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

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 solvents (cyclohexane and water) in the adsorption of three benzene derivatives - phenol (P), 4-nitrophenol (4-NP), and nitrobenzene (NB) - was examined. To understand the mechanism of adsorbent-adsorbate-solvent interactions better 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 systems with water as a solvent compared to those with cyclohexane due to divergent adsorbent-adsorbate-solvent interactions. 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. DSC studies allowed us to find correlations between freezing/melting temperatures and the concentration of the surface oxygen groups in carbons. The increase in water freezing/melting temperatures is mainly related to the removal of the surface oxygen groups in activated carbons (a decrease in the concentration of surface oxygen groups), which also affects the increase in the specific surface area (S_BET).