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Effect of biogeochemical redox processes on the fate and transport of As and U at an abandoned uranium mine site: an X-ray absorption spectroscopy study

Lyndsay D. Troyer A , James J. Stone B and Thomas Borch A C D
+ Author Affiliations
- Author Affiliations

A Department of Chemistry, Colorado State University, 1872 Campus Delivery, Fort Collins, CO 80523, USA.

B Department of Civil and Environmental Engineering South Dakota School of Mines and Technology, 501 East Saint Joseph Street, Rapid City, SD 57701, USA.

C Present address: Department of Soil and Crop Sciences, Colorado State University, 1170 Campus Delivery, Fort Collins, CO 80523, USA.

D Corresponding author. Email: borch@colostate.edu

Environmental Chemistry 11(1) 18-27 https://doi.org/10.1071/EN13129
Submitted: 15 July 2013  Accepted: 30 September 2013   Published: 28 January 2014

Environmental context. Uranium and arsenic, two elements of human health concern, are commonly found at sites of uranium mining, but little is known about processes influencing their environmental behaviour. Here we focus on understanding the chemical and physical processes controlling uranium and arsenic transport at an abandoned uranium mine. We find that the use of sedimentation ponds limits the mobility of uranium; however, pond conditions at our site resulted in arsenic mobilisation. Our findings will help optimise restoration strategies for mine tailings.

Abstract. Although As can occur in U ore at concentrations up to 10 wt-%, the fate and transport of both U and As at U mine tailings have not been previously investigated at a watershed scale. The major objective of this study was to determine primary chemical and physical processes contributing to transport of both U and As to a down gradient watershed at an abandoned U mine site in South Dakota. Uranium is primarily transported by erosion at the site, based on decreasing concentrations in sediment with distance from the tailings. Sequential extractions and U X-ray absorption near-edge fine structure (XANES) fitting indicate that U is immobilised in a near-source sedimentation pond both by prevention of sediment transport and by reduction of UVI to UIV. In contrast to U, subsequent release of As to the watershed takes place from the pond partially due to reductive dissolution of Fe oxy(hydr)oxides. However, As is immobilised by adsorption to clays and Fe oxy(hydr)oxides in oxic zones and by formation of As–sulfide mineral phases in anoxic zones down gradient, indicated by sequential extractions and As XANES fitting. This study indicates that As should be considered during restoration of uranium mine sites in order to prevent transport.


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