The roles of humic substances in the interactions of phenanthrene and heavy metals on the bentonite surface

Purpose: The fate of complex contaminants in soils has aroused increasing attention of environmental scientists. Although heavy metals and polycyclic aromatic hydrocarbons (PAHs) are often encountered together in contaminated sites, the interactions between the co-existing contaminants with clay minerals and humic substances require further investigation. Materials and methods: Phenanthrene, lead (Pb), and cadmium (Cd) were selected as the representatives to investigate their interactions on the surface of bentonite (primarily montmorillonite) and humic acid/fulvic acid-coated bentonite in batch sorption experiments. Their bonding strengths were assessed in terms of extractability by various chemical extraction methods. Results and discussion: The humic acid coating on bentonite enhanced the Pb and Cd sorption capacity, especially at the high Pb loading where the linear sorption coefficient was dramatically increased. The fulvic acid coating had less significant effects because of its higher solubility. The co-existing phenanthrene promoted the Pb sorption to a greater extent on humic acid-coated bentonite due to greater phenanthrene sorption, while the phenanthrene sorption was also promoted by the co-existing Pb. However, the enhancement was less obvious for Cd sorption, possibly due to greater hydration and weaker complexation with oxygen-containing groups despite higher electron polarizability of Cd. On the other hand, the presence of humic acid coating increased both (weakly bound) exchangeable fraction and (strongly bound) residual fraction of sorbed Pb. In contrast, co-existing phenanthrene and Pb resulted in redistribution of each other from weakly bound to strongly bound fractions. Such strengthened bonding of both Pb and phenanthrene indicated the significance of strong cation-π bonding on the bentonite. Yet, such enhancement might be alleviated by the humic acid coating. Conclusions: Understanding the intercorrelated interactions of humic substances, phenanthrene, and heavy metals on clay minerals would help us to maneuver an effective in situ remediation of complex soil contamination.