The effect of pH on the adsorption of phenol and chlorophenols onto natural zeolite

The adsorption isotherms of phenol, 2-chlorophenol, 4-chlorophenol, 2,4-dichlorophenol, and 3,5-dichlorophenol onto natural zeolite were obtained at pH values of 4.0, 6.0, pKa of phenols, and 10.5. A simple two-site Langmuir model was used to fit the experimental data where two types of interaction were quantified by the model. The first is the pH independent interaction of phenols with hydrophobic sites of zeolite. The second is the pH-dependent phenolate complexation with hydrophilic sites of zeolite (metal ions). The adsorption was enhanced with increasing pH values due to the increase of phenolates complexation with metal ions on zeolite surface. The number of hydrophobic sites of zeolite available for phenols (Qm1) was found to be greater than that of the hydrophilic sites (Qm2). On the other hand, the affinity constants of the hydrophilic sites (K2) were found to be much greater than those of the hydrophobic sites (K1). Moreover, the adsorption process of phenols onto zeolite was found to be independent on the ionic strength.