Introduction
Activated carbon is a highly porous carbonaceous material with a well-developed surface structure. Its surface can be modified using various physical and chemical treatments to introduce specific functionalities. The synthesis of activated carbon utilizes a wide range of precursors, including fossil fuels, agricultural residues, and lignocellulosic wastes. Numerous studies have focused on the synthesis, characterization, and diverse applications of activated carbon. This review highlights the fundamental properties of activated carbon, its surface chemistry modifications, adsorption mechanisms, and practical applications across various industries.

Activated Carbon (AC) Activated carbon (AC) is a high-porosity material with exceptional sorption capabilities, produced from sources such as wood, coconut shells, coal, and cones. Due to its effectiveness as an adsorbent, it is widely used in industries for pollutant removal from water and air. The synthesis of AC from agricultural and waste products offers a cost-effective and sustainable alternative to traditional, nonrenewable sources.

Synthesis Process The preparation of AC involves two primary steps: carbonization and activation.

• Carbonization: Precursors are subjected to high temperatures (400-850°C) to remove volatile components, yielding a high-carbon-content char with minimal surface area and porosity.
• Activation: The char undergoes further treatment to enhance porosity and surface area. Pore development occurs through three mechanisms: opening of previously inaccessible pores, creation of new pores via selective activation, and expansion of existing pores.

 

Activation Methods Two primary activation techniques are employed to optimize the porosity and surface characteristics of AC:

1. Physical Activation: This involves treating carbonized char with oxidizing gases such as air, carbon dioxide, or steam at high temperatures (650-900°C).
   • Carbon dioxide activation ensures high pore uniformity and controlled activation but is less effective in increasing surface area compared to steam.
   • Steam activation, preferred for its efficiency, enhances diffusion within the char structure, producing AC with a higher surface area.
2. Chemical Activation: This approach involves mixing the precursor with activating agents such as NaOH, KOH, or FeCl3 before heating at lower temperatures (300-500°C). Chemical activation integrates carbonization and activation in a single step, resulting in a well-developed porous structure, higher carbon yield, and shorter reaction times.
   • Advantages of chemical activation include lower temperature requirements, high microporosity, larger surface area, and faster processing time.

Comparison of AC Prepared by Different Methods
Differences in the microstructure of AC synthesized through steam activation (C6S9) and KOH activation (C6K9) significantly influence its adsorption capabilities. Steam-activated AC generally has a higher surface area, while KOH-activated AC exhibits superior microporosity.

Types of Activated Carbon
Based on particle size and preparation method, AC is classified into three categories:

1. Powdered Activated Carbon (PAC): Consists of fine granules (average diameter 0.15-0.25 mm), suitable for rapid adsorption in liquid-phase applications.
2. Granular Activated Carbon (GAC): Larger particles with lower external surface area, used in both liquid and gaseous phase applications.
3. Bead Activated Carbon (BAC): Spherical particles (0.35-0.8 mm) derived from petroleum pitch, known for high mechanical strength and minimal dust generation. BAC is widely used in fluidized bed applications such as water filtration.

Applications of Activated Carbon
Activated carbon finds applications in various industries due to its exceptional adsorption properties:

• Pollutant Removal: Effective in adsorbing dyes, heavy metal ions, pesticides, pharmaceutical waste, and volatile organic compounds.
• Catalyst Support: Used in catalytic reactions due to its high surface area and chemical stability.
• Food & Pharmaceutical Industries: Utilized for purification processes, decolorization, and removal of impurities from edible oils, beverages, and pharmaceutical products.

Conclusion
Activated carbon, with its highly porous structure and superior adsorption capabilities, is an indispensable material in environmental, industrial, and healthcare applications. Advances in activation methods and precursor selection continue to enhance its performance, making it a sustainable solution for pollution control and resource recovery.