various interactions can contribute to the adsorption of CO₂ on the biochar surface, but the most perceived ones include physisorption on micropores, van der Waals attractions and Lewis-acid base interactions by O and N containing-functional groups, as demonstrated in Fig. 10.

Fig. 10

Possible mechanisms involved in CO₂ adsorption of biochar. Adapted with permission from [195]

The adsorption performance of biochar greatly depends on its pore structure. Here, pore size distribution will dictate the diffusion rate of CO₂ molecules onto the biochar surface, where the surface area determines the number of active sites for adsorption to occur [194]. The micropore filling effect contributes to the physisorption of CO₂ [117, 195]. Biochars having micropores in the range of 0.3–0.8 nm are effective for CO₂ capture; specifically, those with pores below 0.5 nm are the most desirable ones [195]. In addition to this, the highly aromatic structure of biochar could enhance the physical adsorption of CO₂ via van der Waals attractions [195, 196]. Moreover, various functional groups, especially O and N-containing functional groups on biochar surface, contribute to the CO₂ adsorption either via hydrogen bonding and/or Lewis aid-base interactions [197, 198]. In general, the majority of O-containing functional groups are acidic, and hence would inhibit the adsorption of acidic CO₂. According to the acid–base interacting mechanism, the presence of acidic groups on the biochar surface would lead to a negative effect on CO₂ adsorption capacity. However, the inclusion of hydroxyl (-OH) and carboxyl (-COOH) in the biochar matrix may improve the hydrogen bonding interaction with CO₂ molecules [198, 199]. Here, the strongly electropositive H atom in hydroxyl and carboxyl groups interacts with the electronegative O atom in CO₂ to produce a hydrogen bond (O–H···O=C= O) due to the considerable electronegativity difference between O atom (3.5) and H atom (2.1) [200, 201]. This interaction is considered a weak hydrogen bonding compared to the other hydrogen bonding between O–H···O, and N–H···O, where H atom is covalently bonded to strong electronegative atoms (O and N atoms). Another mechanism that contributes to CO₂ adsorption is the interaction of basic N-containing functional groups with acidic CO₂ molecules, known as a Lewis acid–base reaction [201,202,203]. In general, pyridone, pyridine, amine, quaternary-N, pyridine-N-oxide, cyanide, and pyrrole groups are the most common types of N-containing functional groups with different basic strengths. Among the N-containing functional groups, pyridone, pyridine and pyrrole significantly affect the CO₂ adsorption [47]. Lim et al. [197] used density functional theories (DFT) to investigate the interaction of various N-containing functional groups with CO₂. They found that the binding energies (E_ads) estimated from various beginning configurations reveal how the functional groups interact with the CO₂ atom to determine which configuration is most beneficial for adsorption. CO₂ configurations with related N-functional group binding energies are shown in Fig. 11.

Fig. 11

Optimized geometry after CO₂ adsorption and the binding energy (E_ads) values for the different N-containing functional groups. Atom colours: C = grey, H = white, O = red and N = blue. Adapted with permission from [197]

In Fig. 11 (a), the E_ads value is less than -0.10 eV indicating weak van der Waals interactions. Pyridone possesses the highest binding energy of CO₂ (-0.224 eV) compared to the other N-containing functional groups. The adsorption behaviour of a pyridone with CO₂ is mostly determined by two types of interactions: Lewis acid–base and hydrogen-bonding interactions. For pyridine, the E_ads value is almost near to that of pyridine, which explains pyridinic-N is more favourable for adsorption of CO₂ due to its stronger electronegativity. In addition to this, it is possible that pyridinic-N prefers the electron-deficient C atom over the O atom in CO₂ molecules. Pyrrole interacts with CO₂ through hydrogen bonding, whereas the CO₂ interactions occur in pyridine-N-oxide group involving the reaction between the carbon atom of CO₂ with the oxygen atom of the functional group (-NO····C and -NH····O). The lowest E_ads value of -0.110 eV may be attributed to a weaker Lewis acid-basic interaction than that of pyridine group and the hydrogen bonding of pyridone group. Other N-functional groups, including cyanide, quaternary and amines, formed weak Lewis acid–base reaction with CO₂; hence, the presence of these three groups was less significant for adsorption of CO₂.

6 Modified biochar for CO₂ adsorption

Biochar has multifunctional properties that make it a promising adsorbent. The high availability of biomass feedstock is the key parameter for its cost-effectiveness, making it much cheaper than other available CO₂ adsorbents [48]. However, pristine biochar exhibits low CO₂ uptake due to its low microporosity and lack of enriched surface chemistry. Thus, the adsorption of CO₂ molecules on biochar should be enhanced through various modification techniques. From the surface chemistry perspective, the adsorption of CO₂ on pristine biochar is not very efficient as CO₂ is a weak Lewis acidic gas (electron acceptor) [204]. Strong acid–base interaction with the Lewis basic sites (electron donor) will promote the surface affinity and selectivity towards CO₂ molecules [205]. In this regard, biochar modification can be implemented through various methods using different activating agents and activation conditions [206] to produce biochar with desirable surface properties, thus enhancing the adsorption capacity. To obtain biochar with desired properties, the biomass feedstock is normally subjected to treatment before or after the carbonization. The following sub-sections highlight the modification of biochar through physical activations (CO₂ activation and steam activation), chemical activations (metalized-biochar, amino-modified biochar, alkali-modified biochar) and physicochemical activation (ultrasound-assisted amination). A scheme of the implemented modification methods and the routes through which each method affects the physicochemical characteristics of biochar is presented in Fig. 12.

Fig. 12

Different modification methods implemented on the biochar surface

6.1 Physical activation

Physical activation uses several oxidising agents such as steam, CO₂, and air at temperatures above 700 ℃ to increase the porosity of biochar [207]. The penetration of these oxidising agents into the internal surfaces followed by the carbon atom gasification results in the opening and widening of the inaccessible pores [208]. Here, the selection of oxidising agents plays a crucial role in creating microporous biochar [209]. Oxidation with CO₂ is favourable for generating and widening the existing micropores, while steam activation creates micropores and mesopores [207]. These activations can be performed either during pyrolysis or after pyrolysis. Table 5 summarises the related literature on the physical activation of biochar for CO₂ adsorption.

Table 5 Effect of physical activation and operating conditions on the CO₂ adsorption capacity of biochar

6.1.1 Steam activation

Steam activation is utilized to develop the porous structure and introduce oxygen-containing functional groups (i.e., carbonyl, carboxylic, hydroxyl, ether and phenolic groups) onto the carbon surface [169]. For this purpose, steam activation is normally performed at a temperature between 800 and 900 °C for 30 min until 3 h [210] with a steam flow rate of 120 to 300 ml/min [211, 212]. Theoretically, the porous structure of biochar can be improved by devolatilization of trapped products such as aldehydes, ketones, and some acids [213] that result from incomplete combustion during pyrolysis. Pore development in steam activation is related to carbon depletion and the water–gas shift reaction [214]. Therefore, steam activation could develop a variety of pore size distributions and produce micropore and mesopore [208, 215, 216].

The reactions involved in steam activation are explained in Eqs. (1)-(8) [207, 217]. The development of the surface oxide (C(O)) in Eq. (1) comes from the oxygen exchange from the water molecule (H₂O) to the vacant carbon site (C_f) on the surface, which may be devolved as carbon monoxide (CO) (Eq. (2)). The production of CO₂ in Eq. (3) due to the formation of CO increases the rate of carbon gasification by scavenging the C(O). The water–gas shift reaction occurs where CO and H₂O are dissociated to CO₂ and hydrogen (H₂) (Eq. (4)). Simultaneously, carbon gasification occurs where the C_f reacts with H₂O to produce CO₂ and H₂ (Eq. (5)). The presence of CO₂ and H₂ actives C_f and carbon gasification occurs to form CO (Eq. (6)) and CH₄ (Eq. (7)). Further reaction of CH₄ and H₂O produces CO and H₂ (Eq. (8)).

$${C}_{f}+{H}_{2}O\to C\left(O\right)+{H}_{2}$$

(1)

$$C\left(O\right)\to CO+{C}_{f}$$

(2)

$$C{O}_{\left(g\right)}+C\left(O\right)\to C{O}_{2\left(g\right)}+{C}_{f}$$

(3)

$$CO+{H}_{2}O\to C{O}_{2}+{H}_{2}$$

(4)

$${C}_{f}+2{H}_{2}O\to C{O}_{2}+2{H}_{2}$$

(5)

$${C}_{f}+C{O}_{2}\to 2CO$$

(6)

$${C}_{f}+2{H}_{2}\to C{H}_{4}$$

(7)

$$C{H}_{4}+{H}_{2}O\to CO+3{H}_{2}$$

(8)

The overall reaction is presented in Eq. (9) [218].

$${C}_{(s)}+{H}_{2}{O}_{(g)}\to C{O}_{(g)}+{H}_{2(g)} \Delta H=117 kJ/mol$$

(9)

In general, the volume/radius of pore and surface area are positively correlated with steam activation temperature and time due to the continuous removal of carbon atoms from the carbon surface [207]. At high activating temperatures, i.e. around 700–800 ℃, the changes in the surface oxygen-containing groups, aromatic structure, and alkali and alkaline earth metal (AAEM) species contribute to the reactivity of steam activated biochar. Here, at the temperature of 700–800 ℃, the agglomeration of AAEM species increases their particle size and penetration of H radical into the carbon surface leads to changes in the ring condensation reactions, which increases the biochar reactivity. However, raising the temperature to above 800 ℃ makes the biochar more ordered due to the consumption of small aromatic ring structures and reduces its reactivity. Conversely, steam activation below 300 ℃ is not suggested as this low activation temperature cannot remove the strong hydroxyl binding groups [219]. It should be highlighted that the reactivity of biochar depends on the AAEM species, which improve the oxygen-containing groups at the initial gasification process. These AAEM species are further consumed as the activation temperature increases. Generally, a long activation time (> 45–60 min) at a high temperature allows the overactivation phenomenon. Here, more gasification at a faster rate will collapse the wall structure and negatively affect the biochar surface [207]. In this case, pores are still developed, but some pores may exceed the desirable size for CO₂ capture. Here, micropores can be converted to mesopores and macropores, decreasing the volume fraction of micropores [220]. In short, the optimum steam activation temperature and time should be strictly determined to avoid the overactivation phenomenon, which might lead to developing biochar with low surface area and pore volume.

From the previous studies, it should be highlighted that the activation temperature plays an important role in improving the surface area and total pore volume of the modified biochar [137, 221]. The porosity type is greatly influenced by the activating temperature, where steam activation below 750 ℃ generates micropores, while increasing the steam activation temperature up to 750 ℃ may lead to the development of pores distribution consisting of micropores and mesopores [221]. To increase the microporosity and surface area of barley straw-derived biochar, Pallarés et al. [137] used different steam activation temperatures (600, 700 and 800 ℃) for 1 h. Among the activated biochar samples, the one activated at 700 ℃ possessed the higher micropores surface area of 540 m²/g followed by the biochar activated at 800 ℃ (500 m³/g). At the lowest activation temperature of 600 ℃, the micropore surface area could not be detected, as the formation of new pores was not completed due to insufficient activation temperature. Therefore, the employment of suitable activation temperature is critical in producing biochar with high microporosity which is favoured for a high CO₂ uptake.

6.1.2 CO₂ activation

Unlike steam activation, which yields in a pore distribution consisting of micropores and mesopores, CO₂ activation tends to develop micropores. In gas adsorption, where the small molecules, such as CO₂, need to be adsorbed, the occurrence of micropores is more important than mesopores and macropores [222]. Advantageously, the development of highly microporous biochar by CO₂ activation facilitates the adsorption of CO₂ under ambient conditions [200]. As mentioned previously, CO₂ can be used either during the pyrolysis of biomass feedstock, which is referred to as direct activation or after it. The Boudouard reaction explains the mechanism of biochar activation with CO₂ [207, 209]. In this reaction, vacant active sites, denoted as ${C}_{f},$ on the carbon surface undergo dissociative chemisorption of CO₂ to form C(O) and CO, as shown in (Eq. (10)). Next, the pore structure is developed as the surface oxide is desorbed from the surface (Eq. (11)). Finally, CO in the gaseous product is adsorbed on the active carbon site of the char and retards the gasification (Eq. (12)).

$${C}_{f}+C{O}_{2}\to C\left(O\right)+CO$$

(10)

$$C\left(O\right)\to CO$$

(11)

$${C}_{f}+CO \to C(CO)$$

(12)

Among the operating parameters for CO₂ activation, which are activation temperature, CO₂ flowrate, and holding time, most studies reported that activation temperature is the critical parameter in controlling biochar’s textural properties [137, 223]. Zhang et al. [200] employed direct CO₂ activation to develop microporous biochar. They used soya bean straw as a precursor and pyrolyzed it under N₂ gas, and then switched to CO₂ gas after the pre-set temperatures (500, 600, 700, 800 and 900 ℃) were achieved for 30 min. Initially, the micropore surface area for pristine biochar was 250 m²/g. After CO₂ activation at 800 ℃, the micropore surface area increased almost 2 times to 473 m²/g. Here, the hot corrosion on the biochar surface created narrow micropores and weakly improved the mesopores and macropores. Using this activated biochar, a CO₂ uptake of 76 mg/g was obtained. However, an increase in the activation temperature to 900 ℃ reduced the micropore surface area to 455 m²/g. It was discussed that at high temperatures, the hot corrosion became more intense and led to the disruption of coalescence of micropores to mesopores and macropore. As a result, his phenomenon reduced the micropore surface area. Ogungbenro et al. [223] performed CO₂ activation at three different temperatures (600, 700 and 900 ℃) after pyrolysis of date fruit seeds at 800 ℃. Initially, the CO₂ adsorption capacity for pristine biochar was 91.12 mg/g at 20 ℃ and 1 bar. While the biochar activated under CO₂ at 900 ℃ for 1 h, exhibited the highest CO₂ sorption of 141.14 mg/g followed by the ones activated at 700 ℃ (126.21 mg/g) and 600 ℃ (119.11 mg/g) at 20 ℃ and 1 bar. Enhancement of CO₂ adsorption capacity was confidently related to the increment of surface area from 531.33 to 798.38 m²/g and micropore volume from 0.19 to 0.28 cm³/g after CO₂ activation. Apart from activation temperature, holding time also significantly affects the microporosity and surface area of biochar. Studies indicate prolonging the activation time to over 2 h may collapse the pores, and the widening of the micropores is continuously developed, reducing the surface area and micropores volume [137, 223]. Ogungbenro et al. [223] reported that the biochar activated using CO₂ at 800 ℃ for 3 h revealed the lowest surface area and micropore volume of 192.65 m²/g and 0.07 cm³/g, respectively, among the activated biochar samples at different activation times (1, 2 and 3 h). In another investigation, Pallarés et al. [137], carbonized barley straw at 500 ℃ and further activated it using CO₂ at 800 ℃. It was reported that the surface area and micropores volume of barley straw activated for 2 h (769 m²/g and 0.3252 cm³/g, respectively), were lower than those obtained from 1 h activation with values of 789 m²/g and 0.3495 cm³/g, respectively.

It should be highlighted that at high temperatures, a shorter holding time is sufficient to prevent the excessive burn-off of biochar and pores widening. Otherwise, a longer holding time is required at lower activation temperatures so that the CO₂ molecules could penetrate into the carbon matrix to generate more micropores. In studying the effect of CO₂ flow rate, an extreme reduction in surface area and micropore volume from 789 to 160 m²/g and 0.3268 to 0.0657 cm³/g, respectively, was reported when CO₂ flow rate was increased from 2500 to 4000 cm³/min [137]. Here, the insufficient contact of CO₂ molecules with carbon and the shorter residence time reduced the chance for pore development. In summary, in physical activation, CO₂ activation is preferred as steam activation is difficult to control due to the high reactivity of steam [224]. Furthermore, the diffusion rate in steam activation is lower than the reaction rate; hence, carbon atoms and steam can only react on the carbon surface, while CO₂ activation can overcome these limitations [225]. As a result, CO₂ activation produces biochar with higher micropores volume and surface area than steam activation. In a study conducted by Pallarés et al. [137], barley straw was carbonized under nitrogen and activated using CO₂ and steam (in separate experiments) at activation temperatures of 700–900 ℃ for 1–2 h. The authors confirmed that CO₂-activated biochar had higher micropores volume and BET surface area of 0.3268 cm³/g and 789 m²/g, respectively, at the activation temperature of 800 ℃ for 1 h. While, steam-activated biochar produced the maximum micropores volume of 0.2304 cm³/g and the BET surface area of 552 m²/g (700 ℃, 1 h). Notably, the biochar activated with CO₂ showed a 41.84% increment in microporosity than that activated by steam.

6.2 Chemical activation

Chemical activation is applied to pristine biochar to improve its surface chemical properties, mainly surface basicity and surface functional groups. Chemical activation can be implemented via two routes; direct impregnation of biomass feedstock with a chemical agent followed by thermal treatment and activation of synthesized biochar with a chemical agent, which further undergoes heat treatment [207]. The use of different chemical agents will generate various surface functionalities. Specifically, the implementation of chemical activation enhances the surface basicity, which is beneficial for acidic CO₂ adsorption. The following chemical activation section highlights three types of modification as a research hotspot, including metalized-biochar, alkali-modified biochar, and amino-modified biochar, as summarized in Table 6.

Table 6 Effect of chemical activation and operating conditions on the CO₂ adsorption capacity of biochar

6.2.1 Metalized-biochar

Studies have indicated that impregnating pristine biochar with metal or metal oxide can increase its CO₂ capture capacity. According to the reports, impregnation of biochar with metal salt solutions with basic properties such as magnesium, aluminium, iron (III) and calcium resulted in the enhancement of acidic CO₂ gas adsorption by an increment of surface basicity [227, 228]. In a study performed by Zubbri et al. [20], impregnation of biochar by several magnesium salts such as magnesium nitrate, magnesium sulphate, magnesium chloride and magnesium acetate and their effect on the CO₂ adsorption capacity were examined. Firstly, rambutan peel was pyrolyzed at various temperatures (500, 700 and 900 ℃) for 90 min under nitrogen gas. Accordingly, the highest surface area of 569.65 m²/g and micropore volume of 0.201 cm³/g was achieved for biochar prepared at 900 ℃. However, after incorporating 5% magnesium nitrate, the surface area and micropore volume reduced to 505.58 m²/g and 0.182 cm³/g, respectively, due to metal deposition and possible pore blocking. It was reported that the CO₂ adsorption capacity of biochar impregnated with magnesium nitrate was the highest (76.89 mg/g) among all magnesium-loaded biochar samples. The magnesium oxide and magnesium carbonate formation are demonstrated in Eqs. (13)-(15). Here, the decomposition of magnesium nitrate to its oxide form takes place at the temperature of 400 ℃ and above [229] (Eqs. (13)-(14)). Then, the further reaction of magnesium oxide with CO₂ forms carbonate, as shown in Eq. (15) [20]:

$$Mg(N{O}_{3}{)}_{2}6{H}_{2}O\to Mg(N{O}_{3}{)}_{2}+6{H}_{2}O$$

(13)

$$Mg(N{O}_{3}{)}_{2}\to MgO+2N{O}_{2}+\frac{1}{2}{O}_{2}$$

(14)

$$Mg-O+C{O}_{2}\to Mg-O\dots C{O}_{2(ad)}$$

(15)

Finally, after 25 cycles of CO₂ adsorption–desorption, magnesium nitrate loaded-biochar showed excellent stability and its adsorption capacity was retained throughout the process. It was also discussed that the CO₂ adsorption was predominantly governed by physisorption.

In another study, Lahijani et al. [117] introduced various metals such as Mg, Na, Al, Fe, Ni, and Ca on the surface of walnut shell-derived biochar through impregnation to increase the surface basic sites; this was followed by thermal treatment of metalized-biochar at 500 ℃ for 15 min. The performance of the metalized-biochar for CO₂ adsorption was in the sequence of magnesium > aluminium > iron > nickel > calcium > raw biochar > sodium. It was suggested that anhydrous Mg(NO₃)₂ formed at around 110–190℃ from the endothermic dehydration reaction of Mg(NO₃)₂.6H₂O as shown in Eq. (13). Next, the thermal decomposition of anhydrous magnesium nitrate to magnesium oxide occurs at a temperature above 400 ℃ (Eq. (14). The interaction of CO₂ with the basic O²⁻ in the O²⁻-Mg²⁺ (MgO) forms carbonate as represented in Eq. (15). Mg-loaded biochar demonstrated the highest CO₂ adsorption capacity (80.0 mg/g) compared to the pristine biochar (69.1 mg/g) at 30 ℃ and 1 bar. It was discussed that after metal deposition on biochar, chemisorption also contributed to the CO₂ adsorption through carbonate formation. After metal doping on biochar, the contribution of physisorption reduced due to metal deposition on pore entrance and pore blockage (deduced from the reduction in surface area and porosity), yet the interaction of basic metal oxides with CO₂ played an important role in chemisorption. Therefore, the incorporation of magnesium nitrate into the biochar contributed to a 15.7% enhancement in CO₂ adsorption capacity.

Other than that, Creamer et al. [228] investigated the development of various metal oxyhydroxide–biochar composites for carbon dioxide capture. The biochar was prepared using cottonwood and impregnated with three different metal salts (aluminium chloride, iron chloride, and magnesium chloride hexahydrate) at various metal ratios, followed by pyrolysis at 600 ℃ for 3 h. The surface area for the metalized biochar was 289, 367 and 749 m²/g for magnesium-, aluminium- and iron-loaded biochar, respectively. In comparison with the pristine biochar (58 mg/g), the metalized biochar showed higher CO₂ adsorption: 63.69 mg/g for Mg-biochar, 71.05 mg/g for Al-biochar and 66.57 mg/g for Fe-biochar. Interestingly, the authors pointed out that Al-biochar exhibited the highest CO₂ sorption even though Fe-biochar had the highest surface area. Here, the microporosity was a more influential factor than the surface area; the micropore volume of Al-biochar (0.37 cm³/g) was higher than that of Fe-biochar (0.33 cm³/g). In this case, a high volume of small micropores contributed to a high CO₂ uptake.

Apparently, metal loading affects the porosity, surface area, and surface basicity, and the performance of the adsorbents significantly depends on these factors [230]. Therefore, the amount of metal loading should be carefully determined to have the least adverse effect on surface area and microporosity while enhancing the surface basicity for excellent CO₂ capture.

6.2.2 Amino-modified biochar

Numerous studies have suggested that basic nitrogen functional groups increase the basicity and nitrogen functionalities on the biochar surface [231, 232]. Among the nitrogen-containing functional groups, amine has been mostly utilized to be functionalized on biochar surfaces, where CO₂ molecules selectively bind with amine groups via chemisorptive interactions, thus forming carbamate [233,234,235,236]. Although the specific mechanism of this reaction remains unclear, however, some studies discussed that the reaction is in the intermediate formation of zwitterion followed by Brǿnsted base deprotonation [169, 237]. In CO₂ adsorption by primary and secondary amines, the lone pair on the nitrogen atom in the amine molecule attacks CO₂ to form zwitterion. Then, further reaction forms carbamate, while the other amine molecule abstracts the proton from the zwitterion intermediate [238].

Recently, Halem et al. [239] reported that the presence of amine in the development of polyvinyl alcohol (PVA)-based biochar nanofibers was important to assist the adsorption of CO₂ acidic gas through acid–base attraction. In their study, poultry litter was pyrolyzed at various temperatures of 300 to 600 ℃ for 1 h, followed by nitric acid functionalization using the reflux method. Afterwards, the resulting biochars were treated with diethanolamine (DEA), and the mixtures were then heated at 50 ℃ for 1 h. Biochar was immersed in PVA solution (10 wt%) for 30 min, and finally, the mixtures were converted to nanofibers using electrospinning. It was found that, in comparison with the nanofiber biochar pyrolyzed at 500 ℃ without amine treatment (426 mg/g), the one treated with amine displayed a higher CO₂ adsorption capacity of 462 mg/g at 20 ℃. Here, the introduction of amine with basic characteristics was much beneficial for the adsorption of CO₂ acidic gas.

Bamdad et al. [240] attempted to tailor the biochar characteristics by thermal and chemical activation of biochar. They developed microporous biochar from sawdust pyrolyzed at 500 ℃ followed by amination using two different functionalization approaches, namely nitration followed by reduction (denoted as AM-SW500), and condensation of aminopropyl triethoxysilane (denoted as AP-SW500). The prepared biochars were then activated in an air-nitrogen mixture at 560 ℃. A significantly higher CO₂ adsorption capacity of 145.2 mg/g for AM-SW500-A-560 and 167.2 mg/g for AP-SW500-A-560 was obtained compared to that of pristine biochar (110 mg/g), highlighting the contributing effect of amine in CO₂ chemisorption and promotion of the CO₂ uptake capacity.

Liu et al. [241] studied a two-step nitrogen-doping and KOH activation method to modify the biochar surface for superior CO₂ adsorption capacity. First, the coffee ground was used to prepare the pristine biochar by pyrolyzing it at 400 ℃ under N₂ for 1 h. Then, the ammoxidation process was performed via three different methods; (i) dispersion of biochar in 3-Aminopropyltrimethoxysilane (APTES), refluxing at 80 ℃ for 24 h, followed by washing and then drying at 60 ℃ overnight (denoted as SHC), (ii) dispersion of biochar in HCl, then the mixture was treated with poly-condensation of aniline solution by K₂Cr₂O₇ in an ice bath for 6 h followed by washing and then drying in a vacuum oven at 60 ℃ (denoted as PHC), and (iii) sonication of biochar in water with the addition of melamine followed by hydrothermal treatment and drying at 60 ℃ (denoted as MHC). Lastly, all prepared biochars were activated by KOH at 400 ℃ for 1 h and then the temperature was further increased to 600 ℃ for the next hour. MHC possessed the highest CO₂ adsorption capacity of 37.40 mg/g compared to PHC (~ 22 mg/g) and SHC (18.04 mg/g) at 35 ℃ prior KOH activation. Here, the nitrogen content was the factor that influenced the CO₂ adsorption capacity, where the third method developed melamine-modified biochar with the highest nitrogen content (17.4 wt%) compared to the first (4.11 wt%) and second (11.9 wt%) methods. However, the amount of nitrogen content highly decreased in the range of 68–84% after KOH activation due to the decomposition of thermally unstable N species such as nitrile, amide and amine group. It was discussed that the pyrrolic nitrogen content was the highest (5.1%) in KMHC (after KOH activation for the third route) among all the amination reagents. Therefore, an extreme enhancement in CO₂ uptake (117.48 mg/g) at 35 ℃ was obtained. Specifically, in this case, pyrrolic nitrogen was more favourable for CO₂ uptake than pyridinic oxide and pyridinic nitrogen.

On the contrary, few studies reported the decrement of CO₂ adsorption after functionalizing the amine groups on biochar. Ghani et al. [242] investigated the development of amine-modified biochar derived from coconut shells. The biochar was produced from a lab-scale air blown gasifier operated at 800 ℃ and chemically treated using monoethanolamine (MEA) for 20 min. From the ultimate analysis, the nitrogen content of the biochar increased by 77%, from 0.265 to 1.19 wt%, due to the incorporation of nitrogen components after amine treatment. However, the surface area of the biochar decreased from 171.956 to 10.335 m²/g after amine functionalization. This reduction in surface area was consistent with a reduction in CO₂ adsorption capacity from 46.387 to 45.576 mg/g at 30 ℃. This was due to pore blockage by MEA, inhibiting CO₂ adsorption on the biochar surface after the treatment. The authors highlighted that the CO₂ capture at 30 ℃ is more physical related to adsorbent and adsorbate attraction formed by the intermolecular electrostatic forces. The pore blockage thus inhibited the intermolecular forces on the biochar surface and led to the decrement of CO₂ adsorption. While CO₂ capture at 70 ℃ was mostly attributed to chemisorption with the adsorption value of 35.496 mg/g for amine-modified biochar compared to 30.114 mg/g for pristine biochar. Using the similar modification technique, the performance of untreated and amine-treated sawdust was evaluated by Madzaki et al. [85]. The biochar was produced from a lab-scale air blown gasifier reactor at various temperatures (450, 750 and 850 ℃), followed by treatment using MEA for 20 min. The biochar was then subjected to CO₂ adsorption at 30 and 70 ℃. It was reported that the pH of amine-treated biochar was in the range of 6.32–6.93, while that of untreated biochar was between 5.09 and 5.57. The biochar surface pH changed from acidic to basic due to incorporating a strong basic component. However, all amine-treated biochar samples (gasified at different temperatures) displayed a lower CO₂ adsorption capacity than untreated biochar. The surface area of amine-treated biochars, which were gasified at 450, 750 and 850 ℃, reduced from 8.76 to 0.61 m²/g, 11.36 to 0.15 m²/g and 182.04 to 3.17 m²/g, respectively. Again, the decrement in CO₂ adsorption capacity was mainly caused by pore obstruction by amine, during impregnation of biochar by MEA solution. Furthermore, the quinine functional group that appeared near 1600 cm⁻¹ (as observed in FTIR of amine-modified biochars) was likely to be acidic and may result in the decrement of CO₂ capture of amine-treated biochar. To conclude, developing a highly microporous and large surface area adsorbent with high nitrogen content and numerous active sites is critically important for high CO₂ adsorption capacity.

6.2.3 Alkali-modified biochar

Alkali modification is performed by soaking or mixing either biomass or biochar at a specific alkali concentration for about 6–24 h at a temperature range of 25 to 100 ℃ depending on the used raw materials. Alkali reagents commonly used to activate biochar mainly include sodium hydroxide (NaOH) [194, 243, 244], potassium hydroxide (KOH) [245,246,247,248] and potassium carbonate (K₂CO₃) [249,250,251]. Among these reagents, KOH has been widely used as an activating agent for creating small micropores in the carbon skeleton through chemical activation followed by heat treatment [125]. Here, the generation of micro- and meso-porosities results from the separation and degradation of graphitic layers, which is much beneficial for CO₂ uptake [207]. During KOH activation, the presence of oxygen-containing functional groups (i.e., carbonyl, quinone, ether and lactone) increases biochar’s surface basicity [252, 253]. However, these functional groups decompose at different activation temperatures forming CO and CO₂ upon heating. The main products generated for activation temperature below 700 ℃ are K₂O, H₂O, H₂, CO, CO₂ and K₂CO₃, as presented in Eqs. (16)-(19) [254]. Dehydration of KOH forms potassium oxide (K₂O) at 400 ℃, as shown in Eq. (16), while carbon reacts with H₂O to emit hydrogen and carbon monoxide, according to Eq. (17). CO₂ is released from the water–gas shift reaction, as shown in (Eq. (18)). Then, potassium carbonate (K₂CO₃) is generated as K₂O reacts with CO₂ (Eq. (19)). KOH is completely consumed at a temperature above 700 ℃ [255]. For activation temperatures above 700 ℃, the potassium carbonate (Eqs. (19) and (22)) dissociates into K₂O and CO₂ and completely disappears when the temperature reaches 800 ℃. The resulting CO₂ can further react with carbon to form carbon monoxide at high temperatures (Eq. (23)). In addition, the potassium carbonate can be reduced by carbon and hydrogen to produce metallic potassium at a temperature above 700 ℃, as shown in (Eqs. (24) and (25)) [255]. The intercalation of potassium onto the carbon structure develops new micropores and widens the existing pores [256].

$$2 KOH \to {K}_{2}O+{H}_{2}O$$

(16)

$$C+{H}_{2}O\to {H}_{2}+CO$$

(17)

$$CO+{H}_{2}O\to C{O}_{2}+{H}_{2}$$

(18)

$${K}_{2}O+C{O}_{2}\to {K}_{2}C{O}_{3}$$

(19)

$$C+{K}_{2}O\to 2K+CO$$

(20)

$$6KOH+2C\to 2K+3{H}_{2}O+2{K}_{2}C{O}_{3}$$

(21)

$${K}_{2}C{O}_{3}\to {K}_{2}O+{CO}_{2}$$

(22)

$$C{O}_{2}+C\to 2CO$$

(23)

$${K}_{2}C{O}_{3}+2C\to 2K+3CO$$

(24)

$${K}_{2}O+{H}_{2}\to 2K+{H}_{2}O$$

(25)

Hence, the activation temperature beyond 760 ℃ would expand the carbon atomic layers, as the vaporized metallic potassium can diffuse into the carbon matrix [257]. Accordingly, the activation temperature should exceed the boiling point of potassium which is 760 ℃, to ensure its evaporation and diffusion into carbon layers. Therefore, most studies highlighted that the optimum activation temperature is in the range of 800–850 ℃ [243, 258, 259]. In a recent study conducted by Gomez-Delgado et al. [260], Prosopis ruscifolia sawdust was pyrolyzed under N₂ at 500 ℃ for 1 h, followed by KOH activation at 800 ℃ and a high CO₂ adsorption capacity of 264.4 mg/g was attained. However, the use of lower activation temperature was also reported in the literature. Li et al. [261] investigated different KOH activation temperatures (600, 700 and 800 ℃) for CO₂ uptake capacity using mixed sewage sludge and pine sawdust which were initially pyrolyzed at 300 ℃ for 4 h. The KOH-activated biochar at 700 ℃ had the highest surface area and micropore volume of 2623 m²/g and 0.90 cm³/g, respectively, than those activated with KOH at 600 and 800 ℃. It was evident that the high CO₂ uptake capacity of KOH-activated biochar at 700 ℃ (182.0 mg/g) compared to other modified biochars (136.7–141.7 mg/g) was due to the largest surface area and micropore volume as the crucial factors for physical adsorption of CO₂.

Besides the activation temperature, the amount of used KOH affects the textural properties and the functionalities of the modified biochar [262]. The excessive amount of KOH may disrupt the carbon wall structure, leading to a lower surface area and microporosity of the alkali-modified biochar. Considering that KOH modification results in superior CO₂ uptake, in a study carried out by Ding and Liu [262], two different types of seaweed, namely, Sargassum and Enteromorpha, were used to prepare biochar through single-step carbonization and activation. Sargassum and Enteromorpha were mixed at different KOH/biomass ratios (0, 1, 2, and 4) and directly calcined at three different temperatures (400, 600 and 800 ℃) in a fixed-bed adsorption system. The Sargassum seaweed-based porous biochar prepared at 800 ℃ with KOH/biomass mixing ratio of 1:1 demonstrated the highest CO₂ uptake capacity of 46.20 mg/g among the prepared biochars. An excessive activation could disintegrate the carbon wall structure and reduce the surface area. It was noticeable that the total pore volume and surface area of this seaweed-based biochar decreased from 0.16 to 0.07 cm³/g and 60.2 to 16.4 m²/g, respectively, as the KOH/biomass weight ratio increased from 1:1 to 1:4. It was discussed that upon KOH activation, two absorption peaks at 1430 and 1010 cm⁻¹ corresponding to carbonyl C = O and carboxylic C-O stretching, respectively, became more intense, as evidenced in FTIR analysis. These oxygen-containing functional groups thus promoted CO₂ adsorption on the KOH-modified biochar. Apart from that, the optimum activation temperature of 800 ℃ generated more oxygen-containing functional groups due to the maximum activation roles at high temperatures. After ten cycles of CO₂ adsorption–desorption, Sargassum-derived KOH-modified biochar exhibited a 13% reduction in its adsorption capacity.

In another study undertaken by Shao et al. [263], microporous carbons were prepared from poplar wood by three different methods. In the first method, poplar wood was mixed with KOH as an activating agent and carbonized at 600 ℃ in one-step activation carbonization. In the second method, residues of poplar wood after bioethanol fermentation were initially mixed with KOH followed by carbonization at 600 ℃. Finally, in the third method, hydrothermal activation was performed on poplar wood. Here, a dried poplar wood was immersed in a sulfuric acid–water mixture and heated up in a stainless-steel autoclave with Teflon lining for 24 h at 160 ℃. The resulting hydrochar was then activated by KOH at different activation temperatures (600, 700 and 800 ℃) and different mass ratios of 1 and 2. Overall, hydrothermal-KOH activated poplar wood at 800 ℃ at a mass ratio of 1:1 displayed the maximum CO₂ uptake of 126.10 mg/g at 25 ℃ and 1 bar. In comparison, KOH-activated poplar wood at 600 ℃ presented the lowest CO₂ uptake of 48.60 mg/g, while bioethanol-pretreated KOH-activated biochar (at the same temperature of 600 ℃) showed an uptake capacity of 67.90 mg/g. As previously discussed, activation above 760 ℃ results in the formation of new pores as potassium can diffuse into the carbon layers, contributing to a high surface area and micropore volume. Significantly, biochar produced from hydrothermal-KOH activation at 800 ℃ showed the highest BET surface area and micropore volume of 2153 m²/g and 0.85 cm³/g, respectively, compared to the samples obtained from the other two modification methods. The corresponding BET surface area and micropore volume were found to be 511 m²/g and 0.17 cm³/g (for the first method), and 535 m²/g and 0.22 cm³/ (for the second method), respectively. It is important to remember that insufficient activation temperature (< 700 ℃) may result in a lower surface area and microporosity, as KOH is not completely converted to potassium carbonate in which the intercalation of potassium into the carbon matrix generates new pores and widens the existing pores. Hence, the activation temperature is a key factor when impregnating biochar with KOH and should be carefully determined.

Other than KOH, sodium hydroxide (NaOH) has also been used as the activating agent to develop modified biochar. In this regard, Tan et al. [243] impregnated a commercial coconut shell with NaOH solution. They impregnated the coconut shells with NaOH solution at different concentrations (24–32%) and shook the mix for 1–3 h. It was reported that 32% NaOH concentration and 3 h dwelling time resulted in the maximum BET surface area and micropore area of 378.23 m²/g and 277.42 m²/g, respectively. The highest CO₂ uptake capacity of 27.10 mg/g was obtained at 35 ℃ compared to the adsorption temperature of 45 ℃ (24.03 mg/g) and 55 ℃ (16.62 mg/g).

6.2.4 Acid-modified biochar

For developing acid-modified biochar, phosphoric acid (H₃PO₄) [264, 265] and zinc chloride (ZnCl₂) [266, 267] are the most common activating agents, which serve as dehydration agents. Here, one-step activation is normally adopted, where the activation process is carried out through catalyzed condensation, dehydration and cross-linking reactions. As discussed earlier, the optimum activation temperature for KOH is in the range of 800–850 ℃, whereas H₃PO₄ and ZnCl₂ require a lower activation temperature between 450 and 500 ℃ [268]. According to Sevilla and Mokaya [269], a lower activation temperature compared to KOH activation is probably due to the difference in the thermal stability of the cross-links formed during the activation process. H₃PO₄ is a well-known acid activator, which contributes to the introduction of P-containing functional groups and micropore development on the biochar surface. The pores are formed during the cross-linking reactions, including the cyclization and condensation, where H₃PO₄ plays a role as a dehydration agent. Similar to H₃PO₄, ZnCl₂ is one of the outstanding acid activators and has a boiling point of 732 ℃ [49]. ZnCl₂ can penetrate into the carbon structure through the dissolving impact on cellulose, which is beneficial for pore formation. The activation at temperatures below 700 ℃ probably leads to the uniform distribution of ZnCl₂ on the biochar surface, as ZnCl₂ is still in the liquid state. Thote et al. [270] pre-mixed soybean and ZnCl₂ powder at a ratio of 1:1 and pyrolyzed the mixture at 600 ℃ for 2 h. The resultant biochar had a surface area of 811 m²/g and pore volume of 0.33 cm³/g. The CO₂ adsorption capacity of the developed biochar was 41.0 mg/g at 30 ℃ and drastically reduced to 22.4 mg/g at high adsorption temperature (70 ℃). Ahmed et al. [271] examined pre-impregnation and post-impregnation methods for developing ZnCl₂-activated biochar. In pre-impregnation, slash pine wood was added to ZnCl₂ solution and mixed thoroughly for 22 h. Afterwards, the dried mixture was pyrolyzed at 580 ℃ for 2 h. In the second method, slash pine wood was firstly pyrolyzed at 360 ℃ for 2 h; then, the produced biochar was activated with ZnCl₂ at 580 ℃ for 2 h. Accordingly, the biochar developed from post-impregnation route showed slightly higher CO₂ adsorption capacity of 196.24 mg/g compared to the one developed using pre-impregnation method (190.08 mg/g). They discussed that CO₂ adsorption capacities are proportional to the surface area, where large surface area and ultra-micropores (0.57 nm) were beneficial to obtain high CO₂ adsorption. In this case, ZnCl₂ post-impregnated biochar exhibited a larger surface area of 1093 m²/g than that of pre-impregnated biochar (1081 m²/g). The performance of the biochar activated with different activating agents, such as H₃PO₄ and ZnCl₂ was assessed by Heidari et al. [268]. In this study, the Eucalyptus wood was immersed into the H₃PO₄ (ratios of H₃PO₄: Eucalyptus wood = 1.5, 2.0, 2.5:1) or ZnCl₂ (ratios of ZnCl₂: Eucalyptus wood = 0.75, 1.5, 2.5:1) and then dried overnight. The Eucalyptus wood impregnated with H₃PO₄ was then carbonized at 450 ℃ for 1 h, whereas the ones impregnated with ZnCl₂ were subjected to the carbonization at 500 ℃ for 2 h. In addition to this, the effect of the multi-step activation using H₃PO₄ and KOH was also investigated, where the first activation step was carried out using H₃PO₄ followed by KOH activation at 900 ℃ for 1 h. As a result, the CO₂ adsorption capacity of the prepared activated biochars was in the following order: H₃PO₄ + KOH > H₃PO₄ > ZnCl₂. The biochar activated with H₃PO₄ and KOH exhibited a relatively high adsorption capacity of 180.40 mg/g at 30 ℃ and 1 bar compared to those activated using H₃PO₄ (mass ratio of 2.5) and ZnCl₂ (mass ratio of 2.5), which resulted in the CO₂ capture capacity of 82.72 and 58.96 mg/g, respectively. These findings were in line with the maximum BET surface area (2595 m²/g) and micropore volume (1.236 cm³/g) of the biochar developed using multi-step activation. According to their results, the use of KOH enhanced the CO₂ adsorption capacity of biochar by almost 2–3 times compared to those activated by H₃PO₄ and ZnCl₂ at mass ratio of 2.5. Therefore, the selection of appropriate activating agent, and the determination of optimum biomass/biochar: activating agent ratio is important to achieve a high CO₂ uptake capacity.

6.3 Physicochemical activation

Physicochemical activation is a combination of physical and chemical treatment. Table 7 shows a summary of physicochemically activated biochars used for CO₂ adsorption. Recently, a two-step treatment process, ultrasound irradiation-assisted amine functionalization, has been used as an advanced modification technique to modify biochar’s surface area, porosity, and surface chemistry [276, 277]. The oxygen functional groups (i.e., hydroxyl, carbonyl and epoxy) on the graphene oxide layers of biochar are very significant for CO₂ adsorption. However, the graphene oxide layers of pristine biochar remain inaccessible for interaction with CO₂ unless the biochar is exfoliated. By applying the ultrasound waves at a certain frequency, the graphitic layers of biochar could be exfoliated. Here, the exfoliated biochar has a higher surface area and availability of oxygen functionalities; therefore, it is more likely to interact with CO₂. The waves can also open up the clogged pore and prevents the agglomeration of the graphitic sheet by cleaning the surface of biochar. In addition, the cavitation effect induced by ultrasound leads to the generation of more micropores and, at the same time, reduces the pore blockage by removing the lumps from the biochar structure after pyrolysis [277]. A scheme of the mechanism of action of microwave on biochar and its effect on size reduction is presented in Fig. 13 [278].

Table 7 Effect of physicochemical activation and operating conditions on the CO₂ adsorption capacity of biochar

Fig. 13

Schematic diagram showing the effect of ultrasonication on biochar structure. Adapted with permission from [278]

In a study conducted by Chatterjee et al. [279] on the effect of ultrasonication as a physical treatment on the CO₂ adsorption capacity of biochar, an enhancement in micropore surface area from 312.3 to 354 m²/g after 30 s of sonication at the frequency of 20 kHz was observed. The authors discussed during cavitation, the formation of microjet impinges on and penetrates through biochar’s surface; this phenomenon creates more micropores and removes the pore blockage, thus increasing the micropore surface area. However, prolonging the sonication duration to 1 min reduced the micropore surface area to 268.82 m²/g. It was discussed that the intensity of cavitation increased as the sonication duration increased and disrupted the ordered graphitic layer in biochar, which led to pore-clogging. Advantageously, this treatment can be performed at near room temperature for a short while, less than 1 min, which is beneficial for cost and energy saving compared to CO₂ and steam activation methods. Furthermore, ultrasound irradiation can be a techno-economic method applied to large-scale processes without releasing pollutant gases [280].

In an ongoing study, Chatterjee et al. [281] attempted to apply a two-step process to develop biochar with high microporosity and surface area for high CO₂ adsorption. Pinewood-derived biochar was first sonicated for 30 s at ambient temperature, named sono-biochar. In the second step, the sono-biochar was functionalized with five different amines: (i) monoethanolamine (MEA), (ii) diethanolamine (DEA), (iii) piperazine (PZ), (iv) polyethylenimine (PEI), and (v) tetraethylenepentamine (TEPA) and their binary (MEA-TEPA), (DEA-TEPA), (DEA-PEI), (TEPA-PEI) and ternary (DEA-TEPA-PEI) mixtures. Here, sono-biochar, after the incorporation of amine, was known sono-aminated biochar. Finally, all the prepared sono-aminated biochars were activated using two activating agents, namely N-(3-dimethylaminopropyl)-N-ethylcarbodiimide hydrochlo-ride-1-hydroxy benzotri-azole (EDC-HOBt) and KOH. Among the single amines, TEPA led to a CO₂ sorption capacity of 89.76 mg/g followed by MEA (76.56 mg/g), both activated using EDC-HOBt, which were considerably higher compared to that of raw biochar (13.2 mg/g). It was reported that MEA-functionalized biochar demonstrated a micropore volume of 0.12 cm³/g and surface area of 374.66 m²/g. However, TEPA-functionalized biochar showed lower micropore volume and surface area of 0.09 cm³/g and 261.68 m²/g, respectively. Thus, it can be concluded that the synergetic effect of ultrasound-assisted amination was pronounced in the enhancement of CO₂ adsorption capacity of TEPA-functionalized biochar, while in the case of MEA-functionalized biochar, physisorption was dominant. Chatterjee et al. [279] proposed a three-step mechanism for TEPA functionalization of biochar, as depicted in Fig. 14. Firstly, the -COOH group of biochar needed to be activated to react with the amine group. Here, EDC as a coupling agent activated the -COOH group and generated O-acylisourea as an intermediate, followed by a nucleophilic reaction from the amino group. This reaction led to amide production and generated iso-urea as a by-product. Besides, the N-acyl urea could be produced during side reaction on O-N migration of the -COOH group. For the second step, selecting suitable additive such as HOBt was necessary to prevent the formation of these by-products and enhance the product yield. Advantageously, urea is soluble in water, and the filtration process could easily separate the unreacted reagent from its product. In the last step (step 3), the epoxy group's interaction with TEPA formed TEPA-functionalized biochar.

Fig. 14

Overall mechanism of TEPA functionalization of biochar. Adapted with permission from [279]. Step 1 Mechanism of EDC-HOBt coupling with -COOH group of biochar. Step 2 Mechanism of TEPA functionalization of the activated carbonyl group of biochar. Step 3 Mechanism of TEPA functionalization of activated carbonyl group of biochar

In the study of Chatterjee et al. [281], among the blended mixture, the MEA-TEPA-functionalized biochar activated with EDC-HOBt demonstrated the highest CO₂ sorption capacity of 84.04 mg/g with a high micropore volume (0.12 cm³/g) and surface area (375.12 m²/g) among the other mixtures. This result was in line with the highest intensity ratio (I_D/I_G) of 0.95 obtained from the Raman analysis, confirming that more distortion was introduced to the biochar surface from dual amination using MEA-TEPA, which provided a beneficial combination for CO₂ adsorption. For KOH activation, MEA-functionalized biochar exhibited the highest CO₂ uptake of 71.68 mg/g. In comparison, the sorption capacity was lower than the corresponding sample activated by EDC-HOBt (76.56 mg/g). Here, the strong base reaction onto the biochar surface reduced the micropore volume from 0.12 to 0.09 cm³/g, and thus reduced the reaction site for CO₂ adsorption from surface destruction. Therefore, it is important to use the appropriate amine and activating agent, as both factors contribute to the generation of micropores that influence the efficiency of adsorbent for CO₂ uptake.

In another investigation undertaken by Chatterjee et al. [136], various feedstocks, such as miscanthus switchgrass, corn stover and sugarcane bagasse, were subjected to pyrolysis (500, 600, 700 and 800 ℃) and then low-frequency acoustic treatment and amine functionalization for CO₂ adsorption. The SEM images of raw biochar portrayed a non-porous structure, while the creation or opening of new microporous can be observed on sono-activated biochar due to structural degradation (Fig. 15). As can be seen, the structure of all biochars was irregular, rough and bundle like. During pyrolysis at 700 ℃, these structures tend to disrupt and crack, thus releasing volatile matter. In this regard, miscanthus showed the highest micropore volume (0.15 cm³/g) and surface area (324 m²/g) among the prepared biochars pyrolyzed at 700 ℃. Accordingly, the micropore volume and surface area of this biochar increased to 0.21 cm³/g and 532 m²/g, respectively, after 30 s of ultrasonication. In CO₂ adsorption studies, miscanthus-derived biochar synthesised at 700 ℃ and sono-activated biochar presented superior adsorption capacity of 40.92 and 127.16 mg/g, respectively, among the corresponding samples.

Fig. 15

copyright of RSC license (CC-BY 4.0)

SEM images of (a) raw miscanthus, (b) ultrasonicated miscanthus biochar, (c) raw switchgrass, (d) ultrasonicated switchgrass biochar, (e) raw corn stover, (f) ultrasonicated corn stover biochar, (g) raw sugarcane bagasse and (h) ultrasonicated sugarcane bagasse biochar; all biochars were pyrolyzed at 700 °C. Adapted from [136] under the

In summary, various modification methods affect the physicochemical properties of biochar in their own ways to enhance the CO₂ adsorption capacity. In the physical modification, CO₂ activation is more likely to generate micropores in biochar, whereas steam activation contributes to developing a wider range of pores, including mesopores and micropores. In the chemical activation, the use of different activating agents introduces various functional groups, such as O- and N-containing functional groups, on the biochar surface to facilitate CO₂ adsorption. Most chemical modification techniques aim to produce biochar with basic characteristics favourable for the interaction with CO₂ acidic gas. Here, the impregnation of biochar with metal salts solutions such as magnesium, aluminium, iron (III) and calcium increases the surface basicity of biochar, while the introduction of amino groups, especially amine, into biochar increases the N-containing functional groups. Other than that, the introduction of O-containing functional groups on the biochar surface from the alkali modification helps to increase the biochar’s surface basicity. Among the alkaline activators, KOH is widely used in the activation process. In the acid modification, ZnCl₂ and H₃PO₄ play a role as a dehydration agents to initiate pore formation during the cross-linking reaction on the carbon structure. Most recently, the physicochemical activation by ultrasound-assisted amination has gained the attention of researchers to boost the adsorption of CO₂. In this method, ultrasound irradiation exfoliates the graphitic layers of biochar, and the following amine-functionalization facilitates CO₂ adsorption. Therefore, it is important to apply a suitable modification technique to enhance the CO₂ uptake. In modifying the physicochemical properties of biochar, it is also important to consider the cost and environmental impacts so that high-capacity biochar can be developed under optimum process conditions.

7 Selectivity towards CO₂

Selectivity is one of the indicators in determining the successful development of modified biochar to adsorb CO₂. High selectivity highlights the ability of biochar to separate CO₂ from gas mixtures. In this context, a suitable modification method should be implemented on the biochar surface to produce biochar with high adsorption capacity and selectivity towards CO₂. In the selectivity study conducted by Zubbri et al. [20], the magnesium nitrate loaded-biochar revealed an excellent selectivity towards CO₂ compared to other gases with a CO₂ uptake of 76.78 mg/g, which was considerably higher than those obtained for other gases such as air (8.11 mg/g), N₂ (3.76 mg/g) and CH₄ (1.93 mg/g) at 30 ℃ and 1 atm. It was discussed that the selectivity of biochar towards CO₂ depends on the polarizability and quadrupole moment of CO₂ molecules. CO₂ has greater polarizability (29.1 × 10²⁵ cm²) over other gases (CH₄ = 25.9.1 × 10²⁵ cm², N₂ = 17.4 × 10²⁵ cm², and O₂ = 15.8 × 10²⁵ cm²) [285, 286], and this greater polarizability of CO₂ facilitates the adsorbent-adsorbate interaction. Other than that, the higher quadrupole moment of CO₂ molecules (14.3 × 10^–40 C m²) compared to other gases such as CH₄ (0 C m²), N₂ (4.72 × 10^–40 C m²) and O₂ (1.03 × 10^–40 C m²) [287] justifies the stronger affinity of CO₂ towards the biochar surface. In another study, Lahijani et al. [117] observed a high selectivity towards CO₂ over other gases on walnut shell-derived metal-loaded biochar. Accordingly, the adsorbent exhibited superior adsorption performance towards CO₂ (80.0 mg/g) over CH₄ (8.75 mg/g), O₂ (3.25 mg/g) and N₂ (3.24 mg/g). Recently, Zubbri et al. [87] modified hydrochar, obtained from hydrothermal carbonization of biomass using KOH and the resultant biochar showed excellent adsorption performance towards CO₂ with a sorption capacity of 122.37 mg/g. However, the sorbent adsorbed the other gases in a small amount with the sequence of air (7.03 mg/g) > N₂ (3.09 mg/g) > CH₄ (1.93 mg/g).

8 Reusability of biochar

Apart from high adsorption ability, an efficient adsorbent must have a good and stable adsorption performance in regeneration. Reusability of biochar in the operations is crucial when selecting a good adsorbent for CO₂ capture, especially for large-scale applications. Zubbri et al. [20] showed the stable performance of MgO-loaded rambutan peel-derived biochar after 25 cycles of CO₂ adsorption–desorption (30 and 110 ℃, respectively). In another study, Zubbri et al. [87] modified the rambutan peel-derived hydrochar with KOH. The developed adsorbent exhibited stable performance within 10 cycles of adsorption–desorption at previous conditions. Lahijani et al. [117] reported an insignificant loss in CO₂ adsorption capacity of Mg-loaded walnut shell-derived biochar after 10 cycles of adsorption at 30 ℃ and desorption at 110 ℃. In a study conducted by Cao et al. [288], it was shown that pine wood, hickory wood, wheat straw, walnut shell, corn stalk, soybean straw and rape straw-derived biochars presented excellent reusability, where all the adsorbents could recover up to 90% of the adsorption capacity within 10 cycles of adsorption–desorption. However, Shahkarami et al. [226] reported that CO₂ adsorption capacity of steam activated biochar derived from whitewood started to diminish after 20 cycles, indicating that steam-activated biochar may not preserve a sustainable performance in multi-cycle of CO₂ adsorption–desorption.

CO₂ adsorption on the pristine biochar surface is physical adsorption involving weak intermolecular forces (van der Waals forces). This physisorption interaction does not require a major change in the electronic orbital arrangement for each species due to the weak intermolecular interactions [289]. Therefore, CO₂ (adsorbate) tends to move freely over the adsorbent surface, where a minimal amount of energy is necessary to break the weak interactions. However, in amine-functionalized biochar, where chemisorption takes place, CO₂ molecules would be chemically bonded to the amine functionalities on the biochar surface by forming carbamate through a strong covalent bond and occupying the appropriate adsorption sites [289]. Hence, the energy required to desorb CO₂ from the amino-modified biochar is higher than that of pristine biochar. During the desorption process, the presence of orbital overlap and charge transfer makes it difficult to remove carbamates from the adsorbent [290, 291]. This condition resulted in a low generation value of amine-functionalized biochar after several cycles of CO₂ adsorption–desorption [175, 240, 261]. Yaumi et al. [175] reported a reduction of 8.8% in the CO₂ adsorption capacity of melamine-modified biochar developed from rice husk after 12 cycles of CO₂ adsorption–desorption at 30 and 110 ℃, respectively. It was discussed that the reduction in the adsorption capacity was due to the fact that the chemically bonded CO₂ molecules on the biochar surface were not completely released during the desorption process. This finding was in agreement with the reports of Li et al. [261], who produced biochar from a mixture of sewage sludge and pine sawdust. The resultant biochars were then activated using KOH at different activation temperatures (600,700 and 800 ℃). The biochars activated at 700 and 800 ℃ exhibited slight reductions of 3 and 2%, respectively, after 10 cycles of the cyclic test. Overall, from the technical and economic viewpoints, a stable adsorbent with high CO₂ capture capacity and sustainable performance in multi-cycle adsorption–desorption is required to ensure the viability of the adsorption process.

9 Challenges in using biochar as CO₂ adsorbent

Biochar as a sustainable and relatively low-cost CO₂ adsorbent has gained attention for CO₂ removal over the past years. However, despite all the claimed advantages, there are still several challenges related to the large-scale application of biochar as the CO₂ adsorbent.

First, the regeneration characteristic of CO₂ adsorbent after using a few times at a specific adsorption–desorption temperature is important for economic efficiency determination. Therefore, it is necessary to understand the regeneration principles, including the disposal of biochar. Nguyen and Lee [275] experienced a reduction of 12% in CO₂ adsorption capacity after 10 cycles for nitrogen-doped biochar. Ding and Liu [262] found that the CO₂ adsorption capacity for Sargassum and Enteromorpha seaweed-derived biochars decreased 15 and 13%, respectively, after 10 cycles. Bamdad et al. [240] showed that the regeneration capacity of nitrogen-functionalized biochar derived from sawmill residue decreased in the range of 4–8% after 5 cycles and by 20% after 10 cycles. Although such regeneration capacity might be reasonable and satisfactory for research studies, biochar’s capability as a carbon sequestering material may be rendered by a large reduction in adsorption capacity after several cycles of CO₂ uptake and release, especially at large-scale operations. Hence, developing more sustainable and robust biochar-based adsorbents should be considered in future studies.

Secondly, the performance of some developed biochar after certain modifications is still questionable and not fully demonstrated. For sure, high adsorption capacity at ambient temperature is desirable to guarantee the practicability of the proposed modification method in a large-scale application. Nevertheless, some studies reported that their adsorption capacity increased when higher adsorption temperatures (for example 70 °C) were applied [279]. From a large-scale perspective, high adsorption temperature implies huge energy consumption for the desorption process. Hence the modification mechanism and its effect on the adsorbent performance should be fully understood to avoid producing biochar that favours high adsorption temperature.

Thirdly, the availability of biomass feedstocks in huge amounts should be taken into account to produce biochar for large-scale operations. The usage of seasonal crop residues such as paddy straw, paddy husk, wheat straw, etc., might be an issue as these types of feedstocks are not readily available throughout the year. Other than that, several biomass materials might not be suitable for the massive production of biochar. For example, tamarind seeds, orange peels and sunflower seed shells might only be consumed on a small scale at the respective period of time. With the variety of biomass feedstocks throughout the world, selecting a suitable source is important to ensure its availability at a low cost.

10 Conclusions

In this review, the adsorption of CO₂ on several modified biochars was reviewed and discussed. The literature survey indicates that the CO₂ adsorption is significantly affected by biochar’s surface area and microporosity and its surface functional groups. In this regard, operating conditions such as pyrolysis temperature, holding time and heating rate play a vital role in developing highly microporous biochar. Additionally, the basicity of the biochar surface has fundamental impacts on the adsorption of acidic CO₂ molecules through acid–base interaction. Based on the literature survey, a combination of physical and chemical activation is beneficial in enhancing the CO₂ uptake capacity of biochar. This needs a careful determination of the optimum process condition and suitable activating agents. Therefore, future research should focus on the physicochemical treatment methods to obtain microporous biochar with enriched surface functionality to achieve a high CO₂ adsorption capacity. In addition, a deep understanding of CO₂ adsorption mechanism is crucial in dealing with various modification techniques. Comprehensive studies on the resistance to impurities (such as H₂S as a co-occurring acidic component in biogas), long-term stability as well as cost considerations are required to ensure the successful exploitation of biochar for CO₂ adsorption at large-scale operations.