GEOC 171

The direct interpretation of spectra is possible for some small inorganic anions on uniform surfaces (e.g. sulfate on hematite). The data analysis and interpretation becomes more demanding when several species contribute (e.g. solution and surface species and differently protonated species). A systematic variation of parameters and multicomponent fitting can often separate the contributions from individual species. However, the translation of IR-spectra into structures is not generally possible and empirical rules are often inadequate. A long-term goal is the reliable interpretation of vibrational spectra with molecular orbital calculations. IR-spectra of formate, deuterated formate and acetate on rutile and anatase were analyzed with ab initio calculations at the Hartree-Fock level. The computed vibrational frequencies of formate adsorbed in a bridging bidentate mode onto clusters with 2– 5 Ti centers, representing the rutile (110) surface, agreed with experiment, while other structures did not lead to good agreement. This study thus showed that this methodology can be used for the determination of the surface structures of simple adsorbates.

ATR-FTIR is also well suited to follow chemical transformations of adsorbed species. For example, it was shown that DSPB, an optical whitening agent, is rapidly degraded photocatalytically on rutile and anatase, while the photochemical transformation was slow and occurred by direct photochemical transformation on alumina and hematite. As another example, the surface catalyzed oxidation of As(III) on the surface of ferrihydrite by hydrogen peroxide was studied in situ and rate laws expressing the As(III) oxidation as a function of pH and hydrogen peroxide concentrations have been derived.

In presenting various examples, the possibilities and limitations of IR-spectroscopy in the study of structures and reactions on aqueous mineral surfaces are discussed.