Well-developed surface areas and porous structures that render high adsorption capacity are necessary for pollutant removal from wastewater by activated carbons. Activated carbons from natural resources, and agricultural and industrial waste materials are produced using chemical agents, including KOH, H3PO4, K2CO3, ZnCl2 and NaOH. This study is intended to highlight the effects of those agents on the physical properties of the activated carbons. The operating conditions, i.e., temperature, time and ratio, show an interplay towards the physical properties at varying degree. The yield, pore size, mesoporosity and surface area of activated carbons derived using different chemical agents correlate well with the impregnation ratio. Generally, the pore size, mesoporosity and surface area increase, while the yield decreases with increasing ratio (over a given range). Higher ratio and temperature are recommended for KOH, K2CO3 and NaOH activation, to endow activated carbons with greater surface area.

  • Physical properties of activated carbons by different chemical activation strategies.

  • H3PO4 activation produces higher carbon yield.

  • Relationships between activation conditions and physical properties of activated carbons were presented.

  • Interplay of activating agents and operating conditions towards excellent physical properties of activated carbon.

Water is the foundation of life. However, the rapid growth of the world's population and poor wastewater management have led to increased wastewater production and associated implications for the environment. Wastewater generally contains suspended solids, dyes, heavy metals and/or salts. According to Sutisna et al. (2017), the stability of such materials makes them difficult to degrade. At high concentration, they are harmful to the environment and aquatic ecosystems, as they can raise the chemical oxygen demand of water. Therefore, wastewater is a serious problem that requires effective treatment.

Adsorption is an effective and preferred method to abate water contaminants, even at low concentrations. The process is simple and easy to scale up, is often based on low-cost adsorbents, and produces small quantities of sludge or by-products (Ismail et al. 2013; Rangabhashiyam et al. 2013; Reza & Ahmaruzzaman 2015). Activated carbon (AC), and ion exchange materials such as zeolite and bentonite clay are widely used in adsorption studies. AC is a versatile adsorbent with a large specific surface, highly porous structure, and high adsorption capacity and surface reactivity. Commercial AC is often used to remove water pollutants but can be expensive, so the quest for economical and high performance materials has become a subject of considerable interest. New ACs from different precursors such as natural resources, or agricultural or industrial residues via various activation strategies, have been developed and tested for wastewater treatment.

AC is generally prepared by physical and/or chemical activation. Chemical activation is advantageous in producing ACs with high carbon yield and surface area, and rich surface functional groups for high adsorption capacity. The specific surface usually exceeds 1,000 m2/g, with 5 g equivalent to a football field (Arvanitoyannis et al. 2008). The wide pore size distribution and well-developed pore structure bring about a high specific surface to capture and entrap the target molecules. Pore sizes are classified as micropore (<2 nm), mesopore (between 2 and 50 nm) and macropore (>50 nm). Microporous AC is normally used to adsorb gases and vapours, while the mesoporous material is preferred for larger molecules such as dyes and organic pollutants. A relatively high number of micropores is required for gas adsorption because most gas molecules have diameters below 1.0 nm. On the other hand, plentiful mesopores are necessary for liquid adsorption due to the larger molecular size of water contaminants. Thus, adsorption capacity is influenced significantly by pore size distribution as well as the size of adsorbate molecules.

ACs have been chemically activated from various carbonaceous materials. However, there is still limited literature on the effects of the chemical activators associated with various activation parameters and the physical properties of the ACs. The aim of this study is to summarise and correlate the activation parameters (temperature, time, ratio) with the ACs' physical properties to give deeper insight into the possible interactions between the chemical agents and carbon materials.

AC production has two steps: carbonization and activation. Carbonization is pyrolysis of the material at high temperature (500 to 1,000 °C) in an inert atmosphere. The aim is to liberate tar, hydrocarbons and volatiles to enrich the carbon content, at the same time creating an initial char porosity (Ma et al. 2017). Activation has significant effects on the AC matrix, especially in the formation of the pore structure, giving rise to a high specific surface. AC can be produced from a variety of carbonaceous materials by physical and/or chemical activation.

Physical char activation is performed using steam and/or carbon dioxide as the oxidising gas. Porosity is developed by controlling gasification of the carbon material. Carbon dioxide has low reactivity and is preferable to steam to develop uniform porosity by regulating the oxidation rate (Contescu et al. 2018). The activation temperature is normally in the range of 600 to 1,000 °C (Nandi et al. 2012; Guan et al. 2013; Ghouma et al. 2015). Unlike chemical activation, this process has no chemical mixing; it is preferable in term of environmental safety, but requires high operating temperature and often results in low product yield (Yang et al. 2010; Zhou et al. 2018).

Chemical activation is a single-step process where carbonization and activation happen simultaneously. In several cases, a pre-carbonization step is introduced to produce char prior to chemical impregnation and activation (Fu et al. 2019; Sangachini et al. 2019; Yang et al. 2019). Undergoing pre-carbonization enables initial pore development with a greater specific surface on activation (Zaini & Kamaruddin 2013). Two-step activation allows more activating agent to react with the carbon compound, yielding greater pore volume and a larger AC specific surface (Saad et al. 2019).

Chemical activation is more advantageous than physical activation because it produces higher carbon yield, rich in micro- and meso-pores with better specific surface and adsorption capacity. The chemical agents possess dehydrogenation properties to inhibit tar formation and avoid excessive volatile production to give high carbon yields (Zhang et al. 2008). It usually requires lower activation temperature (400 to 800 °C) and shorter activation time (30 minutes to 3 hours), depending on the raw materials and activating agents used (Zhang et al. 2010; Gumus & Okpeku 2015; Borhan et al. 2018). At high temperatures and with prolonged activation, the pore walls tend to collapse due to sintering and realignment of the carbon structure, thus decreasing the pore characteristics and specific surface needed for efficient adsorbent-adsorbate interactions (Hock & Zaini 2018).

Selection of the ratio of the chemical agent to precursor is important for producing an AC with good yield, specific surface and adsorption properties. Typical values are in the range 0.5 to 3 (Tang & Zaini 2016; Astuti et al. 2019; Sangachini et al. 2019). Activating agents oxidise a weak part of the carbon matrix to generate pores. A large agent:precursor ratio often results in a limited specific surface because of collapse of the pore structure, although a high ratio that produces a high specific surface could arise from high molecular weight precursor material, which could minimise the loss.

Alkali and alkaline earth metals, and some acids, are normally used as activating agents. Examples include potassium hydroxide (KOH) (Fu et al. 2019; Seo et al. 2019; Yang et al. 2019), phosphoric acid (H3PO4) (Kang et al. 2018; Wu et al. 2018; Baek et al. 2019), potassium carbonate (K2CO3) (Zhou et al. 2012; Garba et al. 2015; Oliveira et al. 2018), zinc chloride (ZnCl2) (Vunain et al. 2018; Laverde et al. 2019; Ma et al. 2019) and sodium hydroxide (NaOH) (Laverde et al. 2019; Hasanzadeh et al. 2020; Zhang et al. 2020).

Hydroxides like KOH and NaOH have been used widely to produce ACs with well-developed porosity and high specific surface. However, KOH is poisonous, and toxic to humans and aquatic creatures, so its role as a chemical agent should be considered carefully in relation to environmental sustainability. According to Hui & Zaini (2015), KOH is not completely vaporized in the impregnated precursor, as the activation temperature is generally below its boiling point (1,327 °C). Hence, the washing step must allow for the recovery of spent KOH to prevent its release to the environment. In comparison, NaOH is cheaper, more environmentally friendly and less harmful than KOH. K2CO3 is also non-hazardous and brings fewer adverse effects from activation.

H3PO4 is relatively non-polluting in character compared to ZnCl2. ACs produced by H3PO4 activation are typically used to remove colour or organic pollutants from water. ZnCl2, like KOH, is toxic and requires additional measures to recover the spent agent for subsequent use in activation. H3PO4 activation (375 to 500 °C) always causes corrosion in steel equipment, while zinc deposits in ZnCl2 ACs (550 to 650 °C) are difficult to remove (Buczek 2016). Tables 15 summarise the preparation and properties of ACs using KOH, H3PO4, K2CO3, ZnCl2 and NaOH, respectively. The ratio is given as the chemical agent to precursor.

Table 1

Preparation and properties of KOH ACs

MaterialActivation strategies (ratio, temp, time)Yield (%)Specific surface (m2/g)Mesoporosity (%)Pore size (nm)Adsorption pollutant modelReference
Petroleum-based pitch 1:1, 800 °C, 1 h 76.8 1,280 – 1.3 Argon Seo et al. (2019)  
Petroleum-free pitches 67.2 1,160 – 1.3 
Petroleum-based pitch 2:1, 800 °C, 1 h 70.4 2,170 – 1.4 
Petroleum-free pitches 63.6 2,230 – 1.4 
Petroleum-based pitch 4:1, 800 °C, 1 h 59.3 3,230 – 1.8 
Petroleum-free pitches 61.6 1,960 – 1.5 
Rice husk 1:1, 750 °C, 3 h 2.5 589 57.6 2.14 Phenol Fu et al. (2019)  
Pre-carbonisation: 450 °C, 3 h
Activation: 1:1, 750 °C, 1 h 
19.4 741 33.3 1.98 
Rice husk Pre-carbonisation: 450 °C, 3 h – 133 85.7 1.43 Phenol Shen et al. (2019)  
Rice husk pellets – 173 87.5 1.33 
Rice husk 1:1, 750 °C, 1 h – 779 71.8 1.85 
Rice husk pellets – 859 83.7 1.33 
Rice husk 3:1, 750 °C, 1 h – 1,818 93.3 1.20 
Rice husk pellets – 1,320 69.2 1.62 
Municipal sewage sludge and coconut shell 1.5:1, 800 °C, 1 h – 285 54 4.47 Methylene blue Yang et al. (2019)  
Pre-carbonisation: 500 °C, 45 min
Activation: 1.5:1, 800 °C, 1 h 
– 684 72 3.79 
Walnut shell Pre-carbonisation: 600 °C, 1.5 h
Activation: 1.5:1, 900 °C, 2.5 h 
– 728 94 1.38 CO2 Sangachini et al. (2019)  
Arundo donax 1:1, 600 °C, 2 h – 637 72 0.56 CO2 Singh et al. (2017)  
2:1, 600 °C, 2 h – 1,122 84 0.56 
Rice husk Pre-carbonisation: 450 °C, 3 h – 133 14.3 1.43 Toluene and phenol Shen & Zhang (2019)  
1:1, 750 °C, 1 h – 779 15.3 1.85 
3:1, 750 °C, 1 h – 1,818 12.2 1.20 
Wood residue (white birch) Pre-carbonisation: 800 °C, 2
Activation: 3:1, 900 °C, 2 h 
– 1,700 100 – Copper Braghiroli et al. (2019)  
Wood residue (black spruce) – 1,662 100 – 
Polyacrylonitrile (PAN) Pre-carbonisation: 800 °C, 2 h
Activation: 1:1, 800 °C, 2 h 
– 1,156 48.5 – CO2 Singh et al. (2019)  
Pre-carbonisation: 800 °C, 2 h
Activation: 3:1, 800 °C, 2 h 
– 1,884 31.3 – 
MaterialActivation strategies (ratio, temp, time)Yield (%)Specific surface (m2/g)Mesoporosity (%)Pore size (nm)Adsorption pollutant modelReference
Petroleum-based pitch 1:1, 800 °C, 1 h 76.8 1,280 – 1.3 Argon Seo et al. (2019)  
Petroleum-free pitches 67.2 1,160 – 1.3 
Petroleum-based pitch 2:1, 800 °C, 1 h 70.4 2,170 – 1.4 
Petroleum-free pitches 63.6 2,230 – 1.4 
Petroleum-based pitch 4:1, 800 °C, 1 h 59.3 3,230 – 1.8 
Petroleum-free pitches 61.6 1,960 – 1.5 
Rice husk 1:1, 750 °C, 3 h 2.5 589 57.6 2.14 Phenol Fu et al. (2019)  
Pre-carbonisation: 450 °C, 3 h
Activation: 1:1, 750 °C, 1 h 
19.4 741 33.3 1.98 
Rice husk Pre-carbonisation: 450 °C, 3 h – 133 85.7 1.43 Phenol Shen et al. (2019)  
Rice husk pellets – 173 87.5 1.33 
Rice husk 1:1, 750 °C, 1 h – 779 71.8 1.85 
Rice husk pellets – 859 83.7 1.33 
Rice husk 3:1, 750 °C, 1 h – 1,818 93.3 1.20 
Rice husk pellets – 1,320 69.2 1.62 
Municipal sewage sludge and coconut shell 1.5:1, 800 °C, 1 h – 285 54 4.47 Methylene blue Yang et al. (2019)  
Pre-carbonisation: 500 °C, 45 min
Activation: 1.5:1, 800 °C, 1 h 
– 684 72 3.79 
Walnut shell Pre-carbonisation: 600 °C, 1.5 h
Activation: 1.5:1, 900 °C, 2.5 h 
– 728 94 1.38 CO2 Sangachini et al. (2019)  
Arundo donax 1:1, 600 °C, 2 h – 637 72 0.56 CO2 Singh et al. (2017)  
2:1, 600 °C, 2 h – 1,122 84 0.56 
Rice husk Pre-carbonisation: 450 °C, 3 h – 133 14.3 1.43 Toluene and phenol Shen & Zhang (2019)  
1:1, 750 °C, 1 h – 779 15.3 1.85 
3:1, 750 °C, 1 h – 1,818 12.2 1.20 
Wood residue (white birch) Pre-carbonisation: 800 °C, 2
Activation: 3:1, 900 °C, 2 h 
– 1,700 100 – Copper Braghiroli et al. (2019)  
Wood residue (black spruce) – 1,662 100 – 
Polyacrylonitrile (PAN) Pre-carbonisation: 800 °C, 2 h
Activation: 1:1, 800 °C, 2 h 
– 1,156 48.5 – CO2 Singh et al. (2019)  
Pre-carbonisation: 800 °C, 2 h
Activation: 3:1, 800 °C, 2 h 
– 1,884 31.3 – 
Table 2

Preparation and properties of H3PO4 ACs

MaterialActivation strategies (ratio, temp, time)Yield (%)Specific surface (m2/g)Mesoporosity (%)Pore size (nm)Adsorption pollutant modelReference
Cotton stalk (Gossypium hirsutum L.) 0.3:1, 500 °C, 2 h 56.8 330 6.25 3.4 – Nahil & Williams (2012)  
0.75:1, 500 °C, 2 h 55.4 1,200 11.5 3.4 – 
1.5:1, 500 °C, 2 h 53.7 1,720 20.2 3.4 – 
Peach stone 0.22:1, 500 °C, 2 h 42.6 1,153 – 1.0 Methylene blue Attia et al. (2008)  
0.43:1, 500 °C, 2 h 41.8 1,393 – 0.99 
1:1, 500 °C, 2 h 43.8 1,153 – 0.99 
Globe artichoke leaf 1:1, 500 °C, 1 h 37 1,745 37.5 2.9 Methylene blue Benadjemia et al. (2011)  
2:1, 500 °C, 1 h 25 2,038 73.0 3.0 
3:1, 500 °C, 1 h 31 1,607 73.9 3.0 
Humin 6.7:1, 300 °C, 2 h 73.5 1,358 – – Methylene blue Kang et al. (2018)  
6.7:1, 400 °C, 2 h 51.4 2,375 – 1.97 
6.7:1, 600 °C, 2 h 55.0 1,774 – – 
Bacterial cellulose 1:1, 400 °C, 1 h 40.2 1,540 – 2.25 Methylene blue Khamkeaw et al. (2018)  
1:1, 500 °C, 1 h 33.4 1,734 – 2.33 
1:1, 600 °C, 1 h 26.4 1,702 – 2.37 
Kenaf stem 1:1, 600 °C, 1.5 h 43 610 47.4 2.48 – Baek et al. (2019)  
2:1, 600 °C, 1.5 h 45 1,020 56.8 3.15 – 
3:1, 600 °C, 1.5 h 37 1,570 70.3 4.63 – 
Peanut shell Pre-carbonisation: 450 °C, 3 h – 591 22.0 1.83 Reactive brilliant blue X- BR Wu et al. (2018)  
3:1, 450 °C, 3 h – 1,138 30.0 2.34 
MaterialActivation strategies (ratio, temp, time)Yield (%)Specific surface (m2/g)Mesoporosity (%)Pore size (nm)Adsorption pollutant modelReference
Cotton stalk (Gossypium hirsutum L.) 0.3:1, 500 °C, 2 h 56.8 330 6.25 3.4 – Nahil & Williams (2012)  
0.75:1, 500 °C, 2 h 55.4 1,200 11.5 3.4 – 
1.5:1, 500 °C, 2 h 53.7 1,720 20.2 3.4 – 
Peach stone 0.22:1, 500 °C, 2 h 42.6 1,153 – 1.0 Methylene blue Attia et al. (2008)  
0.43:1, 500 °C, 2 h 41.8 1,393 – 0.99 
1:1, 500 °C, 2 h 43.8 1,153 – 0.99 
Globe artichoke leaf 1:1, 500 °C, 1 h 37 1,745 37.5 2.9 Methylene blue Benadjemia et al. (2011)  
2:1, 500 °C, 1 h 25 2,038 73.0 3.0 
3:1, 500 °C, 1 h 31 1,607 73.9 3.0 
Humin 6.7:1, 300 °C, 2 h 73.5 1,358 – – Methylene blue Kang et al. (2018)  
6.7:1, 400 °C, 2 h 51.4 2,375 – 1.97 
6.7:1, 600 °C, 2 h 55.0 1,774 – – 
Bacterial cellulose 1:1, 400 °C, 1 h 40.2 1,540 – 2.25 Methylene blue Khamkeaw et al. (2018)  
1:1, 500 °C, 1 h 33.4 1,734 – 2.33 
1:1, 600 °C, 1 h 26.4 1,702 – 2.37 
Kenaf stem 1:1, 600 °C, 1.5 h 43 610 47.4 2.48 – Baek et al. (2019)  
2:1, 600 °C, 1.5 h 45 1,020 56.8 3.15 – 
3:1, 600 °C, 1.5 h 37 1,570 70.3 4.63 – 
Peanut shell Pre-carbonisation: 450 °C, 3 h – 591 22.0 1.83 Reactive brilliant blue X- BR Wu et al. (2018)  
3:1, 450 °C, 3 h – 1,138 30.0 2.34 
Table 3

Preparation and properties of K2CO3 ACs

MaterialActivation strategies (ratio, temp, time)Yield (%)Specific surface (m2/g)Mesoporosity (%)Pore size (nm)Adsorption pollutant modelReference
Brachystegia eurycoma seed hull 1:1, 760 °C, 2 h 19.2 1,218 – 3.25 Malachite green Garba et al. (2015)  
Palm shell 2:1, 700 °C, 2 h 22.2 425 – 0.72 – Adinata et al. (2007)  
2:1, 800 °C, 2 h 18.9 1,170 – 0.73 – 
2:1, 900 °C, 2 h 16.8 544 – 0.75 – 
Palm shell Pre-carbonisation: 450 °C, 1 h 36 815 43.1 2.85 Methylene blue Hao & Xianlun (2013)  
1:2, 850 °C, 1 h 30 982 38.5 1.65 
Paper pulp (bleached pulp) Pre-carbonisation: 800 °C, 2.5 h 25 66.7 7.9 Pharmaceuticals (anti-epileptic carbamazepine and antibiotic sulfamethoxazole) Oliveira et al. (2018)  
Paper pulp (raw pulp) 18 100 9.43 
Paper pulp (bleached pulp) 1:1, 800 °C, 2.5 h 855 72.3 2.69 
Paper pulp (raw pulp) 814 73.2 2.66 
Lentil waste 1:1, 800 °C, 1 h – 1,253 55.3 1.85 Methylene blue and Methyl orange Saygili & Saygili (2019)  
2:1, 800 °C, 1 h – 1,563 83.2 2.21 
3:1, 800 °C, 1 h 38 1,875 63.9 1.97 
Oxytetracycline bacterial residue 1:1, 800 °C, 3 h – 874 29 3.16 Phenol Zhou et al. (2012)  
2:1, 800 °C, 3 h – 1,084 29.6 2.62 
3:1, 800 °C, 3 h – 1,593 31.0 2.18 
MaterialActivation strategies (ratio, temp, time)Yield (%)Specific surface (m2/g)Mesoporosity (%)Pore size (nm)Adsorption pollutant modelReference
Brachystegia eurycoma seed hull 1:1, 760 °C, 2 h 19.2 1,218 – 3.25 Malachite green Garba et al. (2015)  
Palm shell 2:1, 700 °C, 2 h 22.2 425 – 0.72 – Adinata et al. (2007)  
2:1, 800 °C, 2 h 18.9 1,170 – 0.73 – 
2:1, 900 °C, 2 h 16.8 544 – 0.75 – 
Palm shell Pre-carbonisation: 450 °C, 1 h 36 815 43.1 2.85 Methylene blue Hao & Xianlun (2013)  
1:2, 850 °C, 1 h 30 982 38.5 1.65 
Paper pulp (bleached pulp) Pre-carbonisation: 800 °C, 2.5 h 25 66.7 7.9 Pharmaceuticals (anti-epileptic carbamazepine and antibiotic sulfamethoxazole) Oliveira et al. (2018)  
Paper pulp (raw pulp) 18 100 9.43 
Paper pulp (bleached pulp) 1:1, 800 °C, 2.5 h 855 72.3 2.69 
Paper pulp (raw pulp) 814 73.2 2.66 
Lentil waste 1:1, 800 °C, 1 h – 1,253 55.3 1.85 Methylene blue and Methyl orange Saygili & Saygili (2019)  
2:1, 800 °C, 1 h – 1,563 83.2 2.21 
3:1, 800 °C, 1 h 38 1,875 63.9 1.97 
Oxytetracycline bacterial residue 1:1, 800 °C, 3 h – 874 29 3.16 Phenol Zhou et al. (2012)  
2:1, 800 °C, 3 h – 1,084 29.6 2.62 
3:1, 800 °C, 3 h – 1,593 31.0 2.18 
Table 4

Preparation and properties of ZnCl2 ACs

MaterialActivation strategies (ratio, temp, time)Yield (%)Specific surface (m2/g)Mesoporosity (%)Pore size (nm)Adsorption pollutant modelReference
Liquefied wood Pre-carbonisation: 700 °C, 1 h
Activation: 3:1, 700 °C, 1 h 
44.4 762 42.8 13.3 Methylene blue Ma et al. (2019)  
3:1, 700 °C, 1 h 53.8 1,086 59.4 13.3 
Sunflower (Helianthus annuus) seed hull Pre-carbonisation: 800 °C, 1 h 24.9 206 21.3 1.65 Catechol and resorcinol Vunain et al. (2018)  
1.5:1, 800 °C, 1 h 73.6 1,374 3.34 1.92 
Guava seed Pre-carbonisation: 300 °C, 1 h
Activation: 3:1, 500 °C, 2 h 
24.0 919 – 2.37 2,4-dichlorophenol Anisuzzaman et al. (2016)  
Oil palm shell Pre-carbonisation: 300 °C, 1 h
Activation: 4:1, 500 °C, 2 h 
56.8 1,020 – 2.44 Phenol Anisuzzaman et al. (2018)  
Rice husk 3:1, 500 °C, 1 h 33.1 138 100 3.04 Acetaminophen Laverde et al. (2019)  
Coffee husk 51.4 613 41.4 1.91 
Fox nutshell (Euryale ferox1:1 600 °C, 1 h 42.0 1,601 4.49 2.22 – Kumar & Jena (2015)  
1.5:1 600 °C, 1 h 37.5 2,028 5.08 2.32 – 
2:1 600 °C, 1 h 33.0 2,869 14.3 2.73 – 
Mangosteen peel 2:1, 600 °C, 30 min – 1,128 84.0 2.22 Methylene blue Nasrullah et al. (2018)  
4:1, 600 °C, 30 min 51.9 1,622 100 4.85 
6:1, 600 °C, 30 min – 1,115 96.0 4.47 
Sugar cane bagasse 3:1, 500 °C, 2 h – 1,145 88.5 2.65 Diclofenac sodium Naga et al. (2019)  
Capparis scabrida sawdust 1:1, 600 °C, 2 h – 1,676 21.8 3.75 Tartrazine, Brilliant scarlet 4R, Brilliant blue Valladares et al. (2019)  
Aegle marmelos fruit shell 0.5:1, 500 °C, 2 h – 346 3.68 0.78 Chromium (VI) Gottipati & Mishra (2016)  
1:1, 500 °C, 2 h – 592 5.18 1.87 
2:1, 500 °C, 2 h – 872 10.0 2.95 
Peanut shell 5:1, 480 °C, 2 h – 1,025 25.1 0.70 Toluene, ethyl benzene Bedane et al. (2018)  
MaterialActivation strategies (ratio, temp, time)Yield (%)Specific surface (m2/g)Mesoporosity (%)Pore size (nm)Adsorption pollutant modelReference
Liquefied wood Pre-carbonisation: 700 °C, 1 h
Activation: 3:1, 700 °C, 1 h 
44.4 762 42.8 13.3 Methylene blue Ma et al. (2019)  
3:1, 700 °C, 1 h 53.8 1,086 59.4 13.3 
Sunflower (Helianthus annuus) seed hull Pre-carbonisation: 800 °C, 1 h 24.9 206 21.3 1.65 Catechol and resorcinol Vunain et al. (2018)  
1.5:1, 800 °C, 1 h 73.6 1,374 3.34 1.92 
Guava seed Pre-carbonisation: 300 °C, 1 h
Activation: 3:1, 500 °C, 2 h 
24.0 919 – 2.37 2,4-dichlorophenol Anisuzzaman et al. (2016)  
Oil palm shell Pre-carbonisation: 300 °C, 1 h
Activation: 4:1, 500 °C, 2 h 
56.8 1,020 – 2.44 Phenol Anisuzzaman et al. (2018)  
Rice husk 3:1, 500 °C, 1 h 33.1 138 100 3.04 Acetaminophen Laverde et al. (2019)  
Coffee husk 51.4 613 41.4 1.91 
Fox nutshell (Euryale ferox1:1 600 °C, 1 h 42.0 1,601 4.49 2.22 – Kumar & Jena (2015)  
1.5:1 600 °C, 1 h 37.5 2,028 5.08 2.32 – 
2:1 600 °C, 1 h 33.0 2,869 14.3 2.73 – 
Mangosteen peel 2:1, 600 °C, 30 min – 1,128 84.0 2.22 Methylene blue Nasrullah et al. (2018)  
4:1, 600 °C, 30 min 51.9 1,622 100 4.85 
6:1, 600 °C, 30 min – 1,115 96.0 4.47 
Sugar cane bagasse 3:1, 500 °C, 2 h – 1,145 88.5 2.65 Diclofenac sodium Naga et al. (2019)  
Capparis scabrida sawdust 1:1, 600 °C, 2 h – 1,676 21.8 3.75 Tartrazine, Brilliant scarlet 4R, Brilliant blue Valladares et al. (2019)  
Aegle marmelos fruit shell 0.5:1, 500 °C, 2 h – 346 3.68 0.78 Chromium (VI) Gottipati & Mishra (2016)  
1:1, 500 °C, 2 h – 592 5.18 1.87 
2:1, 500 °C, 2 h – 872 10.0 2.95 
Peanut shell 5:1, 480 °C, 2 h – 1,025 25.1 0.70 Toluene, ethyl benzene Bedane et al. (2018)  
Table 5

Preparation and properties of NaOH ACs

MaterialActivation strategies (ratio, temp, time)Yield (%)Specific surface (m2/g)Mesoporosity (%)Pore size (nm)Adsorption pollutant modelReference
Rice husk Pre-carbonisation: 500 °C, 1 h
Activation: 3:1, 800 °C, 1.5 h 
14.3 2,786 8.44 2.40 Lead ions Zhang et al. (2020)  
Soybean shell Pre-carbonization: 500 °C, 1 h
Activation: 4:1, 800 °C, 1.5 h 
10.2 2,628 9.95 2.60 
Date press cake Pre-carbonisation: 500 °C, 2 h
Activation: 3:1, 700 °C, 1.5 h 
26.0 2,623 22.3 2.04 Cefixime Hasanzadeh et al. (2020)  
Rice husk Pre-carbonisation: 500 °C, 2 h
Activation: 3:1, 500 °C, 1 h 
59.3 626 65.7 2.20 Acetaminophen Laverde et al. (2019)  
Pre-carbonisation: 500 °C, 2 h
Activation: 3:1, 650 °C, 1 h 
57.1 709 43.4 2.19 
Pre-carbonisation: 500 °C, 2 h
Activation: 3:1, 800 °C, 1 h 
56.6 1,004 31.5 2.13 
Argan hard shell Pre-carbonisation: 700 °C, 1 h
Activation: 4:1, 850 °C, 1 h 
– 1,827 76.0 3.00 CO2 Boujibar et al. (2018)  
Tofu 2:1, 800 °C, 2 h – 1,143 37.5 3.25 – Lei et al. (2019)  
Coconut coir pith Pre-carbonisation: 650 °C, 2 h
Activation: 1:1, 700 °C, 1 h 
28.5 633 15.4 1.30 – Sesuk et al. (2019)  
Pre-carbonisation: 650 °C, 2 h
Activation: 2:1, 700 °C, 1 h 
27.4 860 13.7 1.30 – 
Pre-carbonisation: 650 °C, 2 h
Activation: 3:1, 700 °C, 1 h 
26.3 2,056 9.40 1.30 – 
MaterialActivation strategies (ratio, temp, time)Yield (%)Specific surface (m2/g)Mesoporosity (%)Pore size (nm)Adsorption pollutant modelReference
Rice husk Pre-carbonisation: 500 °C, 1 h
Activation: 3:1, 800 °C, 1.5 h 
14.3 2,786 8.44 2.40 Lead ions Zhang et al. (2020)  
Soybean shell Pre-carbonization: 500 °C, 1 h
Activation: 4:1, 800 °C, 1.5 h 
10.2 2,628 9.95 2.60 
Date press cake Pre-carbonisation: 500 °C, 2 h
Activation: 3:1, 700 °C, 1.5 h 
26.0 2,623 22.3 2.04 Cefixime Hasanzadeh et al. (2020)  
Rice husk Pre-carbonisation: 500 °C, 2 h
Activation: 3:1, 500 °C, 1 h 
59.3 626 65.7 2.20 Acetaminophen Laverde et al. (2019)  
Pre-carbonisation: 500 °C, 2 h
Activation: 3:1, 650 °C, 1 h 
57.1 709 43.4 2.19 
Pre-carbonisation: 500 °C, 2 h
Activation: 3:1, 800 °C, 1 h 
56.6 1,004 31.5 2.13 
Argan hard shell Pre-carbonisation: 700 °C, 1 h
Activation: 4:1, 850 °C, 1 h 
– 1,827 76.0 3.00 CO2 Boujibar et al. (2018)  
Tofu 2:1, 800 °C, 2 h – 1,143 37.5 3.25 – Lei et al. (2019)  
Coconut coir pith Pre-carbonisation: 650 °C, 2 h
Activation: 1:1, 700 °C, 1 h 
28.5 633 15.4 1.30 – Sesuk et al. (2019)  
Pre-carbonisation: 650 °C, 2 h
Activation: 2:1, 700 °C, 1 h 
27.4 860 13.7 1.30 – 
Pre-carbonisation: 650 °C, 2 h
Activation: 3:1, 700 °C, 1 h 
26.3 2,056 9.40 1.30 – 

Table 1 shows the properties of KOH-activated ACs. These have specific surfaces in the range of 285 to 3,230 m2/g. Using operating conditions comprising ratio 1.5, 800 °C and 1 hour yields an AC with 285 m2/g specific surface (Yang et al. 2019), while 3,230 m2/g is achieved with ratio 4, 800 °C and 1 hour (Seo et al. 2019). Further comparison has been made because both precursors have carbon content >50% (Seo et al. 2019; Yang et al. 2019). In general, as the ratio is increased, more of the chemical agent can move into the carbon matrix, increasing pore development during activation, thus boosting the specific surface. Above some upper threshold, however, the precursor's structural stability is inadequate and the pores tend to collapse. Similarly, increasing the chemical ratio produces a positive trend in mesoporosity and pore size (Singh et al. 2017; Seo et al. 2019; Shen & Zhang 2019). Use of large amounts of KOH damages the carbon walls, yielding a continuous pore structure, and increases the pore size and volume, as well as the specific surface, by oxidation and gasification at high temperature. Carbon gasification rates can also be improved to promote pore formation, although mesoporosity may decrease because excess pore widening destroys mesopores (Shen et al. 2019; Singh et al. 2019).

Generally, direct chemical activation is less effective than two-step activation because the former is concentrated mainly on the carbon surface and the interior matrix does not react with the agent. Too low an activation temperature could result in less polymerization, with more carbon atoms escaping in gases. Precursors that have been pre-carbonised have undergone condensation and rearrangement reaction, releasing gases and then forming amorphous structures with preliminary porosity. The resulting specific surface for direct and two-step KOH activation increased from 589 to 741 m2/g (Fu et al. 2019). Similarly, KOH-activated municipal sludge mixed with coconut shell shows a specific surface increase from 285 to 684 m2/g (Yang et al. 2019). During activation, the activator promoted pore formation and/or enlargement of small pores within the carbon matrix. Most KOH-activated ACs demonstrate good mesoporosity and pore size development, but the yield is relatively low probably due to KOH's dehydrating effect (around 2.5 to 24.3%).

Table 2 shows the properties of H3PO4-activated ACs. These are reported with specific surfaces ranging from 330 to 2,375 m2/g and pore sizes from 2.33 to 4.63 nm, suitable for large adsorbate molecules. ACs show increases in mesoporosity, pore size and specific surface with increasing ratio, but the yield decreases (Nahil & Williams 2012; Baek et al. 2019). H3PO4-activated humin has a high specific surface of 2,375 m2/g and the yield was 51.4% (Kang et al. 2018). H3PO4 promotes bond cleavage and crosslink formation; the phosphate groups enhance pore formation in an expanded state on the carbon material. Its use could limit damage to the humin structure, with a high AC yield. There are also reports of decreasing specific surface and yield as the activation ratio and temperature increase (Attia et al. 2008; Benadjemia et al. 2011; Kang et al. 2018; Khamkeaw et al. 2018). H3PO4 loses water at temperatures above 300 °C, potentially leading to pore destruction due to poor resistance to acid catalytic activity. Pore blockage and shrinkage, and partial collapse of the carbon structure could also occur, leading to decreased pore volume and specific surface.

Table 3 summarises the properties of K2CO3-activated ACs. The specific surfaces of these is significantly smaller than those from other activators. K2CO3-activated lentil waste produced with ratio 3, 800 °C, 1 hour has 1,875 m2/g specific surface and 38% yield (Saygili & Saygili 2019). The specific surface increased as the ratio was increased from 1 to 3 (Zhou et al. 2012; Saygili & Saygili 2019), but fell as the temperature was increased from 700 to 900 °C (Adinata et al. 2007). Potassium from K2CO3 reduction diffuses into the inner carbon structure, widening the existing pores to increase the pore volume. However, too high a temperature increases the carbon-K2CO3 reaction rate, resulting in increasing carbon burn-off and decreasing specific surface. Oliveira et al. (2018) reported 100% mesoporosity of AC from pre-carbonised paper pulp at 800 °C and 2.5 h, which implies that the char released more volatiles and had a high potential for porosity development despite the lower specific surface. The literature for this activator is limited because it does not work well in AC production and offers low yields (4 to 5%).

Table 4 shows the properties of ZnCl2-ACs. A specific surface of 2,869 m2/g was recorded for ZnCl2-activated fox nutshell at ratio 2, 600 °C and 1 hour (Kumar & Jena 2015). The specific surface, mesoporosity and pore size increased as the ratio was increased from 0.5 to 2 (Kumar & Jena 2015; Gottipati & Mishra 2016). The trend changed as the ratio was increased to 6 (Nasrullah et al. 2018). Generally, ZnCl2 activation causes the cellulose molecular structure to swell, breaking lateral bonds in cellulose molecules, and increases the inter- and intra-micelle voids for higher specific surface. A high ratio amplifies the collapse of existing pores within the carbon matrix. Accordingly, two-stage ZnCl2 activation reduced pore size, specific surface and yield relative to direct activation (Ma et al. 2019). Carbonisation and activation occurred simultaneously on the carbon material at optimum temperature to produce a relatively stable carbon structure, with lower incidence of volatiles and by-products.

Table 5 shows the properties of NaOH-activated ACs. Rice husk-based, pre-carbonised AC has a 2,786 m2/g specific surface but relatively low 14.3% yield (Zhang et al. 2020). However, Laverde et al. (2019) reported a specific surface of 1,004 m2/g for the same material and conditions apart from 0.5 hour activation time. High mesoporosity implies that micropores merge into mesopores during pore formation, reducing the number of active sites and specific surface. Varying the impregnation ratio from 1 to 3 increased the specific surface from 633 to 2,056 m2/g (Sesuk et al. 2019).

Figures 15 illustrate the correlation between the AC activation parameters (temperature, time, ratio) and their physical properties. The figures are arbitrarily plotted from data in Tables 15.

Figure 1

Effects of KOH activation on AC properties; (a) yield and pore size, and (b) specific surface and mesoporosity.

Figure 1

Effects of KOH activation on AC properties; (a) yield and pore size, and (b) specific surface and mesoporosity.

Close modal
Figure 2

Effects of H3PO4 activation on AC properties; (a) yield and pore size, and (b) specific surface and mesoporosity.

Figure 2

Effects of H3PO4 activation on AC properties; (a) yield and pore size, and (b) specific surface and mesoporosity.

Close modal
Figure 3

Effects of K2CO3 activation on AC properties; (a) yield and pore size, and (b) specific surface and mesoporosity.

Figure 3

Effects of K2CO3 activation on AC properties; (a) yield and pore size, and (b) specific surface and mesoporosity.

Close modal
Figure 4

Effects of ZnCl2 activation on AC properties; (a) yield and pore size, and (b) specific surface area and mesoporosity.

Figure 4

Effects of ZnCl2 activation on AC properties; (a) yield and pore size, and (b) specific surface area and mesoporosity.

Close modal
Figure 5

Effects of NaOH activation on AC properties; (a) yield and pore size, and (b) specific surface area and mesoporosity.

Figure 5

Effects of NaOH activation on AC properties; (a) yield and pore size, and (b) specific surface area and mesoporosity.

Close modal

As can be seen in Figure 1, there is a positive correlation between AC properties and KOH ratio. The regression line for activation conditions of 500 to 600 °C for 1 to 2 hours is steeper than that of 700 to 800 °C, 1 to 2 hours, implying that a high ratio would be required to produce an AC rich in mesopore content and surface area. A temperature of 700 to 800 °C for 1 to 2 hours is also recommended because the properties of ACs produced under these conditions are better, assuming that all carbonaceous materials have the same characteristics. In many published works related to AC production, carbonaceous materials with high carbon content – of between 40 and 60% – are often used in chemical activation (Singh et al. 2017; Braghiroli et al. 2019; Singh et al. 2019).

H3PO4 activation requires temperatures in the range 300 to 600 °C. The conditions show good correlation with the AC properties, except for yield. ACs derived at 401 to 500 °C show a rising property trend as the ratio increases from 1 to 3. However, the mesoporosity and specific surface decrease as the time is increased to 2 to 3 hours for the same ratio. The negative linear regression of yield versus ratio in Figure 2(a) arises because high ratios accelerate pore development, resulting in carbon loss and lower yield. However, the 31% yield for ratio 3 is still acceptable.

Figure 3(a) shows negative linear regression for K2CO3 activation at 700 to 800 °C for 1 to 2 hours. For 1 to 2 and 2 to 3 hours' activation (same ratio and temperature), the yield and pore size are 19% and 33 nm, and 5% and 27 nm, respectively. For ratios in the range of 0.5 to 2.0, there was a significant decrease as activation time and temperature increased. This could suggest that the yield and pore size would be below 5% and 0.5 nm, respectively, for ratio 3. However, the mesoporosity and specific surface (up to 1,875 m2/g) increase as the ratio increases from 1 to 3. The specific surface at 700 to 800 °C for 2 to 3 hours decreased from 1,400 to 800 m2/g; despite this, the extended time is recommended as it improves the mesoporosity of K2CO3-AC at low impregnation ratios.

Figure 4 shows the relationships between ZnCl2 activation conditions and AC properties. The yield fell from 73.6 to 44.4%, while pore size increased from 1.92 to 13.3 nm as the ratio increased from 1.5 to 3 at >600 °C for 0 to 1 hour. Similarly, activation at 501 to 600 °C for 0 to 1 hour showed the same trend as the ratio increased from 1 to 2. Both activation series exhibit a decline in specific surface with respect to ratio, as shown in Figure 4(b). However, the mesoporosity increased at all activation conditions of ratio 1.5 to 3, >600 °C for 0 to 1 hour, with linear correlation. ZnCl2 activation should be done at a suitable temperature (400 to 600 °C) and time (1 to 2 hours); adequate time gives better swelling, which breaks lateral bonds in carbon material, thus maximizing the formation of inter- and intra-pores for a higher specific surface.

The falling trend of yield for conditions of 601 to 700 °C for 1 to 2 hours – Figure 5(a) – could be correlated with ratio, the gradient increasing with temperature. However, the mesopores – from 2.4 to 2.6 nm – are developed much better at higher temperature and ratio. NaOH activation generates about 14% mesoporosity quite consistently, while the specific surface increases from 633 to 2,623 m2/g (see Figure 5(b)). NaOH activation conditions of 701 to 800 °C for 1 to 2 hours at high ratio are suggested as yielding ACs with greater pore size, rich mesoporosity and high specific surfaces.

The physical properties of ACs activated with different chemical agents – that is, KOH, H3PO4, K2CO3, ZnCl2 and NaOH – are summarised, and the correlations between activation conditions (temperature, time and ratio) and physical properties presented. Linear regression shows that the activation ratio generally correlates well with yield, pore size, mesoporosity and specific surface. Pore size, mesoporosity and specific surface increased, while yield decreased with increasing ratio from 0.5 to 3. The optimum temperature depends, however, on the chemical agent used: 700 °C and above could be recommended for KOH, K2CO3, and NaOH, with activation time in the range of 1 to 2 hours.

This work was funded by Universiti Teknologi Malaysia through SHINE Signature Grant No. 07G80.

All relevant data are included in the paper or its Supplementary Information.

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