Microbial reduction of uranium under iron- and sulfate-reducing conditions: Effect of amended goethite on microbial community composition and dynamics

Abstract

There is a growing need for a better understanding of the biogeochemical dynamics involved in microbial U(VI) reduction due to an increasing interest in using biostimulation via electron donor addition as a means to remediate uranium contaminated sites. U(VI) reduction has been observed to be maximized during iron-reducing conditions and to decrease upon commencement of sulfate-reducing conditions. There are many unknowns regarding the impact of iron/sulfate biogeochemistry on U(VI) reduction. This includes Fe(III) availability as well as the microbial community changes, including the activity of iron-reducers during the uranium biostimulation period even after sulfate reduction becomes dominant. Column experiments were conducted with Old Rifle site sediments containing Fe-oxides, Fe-clays, and sulfate rich groundwater. Half of the columns had sediment that was augmented with small amounts of Fe(III) in the form of 57Fe-goethite, allowing for a detailed tracking of minute changes of this added phase to study the effects of increased Fe(III) levels on the overall biostimulation dynamics. Mössbauer spectroscopy showed that the added 57Fe-goethite was bioreduced only during the first thirty days of biostimultuion, after which it remained constant. Augmentation with Fe(III) had a significant effect on the total flux of electrons towards different electron acceptors; it suppressed the degree of sulfate reduction, had no significant impact on Geobacter-type bacterial numbers but decreased the bacterial numbers of sulfate reducers and affected the overall microbial community composition. The addition of Fe(III) had no noticeable effect on the total uranium reduction.