Competitive sorption of pyrene and pyridine to natural clay minerals and reference clay standards

Abstract

Sorption processes are known to be the controlling factors in mobility of organic contaminants in soils and sediments. It has been generally assumed that the organic matter fraction is the dominant sorbent of organic compounds. Mineral-organic interactions were thought to be negligible or nonexistent. Recent studies have shown that the mineral contribution in sorption of Polycyclic Aromatic Hydrocarbons (PAH) has been underestimated. Sorption mechanisms between minerals and PAH are poorly understood. This study followed a mechanistic approach to confirm suspected specific mineral-PAH interactions. The focus was on the sorption of pyrene to three clay sorbents, a soil clay fraction and two reference clay standards, in the presence of a competing solute, pyridine. Past studies have shown that pyridine can be directly coordinated to exchangeable cations; therefore, it can theoretically compete with pyrene for specific sites. Equilibrium sorption isotherms were modeled with the Freundlich Equation using nonlinear regression. Results showed that pyrene adsorbed to highly charged and polar phyllosilicates minerals. Macroscopic sorption experiments revealed a trend of increased pyrene sorption with increase in cation exchange capacity (CEC) of sorbents. Competitive behavior was observed when pyridine was introduced in binary solute sorption experiments. Pyridine decreased the amount of pyrene adsorb by all three clay sorbents. Competition was much more significant in the reference clay standards with the lower CEC. These results suggest that a specific interaction between the aromatic rings in pyrene with the exchangeable cations on the clay surface may be present, suggesting that sorption processes can be controlled by the formation of cation-pi bonds.