Integrating structural and thermodynamic mechanisms for sorption of PCBs by montmorillonite

Strong sorption of planar nonionic organic chemicals by clay minerals has been observed for important classes of organic contaminants including polyaromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), and dioxins, and such affinity was hypothesized to relate to the interlayer hydrophobicity of smectite clays. In batch sorption experiments of two trichlorobiphenyls on homoionic Na-, K-, Cs-montmorillonites, considerably greater sorption coefficient (Kw) was observed for coplanar 3,3',5-trichlorobiphenyl (PCB 36); log Kw for Na-, K-, and Cs-montmorillonite were 3.69, 3.72, and 4.53 for coplanar PCB 36 vs 1.21, 1.46, and 0.87 for the nonplanar 2,2',6-trichlorobiphenyl (PCB 19). MD simulations were conducted utilizing X-ray diffraction determined clay interlayer distances (d-spacing). The trajectory, density distribution, and radial distribution function of interlayer cation, water, and PCBs collectively indicated that the hydrophobic nature of the interlayer regions was determined by the hydration status of exchangeable cations and the associated d-spacing. The sorption free energies calculated for both coplanar and nonplanar PCB molecules by adaptive biasing force (ABF) method with an extended interlayer-micropore two-phase model consisting of cleaved clay hydrates and "bulk water" are consistent with the Gibbs free energies derived from the measured log Kw, manifesting enhanced sorption of coplanar PCBs was attributed to shape selectivity and hydrophobic interactions.