Adsorption of phenanthrene on Na-montmorillonite: A model study

Abstract

The role of clay minerals in the fate of polycyclic aromatic hydrocarbons (PAHs) in soil still remains an unresolved and controversial issue. Recent experimental studies report on PAH sorption capacity of clay minerals, which is either much smaller than or comparable to that of soil organic matter with water either inhibiting or not inhibiting PAH sorption. We studied adsorption of phenanthrene – a prominent PAH representative – on Na-montmorillonite – a common member of the smectite group of clay minerals and often dominating component of soil inorganic matter – by means of molecular simulations incorporating free energy calculations. The study also included the effect of surface hydration on sorption. Phenanthrene adsorption site featured by Na +− π bonding was identified as the most favorable one on the dehydrated montmorillonite surface with other inner-sphere adsorption sites being less favorable by 23–32 kJ/mol and the desorption state being less favorable by 74 kJ/mol on the free energy scale. Upon montmorillonite hydration by a water film with a thickness of ~ 1 nm, however, phenanthrene desorption from the montmorillonite surface to the water–air interface becomes more favorable than phenanthrene direct adsorption. Our results support, therefore, experimental studies suggesting that PAHs adsorption on clays predominates in the dry soils but is negligible in the presence of water. Still, a consideration of higher phenanthrene surface coverages (to account for possible aggregation effect) or other exchangeable cations (to account for possible stronger cation − π bonding) in the future model studies may provide additional arguments pro or contra of this suggestion.