Jeon and four co-authors reported in 2005 that abiotic reduction of 0.1 mM U(VI) by Fe(II) in the presence of synthetic iron oxides (biogenic magnetite, goethite, and hematite) and natural Fe(III) oxide-containing solids was investigated in pH 6.8 artificial groundwater containing 10 mM NaHCO3. In most experiments, more than 95% of added U(VI) was sorbed to solids. U(VI) was rapidly and extensively (g80%) reduced in the presence of synthetic Fe- (III) oxides and highly Fe(III) oxide-enriched (18-35 wt % Fe) Atlantic coastal plain sediments. In contrast, long-term (20-60 d) U(VI) reduction was less than 30% in suspensions of six other natural solids with relatively low Fe(III) oxide content (1-5 wt % Fe).
Fe(II) sorption site density was several fold lower on these natural solids (0.2- 1.1 Fe(II) nm-2) compared to the synthetic Fe(III) oxides (1.6- 3.2 Fe(II) nm-2), which might explain the poor U(VI) reduction in the natural solid-containing systems. Addition of the reduced form of the electron shuttling compound anthrahydroquinone-2,6-disulfonate (AH2DS; final concentration 2.5 mM) to the natural solid suspensions enhanced the rate and extent of U(VI) reduction, suggesting that AH2-DS reduced U(VI) at surface sites where reaction of U(VI) with sorbed Fe(II) was limited.
The authors demonstrated that abiotic, Fe(II)-driven U(VI) reduction is likely to be less efficient in natural soils and sediments than would be inferred from studies with synthetic Fe(III) oxides. The authors documented a major contrast in the rate and long-term extent of abiotic Fe(II)-driven U(VI) reduction between synthetic Fe(III) oxides and natural soils and sediments with relatively low Fe(III) oxide abundance (e.g.,
These results demonstrated that the potential for abiotic Fe(II)-driven U(VI) reduction in natural systems cannot be accurately inferred from studies with synthetic Fe(III) oxides (19). A key implication of the authors' findings is that production of Fe(II)- enriched sediments during one-time (or periodic) stimulation of DMRB activity is not likely to permit efficient long-term abiotic conversion of U(VI) to U(IV) in biogenic redox barriers designed to prevent far-field subsurface U(VI) migration (15).
Instead, their results suggest that ongoing DMRB activity will be required to achieve maximal U(VI) reduction efficiency. Development of a detailed understanding of patterns of DMRB growth, colonization, and maintenance in physically and chemically heterogeneous subsurface environments will therefore be required to predict the effectiveness of sub- surface U(VI) bioremediation operations.
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