Model study on sorption of polycyclic aromatic hydrocarbons to goethite

A systematic DFT study of interactions between a set of mono- and polyaromatic hydrocarbons (PAHs) and the (110) goethite surface have been performed in this work. It was found that PAHs form relatively weak surface complexes having their molecular plane practically parallel to the surface plane. The origin of the interactions is in the polarization of the pi-system by polar OH groups and in the formation of weak hydrogen bonds where the pi-system acts as a proton acceptor. The computed perpendicular distances of the molecular plane of PAHs to the hydrogen atoms of the surface OH groups range from 2.3 to 2.7 A. Computed interaction energies regularly increase for the linearly shaped molecules from benzene to anthracene. Two other PAHs with a nonlinear shape, phenanthrene and pyrene, are less strongly bound to the surface although they have a similar (phenanthrene) or even larger size (pyrene) than anthracene. These differences were explained by the specific configuration of the surface hydroxyl groups of goethite. The three types of OH groups, mu-OH, mu(3)-O(II)H, and OH, form a valley, the width of it fits very well the molecular shape of the linear PAHs. It was found that with anthracene as example the linear PAHs can easily slide along the valley of OH groups with practically no barrier. In summary it is concluded from our results that the (110) goethite surface will withhold linear PAHs significantly better than nonlinear ones.