Activated carbon: pores, ash, and what changes when carbon comes from CO₂

A very large surface in a small grain

Activated carbon is carbon with an enormous internal surface. A single gram typically has a surface area of several hundred to well over a thousand square metres, spread across the walls of a network of pores. Molecules from a gas or liquid are held on those walls by weak physical forces. That is how activated carbon cleans drinking water, recovers solvents, removes mercury from flue gas, decolourises sugar and stores charge in supercapacitors.

How pores are classified and measured

The International Union of Pure and Applied Chemistry classifies pores by their width (Thommes et al., 2015):

• micropores, narrower than 2 nanometres, where most adsorption of small molecules happens
• mesopores, between 2 and 50 nanometres, which carry larger molecules and give access to the micropores
• macropores, wider than 50 nanometres, the main transport routes into the grain

The pore structure is measured by gas adsorption, usually nitrogen at minus 196 °C. The amount of gas taken up at increasing pressure gives an isotherm, from which the BET surface area and the pore size distribution are calculated. The same IUPAC report sets out how to do this and where the common methods mislead, for example the BET area of very microporous carbons.

The right balance of pore sizes depends on the job. Drinking water treatment needs mesopores for large natural organic molecules. Gas masks and solvent recovery need many micropores. Supercapacitor electrodes need micropores matched to the size of the electrolyte ions, with mesopores to let them in quickly (Simon and Gogotsi, 2008).

How pores are made

Activation starts from a carbon-rich solid. In physical activation the carbon is heated to roughly 800 to 1,000 °C in steam or carbon dioxide, which gasifies part of the carbon and opens pores. In chemical activation the precursor is mixed with an agent such as potassium hydroxide, phosphoric acid or zinc chloride and heated at lower temperatures, then washed. Both remove carbon to create space, so the yield is well below the starting mass, and everything that is not carbon is left behind and concentrated.

What ash is

Every natural raw material contains minerals: silica, alumina, iron, calcium, magnesium, sodium and potassium compounds. They do not burn away during carbonisation and activation. Because part of the carbon does, their share rises. The result is ash, measured simply by burning a dried sample in a furnace and weighing what remains. The reference method is ASTM D2866, which notes that the amount and composition of ash may influence the properties of activated carbon in specific end uses (ASTM D2866).

Why ash matters

Less working material. Ash adsorbs nothing. Every percent of ash is a percent of material that does no work, and mineral grains in pore mouths can block access to the surface behind them.

Leaching into liquids. In water and liquid treatment, soluble parts of the ash dissolve into the liquid being cleaned. They can shift pH, add metals and raise conductivity. For drinking water, food, beverages and pharmaceuticals that is a concern in itself, and many suppliers acid-wash their carbon to bring ash down, which adds cost, acid consumption and wastewater.

Electrochemistry. In supercapacitors, trace metal and other impurities in the carbon affect self-discharge, ageing and rate performance, because they can take part in redox reactions that the pure carbon does not (Effect of impurities in different activated carbon materials, 2025). Electrode-grade carbons are therefore specified with very low ash and tight limits on iron and other metals.

Catalysis. When carbon is used as a catalyst support, mineral components can poison active sites or catalyse side reactions.

Reactivation. Spent carbon is often thermally reactivated and reused. Ash accumulates with each cycle, so a low starting value leaves more headroom.

How the source decides the ash

Most activated carbon is made from coal, because it is cheap and available in bulk; wood and coconut shell follow (US patent 10947122). Each brings its own mineral load and pore structure.

• Coal (bituminous coal, lignite, anthracite) gives a broad range of pore sizes that suits municipal water treatment. Its ash commonly runs from several percent to above 10%.
• Coconut shell gives hard, dense carbon rich in micropores, preferred for drinking water, gas masks, gold recovery and supercapacitors. Ash is typically a few percent.
• Wood and other biomass give carbon with more meso and macropores, used for decolourising sugar and purifying pharmaceuticals.

In the interlaboratory study behind the ASTM ash method, three commercial activated carbons, one of them wood-based, averaged 7.7%, 1.9% and 4.6% ash, which gives a sense of the range in ordinary products (ASTM D2866, precision statement).

What changes when the carbon comes from CO₂

Carbon can also be built up from a gas. When captured CO₂ is decomposed into solid carbon and oxygen, as explained in from CO₂ to solid carbon, the carbon contains almost none of the minerals that come with coal or plants. Activated in the same way as other carbons, it can reach very low ash without acid washing.

The other differences follow from the same fact:

• Consistency. Natural raw materials vary by mine, season and region; a controlled process varies less.
• Tunable structure. Because the carbon is formed in a controlled way, its starting structure and its activation can be set for a target pore distribution.
• Supply. Coal and coconut are shipped across the world. CO₂ is available wherever there is industry.
• Footprint. Coal-based carbon brings fossil carbon out of the ground. Carbon from captured CO₂ uses carbon that was already emitted. The net climate effect depends on the energy used and on the end of life of the carbon, so it needs a life cycle assessment to ISO 14040 and 14044 for each case.
• Maturity. Coal, coconut and wood carbon are made at the scale of millions of tonnes; carbon from CO₂ is at the start of industrial scale-up.

Where Aeroborn fits

Aeroborn makes activated carbon from captured CO₂ with ash below 0.5%, without acid washing, and tunes the pore structure during activation. A unit at technology readiness level 7 is being commissioned in an industrial Fieldlab. Request the data sheet.