Alkalinity Enhanced Mass Reduction of Uranium Contaminated Sediment

Abstract

Uranium is a toxic and radioactive metal that is naturally occurring in the Earth’s crust. The uranium mining boom of the 1950’s produced many contaminated aquifers throughout the United States. Monitoring of groundwater data suggests that uranium concentrations may take 100’s of years to naturally attenuate to below the maximum contamination limit. This project examines a potential and novel remediation technique of groundwater contaminated with uranium through alkalinity enhanced desorption from the solid phase. Alkalinity enhances the desorption of uranium by complexing uranyl (UO22+) with Ca and CO3 which allows for further desorption. Since remediation is time consuming and expensive, it is vital to understand the mechanisms of uranium transport to enhance the effectiveness of remediation strategies.This study conducted six 1-dimensional flow through experiments using sediment from a contaminated aquifer near Riverton, Wyoming. The columns tests were completed to address four scientific questions: 1) What effect does increasing alkalinity have on the mass transport of uranium? 2) Does increasing alkalinity decrease the flushing time of uranium? 3) What is the dominant mechanism of uranium mobility at the study site 4) Is increasing alkalinity to enhance uranium mobility applicable at other contaminated aquifers? Results indicated that: 1) There is an increase in uranium mass flushed when alkalinity is added to the influent. 2) Increasing alkalinity decreased the flushing time at the field scale but the decrease in flushing time should be viewed as an upper bound. 3) The dominant mechanism of uranium mobility at the site is desorption but where Ca-SO4 minerals are present, dissolution dominates uranium mobility. 4) Under certain conditions alkalinity enhanced desorption is a viable remediation approach. Laboratory based column experiments have shown that alkalinity enhanced flushing could be a promising remediation approach for contaminated aquifers. Field scale flushing is relatively untested and contains large limitations. Future field scale experiments and geochemical modeling need to be completed before alkalinity enhanced flushing of uranium can be considered a viable remediation approach.