Green rusts are widely recognized as an important metastable intermediate phase in Fe biogeochemical
cycling and Fe metal corrosion and are strong reductants capable of reducing a widerange
of contaminants. Here we investigate the effect of interlayer anion on the reaction of green rusts
with hexavalent uranium (U(VI)). We react three synthetic green rusts, including carbonate, sulfate,
and chloride green rust, as well as pyroaurite, a redox-inactive Mg(II)-Fe(III) structural analog of carbonate
green rust with U(VI). The majority of U(VI) (> 80%) was removed from solution in about an hour at pH 8.0 in
0.1 M N-Tris(hydroxymethyl)methyl-3-aminopropanesulfonic acid (TAPS) buffer. Similar kinetics of U(VI) uptake on
green rusts and pyroaurite suggest that the observed uptake kinetics reflect an adsorption step rather than reduction of
U(VI) by structural Fe(II) in the green rusts. X-ray absorption spectroscopy (XAS) of the green rust solids indicates significant
reduction of U(VI) to U(IV) for all three green rusts, with complete reduction observed for sulfate and chloride
green rust and varied extents of reduction (34 to 100%) observed for carbonate green rust depending on the solution conditions.
No reduction of U(VI) was observed in the presence of pyroaurite, consistent with the absence of Fe(II) in the pyroaurite
structure. The decreased extent of U(VI) reduction observed with carbonate green rust in TAPS buffer may be
due to modification of the redox reactivity of U(VI) or green rust due to interaction with the TAPS buffer molecules. XAS
results indicate that U(VI) was reduced to U(IV) and was present as a monomeric-type U(IV) species in the presence of
TAPS buffer. In deionized water, however, carbonate green rust reduced U(VI) to nanoparticulate UO2. Green rusts may
be an important phase in the fate and transport of U(VI) in the contaminated subsurface, or under conditions where it
forms on corroding U-bearing waste containers.
Keywords: Green rust, layered double hydroxide, pyroaurite, uranium, XAS.
Rights & PermissionsPrintExport