Formation of green rust and immobilization of nickel in response to bacterial reduction of hydrous ferric oxide

This investigation documents the formation of Green Rust (GR) and immobilization of Ni2+ in response to bacterial reduction of hydrous ferric oxide (HFO). In the absence of Ni2+, 79% of the total Fe( III) present as HFO was reduced; at 10(3) and 10(4) M Ni2+, 36% of the total Fe( III) was reduced, whereas 45 to 50% of the total Fe( III) was reduced at 10(5) M Ni2+. The inhibitory effect of 10(3) and 10(4) M Ni2+ on Fe(III)-reduction corresponded to a 50% decrease in number of viable cells relative to the Ni2+-free condition, and a 25% decrease at 10(5) M Ni2+. A prominent GR peak at d = 10.9 nm was evident in X-ray diffraction patterns of postreduction residual solids from the cultures. Minor peaks arising for vivianite and magnetite were also present. In samples prepared for scanning electron microscopy, thin hexagonal plates of GR were easily distinguished as a solid phase transformation product of HFO. Small hexagonal sheets and fragments of larger GR plates were also observed in transmission electron microscopy whole mounts together with bacteria that were mineralized by surface precipitates of microcrystalline magnetite. Energy dispersive spectroscopy (EDS) confirmed that GR contained Fe and P, as well as Ni in those samples taken from the Ni2+-amended experiments. EDS detected neither P nor Ni in the magnetite precipitates associated with the bacterial cells. Dissolved Ni2+ concentrations decreased in an exponential fashion with respect to time in all experimental systems, corresponding to an overall first-order rate constant k of -0.030 day(1). At the same time, a strong linear relationship (r(2) = 0.99) between the dissolved and solid phase Ni2+/Fe2+ ratios over the entire period of the Fe(III) reduction experiments provided evidence that the solid-phase partitioning of Ni2+ in GR extended from equilibrium solid-solution behavior.