Reduction of hexavalent chromium by ferrous iron: A process of chromium isotope fractionation and its relevance to natural environments

Stable chromium (Cr) isotopes can be used as a tracer for changing redox conditions in modern marine systems and in the geological record. We have investigated isotope fractionation during reduction of Cr(VI)_aq by Fe(II)_aq. Reduction of Cr(VI)_aq by Fe(II)_aq in batch experiments leads to significant removal of lighter Cr isotopes from solution. Assuming Rayleigh distillation, the fractionation factor α=0.9964 agrees with published results for Cr(VI) reduction by magnetite. In experiments designed to mimic natural aqueous systems, Fe(II)_aq was added at constant rate at pH 6.8 and 8.1. Cr isotope composition of solids and solution displays Rayleigh distillation with α=0.9985. Chemical composition, electron microscopy, X-ray diffraction and Cr isotope composition agree with a model where Cr(VI)_aq reduction occurs through two mechanisms: (a) homogeneous Cr(VI)_aq reduction leads to the formation of unstable Cr(VI)-bearing green rust and results in Cr isotope fractionation of a magnitude similar to that observed during reduction by Fe(II)_aq in our batch experiments, (b) intercalated Cr(VI) is reduced by the Fe(II) in the green rust, resulting in negligible isotope fractionation. The original Cr isotope composition of the reacted Cr(VI)_aq is preserved if >90% of Cr(VI)_aq is reduced by Fe(II)_aq, which supports previous observations that under pH conditions relevant to surface seawater, Cr isotopes in Fe-rich chemical precipitates can provide important information for the interpretation of the paleo-redox state of the seawater from which the sediments precipitated.