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Sulfate is one of the most abundant inorganic ligands in the lithosphere and hydrosphere. It plays a major role in mediating mineral dissolution and precipitation, crystal growth, mineral–water and air–water interfacial reactions, aerosol chemistry and global climate, and biogeochemical cycling of several elements including inorganic and organic toxic contaminants. For several years, macroscopic methods and thermodynamic models have been used in understanding and predicting the geochemistry of sulfate in a variety of systems. As shown by several recent studies, molecular chemistry of chemical species cannot be identified by the macroscopic methods alone (Sposito 1990, Brown et al. 1999). Although researchers have been using different spectroscopic methods (especially infrared spectroscopy, IR) for examining sulfate molecular chemistry, sulfate in geologic materials has not been probed as extensively as some of the other oxoanions, such as chromate, selenate and arsenate. Details of different spectroscopic methods and their applications in geochemical studies are discussed by Hawthorne (1988).
The molecular properties of sulfate, such as coordination environment (types of connecting atoms, their number and bond distances), electronic state, and symmetry, dictate how sulfate reacts in a geochemical system. Several electronic states exist in a molecule, with each electronic state containing several vibrational states, and each vibrational state containing several rotational states (Fig. 1⇓). The energies and probabilities of transitions between different electronic, vibrational, or rotational states can be measured and used in the identification of molecules and their chemical states. However, transitions among all of these states in a molecule are not feasible and the molecule symmetry determines whether a particular transition is allowed or forbidden (Cotton 1971). A variety of electron, X-ray, and optical spectroscopic methods can be used in studying the core (innermost) and valence (outermost) electronic transitions, and infrared and Raman spectroscopic methods can be used to obtain information …