Computational studies of the interaction of pollutants with iron oxide surfaces

The broad aim of the work in this thesis is to apply atomistic simulation techniques to advance our understanding of the surface chemistry of two important iron oxide minerals, hematite (α-Fe2O3) and maghemite (γ-Fe2O3), which have important applications in many fields, including as remedial agents in the soil and catalysts. First we have compared several interatomic potential models to describe the structures and properties of four iron oxide polymorphs, namely α-Fe2O3(hematite), β-Fe2O3, γ-Fe2O3 (maghemite) and e-Fe2O3 to choose a suitable potential for these systems, where we have considered cell volume, angles, Fe-O bond distances and relative stabilities of the four polymorphs. Next, we have investigated the energetic of vacancy ordering in γ-Fe2O3 (maghemite), using a statistical approach to evaluate uptake and ordering as a function of temperature. Our results show clearly that full vacancy ordering, in a pattern with space group P41212, is the thermodynamically preferred situation in the bulk material. This stability arises from a minimal Coulombic repulsion between Fe3+ cation sites for this configuration. Using this ordered model for maghemite, we have studied the surfaces of hematite (α-Fe2O3) and maghemite (γ-Fe2O3) both in dehydrated and hydroxylated form, and their interactions with two organic molecules, namely methanoic acid and hydroxyethanal, as models of organic pollutants. Finally, we have also considered the interaction of the same mineral surfaces with arsenate, another toxic pollutant found in soils and groundwater.