Analytical Advances

A necessary complement to macroscopic studies dealing with sorption, redox, precipitation and dissolution reactions are microscopic measurements capable of providing direct, molecular-level information on the composition and structure of the surface substrate (Reeder, 1991). Many of the recent major advances in understanding the surface chemistry of geochemical systems have been driven by application of surface-sensitive analytical techniques. Application continued on the use of ultra-high vacuum spectroscopic techniques including the use of Auger electron spectroscoppy (AES) and X-ray photoelectron spectroscopy (XPS) for surface characterization. Examples include the characterization of surface chemistry of weathering basalt flows (White and Hochella, 1992), the use of XPS and secondary ion mass spectroscopy (SIMS) to characterize feldspar surfaces (Inskeep et al., 1991) and the use of XPS to study sorption/desorption reactions of Hg(II) on Fe sulfide surfaces (Hyland et al., 1990). Applications of nuclear magnetic resonance (NMR) include Si studies on the characterization of coordination bonding of water on SiO surfaces (Kinney et al., 1993), and the use of Cs to study bonding sites of Cs on clays (Weiss et al., 1990). Also significant advances have been made in high resolution transmission electron microscopy (HRTEM) to characterize submicroscopic isovolumetric reactions associated with the transformation of primary silicates to clay minerals (Banfield and Barker, 1994).

Since the publication of the first paper describing the application of scanning tunneling microscopy (STM) to the semiconducting minerals hematite and galena (Hochella et al., 1989), an increasing number of studies have documented atomic and molecular resolution on geochemical substrates using this technique and the allied technique atomic force microscopy (AFM). Both techniques make use of recent advances in piezoelectric translators that can bring sharp metallic or crystalline tips within several Angstroms of the surface. For STM, the changes in the tunneling current established by a bias voltage between the tip and surface permit mapping of the density and energy of the electron states and topography of conducting and semiconducting minerals. For AFM, the same piezoelectric platform is used but Van der Waals, electrostatic or magnetic forces between the tip and sample are sensed through deflection of a cantilever. Recent specialized implementations of AFM include lateral or frictional force microscopy and magnetic force microscopy. These technologies are currently just being recognized in geochemical applications. The resolution of the STM technique has been demonstrated by a series of papers detailing step formation, oxidation, and sorption on sulfide and Fe oxide minerals (Eggleston and Hochella, 1993; Eggleston and Stumm, 1993); with the later reference documenting real time adsorption-desorption coupled with outer-sphere diffusion of Cr(III) on -FeO.

In AFM analysis, interaction between the tip and sample is more complex than in STM analysis and therefore true atomic resolution is not readily obtained. The principal geochemical applications to date have centered on detailed 3-dimensional topographic mapping of mineral surfaces and resolution of structural features including experimental validation of ledge motion models for the dissolution of quartz (Gratz et al. 1991) and in situ observation of the dynamics of monomolecular growth steps on calcite (Gratz et al.,1993).

The increased availability of synchrotron radiation sources has resulted in a significant increase in application of a number of techniques using X-ray absorption spectroscopy (XAS). These methods have distinct advantages over more established approaches such as XPS and AES in that XAS techniques do not require a vacuum. Techniques applied to geologic materials include extended X-ray absorption fine line (EXAFS) and X-ray absorption near edge structure (XANES). A detailed discussion of these techniques applied to geologic substrates is presented by Brown (1990). Experiments consist of exposing a sample to a monochromatic beam of X-rays that is scanned through a range of energies from below to above the absorption edge of an element. The technique is sensitive to the electronic structure of two or three closest shells of atomic neighbors surrounding the absorbing atom and thus provides information on the composition of solid under in situ conditions in contact with water. The method is a bulk analytical technique in which the structural information is summed over all environments of a given absorber. However characterization of surface species and complexation is commonly done on samples with high surface areas and sorbent concentrations and low internal concentrations.

Recent studies detailing sorption processes using these techniques include the application of XANES and EXAFS to the -Al2O3/water interface. Work of Chisholm-Brause et al.( 1990) demonstrated that Pb(II) bonds directly to the surface as inter-sphere complexes and that the number and size of these complexes increases with surface coverage. EXAFS studies of FeOOH likewise showed that Np (V) bonds as inter-sphere complexes with a lack of any ordered Np oxide or hydroxide structure denoting coprecipitation (Combes et al., 1992). Studies of the surface chemistry of ferrihydrite using EXAFS showed clear evidence for inner-sphere bidentate sorption of arsenate. With increasing As/Fe ratios, the ferrihydrite structure became disordered denoting surface poisoning and a decrease in the extent of crystallization (Waychunas et al., 1993).