Impact of oxygen-containing surface functional groups on activated carbon adsorption of phenols

The adsorptive capacity of activated carbons for phenolic compounds increases significantly in the presence of dissolved oxygen (oxic conditions) due to the oligomerization of these compounds through oxidative coupling reactions. With increased capacity comes an increase in the amount of irreversible adsorption, which is defined as adsorbate that cannot be recovered by solvent extraction. The objective of this study was to determine the impact of oxygen-containing functional groups on activated carbon surface on the irreversible adsorption of phenolic compounds in the presence of dissolved oxygen. The adsorptive capacities and surface functional group (SFG) content were evaluated for seven commercially available activated carbons and an activated carbon whose SFG content was modified by outgassing. This study demonstrated that the presence of acidic surface functional groups hinders the ability of activated carbon to adsorb phenolic compounds under oxic conditions by reducing its effectiveness in promoting adsorption via oxidative coupling reactions. The catalytic ability of activated carbon may be enhanced by eliminating the acidic functional groups and encouraging formation of basic groups by outgassing at 900 degrees C. Re-introduction of oxygen-containing acidic surface functional groups onto the surface of outgassed GAC negates any gains in catalytic ability produced by the outgassing procedure. Therefore, outgassing affects the adsorption of phenolic compounds only by changing the amount and composition of oxygen complexes. Outgassing at higher temperatures (e.g., 1200 degrees C) causes the elimination of oxygen complexes, resulting in a more basic carbon that does not contain oxygenated basic groups. Greater structural ordering and delocalized electrons on the carbon surface may increase the basicity of the carbon but do not enhance its ability to promote irreversible adsorption. The presence of oxygen-containing basic groups (e.g., chromene-type, pyrone-type) is likely a key factor in promoting irreversible adsorption.