Adsorption of Phenolic Compounds from Aqueous Solutions, by Activated Carbons, Described by the Dubinin−Astakhov Equation

It is shown that the adsorption of phenol and 3-chlorophenol from aqueous solutions, by basic active carbons and at an equilibrium solution pH below that of dissociation of the phenols, can be described by an equation of the DRK type with exponent <i>n</i> = 4. The principle of temperature invariance is fulfilled between 283 and 313 K, which means that predictions can be made on the basis of a characteristic energy, <i>E</i>_s, and of affinity coefficients, β_s, relative to phenol. This approach presents an advantage over the traditional Langmuir isotherm. Moreover, the comparison of the limiting amounts adsorbed by the different carbons suggests that phenol and 3-chlorophenol are adsorbed in a monolayer as observed for carbon blacks, except in the case of activated carbons with a low degree of activation in which molecular-sieve effects can take place. Similar conclusions are obtained from the enthalpies of immersion into the aqueous solutions and their comparison with graphitized carbon blacks.