What Is Activated Carbon?

Activated carbon has an incredibly large surface area per unit volume, and a network of submicroscopic pores where adsorption takes place. Activated carbon is a material that is produced from carbonaceous source materials, such as coal, coconuts, nutshells, peat, wood, and lignite. The primary raw material used for activated carbon is any organic material with a high carbon content. The carbon-based material is converted to activated carbon through physical modification and thermal decomposition in a furnace, under a controlled atmosphere and temperature. The finished product has a large surface area per unit volume and a network of submicroscopic pores where adsorption takes place.

What are the Properties of Activated Carbon?

An activated carbon product can be characterized by its activity and physical properties. Activity properties include pore size distribution that defines the available pore volume of a carbon over three pore size regions: the micropore, mesopore, and macropore regions:

  • Micropore region – less than 100 Angstroms
  • Mesopore region – between 100 and 1,000 Angstroms
  • Macropore region – greater than 1,000 Angstroms

Pore size distribution properties are key indicators of a carbon’s potential performance for removing contaminants (adsorbates) from water. The molecules encountered in the gas phase are generally smaller than those in the liquid phase applications; therefore, a gas phase carbon has the majority of its pores concentrated in the micropore region.

A broad range of pore sizes must be available, both for ease of movement of adsorbates through the carbon pores and for the adsorption of particular molecular sizes. Liquid phase carbons often contain a broader pore size distribution to remove color bodies and larger organic materials, while maintaining some microporosity for the removal of taste and odor compounds.

Physical properties include surface area, product density, mesh size, abrasion resistance, and ash content.

Typical measured carbon properties include:

  • Iodine Number – standard testing done to estimate the surface area of the activated carbon by measuring iodine adsorption at a given set of standard testing conditions, reported in mg I2 adsorbed per gram carbon
  • Surface Area – amount of surface available for adsorption for a given mass of carbon, measured using techniques such as BET nitrogen adsorption; reported in units of m2/g
  • Product Density – several properties available, including apparent density which is the density of the carbon at maximum packing efficiency, reported in g/cc or lbs/cf
  • Mesh Size Size – measure of particle range of the granular product, usually reported as a range of sieve openings, such as 12 x 40 for a carbon that passes a 12 mesh screen, but is retained on a 40 mesh screen with a specification on the amount that can be retained on the larger opening screen or passing the smaller opening screen; basis is US sieve sizes Abrasion Number – measure of the ability of the carbon product to resist attrition; this important property permits one to understand how durable the activated carbon is in applications where backwashing is required, carbon will be transferred, or treatment velocities are above average
  • Ash Level– a measure of the non-carbon content of the activated carbon; all base materials have a certain ash constituency, with the content varying from base material to base material; for example, coconut shell carbon tends to have more alkali earth metals, while coal-based carbons have more heavy metals

Contaminants that are organic, have high molecular weights, and are neutral, or non-polar, in their chemical nature are readily adsorbed on activated carbon. For water adsorbates to become physically adsorbed onto activated carbon, they must both be dissolved in water so that they are smaller than the size of the carbon pore openings and can pass through the carbon pores and accumulate.

Besides physical adsorption, chemical reactions can occur on a carbon surface. One such reaction is chlorine removal from water involving the chemical reaction of chlorine with carbon to form chloride ions.