Iodine Number

Activated Carbon is a term given to a variety of carbon-based adsorbants with developed internal pore structures. The most common materials used are wood, coal, lignite, and coconut shell. The base material is subject to a heating process called carbonization in which tiny pores are created. The carbon is then activated by a second heat-steam treatment while regulating oxygen and carbon dioxide levels. This “activation” creates a vast internal pore network and a very large surface area: 1 lb of granular activated carbon (GAC) typically provides a surface area of 125 acres, thereby giving carbon its unique filtering capabilities. Pore structure is classified by the diameter d:

• Micropore: d < 20 Angstroms (Å)
• Mesopore: 20 Å < d < 50 Å
• Macropore: d > 50 Å

This pore structure enables GAC to remove taste- and odor-causing compounds and other volatile organic compounds.

One method of evaluating the adsorption property of GAC is the iodine number. It is described as the milligrams of iodine (I2) that are adsorbed per gram of carbon when the equilibrium concentration of the bulk saturation is 0.02N. Because iodine is a small molecule, it will provide an indication of a particular carbon’s ability to absorb small molecules. The iodine number will decrease over time as adsorption occurs.

Different based carbons have different iodine numbers, i.e. 600 (lignite), 800 (wood), 1000 (bituminous coal), 1200 (coconut). These numbers dictate the benefits and limitations of each carbon. For example, coconut shell activated carbon has a tremendous number of small pores, hence a higher iodine number. Most organics in water are larger than these pores and removal amounts may not be as good as a bituminous-based carbon.

Thor Carbon Technologies, LLC has developed a quicker and more accurate way of determining the iodine number for GAC and PAC for truck deliveries of new carbon and carbon currently in use. This test involves the use of a spectrophotometer and preparation of one standard reagent.