Sorption of polyaromatic compounds by organic matter-coated Ca²⁺- and Fe³⁺-montmorillonite

Sorption of organic contaminants to soil is of growing concern due to the potential risk associated with their persistence in soil and leaching to groundwater. The objective of this study was to evaluate sorption of non-polar (phenanthrene and pyrene) and polar (1-naphthol) polyaromatic compounds on montmorillonite saturated with either Ca²⁺ or Fe³⁺ and coated with natural organic matter, i.e. humic acid (HA), or cuticular waxes. The amount of carbon in both wax and HA-coated montmorillonite was greater for Fe³⁺-montmorillonite than for Ca²⁺-montmorillonite. For all systems, the order of sorption affinities was: pyrene > phenanthrene > 1-naphthol, corresponding to the order of their hydrophobicity. The HA-clay complexes adsorbed 80 to 93% of the added amounts of phenanthrene and pyrene. Sorption affinities (K_OC) of phenanthrene and pyrene to HA-clay were higher than those to wax-clay for all tested systems. In contrast to the HA-clay systems, cation did not significantly affect affinity of phenanthrene to wax-clay surfaces. Sorption of polar 1-naphthol was significantly affected by the type of exchangeable cation. For Ca²⁺-clays, sorption of 1-naphthol was higher on wax-montmorillonite than on HA-montmorillonite. The opposite trend was observed for Fe³⁺-montmorillonite. Calculations eliminating sorbate-water interactions suggested similar changes in chemical potential of molecules of phenanthrene and pyrene upon transfer from pure liquid phase to sorbent, and that neither phenanthrene nor pyrene crystallized during its sorption.