Technical Library

Activated carbon is an engineered amorphous carbonaceous material featuring an exceptionally high internal surface area—typically ranging from 500 to 2,000 m²/g. This means a single gram of activated carbon possesses an internal surface area equivalent to multiple tennis courts, enabling high-capacity adsorption of micropollutants.

Adsorption vs. Absorption Kinetics

A common point of confusion is the difference between adsorption and absorption:

• Adsorption (Surface Phenomenon): The accumulation of molecules (adsorbate) onto the solid surface of the carbon matrix. The adsorbate adheres to the walls of the carbon pores, driven by molecular forces. There is no change in the bulk volume of the carbon block.
• Absorption (Bulk Phenomenon): A fluid process where molecules completely dissolve or penetrate into the bulk volume of the absorbent material (like water soaking into a sponge).

Because activated carbon operates solely through adsorption, the captured pollutants remain on the surface. This allows the adsorption process to be reversed through thermal regeneration, restoring capacity and enabling reuse.

Physisorption vs. Chemisorption

Solute capture inside the active carbon pores occurs through two primary mechanisms:

Converting raw carbonaceous precursors (like coal, coconut shells, and wood) into high-capacity adsorbents requires two distinct, sequential thermal process stages.

1. Carbonization (Thermal Pyrolysis)

The raw precursor is heated in an oxygen-free atmosphere (pyrolysis) to temperatures of 300–500°C (sometimes up to 600°C). This drives off volatile compounds, water, and organic tars, leaving a carbon-rich char with a fixed carbon content of approximately 40–70%. At this stage, the char is dense, with blocked pores and minimal adsorption capacity.

2. Activation (Pore Development)

The carbonized char is activated to etch open the internal pore structure using one of two primary industrial routes:

Physical Activation (Steam & Gasification)

The raw char is treated with steam, carbon dioxide (CO₂), or a mixture of both in a rotary kiln or fluidized bed at high temperatures of 700–1,000°C. The activating gases react endothermically with the carbon matrix:

C + H₂O → CO + H₂   (ΔH = +131 kJ/mol)
C + CO₂ → 2CO   (ΔH = +172 kJ/mol)

This reaction gasifies carbon atoms from the pore walls, clearing blocked channels and developing a balanced pore network with a low ash content. Physical activation is the preferred route for manufacturing granular activated carbons (GAC) for municipal water and air filtration.

Chemical Activation

The raw material is pre-impregnated with a dehydrating or oxidizing chemical agent, such as phosphoric acid (H₃PO₄), potassium hydroxide (KOH), or zinc chloride (ZnCl₂). It is then heated in a kiln at lower temperatures of 400–700°C. The chemical agent acts as a support template, preventing shrinkage and promoting cross-linking, which develops high micropore volumes. After firing, the carbon is washed with water to remove the chemical salts, which are recovered. This route is commonly used for powdered activated carbons (PAC) in food and pharmaceutical applications.

The performance of activated carbon depends on its pore size distribution matching the dimensions of the target contaminant molecules. IUPAC classifies pore structures into three distinct size ranges:

standard ASTM tests are used to specify activated carbon grades for commercial supply contracts. These metrics indicate how the carbon will perform in operation:

Iodine Number (ASTM D4607)

Expressed in mg/g, this measures the adsorption of iodine from a standard solution. It correlates directly with the density of the carbon's **micropore network** (pore width < 2nm). A high iodine number (>1,000 mg/g) is standard for high-performance municipal water filters and gas capture beds.

Ball-Pan Hardness (ASTM D3802)

Expresses mechanical attrition resistance as a percentage. In continuous column filtration, high fluid velocities and backwashing cause carbon particles to rub together. Soft carbons break down, creating black dust that clogs downstream equipment. A hardness rating above 98% is critical for gold recovery CIP/CIL circuits and water filtration.

Molasses Number

Evaluates decolorizing capacity using a standard molasses solution. It measures **mesoporous density** (2–50 nm) for decoloring sugar melts and sweeteners.

Apparent Density (ASTM D2854)

Measured in g/cm³, this determines the weight of carbon required to fill a filter vessel. Higher apparent density (>0.45 g/cm³) is typically associated with high-hardness coal or coconut shell carbons.

Raw carbon precursors contain mineral ash. Acid-washing improves cleanliness but does not create additional adsorption capacity.

Washed vs. Unwashed Parameters

Technical Note: Acid-washing is a post-activation purification step. It removes soluble minerals that poison reaction catalysts or bleed into high-purity pharmaceutical intermediate runs.