Reaction of Uranium(VI) with Green Rusts: Effect of Interlayer Anion

Abstract

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.