Periodic density functional theory calculations of bulk and the (010) surface of goethite

James D Kubicki¹ , Kristian W Paul² and Donald L Sparks³

¹Dept. of Geosciences and the Earth & Environmental Systems Institute, The Pennsylvania State University, University Park, PA 16802, USA

²US Army Research Laboratory, AMSRD-ARL-WM-BD, APG, MD 21005, USA

³Department of Plant and Soil Sciences, 152 Townsend Hall, University of Delaware, Newark, DE 19716, USA

author email corresponding author email

Geochemical Transactions 2008, 9:4doi:10.1186/1467-4866-9-4


Published:	13 May 2008

Abstract

Background

Goethite is a common and reactive mineral in the environment. The transport of contaminants and anaerobic respiration of microbes are significantly affected by adsorption and reduction reactions involving goethite. An understanding of the mineral-water interface of goethite is critical for determining the molecular-scale mechanisms of adsorption and reduction reactions. In this study, periodic density functional theory (DFT) calculations were performed on the mineral goethite and its (010) surface, using the Vienna Ab Initio Simulation Package (VASP).

Results

Calculations of the bulk mineral structure accurately reproduced the observed crystal structure and vibrational frequencies, suggesting that this computational methodology was suitable for modeling the goethite-water interface. Energy-minimized structures of bare, hydrated (one H₂O layer) and solvated (three H₂O layers) (010) surfaces were calculated for 1 × 1 and 3 × 3 unit cell slabs. A good correlation between the calculated and observed vibrational frequencies was found for the 1 × 1 solvated surface. However, differences between the 1 × 1 and 3 × 3 slab calculations indicated that larger models may be necessary to simulate the relaxation of water at the interface. Comparison of two hydrated surfaces with molecularly and dissociatively adsorbed H₂O showed a significantly lower potential energy for the former.

Conclusion

Surface Fe-O and (Fe)O-H bond lengths are reported that may be useful in surface complexation models (SCM) of the goethite (010) surface. These bond lengths were found to change significantly as a function of solvation (i.e., addition of two extra H₂O layers above the surface), indicating that this parameter should be carefully considered in future SCM studies of metal oxide-water interfaces.