Sorption of polycyclic aromatic hydrocarbons to soils enhanced by heavy metals: perspective of molecular interactions

Purpose Combined pollution by polycyclic aromatic hydrocarbons (PAHs) and heavy metals are commonly found in industrial soils. This study aims to investigate the effect of the coexistence of heavy metals on the sorption of PAHs to soils. We focused specifically on the relationship of the sorption capacity with the estimation of the binding energy between PAHs and heavy metals. Materials and methods The sorption of typical PAHs (naphthalene, phenanthrene, and pyrene) to soils coexisting with heavy metals (Cu(II), Pb(II), and Cr(III)) was characterized in batch sorption experiments. The binding energy between PAHs and heavy metals in aqueous solution was estimated by quantum mechanical (QM) method using density functional theory (DFT) at the M06-2x/ def2svp level of theory. Results and discussion Sorption capacity and nonlinearity of the PAHs to the soils were enhanced by the coexisting heavy metals. The extent of increment was positively associated with the hydrophobicity of the PAHs and the electronegativity and radius of the metal cations: Cr(III) > Pb(II) > Cu(II). The cation-pi interaction was revealed as an important noncovalent binding force. There was a high correlation between the binding energies of the PAHs and K-f' (K-f adjusted after normalizing the equilibrium concentration (C-e) by the aqueous solubility (C-s)) (R-2 > 0.906), indicating the significant role of the cation-pi interactions to the improved PAH sorption to soils. Conclusions In the presence of heavy metals, the sorption capacities of naphthalene, phenanthrene, and pyrene to soils were enhanced by 21.1-107 %. The improved sorption capacity was largely contributed from the potent interactions between PAHs and heavy metals.