Activated carbon (AC) is a form of carbon material processed to have an extremely high degree of internal porosity. This unique structure allows it to function as a highly effective adsorbent in various applications, from water purification to air filtration. The goal of activation is to maximize this internal surface area, creating a vast network of microscopic pores—micropores and mesopores—which act like a molecular sponge. A single gram of high-quality activated carbon can possess an internal surface area exceeding 1,000 square meters. This surface area enables the material to trap and hold organic chemicals and contaminants through adsorption, where molecules adhere to the carbon’s surface.
Selecting the Raw Material
The manufacture of activated carbon begins with selecting a suitable carbon-rich source material, known as the precursor. Feedstocks include coconut shells, wood, peat, lignite, coals, agricultural byproducts, and nutshells. The choice of precursor material is important because it dictates the final pore structure and the primary use of the finished product. For instance, carbon derived from coconut shells typically features a high density of micropores, which are excellent for trapping smaller molecules in gas and air purification.
Conversely, wood-based carbon often results in a higher proportion of larger pores (mesopores and macropores), making it suitable for filtering larger organic molecules and decolorizing liquids. Before activation begins, the raw materials must undergo several pre-treatment steps. These steps involve grinding the material to a uniform particle size, followed by washing and drying to remove surface impurities and excess moisture. This preparation ensures that subsequent high-temperature or chemical treatments can uniformly penetrate and process the material.
The Thermal Activation Method
Thermal activation, also known as physical activation, is a two-step process using heat and oxidizing gases to create the porous structure. The first step is carbonization, where the raw material is heated between 600°C and 900°C in an inert atmosphere, such as nitrogen or argon. This pyrolysis drives off non-carbon elements like oxygen, hydrogen, and volatile organic compounds, leaving behind a carbon-rich char with a rudimentary, closed pore structure.
The second step is the activation phase, which develops the extensive internal surface area. The carbonized char is heated to higher temperatures, ranging from 800°C to 1100°C. During this treatment, a controlled stream of an oxidizing gas, such as steam (H2O) or carbon dioxide (CO2), is introduced. These gases react selectively with the disordered carbon atoms, effectively “gasifying” or burning away portions of the structure. This controlled erosion enlarges existing pores and carves out new ones, increasing the volume of micropores and mesopores. The final product is a highly porous material ready for purification applications.
The Chemical Activation Method
Chemical activation provides an alternative method for developing internal porosity, often performing carbonization and activation in a single step. This process begins with the impregnation of the carbon precursor—such as wood or agricultural waste—with a strong chemical agent. Common activating agents include phosphoric acid (H3PO4), zinc chloride (ZnCl2), or potassium hydroxide (KOH). The raw material is soaked in a concentrated solution, allowing the agent to penetrate the internal fiber structure.
Following impregnation, the material is heated in a furnace at a lower temperature range (generally between 400°C and 700°C) compared to thermal activation. The chemical agent acts as a powerful dehydrating and oxidizing agent during heating, promoting the breakdown of the precursor material. These agents chemically restructure the carbon, preventing the natural shrinkage and collapse of the internal structure that would otherwise occur. This chemical control results in the formation of a highly developed and uniform pore network.
Phosphoric acid, for example, forms phosphate bridges that prop open the pore structure during carbonization, leading to a high yield of activated carbon. Once heating is complete, the activated material must undergo a final step of washing and neutralization. This rigorous washing, often using water or acid, removes all residual chemical agents and inorganic impurities, ensuring the finished product is clean and has the desired adsorption properties. Chemical activation is often favored for its efficiency, lower temperature requirements, and the ability to control the resulting pore size distribution.