Date of Award

8-2018

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Environmental Engineering and Earth Sciences

Committee Member

Dr. Stephen Moysey, Committee Co-Chair

Committee Member

Dr. Brian Powell, Committee Co-Chair

Committee Member

Dr. Nicole Martinez

Abstract

Understanding the behaviors of technetium (Tc) during sorption to and desorption from soils amended with titanomagnetite nanoparticles and in redox transition zones is critical for understanding risks from technetium-99 (99Tc) releases into the environment. In this work, experiments were performed under variable redox conditions to evaluate the sorption and desorption behavior of 99Tc in the presence of soil from the Savannah River Site (SRS) and varying concentrations (0.01, 0.1, 1, 10 wt%) of titanomagnetite (Fe3-xTixO4) nanoparticles. Kinetic batch experiments were conducted to investigate how reducing and variable redox conditions and titanomagnetite nanoparticles affect the reduction (sorption) and re-oxidation (desorption) kinetics. There was minimal sorption under oxidizing conditions. Under reducing conditions, there was minimal (<10% sorbed) sorption to solids over 21 days in all percent titanomagnetite systems, except in the 10 wt% titanomagnetite reactors, where there was 50% sorption at 6 hours and 100% sorption at 24 hours. This suggests that reduction of Tc is facilitated by the titanomagnetite nanoparticles above a certain concentration threshold between 1 and 10 wt%. Re-oxidation and desorption of 99Tc occurred relatively rapidly, on the order of hours to days, and the amount of titanomagnetite in the system appeared to have no noticeable effect on the rate of re-oxidation. These findings have important environmental implications about Tc mobility in the environment. It would take Tc weeks to reduce in the subsurface even under reducing conditions, unless there were efficient electron donors, such as titanomagnetite, present in the system. However, it would take relatively short periods of oxidizing conditions for Tc to re-oxidize and become mobile. Experiments were performed under variable redox conditions to evaluate the transport behavior of 99Tc and 99mTc in the presence of soil from the Savannah River Site (SRS) and 1 wt% and 10 wt% titanomagnetite layers. The titanomagnetite was selected for its 99Tc reducing capacity and layer concentrations were selected based on previous batch kinetic work. The experiment was conducted to investigate how redox transitions (reducing to oxidizing conditions) and three concentrations of titanomagnetite nanoparticles (0%, 1%, and 10%) mixed with SRS soil affect the overall mobility of 99Tc and 99mTc as it transitions between mobile Tc(VII) and immobile Tc(IV). A 1D gamma-ray scanner was used to measure 99mTc activity nondestructively within the column and an X-ray computed tomography (CT) imaging system was used to monitor physical processes of NaI transport in situ. These two novel measuring techniques, in addition to measuring fractionated effluent, were used to evaluate the chemical and physical processes of 99Tc and 99mTc transport through the porous media. Tc appeared to behave non-conservatively and became partially and immediately reduced within the column. 1D gamma-ray scanning results revealed 99mTc immobilization in the 1 and 10 wt% titanomagnetite layers, with increased activity in those layers. After switching the system to oxidizing conditions and many displaced pore volumes over the course of weeks, the sorbed 99Tc eventually desorbed and was released from the column, suggesting that the mechanism for 99Tc immobilization was sorption and not incorporation into the crystal structure.

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