Impact of iron and redox chemistry on the environmental fate and transport of metalloids and radionuclides
Date
2014
Authors
Troyer, Lyndsay D., author
Borch, Thomas, advisor
Ladanyi, Branka M., committee member
Levinger, Nancy E., committee member
Henry, Charles S., committee member
Kelly, Eugene F., committee member
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Abstract
Millions of cubic meters of uranium (U) mine tailings worldwide and millions of gallons of contaminated groundwater are the result of U mining and milling activity. Arsenic can occur at up to 10 weight percent in U ore, so both U and As can be released during U mining. Although these elements commonly occur together, little research into their redox behavior when present in the same environmental system has been performed. The goal of this research is to gain an improved understanding of how redox chemistry affects U and As speciation and complexation when the two elements are present together as co-contaminants. The North Cave Hills in Harding County, South Dakota is an abandoned U mine where overburden has been left open to weathering and transport since mining began in 1955. The exposed overburden has resulted in above-background level concentrations of U and As in sediments and groundwater in the surrounding wetlands. We conducted a field-scale study to investigate U and As redox chemistry at the North Cave Hills by taking sediment samples from the tailings pile and the down gradient watershed in order to assess U and As fate and transport. As sediments pass through anoxic zones at the field site, U is immobilized as reduction takes place but As can simultaneously be released into surface waters as reductive dissolution of Fe minerals also occurs. A laboratory-based study was conducted in order to examine the redox chemistry of U and As in North Cave Hills sediments under controlled conditions. Upon microbial reduction of sulfate and formation of mackinawite in batch systems, U(VI) and As(V) were reduced to nano- UO2 and a reduced As-sulfide mineral phase respectively during biostimulation by three different electron donors. When these systems were exposed to air for 24 hours, mackinawite protected U and As from oxidation and little change in their solid-phase speciation was observed. While mackinawite was shown to play a role in reduction, we could not determine if direct microbial reduction of U and As was also taking place in the systems. In order to further explore the reduction of U(VI) and As(V) by mackinawite, an experiment was set up to determine if As(V) prevented U(VI) reduction, especially following the formation of uranyl arsenate precipitates. As(V) only had an impact on the extent of U reduction at concentrations higher than would occur in most environmental systems. When As(V) concentrations were high, U(VI) was shown to be resistant to reduction because of the precipitation of a uranyl arsenate mineral phase. The findings in this dissertation contribute important information that will improve our current understanding of U and As redox behavior that will lead to improved remediation strategies to effectively prevent the mobilization of both elements in environmental systems.
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Subject
redox
mining
arsenic
tailings
uranium