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Uranium fate in wetland mesocosms: Effects of plants at two iron loadings with different pH values
Koster van Groos, P., D. Kaplan, H. Chang, J. Seaman, D. Li, A. Peacock, K. Scheckel, AND P. R. Jaffé. Uranium fate in wetland mesocosms: Effects of plants at two iron loadings with different pH values. Jacob de Boer, and Shane Snyder (ed.), CHEMOSPHERE. Elsevier Science Ltd, New York, NY, 163:116-124, (2016).
There are significant uncertainties regarding the environmental fate of uranium (U) and efforts to minimize U exposures require understanding of its mobility in environmental systems. Much research has focused on sequestering U as solids within groundwater aquifers, where localized risks can be controlled.1 Subsurface sequestration limits transport to surface waters and subsequent receptor exposures. However, the fate of U in other systems is important, including wetlands where contaminated groundwater may interface with surface water and associated receptors. For example, groundwater U has entered and in some cases been concentrated in wetlands on the Department of Energy’s Savannah River Site (SRS) in South Carolina.(2,3) Wetlands have been suggested to be locations where U is naturally sequestered, much as other trace metals including chromium and arsenic appear to be.(4–7) We performed experiments examining the mobility of U in wetland mesocosms with particular focus on the rhizosphere of wetland plants, where strong biogeochemical gradients and redox cycling of iron (Fe) are hypothesized to lead to lower U mobility. The hypothesis of this study was that increased Fe cycling in the rhizosphere of wetland plants would reduce U mobility. We describe experiments using small wetland plant mesocosms to test their affects on U mobility and discuss the role of immobilizing mechanisms. The objective of this wetland mesocosm study was to examine U in rhizosphere sediments and in root-excluded sediments.
Small-scale continuous flow wetland mesocosms (~0.8 L) were used to evaluate how plant roots under different iron loadings affect uranium (U) mobility. When significant concentrations of ferrous iron (Fe) were present at circumneutral pH values, U concentrations in root exposed sediments were an order of magnitude greater than concentrations in root excluded sediments. Micro X-ray absorption near-edge structure (µ-XANES) spectroscopy indicated that U was associated with the plant roots primarily as U(VI) or U(V), with limited evidence of U(IV). Micro X-ray fluorescence (µ-XRF) of plant roots suggested that for high iron loading at circumneutral pH, U was co-located with Fe, perhaps co-precipitated with root Fe plaques, while for low iron loading at a pH of ~4 the correlation between U and Fe was not significant, consistent with previous observations of U associated with organic matter. Quantitative PCR analyses indicated that the root exposed sediments also contained elevated numbers of Geobacter spp., which are likely associated with enhanced iron cycling, but may also reduce mobile U(VI) to less mobile U(IV) species.