Reversibility of adsorption of aromatic compounds onto powdered activated carbon (PAC)

Desorption of sorbed compounds is an important process in the powdered activated carbon-activated sludge (PAC-AS) wastewater treatment system, where sorption and biodegradation interact. To assess the extent of desorption to be expected in the PAC-AS system, reversibility of adsorption was investigated using isotherm studies and a leaching technique under the conditions prevalent in the PAC-AS system. Two aromatic compounds, o-cresol and 3-chlorobenzoic acid (3-CB), and two types of powdered activated carbon (PAC) were used to study desorption. High degrees of irreversible adsorption were observed with both desorption techniques, depending on the type of PAC and the compound. The thermally activated, peat-based PAC (SA4) showed a high degree of irreversible adsorption compared to the chemically activated, wood-derived PAC (CA1). The phenolic compound, o-cresol, could be desorbed to a lesser extent than 3-CB from both types of PAC. Contact time between the PAC and the compound was found to affect the desorption of o-cresol adversely. Oxygen availability enhanced the adsorptive capacity of SA4 for o-cresol. Sorption of 3-CB proved unaffected by either contact time or oxygen. Oxidative polymerization as a probable mechanism for the irreversible adsorption observed is also discussed. Desorption kinetics from SA4 showed a first phase of rapid desorption followed by a second phase of slow desorption. CA1 displayed the first rapid desorption phase only.