Supercritical Fluid Extraction and Temperature-Programmed Desorption of Phenol and Its Oxidative Coupling Products from Activated Carbon

Abstract

Activated carbon remains one of the most economical adsorbents for the removal of contaminants from water. In particular, activated carbon is known to have an extremely high affinity for phenol and its derivatives. This has been shown to be the result of a catalytic process wherein activated carbon catalyzes the oxidative coupling reactions of phenol in aqueous solution when molecular oxygen is present. These reactions are believed to be the source of the difficulty of regenerating activated carbon loaded with phenol. This paper reports on our efforts toward using supercritical fluids to regenerate activated carbon combined with a concurrent temperature-programmed desorption study to identify reaction products and their binding strength to the carbon surface. The results show unequivocally that part of the phenol is chemisorbed on the surface and part of it undergoes polymerization. Dihydroxybiphenyls and phenoxyphenols are the major reaction products present on the surface. Isotope studies showed that surface carbon atoms do not directly participate in these reactions. Supercritical extraction was found to perform as well as solvent extraction for the regeneration of activated carbon loaded with phenol. However, due to the chemisorbed nature of these oxidative coupling products, the reduced mass-transfer limitations afforded by supercritical extraction cannot improve the overall extent of extraction even though the rate is improved with the addition of cosolvents.

Oxidative coupling and the irreversible adsorption of phenol by graphite

“…1 [8,11], including dihydroxybiphenyls and phenoxyphenols, in good agreement with proposed theory [12]. Such products have been identified in organic solvent extracts of phenol-loaded carbons [6,[8][9][10]16]. All proposed mechanisms for oxidative coupling of phenols include two steps [12][13][14][15].…”

“…Previous work has confirmed the presence of oxidative coupling products (with the composition C 6n H 4n+2 O n ) in acetone [6,8], methylene chloride [9,10], and supercritical carbon dioxide [16] extracts of loaded carbons. In this work we quantify, for the first time, both oxidative coupling product formation and irreversible phenol sorption.…”

Recovery, concentration and purification of phenolic compounds by adsorption: A review

“…Carbons regenerated under nitrogen exhibited greater adsorption capacities for smaller size molecules than steam regenerated ones (San Miguel et al, 2002). Temperature-programmed thermal desorption of model phenols adsorbed onto ACs is a technique used to study the surface composition, the binding states of volatile adsorbates and the desorption kinetics (Salvador and Merchán, 1996;Humayun et al, 1998;Nevskaia et al, 1999). Thermal regeneration is also suited for zeolites (Khalid et al, 2004;Roostaei and Tezel, 2004;Koubaissy et al, 2008).…”

Supercritical Regeneration of an Activated Carbon Fiber Exhausted with Phenol

“…The regeneration of ACs saturated with inorganic compounds is less usual and co-solvents must be used to efficiently extract these adsorbates [16]. The employment of SCCO 2 has also been studied to regenerate ACs saturated with low-volatile organic compounds, e.g., phenol [17][18][19], pesticides [20], herbicides [21], or insecticides [22]. Phenol is the most common model compound in the field of regeneration of ACs [3]: it is a pollutant frequently found in wastewater and drinking water that remains strongly retained on the carbon surface of ACs.…”

“…Phenol is the most common model compound in the field of regeneration of ACs [3]: it is a pollutant frequently found in wastewater and drinking water that remains strongly retained on the carbon surface of ACs. The limited number of works that have studied the SCCO 2 regeneration of ACs exhausted with phenol state that, at regeneration pressures and temperatures below 200 bar and 60 • C, respectively, phenol is not properly extracted [19] and the ACs are not efficiently regenerated [18].…”