IMPACT OF OXYGEN-CONTAINING SURFACE FUNCTIONAL-GROUPS ON ACTIVATED CARBON ADSORPTION OF PHENOLS

The adsorptive capacity of activated carbons for phenolic compounds in creases significantly in the presence of dissolved oxygen (oxic condit ions) due to the oligomerization of these compounds through oxidative coupling reactions. With increased capacity comes an increase in the a mount of irreversible adsorption, which is defined as adsorbate that c annot 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 co mpounds in the presence of dissolved oxygen. The adsorptive capacities and surface functional group (SFG) content were evaluated for seven c ommercially available activated carbons and an activated carbon whose SFG content was modified by outgassing. This study demonstrated that t he presence of acidic surface functional groups hinders the ability of activated carbon to adsorb phenolic compounds under oxic conditions b y reducing its effectiveness in promoting adsorption via oxidative cou pling reactions. The catalytic ability of activated carbon may be enha nced by eliminating the acidic functional groups and encouraging forma tion of basic groups by outgassing at 900 degrees C. Re-introduction o f oxygen-containing acidic surface functional groups onto the surface of outgassed GAC negates any gains in catalytic ability produced by th e outgassing procedure. Therefore, outgassing affects the adsorption o f phenolic compounds only by changing the amount and composition of ox ygen complexes. Outgassing at higher temperatures (e.g., 1200 degrees C) causes the elimination of oxygen complexes, resulting in a more bas ic carbon that does not contain oxygenated basic groups. Greater struc tural ordering and delocalized electrons on the carbon surface may inc rease the basicity of the carbon but do not enhance its ability to pro mote irreversible adsorption. The presence of oxygen-containing basic groups (e.g., chromene-type, pyrone-type) is likely a key factor in pr omoting irreversible adsorption.