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 immobiliz ation of Ni2+ in response to bacterial reduction of hydrous ferric oxide (H FO). In the absence of Ni2+, 79% of the total Fe( III) present as HFO was r educed; at 10(3) and 10(4) M Ni2+, 36% of the total Fe( III) was reduced, w hereas 45 to 50% of the total Fe( III) was reduced at 10(5) M Ni2+. The inh ibitory 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 condi tion, and a 25% decrease at 10(5) M Ni2+. A prominent GR peak at d = 10.9 n m was evident in X-ray diffraction patterns of postreduction residual solid s from the cultures. Minor peaks arising for vivianite and magnetite were a lso present. In samples prepared for scanning electron microscopy, thin hex agonal plates of GR were easily distinguished as a solid phase transformati on product of HFO. Small hexagonal sheets and fragments of larger GR plates were also observed in transmission electron microscopy whole mounts togeth er with bacteria that were mineralized by surface precipitates of microcrys talline magnetite. Energy dispersive spectroscopy (EDS) confirmed that GR c ontained Fe and P, as well as Ni in those samples taken from the Ni2+-amend ed experiments. EDS detected neither P nor Ni in the magnetite precipitates associated with the bacterial cells. Dissolved Ni2+ concentrations decreas ed in an exponential fashion with respect to time in all experimental syste ms, 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 F e(III) reduction experiments provided evidence that the solid-phase partiti oning of Ni2+ in GR extended from equilibrium solid-solution behavior.