Correlations and prediction of adsorption capacity and affinity of aromatic compounds on activated carbons

Correlations capable of predicting organic compound adsorption by activated carbons (ACs) are essential to the applications of ACs as environmental adsorbents in water treatment. Adsorption isotherms of 21 aromatic compounds on 11 ACs both with various physicochemical properties were conducted and fitted by Dubinin-Ashtakhov model to develop the predictive correlations in this study. In addition to the correlations of adsorption capacity with total surface area of ACs, micropore surface area ratios (Rmicro) of ACs and chemical molar volume reported in previous studies, the negative correlation of adsorption capacity with chemical melting point was newly observed in this study. This negative correlation could be attributed to expansion of chemicals adsorbed on the mesopore or external surface of ACs. Meanwhile, in addition to the positive correlations of adsorption affinity with Rmicro of ACs, chemical polarity/polarizability and hydrogen bonding donor ability reported also in previous studies, the negative correlation of adsorption affinity with H/C of ACs was newly observed in this study, which should be attributed to that ACs with higher aromaticity could have stronger π-π interaction potential, hydrogen bonding interaction potential and hydrophobic effects for aromatic compounds. These observed correlations can be used to predict aromatic compound adsorption by ACs with readily available properties of both ACs (i.e., surface area, Rmicro and H/C) and aromatic compounds (i.e., molar volume, melting point and solvatochromic parameters). Moreover, these predictive correlations, incorporating various adsorptive forces, steric hindrance effect and packing efficiency in adsorption and having clearly physicochemical significance, are important for exploring the adsorption mechanisms, and guiding the synthesis of ACs with desired physicochemical properties, and selecting ACs as adsorbents in water treatment applications.