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U(VI) speciation and its influence on microbial reduction in presence of humic substances


EMSL Project ID
8202

Abstract

This study is to investigate aqueous U(VI) speciation in presence of humic substances (HS) and its influence on microbial uranyl reduction. Our experimental results indicated that HS enhances the microbial reduction of U(VI), particularly in the presence of relatively high concentrations of calcium and other toxic metals such as nickel. The mechanism of such an enhancement is, however, unclear. We speculate that the presence of HS may alter the aqueous uranyl speciation to facilitate microbial reduction. Previous studies suggested that the rate and extent of microbial reduction of U(VI) was much slower in solutions dominated by a ternary uranyl-calcium-carbonate species than a binary uranyl-carbonate species. The inclusion of HS in solutions may shift the dominance of uranyl-calcium-carbonate toward uranyl-carbonate by competitively forming calcium-HS complexes.

The time-resolved, laser-induced fluorescence spectroscopy (TRLIFS) technique appears to be particularly suited for in situ determination of uranyl species and could therefore aid our interpretation of our experimental observations. TRLIFS is especially useful for providing information regarding the coordination and complexation of U(VI) with HS or other aqueous species and may therefore serve as a structural probe for the determination of the U(VI) complexes and species in solutions. The technique is non-destructive and can be performed in situ either at room temperature or cooled in liquid He for enhanced signal to noise ratio (due to suppressed matrix quenching). Experimentally, the time gate could be set at about 50 ns up to 1 s to offset the strong fluorescence or interferences of HS (because of its short lifetime). On the other hand, the He-cooled U(VI) give a much longer lifetime on the order of hundreds of microseconds. Therefore, spectra of various U(VI) species in solution could be readily obtained.

Project Details

Project type
Exploratory Research
Start Date
2004-04-01
End Date
2005-04-15
Status
Closed

Team

Principal Investigator

Baohua Gu
Institution
Oak Ridge National Laboratory

Team Members

Chongxuan Liu
Institution
Pacific Northwest National Laboratory