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Multiple Influences of Nitrate on Uramium Solubility During Bioremediation of Uranium-contaminated Subsurfac Sediments
Understanding the processes controlling the chemical speciation of radionuclide contaminants is key for predicting their fate and transport in aquatic and terrestrial environments, and is a critical consideration in the design of nuclear waste storage facilities and the development of remediation strategies for management of nuclear legacy sites. The redox processes that influence the chemical speciation, and thus mobility, of Tc, U, Np, and Pu in surface and nearsubsurface environments are reviewed, with a focus on coupled biotic-abiotic reactions driven by microbial activity. A case study of UVI reduction under FeIII- and sulfate-reducing conditions is presented, using a laboratory-based experimental system to simulate potential electron transfer pathways in natural systems. The results suggest that UVI was reduced to nanoparticulate uraninite (UO2) and complexed mononuclear UIV via multiple pathways including direct microbial reduction and coupled biotic-abiotic processes. These results highlight the potential importance of coupled biotic-abiotic processes in determining the speciation and mobility of Tc, U, Np, and Pu in natural and engineered environments
Deterioration in Concrete Pavements Constructed with Slag Coarse Aggregate
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