Adsorption of toluene on various natural soils: Influences of soil properties, mechanisms, and model

Abstract

This study investigated toluene adsorption on natural soils. The linear partition model was found to represent the adsorption isotherm well (R2 = 0.958–0.994), compared with the Freundlich model (R2 = 0.901–0.991). Therefore, the coefficient, Kd, of the linear model indicated the adsorption capacity of soils A to F. Traditionally, Kd and the total organic carbon (TOC) content have a good linear relationship. However, this relationship was weak (correlation coefficient (r) = 0.689) when TOC values (8.43–12.9 mg/g) were low and close. To correct this deviation, this study investigated the influences of physicochemical properties, such as special surface area, mineral composition, functional groups, pH, and potentials. As soils B and C consisted of a large amount of active clayey minerals (69.4% kaolinite and 79.3% nacrite, respectively) and rich functional groups, they demonstrated the strongest adsorption capacity. Additionally, the r for pH-Kd, zeta potential-Kd, and redox potential-Kd were high, at 0.806, 0.914, and 0.932, respectively. To explore adsorption mechanisms, the adsorption thermodynamic parameter (enthalpy) was used initially to determine the forces. Combined with the analysis of soil properties, the mechanisms identified were hydrophobic interaction and hydrogen-pi bonding, involving co-adsorption with water molecules. Based on all studies, the properties were quantified and simplified by the plastic limit (PL), and TOC was simplified by soil organic matter (SOM). Then, PL and SOM were weighted by the entropy-weight method to obtain the determination factor, DF, a logarithmic parameter to replace TOC. Finally, a new model describing toluene adsorption on natural soils was established and expressed as Kd = 4.80 + 3.53DF. This new model had significantly improved the correlation between Kd and TOC (r = 0.933) and expanded the engineering adaptability.