ABSTRACT
Activated carbon usually contains elements other than carbon. These components are attached to the carbon surface in various arrangements, compositions, and quantities, affecting the chemical nature of the carbon surface [69]. Two basic types of noncarbon impurities in AC are mineral components or ash and heteroatoms. Ash does not chemically combine with the carbon surface; rather it occupies some of the open pores. The amount of ash present in any AC depends primarily on the type of raw material used. Heteroatom impurities are mainly oxygen, hydrogen, sulfur, and halogens. For relatively pure carbons, these are mainly oxygen atoms located at the edges of the carbon layer [37]; in less pure carbons, other heteroatoms, like nitrogen, could also contribute to the polar sites. These sites, especially those containing oxygen, are responsible for the adsorption of polar adsorbates. Hydrogen could be present as a residue of incomplete carbonization and could be distributed throughout the volume. The concentration of heteroatoms, especially oxygen, combined with the carbon could be changed considerably by different treatments. For example, the burn-off of carbon at temperatures above 1000°C could decrease, and oxidation of carbon could increase the amount of oxygen attached to the surface. The heteroatoms which combine with peripheral carbon atoms at the corners and edges of crystallite basal planes, form some functional groups that are very reactive to many reagents. These functional groups usually consist of more than one type of heteroatom, e.g., oxygen and hydrogen together as - O H or —COOH (see Chapter 2). The nature of these functional groups depends to a large extent on the method of activation as well as on the raw material from which the carbon is produced. The heteroatoms could also combine with the carbon atoms in the space between the basal planes and even in the highly defective zones of these planes. These heteroatoms are not reactive to other reagents due to both their inaccessibility and their mode of combination with the host carbon atoms [69,120]. Although
the surface sites associated with functional groups constitute only a small portion of the total surface area, they may affect considerably the adsorption capacity [69,120].
