Methods for preparation and activation of activated carbon: a review

Abstract

Activated carbon refers to a wide range of carbonised materials of high degree of porosity and high surface area. Activated carbon has many applications in the environment and industry for the removal, retrieval, separation and modification of various compounds in liquid and gas phases. Selection of the chemical activator agent is a major step controlling the performance and applicability of activated carbon. Here, we review chemical activators used to produce activated carbon. We compare the impregnation method with the physical mixing method used in activating with alkali hydroxides. We selected 81 articles from Google Scholar, PubMed, Scopus, Science Direct, Embase and Medlin databases. Eighteen articles report the activation with potassium hydroxide, 17 with phosphoric acid, 15 with zinc chloride, 11 with potassium carbonate, nine with sodium hydroxide, and 11 with new activating agents. Activation with phosphoric acid is commonly used for lignocellulosic material and at lower temperatures. Zinc chloride generates more surface area than phosphoric acid but is used less due to environmental concerns. Potassium carbonate, in comparison with potassium hydroxide, produces higher yields and a higher surface area for the adsorption of large pollutant molecules such as dyes. Activating with potassium hydroxide in terms of surface area and efficiency shows better results than sodium hydroxide for various applications. Also, the comparison of the physical mixing method and the impregnation method in activation with alkali metals indicates that the activated carbon obtained through physical mixing had a higher porosity than the activated carbon produced by the impregnation method.

Introduction

Activated carbon is a porous carbonaceous material with continually expanding applications in water treatment and desalination, wastewater treatment and air purification due to its unique characteristics (Fig. 1) (Kosheleva et al. 2019; Samsuri et al. 2014; Yousefi et al. 2019). Activated carbon is a very diverse adsorbent material including a high degree of porosity and high surface area, while up to 90% of it may be constituted from carbon (Gopinath and Kadirvelu 2018; Morin-Crini et al. 2019; Samsuri et al. 2014). Also, carbon structures contain the main functional groups such as carboxyl, carbonyl, phenol, lactone and quinone that are responsible for adsorbing contaminants. Oxygen, hydrogen, sulphur and nitrogen are also present in the form of functional groups or chemical atoms in the activated carbon structure. The unique adsorption properties depend on the existing functional groups of activated carbon, which are derived mainly from activation processes, precursors and thermal purification (Bhatnagar et al. 2013; Yousefi et al. 2019).

Fig. 1

Application of activated carbon as an adsorbent for environmental remediation

The advantages of activated carbon for zeolites or polymer-based adsorbents are high quality in wastewater treatment, simple process design, easy exploitation of the process, resistance to corrosive (acid and alkali) and toxic environments, high adsorption potential in gas and liquid purification and their use as supportive catalysts (Belala et al. 2011; Bhatnagar et al. 2013; Rambabu et al. 2015). Theoretically, all carbon-rich organic materials are commonly known as carbonised materials, which can be used to produce activated carbon. Activated carbon may be produced from agricultural waste, livestock and industrial by-products (Huang and Zhao 2016; Khadhri et al. 2019; Njoku et al. 2014).

The production of activated carbon around the world is estimated to be around 100,000 tonnes annually (Samsuri et al. 2014). The most common sources of activated carbon on a commercial scale are wood, anthracite and bitumen charcoal, lignite, peat shells and coconut. Alternative sources such as olive and almond shells are also used. The carbon content of these materials ranges from 40 to 90% (wt.), with a density of 0.4–1.45 g/m³ (Cui et al. 2011; Huang and Zhao 2016; Khadhri et al. 2019; Mishra et al.2010). Today, much effort has been devoted to exploiting waste as raw materials in activated carbon production (Crini et al. 2019). Activated carbon can also be produced from agricultural residues such as olive corn, biomass, rice rolls, corn stalks, bagasse, fruit stones (cherry and apricot stones, grape seeds), hard shells (pistachio, almond and pecan shell), fruit pulp, bones and coffee beans (Klasson et al. 2013). The raw material used for the preparation of activated carbon should be abundant, cheap and safe (Jolly et al. 2006). The mineral content of this material and its biodegradability during initial storage should be minimal (Prauchner et al. 2016; Samsuri et al. 2014; Zhu et al. 2010).

In addition, for the production of activated carbon, the presence of carbon materials, transportation of raw materials and the availability and seasonal changes in relation to the quality and availability of raw materials should be considered (Samsuri et al. 2014; Thitame and Shukla 2016). Also, to obtain good results, high adsorption of carbon and oxygen in adsorbents is very necessary. Other features include high abrasion resistance, high thermal strength and small pore diameters, which result in an increased exposure surface and thus an increase in adsorption capacity (Cui et al. 2011; Prauchner et al. 2016; Soleimani and Kaghazchi 2008). Also, the properties of the prepared activated carbon mainly depend on the type of activation agent (Sawant et al. 2017; Uysal et al. 2014).

Considering that the use of different activator substances culminates in producing different properties in activated carbon, and in many studies the selection of the best activated carbon is a priority for many researchers, a systematic study seems necessary to reach an overall conclusion. The purpose of this study was therefore to compare the properties of activated carbon, which were produced by various chemical activators and in two precursor/activator mixing modes—impregnation and physical mixing, through a systematic review of studies. This study will also mention the advantages and disadvantages of activating agents in selecting the best activated agent according to the potential of the activated carbon produced to adsorb various adsorbates from aqueous solutions.

Methodology

In order to conduct this study, international databases including Google Scholar, PubMed, Scopus, Science Direct, Embase and Medline were searched using the following Medical Subject Headings (MESH) keywords: activated carbon, chemical activation, impregnation, physical mixing, new activating agents, phosphoric acid, zinc chloride, potassium carbonate, potassium hydroxide, sodium hydroxide.

In this study, research articles published during 2000–2019 in valid journals in the field of activated carbon production from various agricultural bio-wastes through different activation procedures and used for the adsorption of various pollutants from aqueous solutions were included. On the other hand, other literatures including books, lectures, letter to editors, review articles and research articles published before 2000 were excluded. The articles with the following contents were excluded: i) activated carbon amended with nanoparticles, organic matters and bases/acids, ii) activated carbon used for application other than water and wastewater treatment and iii) accelerated adsorption onto activated carbon by ultrasound.

Study of article quality

The resources utilised by the researchers were studied. At first, the title, abstract and method of work were examined if needed. Also, for further review of the full text of the related articles, the final articles were selected and studied. Finally, all the articles were studied by an expert group in the field of the adsorption of various pollutants with activated carbon produced by various activation agents. As shown in Fig. 2, a total of 201 articles were retrieved in the primary search, from which 156 articles were left after removing repetitive and completely unrelated articles. The focus of research was restricted to studies on the preparation of activated carbon with chemical activators and using the obtained adsorbents for the removal of various pollutants from aqueous solutions. After this filtering process, 76 articles were left for further investigation. The initial list of the articles was also manually reviewed, and five additional articles were selected. Finally, 81 articles were approved for full text review. Among the selected articles, the number of articles based on the chemical activator used for the preparation of activated carbon from agricultural residues is presented in Fig. 3.

Fig. 2

Flow diagram of the search and selection of articles

Fig. 3

Number of articles based on the chemical activator used for the preparation of activated carbon from agricultural residues

Extracting the data

A checklist of the required information includes the name of the adsorbent, activation type, activation time, activation temperature (T), impregnation rate (IR), Brunauer, Emmett and Teller surface area (S_BET), total pore volume (V_t), adsorption rate and maximum adsorption capacity (Q_max). The name of the adsorbent and the used precursor was prepared and completed. Comparison of the performance and properties of activated carbon with different chemicals showed that among different activation factors used in the process of the removal of large molecules such as dye, metal hydroxides produced activated carbon with a higher surface area than other activating agents, so the capacity adsorption of the adsorbent is higher with metal hydroxides (Ahmed and Theydan 2014; Njoku et al. 2013; Tongpoothorn et al. 2011).

In chemical activation, phosphoric acid and zinc chloride are used to activate lignocellulosic materials that have not previously been carbonised, while metal compounds such as potassium hydroxide are used to activate the precursors of charcoal and char. Phosphoric acid, in comparison with zinc chloride, has fewer restrictions in environmental and toxicological contamination and requires a lower activation temperature than potassium hydroxide (Al Bahri et al. 2012; de Yuso et al. 2014; Demiral and Şamdan 2016; Nowicki et al. 2013). The common ranges of activation temperatures in the process of producing activated carbon with phosphoric acid, zinc chloride, potassium carbonate, sodium hydroxide and potassium hydroxide were 450–600 °C, 400–900 °C, 700–1000 °C, 550–850 °C and 450–850 °C, respectively (Angın et al. 2013; Basta et al. 2009; Ibrahim et al. 2014; Liu et al. 2010a, 2012; Youssef et al. 2012). In chemical activation with alkali, in general activation with potassium hydroxide shows better results than sodium hydroxide in terms of surface area and performance in different applications. However, sodium hydroxide is cheaper and more environmentally friendly and does less harm than potassium hydroxide, and it is clear that it has more industrial applications because of its superiority to potassium hydroxide (Byamba-Ochir et al. 2016; Huang et al. 2014; Yahya et al. 2015). Also, the results of the comparison of the physical mixing method and the impregnation method in activating with metal hydroxides in different studies indicated that the activated carbon obtained through physical mixing had a higher porosity than the activated carbon produced by the impregnation method (Alabadi et al. 2015; Byamba-Ochir et al. 2016; Lee et al. 2015; Nowicki et al. 2013).

Activation of activated carbon methods

Activated carbon could be prepared through the direct activation of dry raw precursor or through a two-stage process including initial carbonisation and then activation. In the two-stage process, the dried raw organic materials such as walnut hulls, wood, bone and coal should be initially carbonised at high temperatures. In the carbonisation process, the material should be exposed to a red spot (less than 700 °C) temperature in the distillation apparatus in order to evaporate and remove the hydrocarbons from it in the absence of oxygen. Overall, the process of carbonisation is thus a pyrolytic process, and its product is known as carbonised material, char or biochar (Byamba-Ochir et al. 2016; Huang et al. 2014; Yahya et al. 2015). After activating the activated carbon, various activation methods are used to further develop porosity and create structures that lead to the formation of fine solid cavities in activated carbon (Yahya et al. 2015). The pores created on the surface of activated carbon could be categorised as macropores > 25 nm, 1 nm < mesopores < 25 nm, micropores < 1 nm (Huang et al. 2014).

In view of the nature of the activation process, activated carbon could be prepared in two ways: physical and chemical.

Activated carbon production through physical activation

Physical activation used commercially is a two-step process that involves the process of carbonisation (pyrolysis) in a neutral atmosphere and then activation in atmospheric oxidising gases such as steam, carbon dioxide, carbon dioxide and nitrogen or air mixtures with increasing temperature in the range of 800–1100 °C (Bouchelta et al. 2008). This method has the ability to produce activated carbon of porous structure and good physical power, which is an inexpensive method for activated carbon preparation and is considered a green approach because it is chemical-free (Byamba-Ochir et al. 2016; Pallarés et al. 2018; Yahya et al. 2015). However, in the process of the physical activation of activated carbon, the long activation time and low adsorption capacity of prepared activated carbon and its high energy consumption are the main disadvantages (Yahya et al. 2015).

Activated carbon production through chemical activation

Chemical activation, known as wet oxidation, is usually used for raw materials containing cellulose, such as wood, sawdust or fruit pits. These materials are also called biomass resources. In chemical activation for the preparation of activated carbon, organic precursors are activated in the presence of chemicals at high temperatures (Njoku et al. 2014; Samsuri et al. 2014; Yahya et al. 2015). For chemical activation, the raw material, in the first stage, is saturated with oxidising and highly dehydrated chemicals. After impregnation, the suspension is dried and the remaining mixture is heated for a given time. Depending on the activating material and the properties of the final product, activation can take place at temperatures ranging from 400 to 900 °C, at which cellulose is degraded. Eventually, activated carbon is obtained from the repeated washing of the resulting mixture. Another purpose of the final rinse is the recovery of active substances (Samsuri et al. 2014). Chemical activation agents are dehydrating agents that influence pyrolytic decomposition and, by inhibiting the formation of bitumen, increase the activated carbon content and, with subsequent changes in the thermal degradation of precursors, result in the development of the porous structure of carbon materials (Gratuito et al. 2008; Molina-Sabio and Rodrıguez-Reinoso 2004). These activating agents with deep penetration into the carbon structure lead to the development of small pores in the activated carbon, thereby increasing its surface area (Gratuito et al. 2008). Unlike thermal physical activation, the carbonisation and activation phenomena occur simultaneously in the chemical activation so, in contrast to physical activation where carbonisation and activation processes are typically performed in two different furnaces, chemical activation can be performed in a single furnace (Samsuri et al. 2014).

In the process of chemical activation, the variables that affect the characteristics of the final activated carbon are the amount of impregnation and the weight ratio of chemical agents to dry precursor (Gratuito et al. 2008). Compared to physical activation, this type of activation is more economical because it requires a lower activation temperature, shorter processing time and higher carbon efficiency (Rambabu et al. 2015). Also, the activated carbon prepared through chemical activation has a more porous structure than that of physical activation (Cui et al. 2011). Activated chemicals react with carbon matrices and liberate gas products to form a porous structure (Molina-Sabio and Rodrıguez-Reinoso 2004). However, the need for a repeated and long washing step to eliminate the spent activator agent from the final mixture at the end of activation process is one of the disadvantages of this method. In addition, toxic wastewater is produced at the washing step, which causes water pollution and therefore requires secondary treatment (Wang et al. 2016; Yorgun et al. 2016).

The precise selection of the parameters of the chemical activation process is important to the quality of activated carbon production. In addition, in the production of activated carbon, the efficiency of the process is also considered an important factor (Molina-Sabio and Rodrıguez-Reinoso 2004; Wang et al. 2016; Yorgun et al. 2016). In the chemical activation method, the parameters of the chemical agent effect, impregnation ratio and method, temperature, final temperature of carbonisation, carbonisation time, activation space (under atmospheric conditions) and activation method have been investigated (Cui et al. 2011; Wang et al. 2016; Yorgun et al. 2016). Different types of chemicals have different reactions with precursors and thus affect the adsorption behaviour. The main chemicals which have been used as potential activators are alkaline groups such as potassium hydroxide (KOH), sodium hydroxide (NaOH), calcium chloride (CaCl₂) and potassium carbonate (K₂CO₃), acidic groups such as phosphoric acid (H₃PO₄) and sulphuric acid (H₂SO₄), intermediate metal salts such as ZnCl₂ and other activating agents (Balajii and Niju 2019; Yahya et al. 2015). Based on the physical nature of the activating agent, the activator and precursor could be mixed through two approaches: the physical mixing of the activator and precursor in dry conditions and impregnation (Nowicki et al. 2013; Yorgun et al. 2016).

Activated carbon activators

Activated carbon preparation through activation with phosphoric acid

Among the activating agents, phosphoric acid with the chemical formula H₃PO₄ is widely used in the activation of various lignocellulosic materials (Yahya et al. 2015; Yorgun et al. 2016). In the reaction of phosphoric acid with lignocellulose since cellulose is resistant to hydrolysis of acid, at the beginning of the mixing of the compounds the acid first reacts with the cellulose and lignin. Activation with phosphoric acid is used in the preparation of activated carbon from various forms of biomass (Jadhav and Mohanraj 2016; Nowicki et al. 2013). Some studies on activation with phosphoric acid and related details about activated carbon prepared with this acid are listed in Table 1.

Table 1 Comparison of the functional properties and textural characteristics of activated carbon prepared from various agricultural wastes and activation with H₃PO₄

During the impregnation stage and due to the high polarity of phosphoric acid, controlling the physical and chemical interactions occurring in the bulk of the solution and with the substratum is essential. In this regard, the solution concentration is a primary factor of the activation process with this acid agent (Yahya et al. 2015; Yakout and El-Deen 2016; Yorgun et al. 2016). In a study by Demiral and Samdan (2016) on the chemical activation of pumpkin skin by phosphoric acid, it was found that the pores and cavities formed on the active surface of activated carbon are created due to the evaporation of phosphoric acid during the carbonisation process (Demiral and Şamdan 2016; Yakout and El-Deen 2016). The main mechanisms of activation with phosphoric acid are the depolymerisation, dehydration and redistribution of biopolymers in lignocellulosic materials (Abdelnaeim et al. 2016; de Yuso et al. 2014). Also, during the activation process, the reaction of phosphoric acid with the active carbon-based precursor leads to the formation of products in the form of particles or volatile substances, which as a result create pores or increase the number of pores in the sites previously occupied by this material (de Yuso et al. 2014; Liu et al. 2010b). In addition, phosphoric acid leads to the expansion of microporous and mesoporous pores in activated carbon (Demiral and Şamdan 2016), so the activated carbon produced from lignocellulose wastes through activation with phosphoric acid is very porous (de Yuso et al. 2014; Demiral and Şamdan 2016).

Generally, acid refinement leads to an increase in acid groups, eliminates mineral elements and improves the hydrophilic nature of the surface, so the carbon surface will have more access to the aqueous phase (Abdelnaeim et al. 2016; de Yuso et al. 2014). Phosphoric acid has two important functions: It improves the pyrolytic decomposition of the starting material and the formation of a grid structure (net or lattice) (de Yuso et al. 2014; Liu et al. 2010b). However, the excessive amount of phosphoric acid due to the formation of an insulating layer on the activated carbon does not result in the enhancement of porosity on the activated carbon surface (Zhong et al. 2012). According to a study conducted by Liu et al. (2010b), at higher phosphoric acid doses, more potential sites could be created and occupied by the activating agent, which are beneficial to the subsequent pore-opening and -widening processes (Liu et al. 2010b). However, an excessive increase in phosphoric acid leads to the formation of an insulating layer on the activated carbon (Liu et al. 2010b; Zhong et al. 2012).

Vicinisvarri et al. (2014) investigated the effects of phosphoric acid concentration on the morphology of the activated carbon derived from the core and shells of nuts. The results showed that by increasing phosphoric acid (80% by weight), the highest porosity surface in activated carbon was observed due to the increase in the velocity of the formation of cavities. The outer surface of activated carbon has different vents, while the pore size depends on the amount of carbonisation and impregnation. In this case, the whole surface of activated carbon is full of holes and irregular shapes (Vicinisvarri et al. 2014; Yakout and El-Deen 2016). In a study of activated carbon production from olive stone through activation by 60, 70 and 80% w/w of phosphoric acid, Yakout and El-Deen (2016) reported that the surface area of the activated carbon increased by increasing the concentration of the acid. It was found that the activated carbon prepared with 80% phosphoric acid had the highest surface area (1218 m²/g) and pore volume (0.63 cm³/g) (Yakout and El-Deen 2016).

Shamsuddin et al. 2016 carried out a study of the synthesis and characterisation of H₃PO₄-activated carbon prepared from hemp fibre. The results indicated that the increases in the surface area and porosity of activated carbon fibre with acid activation were higher than crude activated carbon, and Fourier-transform infrared spectroscopy (FTIR) showed the significant presence of peaks from different frequencies before and after activation. The results of BET analysis indicated an increase in the surface area and porosity of activated carbon after acid activation (Shamsuddin et al. 2016). It has been shown that the activated carbon prepared with phosphoric acid has less C = O groups than the raw materials. The reduction of carbonyl groups may be due to the effect of H₃PO₄ hydrolysis, which decomposes these groups and other products such as volatile substances (Baccar et al. 2009). Phosphorus groups are among the most important substances for the adsorption of heavy metals from acidic solutions. H₃PO₄-activated carbon may therefore be considered a cation exchanger for the removal of heavy metal cations from aqueous solutions in the future (Xu et al. 2014).

Acidic groups are the most derivative of the reaction between phosphoric acid and activated carbon precursor (Liu et al. 2010b). Activation with phosphoric acid leads to the composition of the phosphorus element in the carbon structure (Baccar et al. 2009). Phosphoric acid is the most commonly used chemical activator, can produce high-porous activated carbon from raw materials and has fewer environmental and toxicological contaminants than potassium hydroxide and zinc chloride. Moreover, phosphoric acid requires a lower activation temperature (Al Bahri et al. 2012; de Yuso et al. 2014), is not volatile and can form a large number of alkaline or acid-soluble phosphates with elements such as iron, nickel and boron and others that can be incorporated into carbon precursors (Zou et al. 2016).

Yorgun et al. (2016) and Yorgun and Yildiz (2015) confirmed that H₃PO₄ was an effective activator agent, and they observed via carbon electron micrograph scanning electron microscope images that the pores created on the surface of activated carbon are tunnel-shaped and generally have a honeycomb structure. The honeycomb holes of the activated carbon have been fully developed as the corners of the cavities were clearly visible (Yorgun and Yildiz 2015). Acidic purification of activated carbon leads to an increase in the adsorption of various pollutants on the surface due to changes in the chemical surfaces of activated carbon as observed in various articles (de Yuso et al. 2014; Shamsuddin et al. 2016; Zhong et al. 2012), so purification can lead to the removal of hydroxides and the creation of reactive oxygen species groups on activated carbon. Also, the number of acidic functional groups is strongly associated with activated carbon capacity to absorb metal compounds (Bhatnagar et al. 2013; Shamsuddin et al. 2016; Yorgun and Yıldız 2015).

Activated carbon preparation through activation with zinc chloride

Among the activating reagents, zinc chloride is widely used to produce activated carbon, especially lignocellulosic and cellulosic precursors (Arami-Niya et al. 2010). Table 2 illustrates the detailed properties of activated carbon prepared through activation with zinc chloride (Zou et al. 2016).

Table 2 Comparison of the functional properties and textural characteristics of activated carbon prepared from various agricultural wastes and activation with ZnCl₂

Zinc chloride acts as a dampening agent for samples impregnated with this material during activation. Movement of volatile substances through zinc chloride-saturated pores is not disrupted, and after that, during the activation process, volatile substances are released from the surface of activated carbon. Increasing the mass ratio of zinc chloride causes easier release of volatile substances, so the absorption of nitrogen increases on the activated carbon (Arami-Niya et al. 2010). Zinc chloride activation induces an electrolytic action called swelling in the molecular structure of cellulose. Inflation causes a breaking down of the cellulose molecules and leads to an increase in different intra- and inter-coated cavities, which causes a higher surface area in the activated carbon (Saka 2012). During the activation process, lignocellulosic materials are converted into carbon, and hydrogen atoms, oxygen, carbon monoxide, carbon dioxide, methane and aldehydes are liberated, and diatomaceous distillates are produced (Anisuzzaman et al. 2016). Zinc chloride prevents the formation of bitumen and other fluids that block the surface of carbon monoxide and prevent the movement of volatile substances, and volatile substances are subsequently released from the surface of activated carbon (Deng et al. 2009).

In activation with zinc chloride, the yield of activated carbon increases due to polymerisation by zinc chloride and the creation of a few large-ring aromatic compounds (Anisuzzaman et al. 2016). Since zinc chloride does not react with carbon, the obtained activated carbon has a higher yield than activated carbon produced with potassium hydroxide. Using of zinc chloride leads to the removal of the hydrogen and oxygen atoms from the activated carbon structure (Gundogdu et al. 2013). The effect of temperature and amount of zinc chloride on various atoms is that the content of hydrogen and oxygen decreases while nitrogen increases (Alothman et al. 2011). Zinc chloride acts as a Lewis acid and enhances the condensed aromatic reactions by facilitating molecular hydrogen deformation from the hydro-aromatic structure of precursors so, through the exclusion of some of the active sites from the adjacent molecules, polymerisation reactions occur and are affected (Alothman et al. 2011; Gundogdu et al. 2013).

By increasing the amount of zinc chloride, more cracks may occur in the structure of activated carbon, so the productivity may decline, resulting in an increase in the mesoporosity of the activated carbon structure. It can be said that by increasing the weight ratio of zinc chloride, the structure breaks down and the micropores deform and become mesopores (Donald et al. 2011; Gundogdu et al. 2013). Increasing the amount of zinc chloride leads to the removal of volatile compounds from the activated carbon structure, so the number of acidic groups is reduced. It has also been reported that by increasing zinc chloride, the phenolic and carboxylic groups are also affected, while the lactone groups are not. Then, during activation with zinc chloride, phenolic and carboxylic groups are reduced, while lactone groups are increased (Angin 2014; Gundogdu et al. 2013). By increasing the amount of zinc chloride activating agent, the percentage of carbon and mesopores in the structure of the prepared activated carbon is increased (Gundogdu et al. 2013). Cavities on the surface of the activated carbon result from the evaporation of spaces occupied by zinc chloride during the carbonisation process, so chloride is an active agent in producing activated carbon with a high surface area and provides higher adsorption capacity (Angin 2014; Gundogdu et al. 2013).

In a study conducted by Arami-Niya et al. (2010), it was observed that the BET surface area and micropore and mesopore volumes of activated carbon prepared increased by increasing the amount of zinc chloride in the initial activator/precursor mixture. Moreover, by increasing the amount of zinc chloride, the removal of tar from the activated carbon structure was increased, as was the release of volatiles (Arami-Niya et al. 2010). Although zinc chloride is an excellent activating agent in activated carbon preparation, it is seldom used in the food and pharmaceutical industries due to its health-related problems (Anisuzzaman et al. 2016; Saka 2012).

Activated carbon preparation through activation with potassium carbonate

Potassium carbonate with the chemical formula K₂CO₃ is a well-known activating agent in the production of activated carbon (Abbas and Ahmed 2016; Budinova et al. 2008). Table 3 shows the performance and properties of activated carbon produced from various agricultural materials and activated with K₂CO₃. Potassium and sodium hydroxide have adverse effects, but potassium carbonate is not harmful if used for food supplements (often used as supplementary food supplements) (Budinova et al. 2008). Potassium carbonate is known to be a better activating agent than potassium hydroxide due to the production of activated carbon with higher yield, higher surface and pore volume, and higher capacity for adsorbing large molecules like methylene blue from aqueous solutions (Abbas and Ahmed 2016).

Table 3 Comparison of the functional properties and textural characteristics of activated carbon prepared from various agricultural wastes and activation with K₂CO₃

A study by Tay et al. (2009) showed that the activated carbon produced by potassium carbonate had higher yields than the activated carbon produced by potassium hydroxide. Also, under the same conditions, the specific surface area of the activated carbon produced from potassium carbonate is more than the carbon produced from potassium hydroxide. In addition, the activated carbon produced from potassium carbonate has lower ash and sulphur content than the activated carbon produced from potassium hydroxide (Tay et al. 2009). Foo and Hameed (2012c) reported that by increasing the ratio of potassium carbonate to char, the adsorptive capability of prepared orange peel-based activated carbon was enhanced (Foo and Hameed 2012c). Moreover, the presence of potassium carbonate prevents the formation of tar and other liquids such as acetic acid and methanol during the process (Adinata et al. 2007). The main reactions that may occur between the activating agent of potassium carbonate and activated carbon under gasification conditions (during the activation process) are presented in Eqs. (1)–(3) as follows (Adinata et al. 2007; Foo and Hameed 2012c; Horikawa et al. 2010):

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

(1)

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

(2)

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

(3)

According to these equations, the development of porosity during activation with K₂CO₃ is attributed to its reduction under inert condition to form K, K₂O, CO₂ and CO. The potassium compound formed during the activation stage penetrates the internal structure of char matrix, expands the existing pores and creates new pores (Adinata et al. 2007; Foo and Hameed 2012c; Horikawa et al. 2010). Also, due to the evaporation of potassium carbonate, the cavities on the activated carbon surface could be produced from the occupied spaces by the activating agent. These cavities create channels that provide the adsorbent molecules with access to the micro- and mesopores of the activated carbon (Abbas and Ahmed 2016). In K₂CO₃-activated carbons, phenolic groups in the surface area are more specific than the other groups and, with the increase in the activation temperature, the number of functional groups decreases (Horikawa et al. 2010). Also, by increasing the concentration of potassium carbonate, the dehydration effect decreases and leads to the degradation of mesopores, which reduces the adsorption efficiency. By increasing the carbonisation temperatures from 600 to 800 °C, the microporous pores increase on the surface of the activated carbon (Tay et al. 2009).

Activated carbon preparation through activation with sodium hydroxide

Studies show that chemical activation using alkaline materials such as sodium hydroxide and potassium hydroxide produces large amounts of microspores on the activated carbon surface (Martins et al. 2015). Sodium hydroxide is widely known as an effective activating agent for the production of activated carbon (Table 4) (Pezoti et al. 2016). The proposed reactions during activation by NaOH are presented in Eqs. (4)–(6) as follows (Martins et al. 2015; Pezoti et al. 2016):

$$6{\text{NaOH}} + 2{\text{C}} \to 2{\text{Na + }}2{\text{Na}}_{2} {\text{CO}}_{3} + 3{\text{H}}_{2}$$

(4)

$${\text{Na}}_{2} {\text{CO}}_{3} \to {\text{Na}}_{2} {\text{O}} + {\text{CO}}_{2}$$

(5)

$$2 {\text{Na + CO}}_{2} \to {\text{Na}}_{2} {\text{O}} + {\text{CO}}$$

(6)

Table 4 Comparison of the functional properties and textural characteristics of activated carbon prepared from various agricultural wastes and activation with NaOH

The possible reactions between active substances and the surface of the organic precursor result in the creation of micropores on the activated carbon surface due to: i) the release of CO, CO₂, H₂ gases [Eqs. (4)–(6)], which are produced by Na₂CO₃ decomposition at high temperature and hydroxyl reduction, respectively, and ii) alkali metal intercalation into the carbon structure (Martins et al. 2015; Pezoti et al. 2016). As a result, the evaporation of sodium hydroxide and other compounds derived from activated carbon gives rise to a rugged surface with different pore sizes, indicating that the porous structure is well developed and that these canals are suitable channels for adsorbent materials to penetrate the surface of the activated carbon (Liou et al. 2016; Martins et al. 2015; Pezoti et al. 2016).

Through the oxidation reduction process, sodium hydroxide causes the separation and destruction of the graphite layers and thereby makes the pores expand (Liou et al. 2016; Pezoti et al. 2016). The production of carbonates and alkali metals in the carbon matrix leads to the sustainability and expansion of the spaces between the carbon atomic layers so, by increasing the sodium hydroxide-to-char ratio in the activation process, it plays a key role in the development of pores and increasing the surface area and pore volume (Foo and Hameed 2012a). However, according to Eq. (7), excess sodium hydroxide promotes a vigorous gasification reaction, which destroys the walls between the pores, leading to a dramatic decrease in accessible area. Moreover, excess sodium hydroxide molecules deposited in the carbon pore wall might cause catalytic oxidation and decomposition, lowering the adsorption capacity and carbon yield (Foo and Hameed 2012a):

$$4 {\text{NaOH}} + {\text{C}} \to 4{\text{Na}} + {\text{CO}}_{2} + 2{\text{H}}_{2} {\text{O}}$$

(7)

In chemical activation with alkali, activation with potassium hydroxide generally shows better results than sodium hydroxide in terms of surface area and performance in different applications. However, sodium hydroxide is cheaper, more environmentally friendly and less harmful than potassium hydroxide, and it is clear that sodium hydroxide has industrial applications due to its superiority to potassium hydroxide (Gratuito et al. 2008). At high temperatures, the reaction between carbon and sodium hydroxide leads to the reduction of sodium cation to a sodium metal, the formation of sodium carbonate and the reduction of hydroxyl anion to hydrogen gas (Mestre et al. 2007). The results of the Byamba-Ochir et al. (2016) study showed that the activation of carbon by sodium hydroxide through physical mixing of precarbonised precursor and solid sodium hydroxide resulted in more porous activated carbon than that prepared using the impregnation procedure (Byamba-Ochir et al. 2016). Also, Foo and Hameed (2012b) studied the application of different activating agents in the preparation of activated carbon from langsat (Lansium domesticum) empty fruit bunch waste. In that study, sodium hydroxide was selected as the best activating agent due to its higher adsorption capacity, lower cost, lower environmental pollution during its life cycle and lower level of corrosiveness (Foo and Hameed 2012b). According to a review of various articles, activation with metal hydroxides such as sodium hydroxide and potassium can be used for the preparation of activated carbon with a high surface area in the range of 3500–2300 m²/g (Tongpoothorn et al. 2011).

Activated carbon preparation through activation with potassium hydroxide

In recent years, potassium salts such as KOH and K₂CO₃ have been widely used in the production of low-cost activated carbon (Hui and Zaini 2015). Among the various activators, potassium hydroxide has been extensively used, due to its ability to produce activated carbon with a high surface area, its distribution of fine pore size under the same conditions, low environmental pollution, less corrosiveness and lower cost (Chayid and Ahmed 2015; Zuo et al. 2016). The chemical activation of phosphoric acid and zinc chloride is used to activate lignocellulosic materials that have not previously been carbonised, while metal compounds such as potassium hydroxide are used to activate the precursors of coal (Yakout and El-Deen 2016). Several reports have been presented on the effectiveness of KOH-activated carbon in the adsorption of various organic chemicals such as phenol, dyes, heavy metals and pesticides (Table 5).

Table 5 Comparison of the functional properties and textural characteristics of activated carbon prepared from various agricultural wastes and activation with KOH

Tounsadi et al. (2016) reported that activated carbon with potassium hydroxide has the highest efficiency in the adsorption of heavy metals compared to other activators (Tounsadi et al. 2016). Cavities formed in activated carbon are the result of the evaporation of potassium hydroxide from places previously occupied by this activator (Njoku et al. 2013). KOH activator is an activating agent rapidly saturated with precursors and does not evaporate completely, so its activation temperature is generally lower than the boiling point of KOH (1327 °C) (Hui and Zaini 2015). KOH-activated carbon has a higher surface area and pore volume, but typically has lower yield (10–40%) compared to other activators such as ZnCl₂ and H₃PO₄ (Ahmed and Theydan 2014). During activated carbon activation with alkali substances, alkali metals and carbonates are created which, in the carbon matrix, are responsible for the stability and expansion of the spaces between the carbon-atom layers and, as a result, increase the efficiency and adsorption capacity of activated carbon (Ahmed and Theydan 2014; Njoku et al. 2013). Activated carbon produced from potassium hydroxide has a higher microporous structure than activated carbon produced from sodium hydroxide (Ahmed and Theydan 2014; Wu et al. 2010). By increasing the dosage of potassium hydroxide, microporous pores develop on the surface of the activated carbon, while the mesoporous pores decrease due to the characteristics of the potassium hydroxide activators (Wu et al. 2010).

The results of a study by Jin et al. (2014) showed that chemical activation for the preparation of activated carbon from municipal waste with potassium hydroxide (2 M) significantly (Jin et al. 2014):

1. Increased the surface area and the total pore volume.

2. Modified the number of functional groups on the surface.

3. Increased the removal of arsenic in a shorter time.

4. Improved the arsenic adsorption capacity.

The possible reactions that may occur during the activation process with potassium hydroxide are presented in Eqs. (8)–(11) as follows (Meng and Park 2010; Jin et al. 2014):

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

(8)

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

(9)

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

(10)

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

(11)

Activated carbon with potassium hydroxide is oxidised in alkaline medium with high oxygen content (Huang and Zhao 2016). In strong activation with a high amount of potassium hydroxide, carbon atoms are eliminated from the internal structure of carbon, and the BET surface area increases with the formation of a porous structure (Vukčević et al. 2015). Oxygenated functional groups as active sites are able to interact with other molecules in adsorption applications (Bedin et al. 2016). By increasing the activation temperature of potassium hydroxide, the surface area of activated carbon and the number of oxygen groups of activated carbon increase (Vukčević et al. 2015). Generally, by increasing the impregnation ratio of potassium hydroxide to char, the surface area of the activated carbon increases, but if the amount of potassium hydroxide is about eight times greater, the walls between pores formed on activated carbon are further degraded so the surface area is reduced (Huang et al. 2015). Increasing the concentration of potassium hydroxide activator, the dehydration and degradation of the mesopores and their conversion to larger pores probably lead to a decrease in the adsorption capacity of activated carbon (Ahmed and Theydan 2014). The reactions of carbon with alkali metal activators are presented in Eqs. (12) and (13) as follows (Cha et al. 2016):

$$6{\text{NaOH}} + 2{\text{C}} \leftrightarrow 2{\text{Na}} + 3{\text{H}}_{2} + 2{\text{Na}}_{2} {\text{CO}}_{3}$$

(12)

$$6{\text{KOH}} + 2{\text{C}} \leftrightarrow 2{\text{K}} + 3{\text{H}}_{2} + 2{\text{K}}_{2} {\text{CO}}_{3}$$

(13)

Correspondingly, K₂CO₃, along with carbon, is reduced to K, K₂O, CO and CO₂ in accordance with the following Eqs. (1)–(3).

Potassium metal is thought to be introduced into the internal structure of the carbon matrix during the gasification process, leading to the expansion of existing pores and the creation of new ones (Ahmed and Theydan 2014). Therefore, increasing the amount of potassium hydroxide plays a key role in porosity modelling. Porosity expands successfully, and micro- and mesopores are formed in the off-centre walls of the pores, which increases the Brunauer, Emmett and Teller surface area and pore volume (Chayid and Ahmed 2015).

Activation with potassium hydroxide can also be accomplished through direct chemical activation (physical mixing) or impregnation with activated chemicals. In direct chemical activation, in the first stage activated carbon precursors get saturated, moisture is removed and activation occurs at the desired ratio (KOH weight is greater than precursor weight). The impregnation is solid and then placed in the furnace at a specified temperature and time to heat it. Precursor carbonisation is often eliminated when the solid impregnation method is considered (Chayid and Ahmed 2015; Hui and Zaini 2015). By increasing activation, a large amount of potassium hydroxide is typically used, and the weight ratio of KOH to carbon is in the range of 3–7 in most cases. This not only increases the cost of preparation of materials but also increases the potential for environmental hazards caused by potassium hydroxide, the corrosiveness of the process of washing with acid solutions, which results in using other appropriate methods (such as potassium induction as an activation agent by ion exchange) for activation with potassium hydroxide (Ahmed and Theydan 2014; Chayid and Ahmed 2015; Hui and Zaini 2015; Wang et al. 2016).

Also, in the char impregnation method with KOH, potassium hydroxide molecules are readily in contact with the surface of the char, thus leading to a higher degree of micro- and mesoporosity. At the stage of washing the activated carbon, a significant amount of potassium hydroxide is introduced into the aquatic media before use. Some studies have focused on the significant concerns regarding the release of spent potassium hydroxide residues during activation, in terms of either environmental risks or their recovery potential after activation (Chayid and Ahmed 2015; Hui and Zaini 2015; Wang et al. 2016).

The mechanism of the potassium hydroxide reaction is described by Radovic and Rodriguez-Reinoso. In this mechanism (Fig. 4), potassium hydroxide is converted to K₂O at the beginning of the dehydration process (step 1), and then K₂O is converted to metallic potassium (step 2) (Tounsadi et al. 2016). The free potassium then penetrates the graphene layers and causes the structural expansion of the graphene layers. Moreover, after a series of reactions during activation with potassium hydroxide, oxidation (step 3) and hydration (step 4), various potassium compounds form. The carbon produced with chloride acid 0.1 N and water is then washed to remove K, K₂O, K₂CO₃ and KOH residues from the graphene layers. However, potassium carbonate will decompose during the activation process and CO₂ will be emitted. The reaction between the activating agent and the precursor of carbon materials results in the decomposition of volatile organic compounds, thus creating a porous surface on the surface of the activated carbon samples (Nur 2015; Tounsadi et al. 2016).

Fig. 4

Adapted by kind permission of Noureddine Barka (Tounsadi et al. 2016)

Mechanisms of KOH activation.

Among the alkali metals, potassium hydroxide and sodium hydroxide are effective activators in producing activated carbon. Among the alkaline metals, KOH is the most effective factor for producing activated carbon. Researchers have been able to provide convincing descriptions of the activated carbon activation process with KOH (Rambabu et al. 2015). The results showed that sodium hydroxide has a lower efficiency than potassium hydroxide in the chemical activation of activated carbon, which is due to the different performances of metal hydroxides in the activation of activated carbon (Rambabu et al. 2015; Tay et al. 2009; Tounsadi et al. 2016).

Although potassium hydroxide increases the pore surface, potassium hydroxide-saturated activated carbon is less efficient than activated carbon saturated with zinc chloride or phosphoric acid, so activating with potassium hydroxide requires a high temperature (greater than 650 °C) and carbon content is less than constant carbon in the precursor. In this condition, the potassium metal is placed in the carbon matrix, with activated carbon efficiency lower than the carbon content of the raw material (Prauchner et al. 2016; Rambabu et al. 2015; Thitame and Shukla 2016; Yakout and El-Deen 2016). The use of KOH as an active agent is to produce activated carbon with a narrow pore size distribution and the development of effective porosity. It is believed that the activation mechanism with alkali metals such as KOH relies on the fact that alkali metals act as an input catalyst in the carbon network, an electron donor, during the reaction to gas (gasification) (Rambabu et al. 2015; Yahya et al. 2015).

Also, using KOH as an activator has been proposed for environmental compatibility with ZnCl₂ (Yahya et al. 2015; Yakout and El-Deen 2016). Generally, chemical activation with alkaline groups leads to an increase in the positive charge on activated carbon, which is desirable to absorb contaminants with a negative charge (Bhatnagar et al. 2013; Thitame and Shukla 2016).

Activated carbon preparation through activation with other activating agents

Table 6 shows the performance and properties of prepared activated carbon from various agricultural materials and activated with various uncommon activating materials.

Table 6 Comparison of the functional properties and textural characteristics of activated carbon prepared from various agricultural wastes and activation with others activating agents

Phytic acid with the chemical formula C₆H₁₈O₂₄P₆ (Fig. 5) is one of the new activator materials. Phytic acid is a strong type of acid with the ability to chemically react with proteins in the form of a partial depolymerisation. In addition, phytic acid can easily enter into the spaces of raw material and cause dehydration on hemicelluloses and cellulose (Cheng et al. 2016).

Fig. 5

Structure of phytic acid

Phytic acid decomposition can simultaneously lead to the release of some of the radicals that can accelerate oxidative decomposition and the process of carbonisation at lower temperatures. As a result, some cracks and pores are generated on the activated carbon surface after activation. By increasing the amount of impregnation, the pores on the activated carbon surface are reduced because they are further degraded by increasing the amount of impregnation, and more organic matter is produced, so it is deposited on the activated carbon surface and the pores are blocked (Al-Qodah and Shawabkah 2009; Cheng et al. 2016). In other studies, it was found that, by increasing the impregnation of activated carbon created with phytic acid, the acid decomposes into di-ester or monoester and eventually turns into phosphorus oxoacids, which can create organic materials with high amounts of free radicals, as well as increasing the efficiency. Sulphuric acid is a chemical activating agent that is able to dissolve many minerals and impurities from activated carbon precursor (Al-Qodah and Shawabkah 2009; Cheng et al. 2016; Olivares-Marín et al. 2012). An alternative method is chemical purification of dry carbonisation with concentrated sulphuric acid. Sulphuric acid is a highly reactive material that can be combined with organic compounds (such as carbohydrates and other organic materials) to remove water and break down organic precursors into carbon elements according to Eq. (14) (Olivares-Marín et al. 2012):

$${\text{C}}_{n} {\text{H}}_{2n} {\text{O}}_{n} + {\text{H}}_{2} {\text{SO}}_{4} \to n{\text{C}} + {\text{H}}_{2} {\text{SO}}_{4} \cdot n{\text{H}}_{2} {\text{O}}$$

(14)

In addition, sulphuric acid reacts with the mineral compounds of lignocellulosic material. In fact, sulphuric acid is used as a cleaning and de-ashing agent from activated carbon precursors, so using H₂SO₄ for carbonisation has some advantages in addition to low cost (Olivares-Marín et al. 2012). Activation with sulphuric acid has been used for various porous structures, in which the sulphuric acid activation process enters into the material and leads to large and medium porosity on the surface of the activated carbon (Al-Qodah and Shawabkah 2009; Cheng et al. 2016; Olivares-Marín et al. 2012).

Di-ammonium hydrogen with a molecular formula of (NH₄)₂HPO₄ obtained from ammonium phosphate ((NH₄)₃PO₄) decomposition at temperatures above 155 °C is one of the activating agents for the production of activated carbons with various pore distribution and surface area (Benaddi et al. 2000). In a study by Li et al. (2016a) conducted on activated carbon preparation with different ammonium phosphate groups, it was found that the activated carbon prepared with (NH₄)₃PO₄ (activated carbon –(NH₄)₃PO₄) had much higher surface area and pore volume than those prepared with (NH₄)₂HPO₄ (activated carbon –(NH₄)₂HPO₄) and NH₄H₂PO₄ (activated carbon –NH₄H₂PO₄), while activated carbon –(NH₄)₂HPO₄ had the largest micropore volume ratio. Activated carbon –NH₄H₂PO₄ had a lower specific surface area. The different micropore sizes distribution in the carbons was attributed to the different molecular sizes of these (NH₄)_x H_yPO₄. In this regard, activated carbon –(NH₄)₃PO4 had the highest pore volume because (NH₄)₃PO₄ can be converted to lower-sized NH₄H₂PO₄. Overall, activated carbon –(NH₄)₂HPO₄ had higher nickel adsorption capacity than other adsorbents (Li et al. 2016a). The results of the study obtained by Benaddi et al. (2000) showed that the maximum obtained surface area in activated carbon prepared by phosphoric acid was 1800 m²/g (Benaddi et al. 2000), while in the results of the study Liodakis et al. (2009), the highest surface area obtained in activated carbon prepared by (NH₄)₂HPO₄ was about 1350 m²/g, but the pore structure of (NH₄)₂HPO₄-activated carbon was mainly composed of micropores (Liodakis et al. 2009).

Ferric chloride is an activating agent that has rarely been used as such. The small size of iron cations enables ferric chloride to produce activated carbon of small pore size. Various reports indicate that impregnation with ferric chloride has a significant effect on increasing the specific surface area and development of micropores (Theydan and Ahmed 2012).

During the activation process of potassium hydroxide with activated carbon, potassium acetate (C₂H₃O₂K) is produced, which is known as a good activator (Auta and Hameed 2011a). Potassium acetate is converted to potassium and acetate ions when dissolved in deionised water. Under optimal conditions, the adsorption of opposite charges in the solution of both potassium acetate and water leads to the formation of new compounds such as KOH from the hydrolysis of metal ions (Auta and Hameed 2011b).

The effect of physical mixing and impregnation methods on chemical activation

In the chemical activation process, the initial mixing of dry precursor and alkali hydroxides (such as sodium hydroxide and potassium hydroxide) could be performed in two ways (Lillo-Ródenas et al. 2007):

• 1. Impregnation In this method, precursors are mixed with suitable volumes of hydroxide solutions and then the samples are filtered and dried at 110 °C.

• 2. Physical mixing In this method, the precursors are mixed directly with solid hydroxide lentils at room temperature and other activation steps are performed (it should be emphasised that this process is done in the absence of water).

These two simple processes are useful for activating with sodium hydroxide and potassium hydroxide. In addition, it has been proven that the method of physical mixing is much more efficient in the case of sodium hydroxide (Lillo-Ródenas et al. 2007). A study by Ros et al. (2006) on the preparation of activated carbon from sewage sludge showed that activation with sodium hydroxide through physical mixing, compared to impregnation, led to a higher Brunauer, Emmett and Teller surface area in the final activated carbon. This finding was attributed to better physical contact between the precursor and the sodium hydroxide powder in the physical mixing method (Ros et al. 2006). Overall, the physical mixing method is good for the development of porosity in activated carbon (Lillo-Ródenas et al. 2007; Ros et al. 2006). The behaviour of NaOH and KOH is very similar, although activation with KOH produces a slightly higher porosity (Lillo-Ródenas et al. 2007; Ros et al. 2006). The general reaction of metal hydroxides during the activation stage of activated carbon production is according to Eq. (15) (Lillo-Ródenas et al. 2007; Ros et al. 2006):

$$6{\text{MOH}} + 2{\text{C}} \to 2{\text{M}} + 3{\text{H}}_{2} + 2{\text{M}}_{2} {\text{CO}}_{3}$$

(15)

$${\text{M}} = {\text{Na}};\;{\text{K}}$$

Activation after physical mixing is rarely studied (Lillo-Ródenas et al. 2001). In a study by Lillo-Ródenas et al. (2001) to compare the impregnation and physical mixing methods of the sodium hydroxide activator, the results indicated that the activated carbon produced by the physical mixing had a higher porosity than the activated carbon produced by impregnation (Lillo-Ródenas et al. 2001). Also, the activated carbon obtained from physical mixing has a higher surface area and greater pore volume than the activated carbon obtained by impregnation. Moreover, since the physical mixing method requires less time and work than the impregnation method, physical mixing is appropriate to produce activated carbon on an industrial scale (Alabadi et al. 2015; Lillo-Ródenas et al. 2001).

In a study by Byamba-Ochir et al. (2016) on the activation of activated carbon induced by anthracite with NaOH, both activation methods (physical mixing and impregnation) were used. The results indicated that the activation process under the same conditions by physical mixing resulted in higher Brunauer, Emmett and Teller surface area and pore volume than by impregnation, so the surface areas of the activated carbon obtained by physical mixing and impregnation were reported to be 2063–1357 and 1763–816 m²/g, respectively. In this regard, the adsorption capacity of activated carbon prepared through physical mixing was higher than that prepared through impregnation (Byamba-Ochir et al. 2016). Studies showed that the activation process by the impregnation method creates both micro- and mesopores, while in the physical mixing method, mesopores were formed on activated carbon. Activated carbon derived from impregnation resulted in more oxygenated functional groups and less carbon bonds than that produced by physical mixing (Alabadi et al. 2015; Byamba-Ochir et al. 2016; Lillo-Ródenas et al. 2001).

Other studies also reported that, through impregnation, more micropores are created on the activated carbon surface and a small fraction of the pores are mesopores. It seems that, in the impregnation method, small micropores are developed during the activation process, because the chemical activating agent is better distributed into the pores of raw material than by the physical mixing method (Byamba-Ochir et al. 2016; Lee et al. 2015; Lillo-Ródenas et al. 2001).

Basically, the two approaches mentioned in the process of activation of carbon precursors using alkali metal hydroxides depend on the physical state of the activating agent (Alabadi et al. 2015). In the mechanism of pore formation by physical mixing, a solid activating agent such as KOH pellet is mixed with activated carbon precursor and it seems that, during the carbon oxidation process, KOH is converted into potassium metal and carbonate. In the impregnation method, where the activating agent is the liquid phase and precursor in solid form such as KOH in the first step, KOH is broken down as Eq. (16) (Alabadi et al. 2015; Nur 2015):

$${\text{KOH}}_{{({\text{s}})}} \to {\text{K}}^{ + }_{{({\text{aq}})}} + {\text{OH}}^{ - }_{{({\text{aq}})}}$$

(16)

The alkali metal ions have the power to bind to different materials. In aqueous solutions, potassium ions are randomly distributed and free to move. During activation of these ions, this leads to the formation of pores in the structure of the activated carbon. Then, in the washing stage, the potassium ions are removed from the activated carbon (Nur 2015; Tounsadi et al. 2016). The pore formation processes at lower and higher KOH concentrations are presented in Figs. 6 and 7.

Fig. 6

Adapted by kind permission of Sanagi (Nur, et al. 2015)

Pore formation processes at lower KOH concentrations (< 2.0M): carbon sphere (CS) surrounded by free K⁺ in aqueous solution (a), penetration of energised potassium ion, *K + into the carbon sphere (b), removal of the ions to produce porous carbon sphere (c).

Fig. 7

Adapted by kind permission of Sanagi (Nur 2015)

Pore formation and structural collapse of carbon spheres (CS) at higher KOH concentrations (above 2.5 M): penetration/intercalation of *K + into the carbon sphere (a); formation of through and blind pores (b); distortion of the sphere structure by the interconnecting through pores (c).

In the study of Nowicki et al. (2013), comparing the activating methods of activated carbon with sodium hydroxide by physical mixing and impregnation shows that the activated carbon produced by physical mixing is of more porosity and larger pore volume than that of activated carbon produced by impregnation. Also, oxygenated functional groups on activated carbon produced by physical mixing are greater in number and have higher adsorption capacity (Nowicki et al. 2013).

Conclusion

Activated carbon contains a wide range of carbonised materials that have a high degree of porosity and surface area. Due to its unique characteristics, it has various applications in water purification, domestic and industrial wastewater treatment, desalination, refining and separating gases, odour and pollutant removal and medical applications in many parts of the world. Today, various industrial wastes are used to produce activated carbon. The activation of activated carbon is done both physically and chemically. Compared with physical activation, chemical activation is more economical because it requires lower activation temperature, shorter processing time and higher carbon efficiency. Also, in the chemical activation method, the development of porous structures in activated carbon is greater.

Various chemicals used as potential activators include alkaline groups such as potassium hydroxide (KOH), sodium hydroxide (NaOH), calcium chloride (CaCl₂), potassium carbonate (K₂CO₃), acidic groups such as phosphoric acid (H₃PO₄) and sulphuric acid (H₂SO₄), intermediate metal salts such as ZnCl₂ and other activating agents. In this study, the role of different activation factors was studied in the performance and efficiency of activated carbon produced from various precursors. Utilising activated carbon in developed countries is very important due to the production cost and adsorption capacity of various pollutants, so the selection of the best activating agents in activated carbon production is of particular importance. The discussion of the present study showed that some activating chemicals create more adsorption capacity in the activated carbon.

Activation with phosphoric acid has less environmental and toxicological contamination than with zinc chloride and also requires a lower activation temperature than potassium hydroxide. Generally, in chemical activation with alkali materials, activation with potassium hydroxide gives a better result than sodium hydroxide in terms of surface area and performance in different applications. However, sodium hydroxide is cheaper, more environmentally friendly and is less harmful than potassium hydroxide, and it is clear that sodium hydroxide has industrial applications due to its superiority to potassium hydroxide. Also, activated carbon with potassium hydroxide is more efficient in adsorption than other activators.

On the other hand, the salts of alkali metals are used on the basis of the two methods of physical mixing and impregnation method for the activation of activated carbon. Comparison of these two approaches has shown that activated carbon produced by physical mixing is of a more porous structure and larger pore volume than activated carbon produced through impregnation. Despite the fact that a large number of articles have been written on the removal of various pollutants using activated carbon, few have compared the performance of different activation factors in the production of activated carbon using various primary precursors. As noted, several factors affect the activation of activated carbon, so lots of studies are necessary to better understand the adsorption mechanism and improve the adsorption of pollutants using activated carbon produced on various scales worldwide.