Uranium Biogeochemistry in An Iron-Rich Rhizosphere of a Native Wetland Plant Under Reducing Conditions.

Discharge of contaminated groundwater to surface water through wetlands is one of the concerns of many DOE sites including the Savannah River Site (SRS). During the transport through the wetlands, numerous biogeochemical reactions promote profound effects on the fate of redox-sensitive trace metals and radionuclides, such as Fe and U. The biogeochemical reactions are also strongly affected by the activities of microorganism communities and plants. Thus, this research is intended to provide new insights on how root-induced organic matter impacts on the biogeochemical processes and microorganisms in iron-rich rhizosphere.

A series of greenhouse microcosm studies was conducted with a native plant from the SRS wetlands, Sparganium americanum. A nutrient solution containing high concentration of Fe(II) was provided prior to the introduction of U(VI) solution to develop iron plaque on the root and a desired microbial communities, including Geobacter spp.. After the 160 days of experiment, formation of iron plaque was observed as reddish mineral deposition on the root surface and the rhizosphere. Characterization of microorganism communities using q-PCR showed that the greatest microbial populations were generally identified on and near the roots. The target microorganisms were only active with the plant root, and their populations were increased after U(VI) addition.

Mineralogical characteristics and uranium speciation in the rhizosphere were investigated using microwave assisted digestion, CBD extraction, and XAFS analysis of the root and the soil near root. The results revealed that the concentration of U and Fe of the root was orders of magnitude higher than the soil samples. The comparison of digestion and extraction data suggested that the removed U was associated with the extractable Fe mineral on the root, which may have an amorphous (less crystalline) structure. According to the initial XANES analysis, the U on the root was the mixture of U(VI) and U(IV).