Activated Carbon

Activated carbon is a carbon-rich porous material processed to develop an extensive internal surface and a high capacity for adsorption. Activation creates or enlarges pores, allowing substances from liquids or gases to accumulate on their surfaces. It is a family of materials rather than a single chemical compound: different precursors and manufacturing conditions produce different pore structures and adsorption properties. Its principal forms include powders, granules, and activated carbon fibres. (goldbook.iupac.org)

Structure and adsorption

The defining property of activated carbon is its accessible internal surface. Its specific surface area—surface area per unit mass—can be very large because much of the surface lies inside pores rather than on the outside of particles. However, not every pore is accessible to every substance: access depends on the size and shape of the adsorbing molecules and the geometry of the pore network. (doi.org)

Pores are conventionally classified by approximate width:

Micropores: up to about 2 nanometres.
Mesopores: between about 2 and 50 nanometres.
Macropores: wider than about 50 nanometres.

In narrow micropores, interactions with opposing pore walls overlap, strengthening adsorption and producing micropore filling. Consequently, adsorption capacity in a microporous carbon is not adequately described by surface area alone; accessible pore volume and pore-size distribution also matter. (doi.org)

Adsorption differs from absorption: adsorption concentrates substances at a surface, whereas absorption involves their entry into the bulk of another phase. In purification equipment, activated carbon transfers contaminants from the passing fluid to the solid. This does not necessarily destroy them, and the carbon eventually loses useful capacity as contaminants accumulate. (poison.org)

Manufacture

Common precursors include wood, coconut shells, bituminous coal, lignite, and peat. Manufacturing develops the precursor’s pore structure through physical or chemical processing. Both the starting material and the treatment conditions influence the resulting carbon’s performance. (epa.gov)

Carbonization converts a precursor into carbon-rich char through heating, generally under oxygen-limited conditions. This thermal decomposition is a form of pyrolysis. Activation then develops the char’s porosity, although carbonization and activation can also be combined in a manufacturing sequence. (nepis.epa.gov)

Two broad routes are used:

Physical activation: char reacts at elevated temperature with gases such as steam or carbon dioxide, partially gasifying the carbon and developing pores.
Chemical activation: chemicals are introduced before, during, or after carbonization to promote the development of an adsorptive structure. Activating agents include zinc chloride, phosphoric acid, and potassium hydroxide. Chemical residues may subsequently be removed by washing. (nepis.epa.gov)

“Activation” therefore refers to deliberate modification of pore structure and adsorption properties, not simply to heating carbon or charging it electrically. Ordinary charcoal and barbecue briquettes are not interchangeable with activated carbon. (goldbook.iupac.org)

Principal forms

Powdered activated carbon (PAC) consists of fine particles that can be mixed into a liquid and subsequently separated. It is used extensively in liquid purification and decolorization. Granular activated carbon (GAC) consists of larger particles used in beds through which water or gas passes; exhausted beds can be replaced or regenerated. (nepis.epa.gov)

Activated carbon can also be produced in fibrous or textile form by controlled carbonization and activation of suitable fibres. Its physical form is distinct from its chemical grade: carbons with similar particle sizes can have different pore structures and adsorption capacities. (goldbook.iupac.org)

Characterization and performance

Gas-adsorption measurements are widely used to characterize porosity and surface area. The Brunauer–Emmett–Teller (BET) method estimates surface area from an adsorption isotherm, often measured using nitrogen. For strongly microporous materials, a BET value is best treated as an apparent surface-area descriptor rather than a complete measure of adsorption performance. (goldbook.iupac.org)

Practical performance also depends on the target substance, fluid composition, flow conditions, and competing contaminants. A carbon that removes one compound effectively may perform poorly for another. In water treatment, other adsorbable substances can consume capacity that would otherwise be available for a target contaminant. (epa.gov)

A particularly important operational concept is breakthrough: the point at which appreciable contaminant concentrations begin appearing at the outlet of an adsorption bed. Breakthrough can occur before the entire bed reaches its maximum loading. In regenerated systems, working capacity accounts for the contaminant that remains on the carbon after regeneration, rather than assuming that every cycle begins with completely clean material. (epa.gov)

Applications

Water treatment

Activated carbon removes many taste- and odour-producing compounds, natural organic matter, volatile organic compounds, synthetic organic contaminants, and precursors of disinfection by-products from water. GAC is used in both drinking-water treatment and contaminated-water remediation. Its effectiveness varies with carbon type and water composition. (epa.gov)

GAC is also used to remove per- and polyfluoroalkyl substances (PFAS) from drinking water. This is a separation process: the contaminants accumulate on the treatment medium, creating a subsequent need to manage the spent carbon. (epa.gov)

Gas purification and solvent recovery

Activated carbon adsorbers capture organic vapours from industrial exhaust streams and can recover a solvent for reuse. Carbon beds also treat contaminated air associated with remediation operations. Suitability depends on the compound: substances that bind weakly may be difficult to capture, while strongly retained substances may be difficult to remove during regeneration. (epa.gov)

High humidity can reduce adsorption performance because water competes for adsorption sites. Modified carbons can improve capture of particular pollutants, including mercury, hydrogen sulfide, and ammonia. (epa.gov)

Industrial processing and electrochemical materials

Activated carbon is used to decolorize and purify sugar solutions and other industrial liquids. It is also used as an electrode material in supercapacitors, where its porous surface supports charge storage. Electrode performance depends on pore accessibility, electrolyte conditions, and surface properties, not simply on maximizing surface area. (nepis.epa.gov)

Medical use

Medical-grade activated charcoal is used in selected cases of poisoning by substances swallowed into the gastrointestinal tract. It adsorbs susceptible substances in the gut and can reduce their entry into the body. Its clinical use depends on the substance involved and the circumstances of exposure; it is not a universal antidote. Recognized adverse effects include nausea, vomiting, and aspiration into the lungs. Industrial carbon, barbecue charcoal, and consumer charcoal products are not equivalent to medical preparations. (poison.org)

Regeneration, limitations, and environmental considerations

Exhausted carbon may be replaced, regenerated, or thermally reactivated. Gas-treatment systems often use heating, steam, or reduced pressure to desorb captured vapours. Higher-temperature reactivation of spent water-treatment carbon can volatilize and oxidize retained impurities under controlled conditions. These processes require equipment to manage recovered substances and emissions. (epa.gov)

Regeneration does not always restore the original capacity: some substances remain strongly retained, and adsorption capacity can decline over repeated cycles. Carbon adsorption also introduces safety considerations. Adsorption and reactions within a bed can release heat, and fires or explosions can occur under unsuitable operating conditions. (epa.gov)

Activated carbon treatment transfers contaminants into another material rather than automatically eliminating them. Depending on the accumulated substances, spent carbon may require special handling or hazardous-waste disposal. Environmental assessment therefore includes manufacture, regeneration, emissions, and final management of the contaminated medium, not only removal efficiency at the treatment outlet. (epa.gov)

Historical development

Charcoal and bone char were used to decolorize liquids before modern activated-carbon manufacture developed. Industrial production expanded in the early twentieth century, initially serving applications such as sugar purification. During World War I, demand for gas-mask adsorbents stimulated the development of granular carbons suitable for gas adsorption. Subsequent applications included odour control, solvent-vapour recovery, and broader liquid purification. (nepis.epa.gov)