The influence of surface chemistry of activated carbons on adsorption and freezing/melting processes

Abstract

In this work, the adsorption and freezing/melting processes on activated carbons of different chemical nature (different concentrations of oxygen surface groups) were investigated. The role of solvent (cyclohexane and water) in the adsorption of three benzene derivatives – phenol (P), 4-nitrophenol (4-NP), and nitrobenzene (NB) was examined. To understand better the mechanism of adsorbent-adsorbate-solvent interactions the thermal analysis (TA) coupled with quadrupole mass spectrometry (QMS) was applied. The water freezing/melting effects of chemically and thermally modified activated carbons in the pore system were investigated using differential scanning calorimetry (DSC). For all used adsorbates, higher values of adsorption capacities were found for the systems with water as a solvent compared to those with cyclohexane due to divergent interactions adsorbent-adsorbate-solvent. Moreover, for cyclohexane solutions, the adsorption capacities are as follows: P < NB < 4-NP, while, for aqueous solutions, they are: P < 4-NP < NB. A much slower kinetic rate was observed in the case of systems with cyclohexane as an effect of the strong interactions of the nonpolar solvent with adsorbates and activated carbons. Thermal analysis confirmed the differentiation of adsorption mechanisms in the studied systems. The DSC studies allowed us to find correlations between the freezing/melting temperatures and the concentration of oxygen surface groups of carbons.