Further insights into the role of carbon surface functionalities in the mechanism of phenol adsorption

The presented study describes the temperature as well as pH dependence of phenol adsorption (and adsorption kinetics) on four carbons with different chemical compositions of the surface layer but almost identical porosity. In the first part, it is shown, applying the most sophisticated method of carbon porosity characterization (i.e., the method of Do and co-workers-ND method), that the porosity does not change much after the chemical modification of carbons. Then it is shown that the ND method leads to the same results as the DFT (density functional theory) does. Next, the TPD results for D43/1 carbons (initial, modified with HNO3, fuming H2SO4, and with NH3) are described. The TPD results for carbon modified with fuming sulphuric acid has not been reported yet by others. The deconvolution of peaks is performed. The obtained results, together with those already published, lead to the chemical structures of surface functionalities for all studied carbons. The thermogravimetric analysis of phenol adsorption shows that the amount of chemically bonded molecules is small. Then it is shown that the adsorption at the acidic pH (1.5) level is lower for all studied carbons than that at the neutral one. The description of the isotherms applying adsorbability, quasi-Freundlich and DA models, together with enthalpy measurements, lead to the mechanism of phenol adsorption at both pH values. The mechanism is, furthermore, confirmed by some empirical correlations. The analysis of the average hysteresis on adsorption-desorption isotherms as well as the comparison of phenol adsorption in oxic and anoxic conditions leads to the mechanism of irreversible phenol adsorption. It is suggested that the irreversibility is caused by two effects: the creation of strong complexes between phenol and surface carbonyl and lactones as well as by the polymerization. The last effect is due to the ability of carbon to adsorb the oxygen from solution and form superoxo ions. Finally, the kinetics is considered. The analytical solution of Fick's law of diffusion for adsorption in cylindrical particles is applied, the diffusion coefficients are calculated. It is shown that phenol diffusion is mixed between a surface process and a pore one. The obtained energy of diffusion is correlated with the values of the physicochemical parameters of studied carbons. As a final point, it is concluded that the mechanism of phenol adsorption is not only determined by so called pi-pi interactions and donor-acceptor complex formation but also by (strongly depending on temperature) the solvent effect balancing the influence of the two mentioned factors on this mechanism. (C) 2003 Elsevier Inc. All rights reserved.