Iron (hydroxide) Charging and Chromium Fate during Rapid Redox Cycling.

Environments such as riparian zones have dynamic water table levels and flow conditions, resulting in variable redox conditions. Changes in redox conditions induce mineral transformations and alter geochemical conditions, controlling the fate of toxic, redox active metals such as chromium (Cr). Hexavalent Cr is mobile and carcinogenic whereas trivalent Cr is immobile and innocuous. Iron (Fe) (hydr)oxides are widespread, redox active minerals that influence Cr mobility and speciation. Fe (hydr)oxides are semi-conductors with conduction bands comprised of 
overlapping, unoccupied Fe(3d) orbitals. This structure 
allows electron density to be delocalized
 and possibly retained during rapid redox 
cycling. Depending on the
 concentration of dissolved Fe(II), the mineral assemblage 
becomes charged by either: 1) electron injection into the
 conduction band and delocalization or 2) incorporation of 
solid Fe(II) not necessarily resulting in stoichiometric mixed 
valence or reduced minerals. A charged, heterogeneous mineral assemblage formed during rapid redox oscillations, may effectively reduce Cr, even during oxidation half cycles. This study assesses the impact of Fe mineral charging and transformations on Cr release and speciation in redox active environments. Cr (VI) is exposed to Fe (hydr)oxides that have undergone multiple rapid redox cycles 18 h after the 3^rd, 4^th and 5^th oxidation half cycles. Mineral transformations and solid Cr speciation are determined using X-ray absorption spectroscopy, specifically EXAFS and XANES respectively. Aqueous Cr and Fe speciation are determined using colorimetric methods and inductively coupled plasma emission spectroscopy. Understanding the impact of Fe mineral charging on Cr mobility and speciation contributes to the body of knowledge used to create solutions for contaminated water resources.