Biomass-based activated carbon for CO 2 adsorption

Abstract

Biomass can be converted into energy, fuels, and value-added products by adopting proper conversion or production methods. For many years, biomass has been considered to be a good candidate for producing biochar or activated carbon. The awareness created on mitigation of carbon dioxide (CO₂), which is the major cause of global warming, necessitated developing potential methods and materials for curbing CO₂ originating from various sources. Adsorption is the most viable option to mitigate CO₂ by using activated carbon which can be derived from various biomass sources. In recent years, activated carbon has been produced from different biomass substances by varying carbonization and activation duration, carbonization and activation temperature, impregnation ratio, and the concentration of the activating agent to improve its surface area and porosity. This review article provides a comprehensive review on utilization, production and characterization of biomass-based activated carbon for CO₂ adsorption. Initially, the article discusses the review of research works carried out on utilization of biomass-based activated carbon for CO₂ adsorption. Furthermore, the article presents the research works carried out on surface textural characteristics, physicochemical properties, and maximum adsorption capacity of activated carbon obtained from different biomass substances. Finally, the article presents the research works carried out related to the biomass-based activated carbon and the parameters significantly enhancing the CO₂ adsorption performance.

Keywords

Activated carbon adsorbent adsorption biomass carbon dioxide capture

Introduction

At the beginning of the pre-industrial revolution, carbon dioxide (CO₂) concentration in the ambient air was 280 ppm (part per million), and at the beginning of 2021, it was about 420 ppm. The rising global atmospheric CO₂ level is observed as a serious threat to the environment. In recent years, mitigation of CO₂ has been considered as a thrust area of research in the world. Three different methods currently available for CO₂ capture are; (a) post-combustion, (b) pre-combustion, and (c) oxy-fuel combustion. CO₂ capture by using different techniques such as adsorption, absorption, and membrane separation has been implemented in some of the major coal-fired power plants, industrial plants like cement and steel manufacture, and gas purification units in the world.¹ Among these methods, adsorption is the most promising method because it offers the following benefits; (i) gives better performance, (ii) stable for a long period, (iii) requires minimum cost for regeneration, (iv) does not regenerate toxic materials and (v) cost-effectiveness.² Adsorption is a surface phenomenon where CO₂ molecules (adsorbate) are adsorbed onto the material's surface (adsorbent). It is mainly categorized into two types; (i) physical adsorption (Van der Waals force of attraction) and (ii) chemical adsorption (chemical bond interaction). Activated carbon obtained from biomass is the most efficient adsorbent used for CO₂ separation and capture due to its advantages: (a) non-expensive and easily available raw material, (b) greater selectivity for various concentrations, (c) high adsorption capacity, (d) quicker adsorption kinetic rate and (e) ease to regenerate original adsorbent. The regeneration of solid activated carbon can be achieved by using pressure swing adsorption (PSA) and temperature swing adsorption (TSA). PSA and TSA are carried out in the regeneration process by reducing the operating pressure and increasing the operating temperature in an adsorbent bed, respectively. Therefore, the accumulated adsorbed CO₂ molecule gets discharged or desorbed from the surface of activated carbon. The regenerated activated carbon can again be used further for adsorption cycle without losing its adsorption capacity.³

Recently, many researchers have focused on their research works for converting biomass residues and agricultural by-products into activated carbon for CO₂ adsorption. The lignocellulosic biomass comprises three major components: (a) lignin, (b) cellulose, and (c) hemicellulose.^4,5 The different categories of biomass are agricultural crop residues, forestry residues, wood processing residues, animal waste, industrial waste and municipal solid waste. There are two pathways to produce activated carbon from biomass substance: (i) the selected biomass raw material is converted into biochar by using carbonization process and the obtained bio-char is converted into activated carbon by using physical and chemical activation methods, and (ii) the selected biomass feedstock is directly converted into activated carbon by adopting pyrolysis method. The selection of appropriate raw materials, preparation mode, and preparation methods including activation methods, type of activator, and preparation conditions are important factors that enhance the textural properties and performance of activated carbon.⁶ The carbon yield, impregnation ratio, the concentration of the activating agent, carbonization temperature and time and activation temperature and time are the essential parameters considered at the time of activated carbon preparation. In this review article, review is categorized into different sections and arranged as follows: Section 2 discusses the potential activated carbon for CO₂ adsorption; Section 3 presents the availability of biomass and its residues for selection of suitable raw material; Section 4 provides the preparation methods of the biomass-based activated carbon; Section 5 describes the evaluation of characteristics and properties of activated carbon by the different characterization and analytical methods; and Section 6 discusses the parameters influencing the performance of activated carbon.

Potential activated carbons for Co₂ adsorption

In the last three decades, many research works have been carried out to explore the possibility of using activated carbon to capture CO₂ from flue gas originating from power plants, biogas plants, and natural gas plants. Activated carbon is obtained from different biomass substances such as biomass shell, biomass waste, stone-type biomass; agro-residues; leaves; woody biomass; algae and fungi, and fruit and vegetable peels. Table 1 provides detailed information about the different adsorbent materials for CO₂ adsorption.

Table 1.

Different adsorbent materials for CO₂ adsorption.

Biomass-based activated carbon	Maximum CO₂ adsorption capacity (mmol/g)	Chemical activating agent	Pr. (bar)	Temp. (K)	Reference
A. Activated carbon produced from shell
Almond shell	2.5	KOH	1	323	Gonzalez et al.⁷
Pine nutshell	5.0, 7.7	KOH	1	298, 273	Deng et al.⁸
Coconut shell	3.9, 5.6	KOH	1	298, 273	Huang et al.⁹
Palm shell	4.4, 6.3	KOH	1	298, 273	Ello et al.¹⁰
Peanut shell	1.54, 4.03, 5.23	KOH	0.15, 1, 1	298,298, 273	Deng et al.¹¹
Sunflower seed shell	1.46, 4.61, 7.06	KOH	0.15, 1, 1	273, 298, 273	Deng et al.¹¹
N-doped porous Chestnut shell	4.5, 6.04	NaNH₂	1, 1	298, 273	Rao et al.¹²
Palm kernel shell	2.14	KOH	1.3	298	Rashidi et al.¹³
Coconut shell	3.71, 5.12	K₂CO₃	1, 1	298, 273	Yue et al.¹⁴
Chestnut shell	4.7, 6.0	KOH	1, 1	273, 298	Wei et al.¹⁵
N-doped Coconut shell	5.0, 7.0	KOH	1, 1	298, 273	Chen et al.¹⁶
N-doped porous Coconut shell	4.26, 6.25	KOH	1	298, 273	Yang et al.¹⁷
N-doped argan fruit shell	5.63	KOH, NaOH	1	298	Boujibar et al.¹⁸
Pine cone shells	2.35, 7.63	KOH	0.15, 1	273, 273	Li et al.¹⁹
Mango seed shell	1.2, 3.3	KOH	1, 1	298, 273	Munusamy et al.²⁰
Waste longan shell	4.3, 5.6	KOH	1, 1	298, 273	Wei et al.²¹
N-doped walnut shell	7.42, 14.03	KOH	1, 10	298, 298	Rouzitalab et al.²²
Macadamia nutshells	3.48	NA	1	298	Bae et al.²³
Cocoa shell	0.24	NA	1	273	Bargougui et al.²⁴
B. Activated carbon produced from husk
Rice husk	3.1	KOH	1	298	Li et al.²⁵
Rice husk	5.35	H₃PO₄	1	303	Yaumi et al.²⁶
Rice husk	6.24	KOH	1	273	Li et al.²⁷
C. Activated carbon produced from dreg
Bean dreg	4.2	KOH	1	298	Xing et al.²⁸
D. Activated carbon produced from bamboo
Granular Bamboo	7.0, 4.5	KOH	1, 1	298, 273	Wei et al.²⁹
E. Activated carbon produced from coffee ground
Spent coffee ground	3.0, 4.8	KOH	1, 1	298, 273	Plaza et al.³⁰
Waste/spent coffee ground	4.4, 23.3	KOH	1, 10	298, 273	Travis et al.³¹
F. Activated carbon produced from waste
Lignin waste	4.6, 17.3	KOH	1, 20	298, 298	Sangchoom et al.³²
G. Activated carbon produced from flour
Wheat flour	3.48, 5.7	KOH	1, 1	298, 273	Hong et al.³³
H. Activated carbon produced from stem
Lotus stem	3.85, 6.17	KOH	1, 1	298, 273	Wu et al.³⁴
I. Activated carbon produced from soya
Soya	3.2	NaOH	1	298	Rana et al.³⁵
J. Activated carbon produced from stone
Olive stone	3.2	KOH	1	323	Gonzalez et al.⁷
Yellow mombin fruit stones	4.9, 7.3, 10.5	KOH	11,1	348, 298, 273	Fiuza et al.³⁶
K. Activated carbon produced from bagasse
Sugarcane bagasse	3.5, 4.28, 5.50	NaOH, H₃PO₄	1, 1, 1	323, 298, 273	Guo et al.³⁷
Sugarcane bagasse	4.8	Urea - KOH	1	298	Han et al.³⁸
L. Activated carbon produced from stalk
Cotton stalk	1.1	NH₃	1	293	Zhang et al.³⁹
M. Activated carbon produced from tannin
Chestnut tannin	2.24, 3.44	NH₃	1, 1	298, 273	Nelson et al.⁴⁰
N. Activated carbon produced from leaves
Celtuce leaves	6.04, 4.36	KOH	1, 1	273, 298	Wang et al.⁴¹
N-doped camphor tree leaves	3.74, 5.86	KOH	1, 1	298, 273	Xu et al.⁴²
N-doped camphor tree leaves	2.28, 3.74, 5.86	KOH	1, 1, 1	323, 298, 273	Xu et al.⁴²
Fern leaves	4.12, 4.52	KOH	1,1	298, 273	Serafin et al.⁴³
Camphor leaves	6.63	KOH	4	298	Guangzhi et al.⁴⁴
Jujun grass & camellia japonica	1.5, 5.0, 21.1	KOH	0.15,1, 20	298, 298, 298	Coromina et al.⁴⁵
Arundo donax	6.3, 15.4	KOH	1, 3	273, 273	Singh et al.⁴⁶
O. Activated carbon produced from wood
Sawdust	4.8, 6.1	KOH	1, 1	298, 273	Plaza et al.⁴⁷
Eucalyptus wood	3.22	H₃PO₄	1	303	Heidari et al.⁴⁸
Mesquite wood	6.5, 26.0	KOH	3, 30	298, 298	Li et al.⁴⁹
Whitewood	1.1	NA	1	298	Shahkarami et al.⁵⁰
Persian ironwood	6.7	H₃PO₄	1	293	Nowrouzi et al.⁵¹
Black locust	5.05, 7.19	KOH	1, 1	298, 273	Zhang et al.⁵²
P. Activated carbon produced from algae/seaweed
Microalgae-chlorella	3.44	KOH	1	298	Shi et al.⁵³
Microalgae-spirulina	3.09	KOH	1	298	Shi et al.⁵³
N-doped microalgae	4.03, 6.68	KOH	1, 1	298, 273	Luo et al.⁵⁴
Seaweed-Sargassum	1.05	KOH	1	298	Ding et al.⁵⁵
Seaweed-Enteromorpha	0.52	KOH	1	298	Ding et al.⁵⁵
Q. Activated carbon produced from fruit and vegetable peels
Pineapple	4.25, 5.32	K₂CO₃, KOH	1, 1	298, 273	Zhu et al.⁵⁶
Strawberries	4.49, 6.35	KOH	1, 1	298, 273	Yue et al.⁵⁷
Pomegranate	4.11, 6.03	KOH	1, 1	298, 273	Ding et al.⁵⁵
Carrot	4.18, 5.04	KOH	1, 1	298, 273	Ding et al.⁵⁵
Garlic	4.22, 6.22	KOH	1, 1	298, 273	Huang et al.⁵⁸

*NA- Data Not Available, Temp. – Temperature, Pr. – Pressure.

Many research groups documented their research works on the utilization of biomass-based activated carbon to capture CO₂. Rouzitalab et al.²² developed the nitrogen-doped Walnut shell-based activated carbon by using nitrogen doping and followed by chemical activation using potassium hydroxide (KOH). The prepared n-doped Walnut shell activated carbon showed a maximum adsorption capacity of 14.03 mmol/g at 10 bar pressure and 298 K temperature. Bargougui et al.²⁴ experimentally investigated activated carbon obtained from the Cocoa shell. At a pressure of 1 bar and temperature of 273 K, activated carbon exhibited a minimum adsorption capacity of 0.24 mmol/g. Travis et al.³¹ investigated the adsorption performance of Waste coffee ground activated carbon which showed the highest CO₂ uptake of 23.3 mmol/g at 10 bar, 273 K; and 4.4 mmol/g at 1 bar, 298 K; whereas Plaza et al.³⁰ reported that the Spent coffee ground activated carbon exhibited the lowest CO₂ uptake of 4.8 mmol/g at 1 bar, 273 K; and 3.0 mmol/g at 1 bar, 298 K. Sangchoom et al.³² developed the Lignin-waste based activated carbon by using KOH and examined its adsorption performance. Activated carbon exhibited a higher CO₂ adsorption capacity of 17.3 mmol/g at 1 bar, and 298 K. Coromina et al.⁴⁵ investigated the Jujungrass and Camellia Japouica based activated carbon by chemical activation using KOH and examined its adsorption capacity. Activated carbon showed a maximum CO₂ adsorption capacity of 21.1 mmol/g at 20 bar and 298 K. Singh et al.⁴⁶ explored the feasibility of activated carbon derived from Arundo donase using KOH. Activated carbon exhibited the highest CO₂ adsorption capacity of 15.4 mmol/g at 3 bar and 298 K. Li et al.⁴⁹ recently reported that activated carbon obtained from Mesquite wood by chemical activation using KOH. At a pressure of 30 bar and a temperature of 298 K, the obtained activated carbon showed maximum CO₂ adsorption of 26.0 mmol/g.

Ding et al.⁵⁵ examined the performance of Seaweeds activated carbon obtained from Sargassum and Enteromorpha. The maximum CO₂ adsorption capacity of 1.05 mmol/g was achieved for Sargassum activated carbon and 0.52 mmol/g for Enteromorpha activated carbon at 1 bar, 298 K. The same research group developed activated carbon from Pomegranate peels and Carrot peels by chemical activation using KOH. The CO₂ adsorption capacities of 4.11 mmol/g at 1 bar, 298 K and 6.033 mmol/g at 1 bar, 273 K for Pomegranate peels based activated carbon; and 4.18 mmol/g at 1 bar, 298 K and 5.04 mmol/g at 1 bar, 273 K for Carrot peels based activated carbon. Wu et al.³⁴ experimentally investigated activated carbon obtained from Lotus stem waste with hydrothermal carbonization followed by chemical activation using KOH and sulphuric acid (H₂SO₄). The three different impregnation ratios (1:0, 1:2, and 1:4) were adopted to examine the activated carbon's surface area and porous volume. At the impregnation ratio of 1:4, activated carbon showed the highest surface area of 2893 m²/g and the greatest total pore volume of 1.59 cm³/g. At the impregnation ratio of 1:2, the same activated carbon exhibited the maximum CO₂ uptake of 3.85 mmol/g at 1 bar, 298 K; and 6.17 mmol/g at 1 bar, 273 K. Wei et al.²⁹ proposed that the Granular Bamboo-based activated carbon can be used for CO₂ adsorption. Activated carbon was derived from bamboo by chemical activation using KOH. Two different impregnation ratios (1:1 and 1:3) were used for the activated carbon preparation. At the activation temperature of 600°C and the impregnation ratio of 1:3, activated carbon exhibited the highest CO₂ adsorption capacity of 5.3 mmol/g at 1 bar, 298 K and 7.0 mmol/g at 1 bar, 273 K. At the activation temperature of 600°C and the impregnation ratio of 1:1, activated carbon showed the adsorption capacities of 4.0 mmol/g at 1 bar, 298 K and 4.5 mmol/g at 1 bar, 273 K. It was concluded from the observation, that the maximum CO₂ adsorption capacity could be achieved by maintaining the minimum temperature and the maximum pressure.

Activated carbon for Co₂ capture application

According to the Environmental Protection Agency (EPA), activated carbon is one of the most value-added and energy-efficient materials derived from waste organic substances. The demand for activated carbon has increased due to its wide range of applications such as drinking water treatment, CO₂ adsorption, air and gas purification, exhaust purification, and sewage treatment. Conversion of an organic waste substance into activated carbon is the most viable waste management and waste minimization option. Nowadays, activated carbon has been obtained from different resources such as agricultural and crop residues, municipal solid wastes, industrial wastes, animal residues, and forestry bio-wastes. Activated carbon derived from the various sources offers the following advantages; (a) can meet the demand, (b) can be produced at low cost, (c) can be easily accessible, (d) can offer a solution to waste minimization, (e) can yield low degradation properties, (f) can be eco-friendly, (g) can increase sustainable resources and (h) can offer a low negative impact to the environment. Activated carbon is an amorphous, tasteless, micro-crystalline, non-graphite and carbon skeleton structure of solid carbonaceous material. It is also known as a solid sponge, adsorbent, activated charcoal.^59,61 Typically, it has the following desirable characteristics such as high internal surface area,⁶² high degrees of porosity,⁶³ high mechanical strength,⁶⁴ high selectivity,⁶⁵ adequate pore size distribution,⁶⁶ high thermal and chemical stabilities,⁶⁷ high surface affinity,⁶⁸ high adsorptive capacity,⁶⁹ high degrees of surface reactivity,⁷⁰ and sufficient surface functional groups.⁷¹ The biomass residues or agricultural by-products would significantly decrease waste disposal and increase economic benefits.

Activated carbon obtained from different forms of biomass

The biomass residue or agricultural by-product is a potential source of raw material or feedstock for activated carbon production. Activated carbon can be produced from different forms of biomass source are leaves, stems, flowers, fruits, seeds, peels, stalks, cores, husks, straws, fibers and woods, fruit and vegetable peels, pomaces, roots, dregs, grass, pods, saw-dusts, bagasse, piths, brans, pulps, kernels, hulls, barks, shells, and stones. The agricultural residues can be classified into two different types; (a) primary residues and (b) secondary residues. The primary or field-based residues are generated at harvesting time (e.g. straws, bunches, stalks, and fronds). The secondary or processing-based residues are produced during processing time (e.g. shells, cobs, husks, and bunches). There are significant quantities of unused biomass residues that remain in the field itself in the form of cob and straw. Some of the crop residues are (i) coconut-fronds, husks, and shells; (ii) coffee-grounds, hulls, and husks; (iii) corn-cobs, leaves, stalks, and stovers; (iv) cotton-stalks, hulls, and shells; (v) rice-hulls, husks, straws, and stalks; and (vi) sugarcane-bagasse. Table 2 presents the different forms of biomass resources.

Table 2.

Different form of biomass resources.

The selection of feedstock mainly depends on the energy density and the physical properties. The preparation method of activated carbon plays a significant role in the aspect of the desirable properties and physicochemical characteristics.^63,72 The biomass-based activated carbon is also known as carbon enriched material. It has a large specific surface area, inherent well-defined porous structure, and surface functional groups. These properties are directly related to adsorptive capacity.⁷³ Table 3 provides different biomass sources and their respective by-products that are significantly available in India.

Table 3.

Different biomass sources and their by-products.^74,76

Sl. No.	Name of the crop	Annual production (Thousand MT)	Type of residue	Total available residue (Thousand MT)
1	Areca nut	330	Fronts; Husks	0.857; 264
2	Arhar	1950	Husks; Stalks	585.3; 4875
3	Bajra	7690	Cobs; Husks; Stalks	2537.7; 2307; 15,308
4	Banana	80,000	Residue	240,000
5	Barley	1200	Stalks	1560
6	Cardamom	8.5	Stalks	0.04687
7	Cassava	6060	Solid waste; starch from roots	3630; 10,980
8	Coconut	13,125.2	Fronts; Husk and pith; Shells	7.6; 6956.356; 2887.544
9	Coffee	300.3	Husk; waste	150.15; 1.328
10	Coriander	250	Stalks	287.5
11	Cotton	3000	Husk; Shells; Stalks	3300; 3300; 34.922
12	Cumin Seed	200	Stalks	310
13	Dry Chilly	800.1	Stalks	1200.15
14	Jute	18.32	Residue	31.51
15	Garlic	598	Sheath; Stalks	149.5; 29.9
16	Ginger	273.333	Stalks	13.667
17	Groundnut	37.86	Residue (skin and hull)	90.86
18	Maize	18,500	Cobs; Stalks	5550; 37,000
19	Millets	6060	Stalks	14,892
20	Rice	145,050	Husks; Stalks; Straws	29,010; 217,515; 217,515
21	Sugarcane	276,250	Bagasse; Top and leaves	91,1625; 13,8125
22	Rubber	825	Primary wood; Secondary wood	1.743; 1.162
23	Wheat	78,000	Pod; Stalks	23,400; 117,000
24	Urad dal	750	Husk; Stalks	150, 825
25	Rapeseed and mustard	7.20	Residue	17.28

Biomass lignocellulosic components

Biomass substances are mainly composed of three main natural fiber components viz., (i) cellulose, (ii) hemicellulose, and (iii) lignin. The composition of them varies from material to material. Generally, the composition of cellulose, hemicellulose and lignin is in the range of 20–55 wt %, 20–45 wt % and 15–35 wt %, respectively. An effective activated carbon as a potential CO₂ adsorbent can be produced from distinct materials such as (i) carbonaceous raw material, (ii) biomass-based residues and (iii) lignocellulosic agricultural by-products. It possesses a high degree of porosity, good adsorption capacity, excellent surface textural properties and physicochemical characteristics.^77,78 Figure 1 shows the lignocellulosic chemical structures of cellulose, hemicellulose, and lignin.

Figure 1.

Lignocellulosic chemical structures of cellulose, hemicellulose, and lignin.⁷⁹

Table 4 gives the lignocellulosic composition of various agricultural by-products or biomass residues.

Table 4.

Lignocellulosic composition of different biomass residues.

Biomass component	Lignin (wt %)	Cellulose (wt %)	Hemicellulose (wt %)	Extractives and Others* (wt %)	Reference
A. Activated carbon produced from shell
Fox nutshell^a	33.4	20.4	46.3	0.00	Kumar et al.⁶⁶
Cocoa shells	16.43	13.24	10.81	59.52	Goncalves et al.⁸⁰
Walnut shell^a	37.14	23.55	29.28	10.03	Yang et al.⁸¹
B. Activated carbon produced from husk
Lemont LG rice husk^a	20.00	29.20	20.10	30.7	Mansaray et al.⁸²
ROK 14 rice husk^a	18.80	33.47	21.03	26.7	Mansaray et al.⁸²
CP 4 rice husk^a	24.60	25.89	18.10	31.41	Mansaray et al.⁸²
Pa potho rice husk^a	18.20	35.50	21.35	24.95	Mansaray et al.⁸²
C. Activated carbon produced from stone
Date stone	15	57	23	5.00	Danish et al.⁸³
Peach stone	33	46	14	7	Uysal et al.⁸⁴
Olive stone^a	27.2	40.4	32.2	0	Saleem et al.⁸⁵
D. Activated carbon produced from trunk
Banana trunk	9.00	43.46	38.54	9.00	Justiz-Smith et al.⁸⁶
E. Activated carbon produced from coir
Coconut coir	59.40	32.65	7.95	0.00	Justiz-Smith et al.⁸⁶
F. Activated carbon produced from bagasse
Sugarcane bagasse	13.00	30.27	56.73	0.00	Justiz-Smith et al.⁸⁶
G. Activated carbon produced from tea waste
Tea waste	47.09	17.19	19.90	15.82	Gurten et al.⁸⁷
H. Activated carbon produced from cake
Soybean oil cake^a	2.80	21.58	52.51	17.86	Tay et al.⁸⁸
I. Activated carbon produced from cob
Corn cob	13.01	35.41	36.68	9.87	Cao et al.⁸⁹
J. Activated carbon produced from shell
Siriguela seeds	16.87	18.87	9.01	55.25	Goncalves et al.⁸⁰
K. Activated carbon produced from root
Vine roots	14.8	33.50	21.70	30	Nabais et al.⁹⁰
L. Activated carbon produced from waste
Casuarina equisetifolia fruit waste^a	46.06	25.52	18.84	9.85	Ravichandran et al.⁹¹
M. Activated carbon produced from stalk
Cotton stalk	19.87	35.50	21.98	22.65	Deng et al.⁹²

Others*: ash, moisture, total fat, protein and starch.

weight percent on dry basis.

It is understood from the table that the different lignocellulosic-based agricultural by-products or biomass residues are used to produce activated carbon for adsorption purposes. It is reported that biomass sources are used as potential feedstocks to prepare activated carbon.^4,6 The outer surface of the biomass is covered by lignin. The inner part of the biomass is filled with cellulose. The random amorphous structure is hemicellulose located in between lignin and cellulose. These lignocellulosic components significantly enhance the adsorption capacity and specific porous volume of the activated carbon.

Methods adopted for the preparation of activated carbon

This section discusses the methods involved in the preparation of activated carbon.

Pre-treatment

Initially, unwanted impurities, dust, dirt and foreign particles, and unevenly distributed materials are first removed from the selected raw material. Then, it is subjected to a drying process for removing the moisture content present on it.

De-ashing/demineralization

Biomass substances generally consist of volatile matter, fixed carbon, moisture, ash, and some mineral components. The percentage composition of them affect the chemical properties and surface functional groups of the prepared activated carbon.⁶¹ Therefore, the moisture and undesirable components should be removed by drying or heating and using a suitable acidic or basic solution.⁶ Figure 2 illustrates the process flow diagram of the bio-products obtained from different biomass residues.

Figure 2.

Flow process of activated carbon preparation.

Two main methods are involved in the preparation of activated carbon: (i) carbonization and (ii) activation. Carbonization is a process of thermally decomposing a selected material at an elevated temperature with an inert atmosphere and obtaining a solid form of carbon enriched product. Pyrolysis is the process of thermally decomposing of an organic substance in the absence of oxygen at high temperature in the conversion of organic material to form a liquid (bio-oil), combustible gas (syngas), and solid residues (bio-char). The physical activation, chemical activation, and combination of both activation methods are used to activate the material for developing the higher porosity and enhancing the larger specific surface area. The characteristics and properties of activated carbon purely depend on the preparation methods and nature of the raw material. Figure 3 shows the schematic diagram for the activated carbon preparation methods such as carbonization and activation (physical and chemical).

Figure 3.

Schematic representations for activated carbon production.⁸⁶

Carbonization

The important parameters affecting carbonization are carbonization temperature and duration, heating rate, and the presence or absence of an inert atmosphere. The moisture content and low molecular weight of the volatile matter are first reduced from the raw material. The thermal decomposition of a raw material eliminates non-carbon elements like hydrogen (H₂), nitrogen (N₂), oxygen (O₂) and sulfur (S) in the form of gas and tar. This process is more intended to develop the carbon enriched skeleton structure. Usually, the carbonization temperature maintains at a temperature above 600°C and obtains a better carbon yield. High temperature reduces the volatile matter and increase the fixed carbon content in the material. The obtained carbonized product (biochar) has a low surface area and less porosity. Therefore, it is subjected to activation. The main objective of the activation process is to develop the specific surface characteristics and inherent porosity of activated carbon.⁵⁹

Activation

The activation method dramatically influences the physical, and chemical properties of activated carbon. There are two different methods involved in activation, namely physical activation and chemical activation. It is mainly used for pore development based on the following three categories;

Opening of developed inaccessible pores,

Developing and formation of new pores and

Enlarging or widening of existing pores.

Physical activation

Dry oxidation is also referred to as a physical treatment method. It is performed by two steps; the first one is carbonization and the second one is activation. In this method, the sample is first subjected to carbonization, followed by activation which is carried out at an elevated temperature. Generally, the carbonization temperature ranges between 400 and 850°C, while the activation temperature ranges between 600 and 900°C and sometimes the maximum temperature reaches up to 1000°C. Suitable oxidizing gases such as steam, CO₂, air, or mixtures are used. CO₂ is the most preferably used gas for the physical treatment of oxidation due to its advantages such as cleanness, ease to handle, and achieving uniform pore development on the surface of activated carbon.⁷⁰ There are different physical activating agents and their reactions are given below;

Oxygen

C + O_{2} \to C O_{2}

(1)

2 C + O_{2} \to 2 C O_{2}

(2)

Carbon dioxide

C + O_{2} \to 2 CO

(3)

Steam

C + H_{2} O \to H_{2} + 2 CO

(4)

CO + H_{2} O \to H_{2} + C O_{2}

(5)

C + 2 H_{2} \to C H_{4}

(6)

Chemical activation

Wet-oxidation is also known as chemical treatment method. The carbonization and activation processes are carried out simultaneously in this method, so it is also termed as a single-step method. Before being subjected to carbonization, the sample is directly impregnated with suitable chemical activating agents. Then, it is heated under an inert atmospheric condition and is maintained in the temperature range between 300–800°C. The activating agent dehydrates and degrades the cellulose found in the material.⁴ During thermal decomposition, the activating agent is supported to inhibit the ash and tar formation. The activating agent is also known as oxidizing agent or oxidant, activator, chemical catalyst, and dehydrating agent. The activating agent is mainly used to trigger the oxygen functional group on the surface of activated carbon by the impregnation method. Gonzalez-Garcia et al.⁵⁹ reported that the use of activating agent significantly exhibits the uniform pore formation and wide pore distribution. It was reported that the concentration of the activating agent plays a vital role in pore formation on the surface of activated carbon. Table 5 provides the different chemical activating agents used for the chemical activation.

Table 5.

Different chemical activating agents.

Sl. No.	Compounds or elements	Chemical activating agents
1	Alkali compounds	Sodium carbonate (Na₂CO₃) Sodium hydroxide (NaOH) Potassium carbonate (K₂CO₃) Potassium hydroxide (KOH)
2	Alkali earth metal salts	Aluminum chloride (AlCl₃) Calcium dichloride (CaCl₂) Ferric chloride (FeCl₃) Zinc chloride (ZnCl₂)
3	Acid elements	Nitric acid (HNO₃) Nitrous acid (HNO₂) Sulfuric acid (H₂SO₄) Phosphoric acid (H₃PO₄)
4	Others	Ammonium persulfate ((NH₄)₂S₂O₈) Formamide (CH₃NO) Hydrogen peroxide (H₂O₂) Potassium permanganate (KMnO₄) Potassium thiocyanate (KCNS) Sodium amide (NaNH₂) Urea (CH₄N₂O)

The main advantages of chemical activation are minimum temperature, shorter duration, high yield of the target product, uniform heating process, and reduction of the mineral content than physical activation. The demerits of chemical activation are (i) formation of impurities and (ii) corrosive properties of activating agents. Girgis et al.⁷² discussed the use of the activating agent in chemical activation. Some of the considered factors are the impregnation ratio, selection of activating agent, the concentration of the activating agent, and activation time and temperature. During activation, the mentioned factors significantly (i) control the tar formation, (ii) inhibit the formation of unwanted impurities, and (iii) restrict any by-products. Finally, the chemically treated sample is washed with the appropriate acid or alkali compounds followed by distilled water. In the chemical activation method, washing is essentially done as the final step of the process. As a result, it helps to remove the occupied chemical compound in the carbon structure and enhances the porosity in activated carbon. Ioannidou et al.⁹³ reported that the porous activated carbon obtained from agricultural by-products would be more effective for adsorption. It should possess better characteristics that increase the adsorptive properties. Table 6 provides the important parameters of physical activation and chemical activation.

Table 6.

Physical activation and chemical activation.

Sl. No.	Parameter	Physical activation	Chemical activation
1	Method	Dry oxidation	▪ Wet oxidation
2	Process	Dual-step activation process	▪ Single-step activation process
3	Activation temperature	400-1000°C	▪ 300-800°C
4	Activating agent	Steam, CO₂, O₂, a combination of steam and CO₂ and some other gases	▪ NaOH, K₂CO₃, KOH, Na₂CO₃, ZnCl₂, H₃PO₄, HNO₂, H₂SO₄ and HNO₃

Table 7 presents the advantages and disadvantages of physical activation and chemical activation.

Table 7.

Advantages and disadvantages of physical activation and chemical activation.

Sl. No.	Parameter	Physical activation	Chemical activation
1	Advantages	Well-developed microporosity Desirable physicochemical characteristics Good physical strength	▪ Better porous structure and optimum surface textural characteristics ▪ Minimum activation temperature and activation duration ▪ Inhibit the tar formation and reduce other volatile products ▪ Develop more carbon yields ▪ Reduction of mineral content
2	Disadvantage	Maximum activation temperature and activation duration	▪ Chemical activating agents associated with environmental issues such as corrosion and degradation

Factors involved in the selection of activated carbon

The following factors are considered for the selection of suitable activated carbon: (a) high carbon and low inorganic contents, (b) superior adsorption capacity, (c) well-developed porosity and higher surface area, (d) good resistance to moistures and impurities, (e) high CO₂ intake and selectivity against other molecules, (f) adequate mechanical strength and stable CO₂ adsorption capacity in the multi adsorption-desorption cycle, (g) minimal energy consumption and operational cost, and (i) easy modification in terms of physicochemical characteristics, desirable properties, and surface chemistry without imposing high energy penalty.

Characterization and analytical methods

Characterization is the important and foremost step to confirm the suitability of activated carbon for adsorption. It is necessary to identify the surface textural characteristics and physical/chemical properties of activated carbon obtained from biomass residues or agricultural by-products. There are different techniques used to perform the proximate, ultimate and thermal analyses, and also determine physical, chemical, physiochemical, surface-chemical, surface structural and morphology, textural characteristics, adsorptive capacity, adsorptive behavior, and adsorptive properties. Table 8 provides the different characterization and analytical techniques used to determine the activated carbon's surface textural and physicochemical characteristics and physical/chemical properties.

Table 8.

Characterization techniques and their determined characteristics and properties^6,93,97

Characterization techniques	Determination of characteristics and properties
Proximal or quantitative analysis	◦ Physicochemical properties of activated carbon are moisture content, ash content, volatile matter, fixed carbon content and apparent density
Ultimate or elemental analysis (CHNS-O analyzer)	◦ Elemental compositions of activated carbon are carbon, hydrogen, oxygen, nitrogen and sulfur
Lignocellulosic analysis	◦ Chemical/biological composition of activated carbon are lignin, hemicellulose, cellulose and extractives
Ultrapycnometer	◦ Solid and apparent density of activated carbon
Brunauer-Emmet-Teller (BET) Dubinin-Radushkevich (DR) method Non-local density functional theory (NLDFT)	◦ Textural features such as specific surface area, pore size distribution, micropore surface area, micropore volume, adsorption capacity and adsorptive properties
Barrett-Joyner-Halenda (BJH) Horvath-Kawazoe (HK) method Mercury porosimeter	◦ Pore size distribution, total pore size area, specific pore volume and particle size and shape
Nitrogen adsorption/desorption isotherms	◦ Textural properties of activated carbon, surface area, total pore volume and pore size distribution
Boehm titration Potentiometric titration pH drift - zero point charge (pHZPC) Isoelectric point (pHIEP)	◦ Acidic or basic oxygenated surface functional groups of activated carbon ◦ Surface chemistry characterization by both qualitatively and quantitatively ◦ Surface chemical properties of activated carbon
pH measurement	◦ Interaction between the adsorbate and the adsorbent ◦ Estimate the other non-carbon constituents
Phenol Methylene blue index Mercury intrusion porosimeter Molasses number Iodine number	◦ Adsorption capacity of activated carbon ◦ Number of pores present on the surface of activated carbon
Quantachrome Isorb HP Immersion calorimetry	◦ At high-pressure CO₂ adsorption isotherms ◦ Study of adsorbate and adsorbent interaction in the adsorption process and adsorption capacity
Infrared absorption spectroscopy (IR) Attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) Diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) Boehm titration and Raman spectra (RS)	◦ Active surface functional groups ◦ Surface chemistry ◦ Chemical structure of activated carbon
Field emission scanning electron microscope (FE-SEM) Transmission electron microscope (TEM) Atomic force microscopy (AFM)	◦ Visual validation of high-resolution microscopic images of microstructure and micro-porosity of activated carbon ◦ Surface texture and physical morphology/topography ◦ Analysis of void pores, cracks, crevices, flaws, cavities and some grains at various sizes present on the surface of activated carbon
Energy-dispersive X-ray spectroscopy (EDX) Scanning electron microscopy with energy-dispersive X-ray (SEM-EDX) X-ray fluorescence (XRF) X-ray diffraction (XRD) X-ray photoelectron spectroscopy (XPS)	◦ Elemental analysis (elemental composition) of activated carbon ◦ Chemical composition of activated carbon ◦ Distribution of elements on activated carbon ◦ Crystallographic characteristics or amorphous structure of the activated carbon ◦ Surface chemical properties of activated carbon
Vibrating sample magnetometer (VSM)	◦ Magnetic properties of activated carbon
Point of zero charge (pzc)	◦ Physicochemical properties, surface charges on the activated carbon, cationic exchange capacity, hardness and bulk density
Nuclear magnetic resonance (NMR)	◦ Compound mixture of activated carbon ◦ Molecular structure of activated carbon ◦ Active sites of activated carbon
Thermogravimetric analysis (TGA or TG) Differential thermogravimetry (DTG) Differential thermal analysis (DTA) Differential scanning calorimetry (DSC)	◦ Physical phenomena-absorption and desorption ◦ Chemical phenomena-chemisorption, thermal decomposition and solid-gas reactions (oxidation/reduction) ◦ Mass change of sample is measured as a function of temperature/time ◦ Temperature difference between the test sample and reference sample as a function of temperature/time ◦ Particle size, thermal and oxidative stabilities of the sample ◦ Composition of multi-component, molecular structure, decomposition temperature and rate of degradation ◦ Thermal phase change, transition phenomena and solid-state structure
Temperature-programmed desorption (TPD) Thermal desorption spectroscopy (TDS)	◦ Chemical composition, oxygen-containing surface functional groups and distribution of elements ◦ Monitor the surface interactions between adsorbed molecules and substrate surface ◦ Heat of adsorption, multiple adsorption sites and adsorbate-adsorbent interactions ◦ Rate of desorption, kinetic order, surface reactions and tribological performance of materials ◦ Thermal stability of the surface functional groups

Proximate analysis

Proximate analysis is the quantitative analysis used to identify the percentage of volatile matter, moisture, ash, and fixed carbon contnent.⁹⁸ According to the American Society of Testing and Materials (ASTM standard), the quantitative analysis of the compound determines the chemical properties found in a given sample. Table 9 provides the proximate analysis of activated carbon obtained from different biomass residues.

Table 9.

Proximate or quantitative analysis of different activated carbons.

Biomass residues	Ash content (wt %)	Moisture content (wt %)	Volatile matter (wt %)	Fixed carbon content (wt %)	Reference
A. Activated carbon produced from shell
Coconut shell^a	0.74	10.53	78.30	20.96	Guo et al.⁶⁴
Wood apple outer shell	2.60	7.40	10.01	80.72	Bhadusha et al.⁶⁵
Fox nutshell^a	5.0	4.0	70.1	20.9	Kumar et al.⁶⁶
Walnut shell^a	0.53	3.42	78.44	17.61	Yang et al.⁸¹
Oil palm shell^a	7.96	1.1	72.47	18.7	Arami-Niya et al.⁹⁹
Pistachio nutshell^a	1.0	4.0	73.4	21.6	Lua et al.¹⁰⁰
Peanut shell^a	1.74	8.71	67.7	21.9	Wu et al.¹⁰¹
Palm shell^a	2.1	0	76.7	21.2	Lim et al.¹⁰²
Coconut shell^a	1.28	3.52	74.60	20.60	Cazetta et al.¹⁰³
Pumpkin seed shell^a	3.90	7.60	70.97	17.53	Demiral et al.¹⁰⁴
Palm shell^a	1.1	7.96	72.47	18.7	Daud et al.¹⁰⁵
Groundnut shell	8.2	9.0	11.4	71.4	Malik et al.¹⁰⁶
B. Activated carbon produced from pseudo stem
Raw banana tree pseudo stem^a	10	5	75	10	Romero-Anaya et al.¹⁰⁷
Enzymatic hydrolysis of the banana tree Pseudo stem^a	14	18	65	3	Romero-Anaya et al.¹⁰⁷
C. Activated carbon produced from rice husk
Lemont LG rice husk^a	20.00	0	66.40	13.60	Mansaray et al.⁸²
ROK 14 rice husk^a	18.80	0	67.30	13.90	Mansaray et al.⁸²
CP4 rice husk^a	24.60	0	63.00	12.40	Mansaray et al.⁸²
Pa potho rice husk^a	18.20	0	67.60	14.20	Mansaray et al.⁸²
D. Activated carbon produced from stone
Olive stone^a	2.3	8.8	72.7	16.2	Saleem et al.⁸⁵
Cherry stones^a	0.17	0	78.50	21.33	Angin et al.¹⁰⁸
E. Activated carbon produced from wood
Paulownia wood^a	1.05	3.50	76.54	18.91	Yorgun et al.⁷¹
Invasive wood^a	2.4	0	75.4	22.2	Celis et al.¹⁰⁹
Tamarind wood	4.55	10	36.48	48.97	Sahu et al.¹¹⁰
F. Activated carbon produced from peel
Jackfruit peel^a	4	10	50	36	Prahas et al.⁶³
Cassava peel^a	0.3	11.4	59.4	28.9	Sudaryanto et al.¹¹¹
Durian peel^a	4.22	4.54	69.82	21.42	Nuithitikul et al.¹¹²
G. Activated carbon produced from microalgae
Microalgae-chlorella^a	6.73	5.24	71.78	16.25	Shi et al.⁵³
Microalgae-spirulina^a	5.87	7.12	71.18	15.83	Shi et al.⁵³
H. Activated carbon produced from cake
Soybean oil cake^a	5.83	9.92	69.81	14.44	Tay et al.⁸⁸
I. Activated carbon produced from waste
Casuarina equisetifolia waste^a	2.65	2.10	75.87	19.38	Ravichandran et al.⁹¹
J. Activated carbon produced from residue
Tobacco residues^a	11.73	8.13	67.55	12.59	Kilic et al.¹¹³
K. Activated carbon produced from stalk
Grape stalk^a	8.7	7.0	63.13	21.17	Ozdemir et al.¹¹⁴
L. Activated carbon produced from pulp
Apple pulp^a	2.63	5.70	79.93	11.77	Suarez-Garcia et al.¹¹⁵

weight percent on dry basis.

It is necessary to identify the compositions of raw material and activated carbon for adsorption application. Volatile matter and fixed carbon content are the primary elements that greatly influence activated carbon for adsorption. The high volatile matter favors activated carbon production while decreasing the amount of ash content. Therefore, it significantly increases the amount of carbon yield and bio-char formation. It is apparent from the table that the maximum carbon content and minimum ash content are found in the activated carbon. Meanwhile, a significant quantity of carbon content will enhance the adsorption process. It is noticed that the minimum ash content (i.e. 0.17 wt %) and the maximum ash content (i.e. 24.60 wt %) are found in Cherry stone and CP4 rice husk, respectively. The higher and lower moisture contents are noticed about 11.4 wt % and 0 wt % for Cassava peel and Rice husks, respectively. The highest volatile matter and fixed carbon content percentages are noticed at about 78.50 wt % and 80.72 wt % for Cherrystone and Wood apple outer shell, respectively. On the other hand, the high volatile content present in the biomass feedstock can produce a significant porous structure of activated carbon.

Ultimate analysis

The ultimate analysis determines C, H₂, N₂, O₂, and S.⁹⁸ These elemental compositions are the most fundamental chemical elements considered for the chosen raw material and activated carbon. A CHNS/O analyzer or elemental analyzer is an instrument used to examine the elements present in a given sample. Based on the biomass substances, the elemental compositions significantly vary from material to material. Table 10 presents the information of the ultimate analysis of activated carbons obtained from biomass residues.

Table 10.

Ultimate or elemental analysis of different activated carbons.

Biomass residues	Carbon (wt %)	Hydrogen (wt %)	Oxygen (wt %)	Nitrogen (wt %)	Sulfur (wt %)	Others (wt %)	Reference
A. Activated carbon produced from shell
Fox nutshell^a	42.30	4.30	52.51	0.82	0.002	0	Kumar et al.⁶⁶
Oil palm shell^a	50.01	6.85	41.15	1.19	0	0.8	Gonzalez-Garcia et al.⁵⁹
Walnut shell^a	48.07	7.11	42.84	0.76	1.22	0	Yang et al.⁸¹
Peanut shell^a	45.3	6.12	47.8	0.767	0	0	Wu et al.¹⁰¹
Palm shell^a	49.5	6.3	43.6	0.5	0	0	Lim et al.¹⁰²
Pumpkin seed shell^a	48.79	7.52	39.72	3.97	0	0	Demiral et al.¹⁰⁴
Palm shell^a	50.10	6.85	41.15	1.19	0	0	Daud et al.¹⁰⁵
Groundnut shell	75.8	2.95	19.4	1.53	0.32	0	Malik et al.¹⁰⁶
B. Activated carbon produced from pseudo stem
Raw banana tree pseudo stem^a	47.66	6.48	45.21	0.65	0	0	Romero-Anaya et al.¹⁰⁷
Enzymatic hydrolysis of the banana tree pseudo stem^a	45.42	8.81	44.89	0.88	0	0	Romero-Anaya et al.¹⁰⁷
C. Activated carbon produced from husk
Lemont LG rice husk^a	37.60	5.42	36.56	0.38	0.03	20.0	Mansaray et al.⁸²
ROK 14 rice husk^a	42.10	4.98	33.66	0.40	0.02	18.80	Mansaray et al.⁸²
CP 4 rice husk^a	38.70	4.70	31.37	0.51	0.01	24.60	Mansaray et al.⁸²
Coconut husk	81.11	1.10	16.21	1.48	0.10	0	Foo et al.¹¹⁶
D. Activated carbon produced from stone
Olive stone^a	46.5	6.40	46.7	0.4	0	0	Saleem et al.⁸⁵
Cherry stones^a	48.72	6.41	43.2	1.85	0	0	Angin et al.¹⁰⁸
E. Activated carbon produced from wood
Paulownia wood^a	45.83	6.29	47.48	0.40	0	0	Yorgun et al.⁷¹
Invasive wood^a	47.1	6.2	45.3	0.40	0	0	Celis et al.¹⁰⁹
Olive wood^a	45.6	6.19	40.11	0.00	0	8.1	Zabaniotou et al.¹¹⁷
F. Activated carbon produced from stalk
Grape stalk^a	41.58	4.92	52.16	1.18	0.16	0	Ozdemir et al.¹¹⁴
Grape stalk	92.41	0.284	5.869	1.266	0.171	0	Ozdemir et al.¹¹⁴
G. Activated carbon produced from pulp
Apple pulp^a	49.56	8.43	38.22	0.97	0.05	0	Suarez-Garcia et al.¹¹⁵
Oak cup pulps^a	48.91	6.85	42.81	1.31	0.12	0	Timur et al.¹¹⁸
H. Activated carbon produced from peel
Cassava peel^a	59.31	9.78	28.74	2.06	0.11	0	Sudaryanto et al.¹¹¹
Durian peel^a	42.86	5.71	51	0.18	0.25	0	Nuithitikul et al.¹¹²
Jackfruit peel	79.57	1.02	12.86	6.47	0.08	0	Foo et al.¹¹⁹
I. Activated carbon produced from microalgae
Microalgae-chlorella^a	45.03	6.89	26.37	8.43	1.31	11.97	Shi et al.⁵³
Microalgae-spirulina^a	44.76	6.80	24.25	10.10	1.10	12.94	Shi et al.⁵³
J. Activated carbon produced from cob
Corn cob	76.87	2.99	18.48	1.66	0	0	Tsai et al.⁷³
K. Activated carbon produced from waste
Tea waste^a	52.72	6.34	38.15	2.61	0.18	0	Gurten et al.⁸⁷
Casuarina equisetifolia fruit waste^a	43.87	5.34	49.07	1.61	0.11	0	Ravichandran et al.⁹¹
L. Activated carbon produced from cake
Soybean oil cake^a	44.48	6.28	40.49	8.21	0.54	0	Tay et al.⁸⁸
M. Activated carbon produced from residue
Tobacco residues^a	40.95	5.21	47.85	5.99	0	0	Kilic et al.¹¹³
N. Activated carbon produced from sorghum
Grain sorghum^a	43.43	7.10	48.52	0.95	0	0	Diao et al.¹²⁰
O. Activated carbon produced from bamboo
Bamboo	79.96	2.01	17.19	0.39	0.50	0	Mui et al.¹²¹

weight percent on dry basis.

Others-some other compounds.

Many researchers have documented their research works on the biomass-based activated carbon. Activated carbon originating from biomass substances shows distinct elemental compositions. It is seen from the table that the carbon content for all activated carbon is found to be maximum when compared to other elements. This is mainly due to the inherent carbonaceous nature of the chosen feedstock. Therefore, it indicates that activated carbon has a great potential to adsorb CO₂. It is also noted that the amount of carbon is higher (i.e. 92.41 wt %) and lower (i.e. 31.60 wt %) in Coconut husk and Rice husk, respectively. The average percentage of H₂, O₂, S and N₂ components ranges are 4–12 wt %, 20–45 wt %, 2–8 wt % and 0.05-1 wt %, respectively. From the ultimate analysis, it is easy to calculate the amount of CO₂ and steam used for physical activation (i.e. gas/carbon molar ratio) and the amount and concentration of chemical agent (i.e. activating agent/carbon molar ratio) used for chemical activation. Therefore, a suitable impregnation ratio gives better surface characteristics of activated carbon.

Adsorption-desorption isotherms

Adsorption isotherm is a process in which the gas molecules are adsorbed on the surface of adsorbent by surface phenomena (i.e. adsorbate and adsorbent interaction). The adsorption isotherm is a function of pressure at a given temperature. By increasing the pressure and decreasing the temperature in the adsorption column; activated carbon will tend to adsorb more CO₂. The desorption isotherm is a process to remove the adsorbed gas molecules from the adsorbent's surface by decreasing the pressure and increasing the temperature in the desorption column.

Adsorbate + Adsorbent \to Adsorption

(7)

Adsorbate + Adsorbent \leftarrow Desorption

(8)

According to the International Union for Pure and Applied Chemistry (IUPAC), the adsorption-desorption isotherms are classified into six types and represented in the forms of curves. These curves mainly indicate the various surface layers of the adsorption mechanism viz., monolayer adsorption, multilayer adsorption, and pores filled by capillary condensation. Figure 4 portrays the adsorption-desorption isotherm curves of activated carbon. It mainly indicates the kinetic behavior of adsorption and desorption. It reveals that the rate of adsorption is equal to the rate of desorption.

Figure 4.

adsorption-desorption isotherm curves.^122,124

Table 11 gives the adsorption isotherms and their characteristics are classified according to the IUPAC.

Table 11.

Adsorption isotherms and their characteristics.

Type of isotherm	Porous form	Adsorption isotherm
Type I	Microporous	◦ Monolayer of adsorbate molecules get adsorbed via adsorption ◦ Concave to relative pressure axis ◦ No multilayer formation
Type II	Non-porous and macro porous adsorbent	◦ Completion of monolayer and the initialization of multilayers formation ◦ Pore condensation and no hysteresis
Type III	Non-porous and macro porous adsorbent	◦ Hyperbolic graph (Convex to relative pressure axis) ◦ Weak interaction between adsorbate-adsorbent
Type IV	Mesoporous	◦ Due to capillary condensation hysteresis loop formed between the adsorption and the desorption ◦ Mono-multilayer adsorption
Type V	Mesoporous	◦ Elongated ‘S’ type graph ◦ Condensation may occur
Type VI	Non-porous and macro porous adsorbent	◦ Stepwise multilayer formations on adsorption in uniform non-porous surface

Specific surface area

Surface area is one of the most important properties of activated carbon that is considered for real-time adsorption applications Because it strongly influences the reactivity and behavior of activated carbon. It is mainly related to the surface texture, particle size and morphology of the sample. A larger surface area and higher pore volume of activated carbon play an important role in adsorption. Similarly, the active sites and surface functional groups are directly proportional to the adsorption capacity of activated carbon. It also influences strong interaction between the adsorbate and the adsorbent.⁸⁵ Table 12 provides the different adopted adsorption methods, assumptions, and determined characteristics.

Table 12.

Characterization of adsorption methods and their determined characteristics.¹²²

Sl. No.	Method	Assumption	Determine parameters
1	▪ Brunauere-Emmette-Teller (BET) method	Based on Langmuir theory	○ Total specific surface area (S_BET)
2	▪ Barrette-Joyner-Halenda (BJH) method	Based on Kelvin equation	○ Pore size distribution of mesopores
3	▪ Dubinine-Radushkevich (DR) method	Gaussian-type distribution of pore sizes Based on Polanyi potential Theory	○ Micropore volume
4	▪ Horvathe-Kawazoe (HK) method	Presence of micropores N₂ adsorption at 77K	○ Pore size distribution of micropores
5	▪ Density functional theory (DFT) method	Based on density functional theory	○ Pore size distribution in a wide range of sizes
6	▪ α_s Plot	Comparison to the reference material (non-porous carbon)	○ Surface areas and pore volumes

Surface porosity - pore size, pore volume, and pore size distribution

A porous structure of activated carbon plays a vital role in the adsorption kinetics and dynamic mechanism. It means that the adsorbate molecules can easily access available pores in the adsorbent. The size of the adsorbent pore should be greater than the size of the adsorbate molecule. Then only adsorbate molecules can accumulate and adhere to the surface of the adsorbent. According to the IUPAC classification, the pores are classified based on their sizes of material.¹²⁵ Table 13 presents the different pore sizes and their ranges.

Table 13.

Different pore sizes and their range.

Sl. No.	Name of pore size	Size range
1	Macropores	More than 50 nm
2	Mesopores	Range between 2-50 nm
3	Micropores	Less than 2 nm
4	Supermicropores-wide	Range between 0.7-2 nm
5	Ultramicropores-narrow	Less than 0.7 nm

The pore size of activated carbon plays an important role in adsorptive behavior. It is noticed that the porosity is mainly related to pores size, pores shape, pores volume and pore size distribution. Figure 5 shows the different pore sizes present in the material.

Figure 5.

Different pores sizes.⁶⁰¹²⁶

Surface morphological aspects

Scanning electron microscopy (SEM) is used to study the surface morphology, topography, tortuosity, pore size and shape of the material. It provides the high resolution of micrograph images, which elucidate the uneven and even cavities, cracks, holes, open and filled pores and some grains are present on it. The smoother surface structure, high degree of porosity and active adsorptive pores in activated carbon can be achieved by adopting suitable activation methods and optimum impregnation ratio.^6,127 Figure 6 depicts the porous form of carbonaceous materials and their different characterization techniques.

Figure 6.

(a) Different pore sizes and their characterization techniques.¹²²^,126 (b) classification of pore sizes.

According to the IUPAC classification, the pore size is categorized into three main types: (i) microporous (1–100 µm), (ii) millipore (0.1–100 mm), and (iii) nanoporous (0.1–100 nm). Figure 6(b) shows the classification of main pore sizes and sub-division of the pore sizes.

The critical factors directly related to high adsorption capacity are the surface area, internal porosity, surface structure, pore volume, and pore-size distribution. The activation treatment of activated carbon can significantly enhance the porous structure and adsorption performance.^59,93

Surface chemistry or surface functional groups

Fourier transform infrared spectroscopy (FTIR) analysis provides detailed information about the surface functional groups and surface chemistry present in activated carbon. Generally, the adsorption capacity of activated carbon is directly linked to the surface area, porosity, and especially surface functional groups. The surface functional groups significantly contribute to the activated carbon's adsorption behavior and adsorption capacity. Activated carbon is characterized by infrared spectroscopy (400–4000 cm⁻¹) to examine surface functional groups. Mainly, activated carbon is associated with the different heteroatoms: H₂, N₂, O₂, S, halogen, phosphorous, and other elements. These elements are chemically bonded to the chemical structure of the activated carbon.^85,128 Figure 7 shows the chemical structure of the active surface functional groups present in activated carbon.

Figure 7.

Chemical structures of surface functional groups.⁶⁰

The different functional groups present in activated carbon are as follows, (a) aromatic C = C stretching; (b) and (c) carboxyl carbonates; (d) carboxylic acid; (e) lactone (4-membered ring); (f) lactone (5-membered ring); (g) ether bridge; (h) cyclic ether; (i) cyclic anhydride (5-membered ring); (j) cyclic anhydride (6-membered ring); (k) quinine; (l) phenol; (m) alcohol; and (n) ketene. Table 14 gives the details of the FTIR analysis for different activated carbons with absorption peaks and corresponding surface functional groups.

Table 14.

Different biomass activated carbon and their absorption peaks and surface functional groups^{66,104,116,129,133}

FTIR spectra-Absorption peak number (cm⁻¹)									Suspected surface functional groups
A	B	C	D	F	E	G	H	I	Suspected surface functional groups
3430	3506, 3419	3436, 3247	3441	3457	3528	3373, 3278	3495	3290	OH groups, hydroxyl and carboxyl groups, amine N-H groups
2920, 2848	2929	-	2928, 2847	2924, 2854	2895	-	2995, 2880	2930, 2860	CH stretching, -CH₃ and -CH₂ alkane groups
-	2160	2361	-	-	-	-		-	C≡C alkynes stretch
-	-	-	-	-	1724	-	1745	1740	C = O stretch (esters, aldehydes, ketone groups and acetyl derivatives)
1591	-	1652	1620, 1559	1640	1598	1590	1627, 1584	1640	COOH and C = C stretch, OH groups, NH and NH₂ deformation
1385	1375	1425	-	1430	1368	1390	1362	1420	C = O, C-H, C-O, C-N, OH, COOH and amide group N-H bending, CH₂ deformation, aromatic amine and groups
1100	1019	1053	1080	-	1108	1119, 1065	1260	1240, 1030	C-O-O (ester, ether or phenol), C-O (alcohol, anhydrides), C-N stretch (nitriles), C-H bending (alkanes), P = O phosphates groups, aliphatic amine, Ester functional group, aliphatic amine
468	726	-	790	-	-	609		450-750	C-C stretch, alkynes and alkyl halides, alkanes, aromatic

A = Rice husk; B = Sugarcane bagasse; C = Coconut husk; D = Wheat straw; E = Date seed; F = Coconut shell; G = Foxnut shell; H = Cotton stalk; I = Pumpkin seed shell.

Few researchers examined the absorption peaks and surface functional groups of activated carbon by FTIR analysis, and the details are listed in Table 14. It is noticed that the hydroxyl (-OH), carboxyl (C = O), and carbonyl (COOH) surface functional groups (i.e. oxygen-containing groups) are influential groups for adsorption. These three oxygen-containing surface functional groups are generally present in activated carbon from the various feedstock. The nature of the activating agent, the concentration of the impregnant and the impregnation ratio are directly linked to the oxygenated surface groups of activated carbon.^{66,104,116,129,130} In addition to that, the nitrogen functional groups present in activated carbon also play a significant role in adsorption.^131,134

Thermal stability

Thermogravimetric analysis (TGA) provides detailed information about the weight loss of the given materials as a function of temperature/time. There are three main stages of decomposition: (i) the first stage represents the decomposition of organic matter into the smaller molar mass to release the volatile gaseous components; (ii) the second stage in which stage intermediate component gets decompose to liberate other volatile species, tar and char; and (iii) the third stage indicates the degradation of materials. Biomass mainly comprises three lignocellulosic components: cellulose, hemicellulose, and lignin as discussed earlier in Section 3. At different temperature ranges, decomposition will occur in lignocellulosic components. It purely depends on the composition of natural fibers. Generally, the decomposition temperature ranges for cellulose, hemicellulose and lignin are 200–500°C, 180–400°C and 150–800°C, respectively.¹³⁵ Table 15 gives the information of TGA analysis for activated carbon obtained from different feedstock and their decomposition temperature range and percentage of weight loss.

Table 15.

TGA analysis of biomass activated carbon at different stages of decomposition.

Biomass-based activated carbon	Thermal characteristic–stepwise decomposition temperature (°C) and Weight loss (wt %)						Reference
Biomass-based activated carbon	1^st stage temp. (°C)	Dehydration stage (wt %)	2^nd stage temp. (°C)	Acute weight loss stage (wt %)	3^rd stage temp. (°C)	Slow weight loss stage (wt %)	Reference
Fox nutshell	65	8	260-365	51.60	600	11.4	Kumar et al.⁶⁶
Date stone (Physical activation)	90-313	3.58	373-473	46.14	473-570	26.31	Danish et al.⁸³
Date stone (Chemical activation)	30-163	10.45	400	5.64	835-900	27.52	Danish et al.⁸³
Sugarcane bagasse	105	3	250	22	350	67.2	Alhassan et al.¹³⁰
Date seed	< 200	2.21	220-400	72.85	450-870	24.94	Ogungbenro et al.¹³¹
Balanite aegyptiaca shell	27-245	12.84	250-350	41.60	350-891	43.23	Wyasu et al.¹³⁴
Detarium microcarpum shell	42-386	15.9	396-536	75.5	540-850	2.58	Wyasu et al.¹³⁴
Acorn shell	30-100	20	100-200	7	400-600	35	Saka et al.¹³⁶
Tea industry waste	75	NA	316	52.6	590	38.5	Gundogdu et al.¹³⁷
Jatropha curcas shell	30-180	NA	180-375	NA	375-650	NA	Tongpoothorn et al.¹³⁸
Reedy grass leaves	35-200	2.01	200-400	72	400-800	8	Xu et al.¹³⁹
Rice bran	200	NA	200-450	60	450-800	< 10	Suzuki at al.¹⁴⁰

*NA – Not Available, Temp. - temperature.

The thermal characteristics of activated carbon (i.e. thermal stability) are determined by the TGA method, and the results obtained in different research works are reported in Table 15. Activated carbon is subjected to thermal decomposition by gradually increasing temperature. At a particular temperature range, the sample starts degrading that helps to evaluate the thermal characteristics. It is inferred from the analysis that the biomass-based activated carbon can be used at a high temperature for better adsorption. In the first stage, activated carbon undergoes decomposition of weight loss (8–15 wt. %) due to the elimination of moisture content and volatile matter. Detarium microcarpum shell and Balanite aegyptiaca shell-based activated carbons reach more than 245°C and the peak temperature of the remaining activated carbon is less than 200°C due to the presence of moisture. In the second stage of decomposition, the significant weight loss (50–70 wt %) occurs at a temperature range of 250–550°C. This is due to the degradation and distillation of tar components. At a temperature range of 750–900°C, the total weight loss (15–30 wt %) is achieved in the sample. Hence, it can be concluded that the activation temperature at above 500°C is suitable for the activated carbon preparation.

Adsorption capacity

Adsorption capacity is one of the vital characteristics of activated carbon. In this context, several natural raw materials (i.e. biomass residues or agricultural by-products) are used to produce efficient activated carbon. The developed activated carbon is treated with methylene blue at the optimum operating conditions to examine its maximum adsorption capacity. Table 16 presents the operating condition and maximum adsorption capacity of the biomass-based activated carbon.

Table 16.

Different biomass based-activated carbons and their maximum adsorption capacity.

Biomass-based activated carbon	Chemical activating agent	Adsorbent dosage (g)	pH	Temp. (K)	q_max (mg/g)	Reference
Coconut shell	H₂SO₄	0.1	8	303	50.6	Jawad et al.¹⁴¹
Coconut leaves	H₂SO₄	0.2–2.5	3–11	303, 313, 323	126.9,137.0, 149.3	Jawad et al.¹⁴²
Waste banana peels	H₂SO₄	0.08	12	303	250	Jawad et al.¹⁴³
Dragon fruit peel	KOH	0.04–0.12	3–10	323	195.2	Jawad et al.¹⁴⁴
Coconut leaf	H₃PO₄	0.03-0.4	3-11	303, 313, 323	357.14, 370.37, 370.37	Jawad et al.¹⁴⁵
Corn cob	H₃PO₄	0.1	9.4	313	183.3	Jawad et al.¹⁴⁶
Sugarcane bagasse waste	KOH	0.08	8	303	136.5	Jawad et al.¹⁴⁷
Coconut leaf	H₃PO₄	0.2-10	3-10	303	250	Jawad et al.¹⁴⁸
Pomegranate peel	H₂SO₄	0.18	9.7	322	161.9	Jawad et al.¹⁴⁹
Bamboo chips	KOH	0.02-0.1	3-10	313	305.3	Jawad et al.¹⁵⁰
Betel nut husks	NaOH	0.025–0.250	2–12	303, 313, 323	381.6, 339.8 235.2	Bardhan et al.¹⁵¹
Coconut leaves	KOH	0.02-0.25	3-11	303, 313, 323	147.1, 151.5, 151.5	Rashid et al.¹⁵²
Betel nut husk	NaOH	NA	2-12	303, 313, 323	324.4, 387.4, 429.6	Tabassum et al.¹⁵³
Grass	K₂CO₃	0.04-0.06	4-10	318	364.2	Abdulhameed et al.¹⁵⁴

*NA – Not Available, Temp. – Temperature, q_max – maximum adsorption capacity.

Jawad et al.^141,150 explored the feasibility of activated carbon obtained from different feedstocks such as used coconut (Cocos nucifera) shell, coconut leaves, waste banana peels, dragon fruit peels, corn cob, sugarcane bagasse waste, pomegranate peel, and bamboo chip. The chemical activating agent, operating condition, and finding of the maximum adsorption capacity of each activated carbon are given in Table 16. Bardhan et al.¹⁵¹ prepared activated carbon from agricultural by-products of betel nut husks with NaOH chemical activation. The adsorption isotherm of methylene blue was performed at three different temperatures viz., 30, 40, and 50°C and their corresponding maximum adsorption capacities of activated carbon were about 381.6, 339.8, and 235.2 mg/g, respectively. Rashid et al.¹⁵² developed activated carbon from coconut leaves with the chemical activation using KOH. Activated carbon was treated with methylene blue at operation conditions: a dosage of 0.02–0.25g/100 mL and solution pH of 3–11. It was found that the maximum adsorption capacities of activated carbon were147.1, 151.5, and 151.5 mg/g at 303, 313, and 323 K, respectively. Tabassum et al.¹⁵³ examined the samples of activated carbon obtained from betel nut husks with chemical activation using NaOH. The sample was processed with methylene blue to examine its maximum adsorption capacity. It was reported that the maximum adsorption capacities were about 324.4, 387.4, and 429.6 mg/g attained at the temperature of 30, 40 and 50°C, respectively. Abdulhameed et al.¹⁵⁴ investigated the grass-based activated carbon with impregnating K₂CO₃ for activation at a temperature of 700°C. The activated carbon sample was subjected to the methylene blue treatment at a dosage of 0.06g to examine its maximum adsorption capacity. It was reported that the feasibility of activated carbon has a maximum adsorption capacity of 364.2 at a temperature of 45°C.

Parameters influencing the characteristics of the activated carbon

Effect of activating agents

The surface characteristics of activated carbon can be modified by using either a physical or chemical activating agent. The pores development on the surface of activated carbon is mainly achieved by three significant steps viz., (i) opening of already inaccessible pores, (ii) generating new pores by using suitable activating agents and activation methods, and (iii) enlarging and magnifying of the existing available pores.¹²⁷ The carbon yield and well-developed porosity are superior and effective in the chemical activation method when compared to the physical activation method for the preparation of the activated carbon.¹⁵⁵

Activating agents used in physical activation

Air, CO₂ and steam are excellent activating agents that are most commonly used in the physical activation method. The carbonized bio-char reacts with air (10 times) and steam (8 times) are faster than that of CO₂ (slow reaction rate).⁴ Ruiz-Fernandez et al.¹⁵⁵ experimentally investigated the effects of steam activation on Vine shoot activated carbon produces meso and macro-porosity than CO₂ activation. It is found that activated carbon exhibited large pore size distributions. The CO₂ activation developed the microporous surface texture compared to steam activation. For physical activation, the performance of the physical activating agents is given in the following order: steam > air > CO₂. Similarly, the carbon yield also depends on the activating agents is shown in the following order: CO₂ > air > steam.

Activating agents used in chemical activation

The chemical activating agent is also known as an activator or oxidizing agent or activating catalyst or dehydrating agent. It is mainly used for chemical activation purposes. The chemical activating agents are used to develop more pores on activated carbon. It is non-toxic, non-corrosion, and has non-harmful properties.⁵³ It mainly promotes decomposition and induces the catalytic bond cleavage by forming link structures and triggering to develop the pores. Ruiz-Fernandez et al.¹⁵⁵ prepared Vine shoot-based activated carbon by using chemical activation with the different chemical agents. It was reported that a high carbon yield was achieved in the following order of the chemical activating agent: ZnCl₂ > H₃PO₄ > KOH. It was also noticed that the Vine shoot activated carbon developed micro and mesoporosity by using ZnCl₂ and H₃PO₄. The same activated carbon formed macro-porosity by using KOH.

Effect of carbon yield and impregnation ratio

The selection of a suitable raw material and appropriate preparation methods significantly influence the activated carbon's characteristics. The different parameters such as activation temperature and duration, activation method and process, activating agent, heating rate, impregnation time, and concentration and impregnation rate of impregnants greatly influence the carbon yield and quality of activated carbon. As the activation temperature and impregnation ratio increase, activated carbon yield gradually decreases. The parameters significantly influence the maximum adsorption capacity of activated carbon are given in the following order: activation temperature > concentration of activating agent > impregnation ratio > activation time. Similarly, the parameters significantly impact the high amount of carbon yield achieved are given in the following order: activation temperature > activation time > impregnation ratio > concentration of activating agent.⁵

The following expression can be used to calculate the percentage of the carbon yield

Carbon Yield (%) = \frac{(weight of activated carbon)}{(weight of dried raw material)} * 100 %

(9)

Lim et al.¹⁰² investigated the effect of H₃PO₄ impregnated with the palm shell to develop activated carbon. It is found that at a minimum activation temperature, a higher amount of carbon yield is achieved with good surface textural characteristics. Nowicki et al.¹⁵⁶ examined activated carbon derived from plum stone using KOH chemical activation. It is reported that more than 20% of the carbon yield was achieved compared to without chemical activation.

Impregnation ratio = \frac{(weight of activating agent)}{(raw material)}

(10)

The development of porosity and improvement of the surface area mainly depend on the concentration and quality of the impregnant. It is reported that the adsorption capacity is directly linked to an increase in porosity of activated carbon with the optimum impregnation ratio. It also corresponds to magnifying the surface area of activated carbon.^102,157 Table 17 provides the biomass-based activated carbon, appropriate chemical activating agent, suitable impregnation ratio, BET specific surface area and total pore volume.

Table 17.

Characteristics of activated carbons prepared from biomass residues.

Biomass-based activated carbon	Impregnation ratio		S_BET (m²/g)	V_Total (cm³/g)	Reference
Biomass-based activated carbon	Chemical activating agent/ Raw material (w/w)	Suitable impregnation ratio	S_BET (m²/g)	V_Total (cm³/g)	Reference
A. Activated carbon produced from shell
Fox nutshell	ZnCl₂/Fox nutshell	2:1	2869	1.96	Kumar et al.⁶⁶
Walnut shell	ZnCl₂/ Walnut shell	2:1	1800	1.176	Yang et al.⁸¹
Palm shell	H₃PO₄/ Palm shell	3:1	1109	0.903	Lim et al.¹⁰²
Coconut shell	NaOH/Coconut shell	3:1	2825	1.498	Cazetta et al.¹⁰³
Pumpkin seed shell	H₃PO₄ / Pumpkin seed shell	2:1	1421	0.908	Demiral et al.¹⁰⁴
Jatropha curcas fruit shell	NaOH/Jatropha curcas fruit shell	4:1	1873	1.312	Tongpoothorn et al.³²
Fox nutshell	H₃PO₄/Fox nutshell	3:2	2636	1.53	Kumar et al.¹⁵⁸
Licorice residues + Pistachio shells	ZnCl₂ / Licorice residues + Pistachio shells	2:1	1492.4	1.1	Kaghazchi et al.¹⁵⁹
Licorice residues + Pistachio shells	H₃PO₄/ Licorice residues + Pistachio shell	1:1	1013.6	0.6	Kaghazchi et al.¹⁵⁹
B. Activated carbon produced from wood
Paulwonia wood	H₃PO₄/ Paulwonia wood	4:1	2806	1.74	Yorgun et al.⁷¹
Invasive wood	H₃PO₄/ Invasive wood	2:1	2281	1.73	Celis et al.¹⁰⁹
Fir wood	KOH/Fir wood	6:1	2794	1.54	Wu et al.¹⁶⁰
C. Activated carbon produced from bagasse
Hazelnut bagasse	ZnCl₂ / Hazelnut bagasse	3:1	1489	0.932	Demiral et al.¹⁶¹
Hazelnut bagasse	KOH/ Hazelnut bagasse	3:1	1642	0.964	Demiral et al.¹⁶¹
D. Activated carbon produced from peel
Jackfruit peel	H₃PO₄/ Jackfruit peel	4:1	1260	0.733	Prahas et al.⁶³
Cassava peel	KOH/Cassava peel	5:1	1605	0.691	Sudaryanto et al.¹¹¹
E. Activated carbon produced from straw
Wheat straw	ZnCl₂ /Wheat straw	1:1	907	0.511	Ma et al.¹³²
Wheat straw	KOH/ Wheat straw	1:1	552	0.387	Ma et al.¹³²
F. Activated carbon produced from stone
Cherry stone	ZnCl₂/Cherry stone	3:1	1704	1.566	Angin et al.¹⁰⁸
Cherry stone	ZnCl₂/Cherry stone	4:1	1971	1.15	Olivares-Marin et al.¹⁶²
Olive stone	KOH/ Olive stone	2:1	1310	0.58	Ubago-Perez et al.¹⁶³
G. Activated carbon produced from stalk
Grape stalk	ZnCl₂/ Grape stalk	2:1	1411	0.723	Ozdemir et al.¹¹⁴
Cotton stalk	KOH/ Cotton stalk	3:1	1310	NA	El-Hendawy et al.¹⁶⁴
H. Activated carbon produced from stem
Lotus stem waste	KOH/Lotus stem waste	4:1	2893	1.59	Wu et al.³⁴
Date stem	H₃PO₄/data stem	2:1	1455	1.045	Hadoun et al.¹⁶⁵
I. Activated carbon produced from bamboo
Bamboo	H₂SO₄/bamboo	1:1	553	NA	Mui et al.¹²¹
Bamboo	H₃PO₄/bamboo	1:1	1432	NA	Liu et al.¹⁶⁶
J. Activated carbon produced from microalgae
Microalgae-chlorella	KOH/Chlorella	2:1	422.6	0.28	Shi et al.⁵³
Microalgae-spirulina	KOH/Spirulina	2:1	404.8	0.23	Shi et al.⁵³
K. Activated carbon produced from soybean oil cake
Soybean oil cake	K₂CO₃/ Soybean oil cake	3:1	1352.86	0.680	Tay et al.⁸⁸
L. Activated carbon produced from tea waste
Tea waste	ZnCl₂/Tea waste	2:1	1141	0.806	Gundogdu et al.¹³⁷
M. Activated carbon produced from seed
Date seed	KOH/Date seed	2.5:1	1033.9	0.649	Abuelnoor et al.¹⁶⁷
N. Activated carbon produced from root
Vetiver roots	H₃PO₄ / Vetiver roots	1:1	1272	1.19	Altenor et al.¹⁶⁸

*NA-Not Available, BET surface area = S_BET (m²/g); Total pore volume = V_Total (cm³/g).

Many research works were carried out by varying the impregnation ratio to develop activated carbon. Kumar et al.⁶⁶ conducted an experimental investigation of Fox nutshell activated carbon by chemical activation using ZnCl₂. At the impregnation ratio of 2:1, the prepared activated carbon exhibited a maximum surface area of 2869 m²/g and a total pore volume of 1.96 cm³/g. Shi et al.⁵³ developed activated carbon from Microalgae spirulina by using KOH. At the impregnation ratio of 2:1, the obtained activated carbon showed a surface area of 404.8 m²/g and a total pore volume of 0.23 cm³/g. Kilic et al.¹¹³ explored the feasibility of activated carbon prepared from Tobacco residues by using KOH and K₂CO₃. About 75 wt % of KOH and K₂CO₃ were used for impregnation. It was reported that the surface area of activated carbon was 1474 and 1635 m²/g for KOH and K₂CO_3, respectively. De Celis et al.¹⁰⁹ developed the Saw-dust invasive Wood-based activated carbon by chemical activation using H₃PO₄. The impregnation was performed at 50 wt % of H₃PO₄ and Saw-dust invasive wood maintained at 450°C, 0.5 h. Activated carbon exhibited the highest surface area of 2281 m²/g and the total pore volume of 1.7 cm³/g.

Diao et al.¹²⁰ evaluated activated carbon produced from Grain sorghum by using H₃PO₄. The authors experimentally investigated the different impregnation ratios viz., 10 wt %, 20 wt %, 35 wt %, and 50 wt %. Especially, at 35 wt % of H₃PO₄ of the developed activated carbon showed the surface area of 346 m²/g and 547 m²/g for single and double stage activation, respectively. It was also reported that H₃PO₄ concentration was more induced to develop many pores on the surface of activated carbon. Alhassan et al.¹⁶⁹ carried out the performance assessment of activated carbon obtained from a mixture of Jatropha curcas shell treated with KOH and triethanolamine (TEA). Activated carbon was developed from three different impregnation ratios, such as 2.5 wt %, 5 wt %, and 7.5 wt %. The determined CO₂ adsorption capacities of activated carbon were 66 mg/g and 78 mg/g for Jatropha curcas shell and Jatropha curcas shell treated TEA, respectively. It was reported that the Jatropha curcas shell treated with TEA showed the same CO₂ adsorption capacity up to seven turns of cycles.

Effect of activation temperature on surface area and pore size

The carbonization and activation temperatures are the two main factors that significantly influence the activated carbon's surface properties and characteristics. Activated carbon obtained from the high temperature carbonized sample shows the large-specific surface area, well-developed internal porosity, and low carbon yield compared to those of the activated carbon obtained from the low temperature carbonized sample. Similarly, the low carbonization temperature maintained in the sample gives more volatile matter and a less order structure than the high carbonization temperature.⁹⁷ Table 18 gives the information of activated carbon obtained from biomass, the physical and chemical activating agents, appropriate activation temperature and specific surface area and total pore volume.

Table 18.

Characteristics of activated carbons prepared from biomass residues.

Biomass based activated carbon	Activation method and activating agent		Activation temp. (^◦C)	S_BET (m²/g)	V_Total (cm³/g)	Reference
Biomass based activated carbon	Method	Agent	Activation temp. (^◦C)	S_BET (m²/g)	V_Total (cm³/g)	Reference
A. Activated carbon produced from shell
Coconut shell	Physical activation	CO₂ and steam	1000	1700	1.135	Guo et al.⁶⁴
Wood apple outer shell	Chemical activation	ZnCl₂	NA	794	0.47	Bhadusha et al.⁶⁵
Fox nutshell	Chemical activation	ZnCl₂	700	2869	1.96	Kumar et al.⁶⁶
Cocoa shells	Chemical activation	H₃PO₄, ZnCl₂	>500	1077, 642	0.68, 0.37	Goncalves et al.⁸⁰
Walnut shell	Chemical activation	ZnCl₂	450	1800	1.176	Yang et al.⁸¹
Oil palm shell	Chemical activation	H₃PO₄	850	1653	0.94	Arami-Niya et al.⁹⁹
Oil palm shell	Chemical activation	ZnCl₂	850	1118	0.57	Arami-Niya et al.⁹⁹
Oil palm shell	Physical activation	CO₂ and steam	850	1213	0.66	Arami-Niya et al.⁹⁹
Pistachio nutshell	Physical activation	N₂ and steam	500	778.1	0.466	Lua et al.¹⁰⁰
Peanut shell	Physical activation	CO₂	900	1060	0.8021	Wu et al.¹⁰¹
Palm shell	Chemical activation	H₃PO₄	800	1109	0.93	Lim et al.¹⁰²
Pumpkin seed shell	Chemical activation	KOH	700	1421	0.908	Demiral et al.¹⁰⁴
Coconut shell	Physical activation	Steam	900	1926	1.260	Li et al.¹⁰⁴
Acorn shell	Chemical activation	ZnCl₂	600	1289	0.201	Saka et al.¹³⁶
Jatropha curcas fruit shell	Chemical activation	NaOH	800	1873	1.312	Tongpoothorn et al.¹³⁸
Fox nutshell	Chemical activation	H₃PO₄	700	2636	1.53	Kumar et al.¹⁵⁸
Mixture of licorice residues and pistachio nut shells	Chemical activation	H₃PO₄, ZnCl₂	600,700	1492.4, 1036.1	1.1, 0.6	Kaghazchi et al.¹⁵⁹
Coconut shell	Physical activation	Steam	1000	1926	1.26	Laine et al.¹⁷⁰
Coconut shell	Chemical activation	KOH	700	2760	1.4849	Punsuwan et al.¹⁷¹
Pistachio nut shell	Chemical activation	KOH	700	700.53	0.375	Foo et al.¹⁷²
B. Activated carbon produced from wood
Paulownia wood	Chemical activation	H₃PO₄	400	2806	1.746	Yorgun et al.⁷¹
Acacia mangium wood	Chemical activation	H₃PO₄, ZnCl₂	500	1038.77, 957.47	NA	Danish et al.⁹⁸
Invasive wood	Chemical activation	H₃PO₄	450	2281	1.73	Celis et al.¹⁰⁹
Tamarind wood	Chemical activation	ZnCl₂	439	1322	1.042	Sahu et al.¹¹⁰
Fir wood	Physical activation steam + Chemical activation	KOH	900, 780	1131; 2794	0.87; 1.54	Wu et al.¹⁶⁰
C. Activated carbon produced from husk
Coconut husk	Chemical activation	KOH	700	1356.25	0.780	Foo et al.¹¹⁶
Coffee husk	Chemical activation	FeCl₃ + ZnCl₂; FeCl₃; ZnCl₂;	280	965; 1522; 1374	0.51; 0.65; 0.75	Oliveira et al.¹⁷³
Hazelnut husk	Chemical activation	ZnCl₂	700	1092	NA	Imamoglu et al.¹⁷⁴
D. Activated carbon produced from peel
Jackfruit peel	Chemical activation	H₃PO₄	450, 550	1033, 1260	0.644, 0.733	Prahas et al.⁶³
Cassava peel	Chemical activation	KOH	650, 750	1183, 1378	0.520, 0.580	Sudaryanto et al.¹¹¹
Jackfruit peel	Chemical activation	NaOH	700	1286.7	0.764	Foo et al.¹¹⁹
E. Activated carbon produced from stone
Datestone	Chemical activation	H₃PO₄	900	1225	NA	Danish et al.⁸³
Cherry stones	Chemical activation	ZnCl₂	700	1704	1.56	Angin et al.¹⁰⁸
Plum stones	Chemical activation	KOH	500, 800	1154, 3228	0.57, 1.61	Nowicki et al.¹⁵⁶
F. Activated carbon produced from straw
Rice straw	Chemical activation	(NH₄)₂HPO₄	700	1154	0.670	Suzuki et al.¹⁴⁰
Wheat straw	Chemical activation	KOH, ZnCl₂	700, 700	552, 907	0.387, 0.511	Ma et al.¹³²
G. Activated carbon produced from stalk
Grape stalk	Chemical activation	ZnCl₂	700	1411	0.723	Ozdemir et al.¹¹⁴
Cotton stalk	Chemical activation	H₃PO₄	800	1720	1.23	Nahil et al.¹⁷⁵
H. Activated carbon produced from corncob
Corn cob	Chemical activation	KOH and K₂CO₃	800	1806, 1541	0.87, 0.74	Tsai et al.⁷³
Corn cob	Chemical activation	KOH	850	2700	NA	Cao et al.⁸⁹
Corncob	Chemical activation	H₃PO₄	400	2081	1.13	Sych et al.¹⁷⁶
I. Activated carbon produced from root
Vine roots	Chemical activation	ZnCl₂	800	1173	0.53	Nabais et al.⁹⁰
Vetiver roots	Chemical activation	H₃PO₄	600	1272	1.19	Altenor et al.¹⁶⁸
J. Activated carbon produced from seed
Siriguela seeds	Chemical activation	H₃PO₄, ZnCl₂	>500	1549, 956	0.95, 0.48	Goncalves et al.⁸⁰
Date seed	Chemical activation	KOH	600	1033.9	0.649	Abuelnoor et al.¹⁶⁷
K. Activated carbon produced from waste
Waste tea	Chemical activation	K₂CO₃	900	1722	0.946	Gurten et al.⁸⁷
Soybean oil cake	Chemical activation	K₂CO₃	800	1352.86	0.680	Tay et al.⁸⁸
Tea waste	Chemical activation	ZnCl₂	700	1141	0.806	Gundogdu et al.¹³⁷
Spent coffee grounds	Chemical activation	KOH	700	1082	0.51	Plaza et al.³⁰
L. Activated carbon produced from microalgae
Microalgae-chlorella	Chemical activation	KOH	600	422.6	0.28	Shi et al.⁵³
Microalgae-spirulina	Chemical activation	KOH	600	404.8	0.23	Shi et al.⁵³
M. Activated carbon produced from pulp
Oak cup pulps	Chemical activation	H₃PO₄, ZnCl₂	600	1002, 1152	0.09, 0.22	Timur et al.¹¹⁸
N. Activated carbon produced from bagasse
Hazelnut bagasse	Chemical activation	KOH, ZnCl₂	700, 700	1642, 1489	0.964, 0.932	Demiral et al.¹⁶¹
O. Activated carbon produced from grain sorghum
Grain sorghum	Chemical activation	H₃PO₄	500	1522	0.688	Diao et al.¹²⁰
P. Activated carbon produced from leave
Reedy grass leaves	Chemical activation	H₃PO₄	500	1474	0.560	Xu et al.¹³⁹
Q. Activated carbon produced from stem
Date stem	Chemical activation	H₃PO₄	550	1455	1.045	Hadoun et al.¹⁶⁵

*NA-Not Available, Temp. = Temperature, BET surface area = S_BET (m²/g); Total pore volume = V_total (cm³/g).

Several researchers documented their research works on the production of biomass-based activated carbon by using activation methods. Kumar et al.⁶⁶ investigated the Fox nutshell activated carbon by using ZnCl₂. At the activation temperature of 700°C, activated carbon exhibited a maximum surface area of 2869 m²/g and a total pore volume of 1.96 cm³/g. Shi et al.⁵³ developed and examined activated carbon produced from Microalgae spirulina by chemical activation using KOH. At the activation temperature of 600°C, activated carbon showed a surface area of 404.8 m²/g and a total pore volume of 0.23 cm³/g. Diao et al.¹²⁰ investigated the effect of activation temperature on the biomass-based activated carbon. Activated carbon obtained from Grain sorghum was maintained at the activation temperature of 600°C (single-stage activation) and 500°C (double-stage activation). The surface area (547 m²/g) of activated carbon for double activation was higher when compared to the surface area (346 m²/) of activated carbon for a single activation. Maneerung et al.¹²⁷ evaluated activated carbon produced from Woody biomass by using physical activation. Steam was used for the activation to maintain the activation temperature of about 900°C. The sample exhibited a surface area of 776.46 m²/g and an adsorption capacity of 189.93 mg/g. It also reported that the surface functional groups such as hydroxyl ( − OH) and carboxyl ( − COOH) groups present on the surface of activated carbon highly attribute to adsorption. In an investigation, Ogungbenro et al.¹³¹ explored the possibilities of producing activated carbon from Date seeds via physical activation. At the temperature of 600°C, the pyrolyzed date seed-based activated carbon was obtained. At the activation temperature of 900°C, the optimized date seed-based activated carbon was achieved. The pyrolyzed date seed activated carbon and optimized date seed activated carbon exhibited the maximum CO₂ adsorption capacities of 64.96 mg/g and 141.14 mg/g, respectively.

Conclusions and prospects

This review article gives a comprehensive review of activated carbon obtained from different biomass sources that were chosen based on suitability, pre-treatment process, preparation methods, modification methods, possible characterization techniques, and adsorption performance. The findings related to production, characterization, and adsorption characteristics have been reviewed, discussed and summarized. The effect of activating agents, the concentration of the activating agents, impregnation ratio, activation temperature and activation duration greatly influence the adsorption performance of activated carbon that has also been discussed and presented. The following points are the important conclusions of this review;

Inexpensive and renewable biomass-based residues are used as raw materials to produce activated carbon and can be potentially used for CO₂ adsorption

Due to their excellent physicochemical properties, surface textural characteristics and maximum adsorption capacity. Large surface area, greater porosity and active surface functional groups significantly contribute to CO₂ adsorption performance.

Surface modification on activated carbon with remarkable adsorptive properties and finding optimum operating conditions could significantly enhance the adsorption performance.

At the impregnation ratio of 4:1, H₃PO₄ impregnated Lotus stem and Paulownia-wood-based activated carbon showed the maximum surface area of 2893 m²/g and 2806 m²/g; and the most significant total pore volume of 1.59 cm³/g and 1.74 cm³/g, respectively.

Among the chemically impregnated activated carbon, ZnCl₂ impregnated Fox nutshell activated carbon exhibited the highest total pore volume of 1.96 cm³/g. KOH treated Plum stones activated carbon showed the utmost highest surface area of 3228 m²/g.

Waste coffee ground-based activated carbon showed the highest CO₂ uptake of 23.3 mmol/g, at 10 bar and 273 K. Activated carbon produced from Jujun grass and Camellia Japonica exhibited the maximum CO₂ adsorption capacity of 21.1 mmol/g, at 20 bar and 298 K.

Activated carbon obtained from Mesquite wood exhibited the superior CO₂ adsorption capacity of 26.0 mmol/g, at 30 bar and 298 K. Hence, it can be concluded that the maximum adsorption capacity of activated carbon could be achieved by increasing the pressure and decreasing the temperature.

Finally, the above-mentioned findings confirmed that biomass-based activated carbon could be effectively used as advanced adsorbents to remove CO₂ from CO₂ contained gas streams.

Future outlook and scope

More research studies are needed to develop novel adsorbent materials for the application of CO₂ adsorption. The in-depth analyses of characteristics and properties of the adsorbent studies are also required to improve their significance in CO₂ adsorption performance.

Key points/Highlights

Inexpensive biomass residue or agricultural by-product is used as feedstock/raw material to prepare potential activated carbon for CO₂ capture.

Different characterization techniques are used to determine the physicochemical properties, surface textural characteristics and adsorptive properties of activated carbon.

Surface modification, porous structural properties, active surface functional groups, active adsorptive pores and excellent adsorption capacity of the activated carbon gives better CO₂ adsorption.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iDs

R. Maniarasu

S. Murugan

Nomenclature

R. Maniarasu completed his B.E (Automobile Engineering) from Anna University-BIT campus, Tiruchirappalli, TamilNadu, India and M.E-Automotive Engineering from PSG College of Technology, Coimbatore, TamilNadu, India. He is currently pursuing his PhD in the Deparment of Mechanical Engineering, National Institute of Technology, Rourkela, Odisha, India. His research is focused on post-combustion CO₂ capture. His areas of interest are internal combustion (IC) engine, automotive ergonomics, automotive comfort and safety, automotive industry-artificial intelligence (AI) and internet of things (IoT). He has published review and research papers in reputed international journals and presented papers in national and international conferences. He was act as a student coordinator for GIAN course on advanced refrigeration systems, short-term course on CCS-2019, International conference on ICITFES-2020 and AICTE-FDP on FCT-2021, Department of Mechanical Engineering, National Institute of Technology Rourkela, Odisha, India.

Sushil Kumar Rathore completed his B.E (Mechanical Engineering) from Government Engineering College, Bilaspur, India and M. Tech (Thermal Science and Engineering) from Indian Institute of Technology, Kharagpur, India. He completed his PhD (Turbulent flow and heat transfer) from Indian Institute of Technology, Kharagpur, India. Now, he is working as an Assistant Professor in the Department of Mechanical Engineering, National Institute of Technology Rourkela. In addition to his administrative works, he has organized a short-term course. He has been independently working on different research projects and supervising many undergraduate and post-graduate students. Currently, four research scholars are working under his guidance. He has published more than 11 research papers in reputed international journals and presented more than 10 papers in international conferences. His areas of interest are turbulent flow and heat transfer, CFD, natural convection and mixed convection flow, Non-Newtonian fluid mechanics, and solar thermal systems.

S. Murugan is a Professor in the Department of Mechanical Engineering at the National Institute of Technology, Rourkela, Odisha, India. He was a visiting researcher at Cranfield University, UK in 2014, under International Research Staff Exchange Scheme. He pursued his post-doctoral research in Polygeneration Systems at VSB Technical University, the Czech Republic in 2015. He pursued his master's and PhD degrees from Anna University, Chennai in the years 1992 and 2009 respectively. He has published more than 150 research articles in peer-reviewed International Journals and reputed International Conferences.

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Biomass-based activated carbon for CO 2 adsorption–A review

Abstract

Keywords

Introduction

Potential activated carbons for Co2 adsorption

Activated carbon for Co2 capture application

Activated carbon obtained from different forms of biomass

Biomass lignocellulosic components

Methods adopted for the preparation of activated carbon

Pre-treatment

De-ashing/demineralization

Carbonization

Activation

Physical activation

Chemical activation

Factors involved in the selection of activated carbon

Characterization and analytical methods

Proximate analysis

Ultimate analysis

Adsorption-desorption isotherms

Specific surface area

Surface porosity - pore size, pore volume, and pore size distribution

Surface morphological aspects

Surface chemistry or surface functional groups

Thermal stability

Adsorption capacity

Parameters influencing the characteristics of the activated carbon

Effect of activating agents

Activating agents used in physical activation

Activating agents used in chemical activation

Effect of carbon yield and impregnation ratio

Effect of activation temperature on surface area and pore size

Conclusions and prospects

Future outlook and scope

Key points/Highlights

Footnotes

Declaration of conflicting interests

Funding

ORCID iDs

Nomenclature

References

Potential activated carbons for Co₂ adsorption

Activated carbon for Co₂ capture application