Defining the Interfacial Molecular Structure of Goethite: A Key to Understanding Environmental Chemical Reactions

The mineral goethite (a-FeOOH) is one of the most common iron minerals on Earth’s surface. As the chief constituent in many forms of natural rust, it is most obvious as the yellow or brown-red coloring agent in soils and sediments. It is also a microscopic to nanoscale mineral, with a high surface area and high density of chemically reactive atomic sites. As such, it has a strong influence on environmental chemistry, given that it can sorb large quantities of soluble metal complexes and toxic anions, including lead, copper, selenium, and arsenic. Goethite sorption and binding of arsenic in Asia are key components in the contamination problems that have been creating widespread health issues during the last decade.

Due to its importance, goethite’s environmental chemistry has been heavily studied, but almost entirely by “bulk” analyses, where no direct knowledge of the surface structure and reactive sites is employed. Despite this, empirical models for sorption and other reactions have been developed that utilize an assumed structural model, based on what is known to be the average internal structure of the mineral.

Now, for the first time, as reported in recent work by Ghose et al. (2010) (including LBNL Earth Sciences Division’s Glenn Waychunas), investigators have been able to define the surface structure of goethite crystallites, including the details of the chemically reactive sites on the surface (called “functional groups”), and the nature of the water on top of the surface. This shows how the surface is protonated, and reveals what sites on the surface are responsible for particular acid-base and binding (i.e., chemical) characteristics (Figure 1). Further, the structured water, which is at least two layers in thickness, suggests that sorption “sites” might be available within these layers for so-called outer sphere sorption, putting a physical picture in place for a model that has up to now been rather speculative.

Several subtle aspects of the result may be important in developing improved models of the mineral-water interface, and hence better approaches to understanding environmental chemical reactions. For example, the relaxation of the surface atoms with respect to the internal structure changes the acid-base characteristics of these species, and the nature of the ordered water determines in part the electrostatic properties of the electrical double layer (EDL) at the goethite-water interface. The scattering work by Ghose et al. (2010) also revealed the presence of subunit cell-sized steps on the goethite surface (Figure 2) having different chirality. This means that these steps may preferentially sorb ions having distinct “handedness.” The ratio of left to right handed steps is dependent on the way the goethite is cleaved or broken. This could be a crucial factor in the way goethite nanocrystals interact with biological molecules.

Figure 1.  The structure of the goethite-water interface. Oxygens are red, Ferric ions are purple. Water oxygens in the first water layer are blue and in the second layer light blue. Protons are shown as small white spheres. The dashed lines refer to likely hydrogen bonding directions. ^IO and ^IIO refer to two surface functional groups. ^IO is a water molecule bound to a single ferric ion, ^IIO is a hydroxyl bound to a single ferric ion. The first water layer is hydrogen bonded mainly to the surface oxygens, while the second is mainly bonded to the first water layer. Subsequent water layers would only be bonded to other water layers without direct surface influence. The bulk unit cell is outlined with the heavy blue dashed lines.

 

Figure 2. Goethite surface revealed by CTR scattering showing steps of sub unit cell (1/4 C) height. Adjacent steps have different handedness and would interact differently with chiral molecules. In this diagram, the overlying water structure is not shown. Also, the actual termination steps would be many tens or hundreds of nanometers wide.

 

Additional Information:

• Ghose, S.K., G.A. Waychunas, T.P. Trainor, and P.J. Eng (2010), Hydrated goethite (alpha-FeOOH) (100) interface structure: Ordered water and surface functional groups. Geochimica et Cosmochimica Acta, 74, 1943–1953.
• This publication was also a featured highlight in the APS annual science report at Argonne National Laboratory.