Activated Carbon

Activated carbon is a carbon-based adsorbent material characterized by high porosity and a large internal surface area. It is typically produced by subjecting carbon-rich raw materials such as coal, wood, coconut shell or peat to a process called activation at high temperatures. This activation process develops the material’s microscopic pore structure, enhancing its ability to adsorb gases, liquids and dissolved substances on its surface. These unique adsorption properties have made activated carbon an indispensable material in diverse applications including water and air purification, chemical decontamination, gas separation, medical uses and catalysis.

Although activated carbon has an amorphous carbon structure, it contains graphitic microcrystallites. The irregular arrangements and atomic voids between these microcrystallites form the porous structure of activated carbon. The pore structure is the most important property directly influencing adsorption capacity and kinetics.

The pores of activated carbon are classified into three main categories based on size:

1. Micropores: Pores with a diameter smaller than 2 nanometers (nm). These pores serve as the primary sites for adsorption and play a critical role in the adsorption of gas molecules. A large portion of the surface area originates from micropores.
2. Mesopores: Pores with diameters between 2 and 50 nm. These pores function as transport pathways facilitating the diffusion of adsorbate molecules into micropores. They also enable the adsorption of larger molecules.
3. Macropores: Pores with diameters greater than 50 nm. They contribute relatively little to surface area but primarily serve as main channels for the transport of adsorbates into the pore structure.

Activated carbons can be classified into different types based on their physical forms:

1. Granular Activated Carbon (GAC): Consists of irregular particles typically larger than 0.2 mm in diameter. Widely used in gas and liquid phase applications where high flow rates and low pressure drops are important.
2. Powdered Activated Carbon (PAC): Has smaller particle sizes than granular activated carbon (typically <0.18 mm). Often used in short-term applications in suspension or when high adsorption rates are required (Kim et al. 2015).
3. Extruded Activated Carbon (EAC): Produced in cylindrical or pellet form. It exhibits high mechanical strength and low dust generation. Suitable for gas phase applications and continuous column systems.
4. Activated Carbon Fiber (ACF): A flexible material produced from synthetic or natural fibers. It features rapid adsorption/desorption kinetics and a more uniform pore distribution. Used in specialized applications such as gas masks, filters and electrodes.
5. Spherical Activated Carbon (SAC): Composed of regular spherical particles. It provides low pressure drop and high flow rates. Particularly advantageous in sensitive gas separation and chromatography applications.

The production of activated carbon consists of two main stages: carbonization (pyrolysis) and activation.

In this stage, a carbon-rich raw material (e.g. wood, coal, coconut shell) is heated in an inert atmosphere (such as nitrogen or argon) at temperatures between 400 and 850 °C. Carbonization process removes volatile components from the raw material, producing a precursor carbon (char) with high carbon content but an incompletely developed pore structure. This stage lays the foundation for the final pore structure expected in activated carbon.

Activation is the process used to enhance the adsorption capacity and surface area of the precursor carbon. Two primary activation methods exist:

In this method, the carbonized precursor is exposed to oxidizing gases such as steam, carbon dioxide (CO₂) or air at high temperatures (700–1100 °C). The oxidizing gases selectively react with carbon atoms on the surface of the carbonized material, creating new pores and expanding existing ones. Steam activation is generally effective in developing micropores, while CO₂ activation can promote mesopore formation. This method is widely used industrially due to its environmental friendliness and controllability.

In this method, the carbon-rich raw material is mixed with a chemical agent (e.g. phosphoric acid (H₃PO₄), zinc chloride (ZnCl₂), potassium hydroxide (KOH)) before or during carbonization and then pyrolyzed at lower temperatures (400–900 °C). Chemical agents promote dehydration of the raw material, increase volatile release and direct pore formation within the carbon matrix. Phosphoric acid activation, in particular, is widely used to produce activated carbons with high mesopore content. Chemical activation generally provides better pore development at lower activation temperatures and higher yields, but chemical recovery and waste management can be costly.

The adsorption capacity of activated carbon depends not only on its large internal surface area and pore structure but also on its surface chemistry. Adsorption is the process by which adsorbate molecules bind to the surface of activated carbon.

Physical adsorption is a reversible process occurring through weak van der Waals forces or hydrophobic interactions. Adsorbate molecules are attracted to and concentrated within the pores of the activated carbon surface. This mechanism is generally more effective at low temperatures and does not involve chemical bond formation between the adsorbate and adsorbent. Large internal surface area and micropores are critical for physisorption. It is the primary mechanism for removing organic contaminants in water treatment.

Chemical adsorption involves the formation of stronger chemical bonds such as covalent or ionic bonds between adsorbate molecules and the activated carbon surface. This type of adsorption is more specific and generally irreversible or requires high energy for desorption. Surface oxygen groups (carboxyl, hydroxyl, lactone, quinone, etc.) and other heteroatoms (nitrogen, sulfur, phosphorus) present on the activated carbon surface can act as chemisorption sites (Bandosz 2011). Chemisorption mechanisms play a significant role in the removal of heavy metals and certain inorganic contaminants.

The wide adsorption capacity of activated carbon has made it a preferred material in numerous industrial and everyday applications.

Activated carbon is widely used in water and wastewater treatment to remove organic contaminants, chlorine, pesticides, pharmaceutical residues and color- and odor-causing substances. In drinking water treatment plants, activated carbon filters improve water quality by removing aesthetically and health-significant pollutants. In industrial wastewater treatment, it provides an effective solution for eliminating toxic organic compounds.

Activated carbon is used to adsorb gaseous pollutants such as volatile organic compounds (VOCs), sulfur dioxide (SO₂), nitrogen oxides (NO_x) and mercury vapor in gas masks, industrial flue gas treatment systems and ventilation systems. Activated carbon filters also play a significant role in odor control and overall air quality improvement.

Activated carbon is used as a detoxifying agent administered orally to prevent the absorption of poisons or drugs from the digestive system in cases of poisoning. It can also be used to adsorb uremic toxins in patients with kidney failure and to alleviate gas and bloating symptoms.

• Chemical Processes: Used as a catalyst support in various chemical reactions or for purification of reactants and products.
• Energy Storage: Investigated as an electrode material in energy storage devices such as supercapacitors and fuel cells.
• Food and Beverage Industry: Used for decolorization and deodorization in sugar refining, purification of alcoholic beverages and fruit juice production.
• Gold Recovery: Widely used in gold mining to adsorb gold from solution after cyanidation.

The effectiveness of activated carbon decreases when it becomes saturated with adsorbed substances. At this point, the activated carbon must be regenerated or disposed of.

Regeneration is the process of removing adsorbed substances from the surface of saturated activated carbon to enable its reuse. The main regeneration methods are as follows (Ahmed and Hameed 2018; Al-Ghouti and Da'ana 2020):

1. Thermal Regeneration: The most common method. Saturated activated carbon is heated at high temperatures (600–900 °C) in an inert or oxidizing atmosphere to vaporize or pyrolyze the adsorbed organic materials. Steam or CO₂ is often used as the regeneration gas. Although thermal regeneration may slightly alter the structure of activated carbon, it remains one of the most economical and effective methods.
2. Chemical Regeneration: Achieved by washing the adsorbed material with chemical solvents, acids or bases. This method is particularly suitable for inorganic or specific organic contaminants.
3. Biological Regeneration: Based on the principle that microorganisms degrade adsorbed organic substances on the activated carbon surface. Typically used for biodegradable contaminants and offers lower energy consumption compared to thermal regeneration.
4. Electro-regeneration: Uses electric current to induce desorption of adsorbed substances.

Activated carbon that cannot be regenerated or has reached the end of its service life must be disposed of in accordance with environmental regulations. Depending on the nature of the adsorbed substances, it may be classified as hazardous waste and require special handling. Regeneration processes reduce the lifecycle cost of activated carbon and minimize its environmental impact.

Activated carbon technology continues to evolve with increasing environmental concerns and the emergence of new applications. Future research will focus on the use of sustainable raw materials (such as biomass waste) in activated carbon production, the synthesis of functionalized activated carbons (for selective adsorption and catalytic properties), and new applications in energy storage and carbon capture. Nanotechnology's integration with activated carbon science also offers new opportunities for developing more efficient and customized adsorbent materials.