Tc(VII) and Cr(VI) Interaction with Naturally Reduced Ferruginous Smectite from a Redox Transition Zone

Fe(II)-rich clay minerals found in subsurface redox transition zones (RTZs) can serve as important sources of electron equivalents limiting the transport of redox-active contaminants. While most laboratory reactivity studies are based on reduced model clays, the reactivity of naturally reduced field samples remains poorly explored. Characterization of the clay size fraction of a fine-grained unit from the RTZ interface at the Hanford site, Washington, including mineralogy, crystal chemistry, and Fe(II)/(III) content, indicates that ferruginous montmorillonite is the dominant mineralogical component. Oxic and anoxic fractions differ significantly in Fe(II) natural content, but Fe_TOTAL remains constant, demonstrating no Fe loss during its reduction-oxidation cyclings. At native pH of 8.6, the anoxic fraction, despite its significant Fe(II), ∼23% of Fe_TOTAL, exhibits minimal reactivity with TcO₄^- and CrO₄^2- and much slower reaction kinetics than those measured in studies with biologically/chemically reduced model clays. Reduction capacity is enhanced by added/sorbed Fe(II) (if Fe(II)_SORBED > 8% clay Fe(II)_LABILE); however, the kinetics of this conceptually surface-mediated reaction remain sluggish. Surface-sensitive Fe L-edge X-ray absorption spectroscopy shows that Fe(II)_SORBED and the resulting reducing equivalents are not available in the outermost few nanometers of clay surfaces. Slow kinetics thus appear related to diffusion-limited access to electron equivalents retained within the clay mineral structure.