The absolute energy positions of conduction and valence band edges were compiled for about 50 each semiconducting metal oxide and metal sulfide minerals. The relationships between energy levels at mineral semiconductor-electrolyte interfaces and the activities of these minerals as a catalyst or photocatalyst in aqueous redox reactions are reviewed. The compilation of band edge energies is based on experimental flatband potential data and complementary empirical calculations from electronegativities of constituent elements. Whereas most metal oxide semiconductors have valence band edges 1 to 3 eV below the H 2 O oxidation potential (relative to absolute vacuum scale), energies for conduction band edges are close to, or lower than, the H 2 O reduction potential. These oxide minerals are strong photo-oxidation catalysts in aqueous solutions, but are limited in their reducing power. Non-transition metal sulfides generally have higher conduction and valence band edge energies than metal oxides; therefore, valence band holes in non-transition metal sulfides are less oxidizing, but conduction band electrons are exceedingly reducing. Most transition-metal sulfides, however, are characterized by small band gaps (<1 eV) and band edges situated within or close to the H 2 O stability potentials. Hence, both the oxidizing power of the valence band holes and the reducing power of the conduction band electrons are lower than those of non-transition metal sulfides.tion arises of whether semiconducting minerals could promote these same processes. If so, these minerals could play an important role in the fate of contaminants and the chemical compositions of atmosphere and hydrosphere of the early earth.Although all the processes mentioned above are photochemical processes, there is also evidence for non-photolytic catalysis of redox reactions by semiconductors (Xu 1997;Xu and Schoonen 1995;Xu et al. 1996Xu et al. , 1999. Semiconductors can act as a conduit for electrons between the aqueous reactants. Because no illumination is needed for non-photo catalytic processes, this mechanism may be important beneath the photic zone in aquatic systems.In photochemical reactions, as well as the non-photochemical mechanism outlined in our earlier work, the crucial step is the transfer of electrons between the semiconductor and sorbed reactants. As pointed out by Morrison (1990), electrons can only be transferred between those energetic states in the semiconductor and the electrolyte that are at approximately the same energy level. The energy level of energetic states of sorbates undergoing an electron transfer can be approximated by the standard redox potential (E 0 ), whereas relevant energy levels for a semiconductor are the top of the valence band (E V ) and the bottom of the conduction band (E C ). The relative energetics of E V and E C vs. E 0 is the fundamental property of an electrolyte/semiconductor system that dictates whether an electron transfer between the semiconductor and sorbate is feasible.Although band gap (E g ) is well know...

Despite the ubiquity of ferrihydrite in natural sediments and its importance as an industrial sorbent, the nanocrystallinity of this iron oxyhydroxide has hampered accurate structure determination by traditional methods that rely on long-range order. We uncovered the atomic arrangement by real-space modeling of the pair distribution function (PDF) derived from direct Fourier transformation of the total x-ray scattering. The PDF for ferrihydrite synthesized with the use of different routes is consistent with a single phase (hexagonal space group P6(3)mc; a = approximately 5.95 angstroms, c = approximately 9.06 angstroms). In its ideal form, this structure contains 20% tetrahedrally and 80% octahedrally coordinated iron and has a basic structural motif closely related to the Baker-Figgis delta-Keggin cluster. Real-space fitting indicates structural relaxation with decreasing particle size and also suggests that second-order effects such as internal strain, stacking faults, and particle shape contribute to the PDFs.