Iron Mineralogy and Uranium-Binding Environment in the Rhizosphere of a Wetland Soil
Kaplan, D., R. Kukkadapu, J. Seaman, B. Arey, A. Dohnalkova, S. Buettner, D. Li, T. Varga, K. Scheckel, AND P. Jaffe. Iron Mineralogy and Uranium-Binding Environment in the Rhizosphere of a Wetland Soil. D. Barceló (ed.), SCIENCE OF THE TOTAL ENVIRONMENT. Elsevier BV, AMSTERDAM, Netherlands, 569-570:53-64, (2016).
Uranium migration has been significantly attenuated in the Tims Branch wetland on the Savannah River Site (SRS) in South Carolina, a nuclear processing facility. Of the 44 metric tons of depleted U introduced between 1954 and 1985 into the Tims Branch system, 70% remains in the wetland. Based on XAFS and sequential extraction characterization, most of the wetland U exists in association with OM. Furthermore, while this site is moderately acidic (pH ~5.5) and has microbes that have the potential to promote U reduction (including Geobacter spp. and sulfur reducing bacteria) the U at the site exists almost exclusively as U(VI). Greenhouse mesocosm studies, simulating Tims Branch conditions, showed that U concentrations near the root were 3 x greater than these in root-free soils. Furthermore, the rhizosphere had a distinct color difference compared to the bulk soil; it was brick-red, whereas the bulk soil was either brown or yellowish white. The red coloration originated from Fe(III)-mineral formation. The objective of this study was to build upon results from mesocosm studies and to conduct a field investigation to determine how rhizosphere and non-rhizosphere soil differ in terms of mineralogy and geochemistry that might influence uranium binding. Our hypothesis was that wetland plant roots contribute OM and release O2 within the rhizosphere that promote the formation of Fe(III)-(oxyhydr)oxides. In turn, these Fe(III)-(oxyhydr)oxides stabilize organic matter that together contribute to contaminant immobilization. The general approach was to collect soils containing roots from the Tims Branch wetland and characterize subsamples designated as near (rhizosphere) and far (non-rhizosphere) from the roots. The samples were characterized using wet chemistry, and various types of spectroscopy and microscopy. Additionally, soil porewater samples were collected from depth-discrete diffusion samplers to provide information about the aqueous chemical conditions at the study site.
Wetlands mitigate the migration of groundwater contaminants through a series of biogeochemical gradients that enhance multiple contaminant-binding processes. The hypothesis of this study was that wetland plant roots contribute organic carbon and release O2 within the rhizosphere (plant-impact soil zone) that promote the formation of Fe(III)-(oxyhydr)oxides. In turn, these Fe(III)-(oxyhydr)oxides stabilize organic matter that together contribute to contaminant immobilization. Mineralogy and U binding environments of the rhizosphere were evaluated in samples collected from contaminated and non-contaminated areas of a wetland on the Savannah River Site in South Carolina. Based on Mössbauer spectroscopy, rhizosphere soil was greatly enriched with nanogoethite, ferrihydrite-like nanoparticulates, and hematite, with negligible Fe(II) present. X-ray computed tomography and various microscopy techniques showed that root plaques were tens-of-microns thick and consisted of highly oriented Fe-nanoparticles, suggesting that the roots were involved in creating the biogeochemical conditions conducive to the nanoparticle formation. XAS showed that a majority of the U in the bulk wetland soil was in the +6 oxidation state and was not well correlated spatially to Fe concentrations. SEM/EDS confirm that U was enriched on root plaques, where it was always found in association with P. Together these findings support our hypothesis and suggest that plants can alter mineralogical conditions that may be conducive to contaminant immobilization in wetlands.