Adsorption of polycyclic aromatic hydrocarbons on FeOOH polymorphs: A theoretical study

Abstract

In this work we studied interactions of polycyclic aromatic hydrocarbons (PAHs) with surfaces of two iron oxyhydroxide polymorphs, goethite and lepidocrocite, by means of density functional theory (DFT) method. The first focus here was to obtain adsorption energies of selected PAH molecules with the (110) goethite surface and the (010) lepidocrocite surface by performing DFT calculations with the inclusion of dispersion interactions. To this end we have performed assessment of six different methods to show their efficiency and predictability of nonbonding interactions. The second objective was to compare adsorption capability of two FeOOH polymorphs, goethite and lepidocrocite, as common minerals in several soil types and to relate obtained adsorption energies to different surface structure and topology of the most stable surfaces of both polymorphs. The goethite (110) surface has more complicated topology and three different types of OH groups compared to the lepidocrocite (010) surface formed by a plane of OH groups of the same type. For goethite, adsorption energy did not increase monotonically with the increasing molecular size of PAHs and the strongest adsorption was observed for the linearly-shaped anthracene molecule. On the other hand, adsorption of PAHs at the lepidocrocite surface increased energetically with increasing molecular weight, and the strongest adsorption was achieved for the pyrene molecule. The results showed that the goethite (110) surface is more attractive than the (010) lepidocrocite surface, which is explained by the different surface topology, and types and surface density of the OH groups.