Phenol oxidation on the surface of granular activated carbon under water treatment conditions

Chemical activity of granular activated carbon (GAC) toward adsorbed phenol were investigated in a fixed bed column. After adsorption from aqueous solution, phenol was oxidized to carbon-carbon (C-C) and carbon-oxygen (C-O) bonded dimers as well as an oxidized compound of uncertain structure (molecular weight 276) as major products. This phenol coupling oxidation was observed on several types of commercial GAC.Various experimental parameters were tested to further characterize the coupling reaction of phenol on F400 GAC. Total product yields (based on adsorbed phenol) were determined based on the amount of phenol removed by GAC, not the initial concentration of input phenol. The ratio of C-C dimer to C-O dimer increased with the increase of the amount of phenol adsorbed. Also, total product yield was dependent on reaction pH which influenced phenol adsorption onto GAC from aqueous solution.Phenol oxidations on GAC surfaces were also studied in a series of experiments under conditions similar to those commonly met in water treatment. The presence of humic substances from soil or groundwater inhibited phenol oxidation and favored the formation of the C-O dimer relative to the C-C dimer. The lower total product yield was partially due to the competitive adsorption on active sites and partially to the free radical quenching activity of humic substances. The mechanism of the inhibition of humic substance has been discussed according to the results from syringic acid as a model compound of humic substances and ascorbic acid as a hydrophilic free radical scavenger, which were applied along with phenol onto GAC separately. Free chlorine-oxidized GAC showed less capacity for phenol adsorption than virgin GAC. The lower degree of phenol uptake resulted in lower formation of phenol coupling products on the GAC. However, application of free chlorine solution to the humic substance-loaded GAC restored the ability of the GAC to promote oxidation of adsorbed phenol. With or without the presence of humic substances, no data indicated that free chlorine-treated GAC produced higher yield of dimers, in comparison to similar experiments in which virgin carbon was used. In experiments with used carbons taken from the filters of water treatment plants, analyses of phenolic products were hindered by the complex gas chromatograms observed after these carbons were treated with phenol solutions. The removal of adsorbed material on used GAC by CH$\sb3$OH and CH$\sb2$Cl$\sb2$ extraction, however, improved the ability of used carbon to promote phenol oxidation.U of I OnlyETDs are only available to UIUC Users without author permissio