Microbiological reduction of soluble U(VI) to insoluble U(IV) has been proposed as a remediation strategy for uranium-contaminated groundwater. Nitrate is a common co-contaminant with uranium. Nitrate inhibited U(VI) reduction in acetate-amended aquifer sediments collected from a uranium-contaminated site in New Mexico. Once nitrate was depleted, both U(VI) and Fe(III) were reduced concurrently. When nitrate was added to sediments in which U(VI) had been reduced, U(VI) reappeared in solution. Parallel studies with the dissimilatory Fe(III)-, U(VI)- and nitrate-reducing microorganism, Geobacter metallireducens, demonstrated that nitrate inhibited reduction of Fe(III) and U(VI) in cell suspensions of cells that had been grown with nitrate as the electron acceptor, but not in Fe(III)-grown cells. Suspensions of nitrate-grown G. metallireducens oxidized Fe(II) and U(IV) with nitrate as the electron acceptor. U(IV) oxidation was accelerated when Fe(II) was also added, presumably due to the Fe(III) being formed abiotically oxidizing U(IV). These studies demonstrate that although the presence of nitrate is not likely to be an impediment to the bioremediation of uranium contamination with microbial U(VI) reduction, it is necessary to reduce nitrate before U(VI) can be reduced. These results also suggest that anaerobic oxidation of U(IV) to U(VI) with nitrate serving as the electron acceptor may provide a novel strategy for solubilizing and extracting microbial U(IV) precipitates from the subsurface.
In Michigan, sections of an Interstate-type pavement are suffering extensive cracking and joint deterioration after 10 years of service, having been constructed in 1992. An adjacent section constructed in 1993 with comparable design features and materials remains in good condition, with little visual sign of distress. A study was conducted to determine, if possible, the cause of the observed distress in the highway built in 1992. In all, cores from nine different projects were evaluated, all of which were made with iron blast-furnace-slag coarse aggregate and natural fine aggregate containing chert constituents. The analyses conducted included stereo and petrographic microscopy and chemical extractions to determine levels of exchangeable and soluble potassium and sodium, as well as sulfates. The findings indicate that, in distressed pavement sections, the chert constituents in the fine aggregate are deleteriously alkali–silica reactive (ASR), whereas these same constituents are not deleterious in the sections rated as fair. Further, the distressed sections all had sulfate levels significantly higher than predicted by the mixture design. It is hypothesized that, in addition to the ASR in the fine aggregate, dissolution of the calcium sulfide dendrites in the slag coarse aggregate is providing excess internal sulfates, resulting in in-filling of the air-void system with ettringite and potentially sulfate attack. The exact nature of the deterioration mechanisms is not fully understood, but it seems clear that some type of interaction exists between the ASR and excess sulfates.
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