R. Humayun et al., SUPERCRITICAL-FLUID EXTRACTION AND TEMPERATURE-PROGRAMMED DESORPTION OF PHENOL AND ITS OXIDATIVE COUPLING PRODUCTS FROM ACTIVATED CARBON, Industrial & engineering chemistry research, 37(8), 1998, pp. 3089-3097
Activated carbon remains one of the most economical adsorbents for the
removal of contaminants from water. In particular, activated carbon i
s known to have an extremely high affinity for phenol and its derivati
ves. This has been shown to be the result of a catalytic process where
in activated carbon catalyzes the oxidative coupling reactions of phen
ol in aqueous solution when molecular oxygen is present. These reactio
ns are believed to be the source, of the difficulty of regenerating ac
tivated carbon loaded with phenol. This paper reports on our efforts t
oward using supercritical fluids to regenerate activated carbon combin
ed with a concurrent temperature-programmed desorption study to identi
fy 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. Dihydroxybiph
enyls and phenoxyphenols are the major reaction products present on th
e surface. Isotope studies showed that surface carbon atoms do not dir
ectly participate in these reactions. Supercritical extraction was fou
nd to perform as well as solvent extraction for the regeneration of ac
tivated carbon loaded with phenol. However, due to the chemisorbed nat
ure of these oxidative coupling products, the reduced mass-transfer li
mitations afforded by supercritical extraction cannot improve the over
all extent of extraction even though the rate is improved with the add
ition of cosolvents.