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Uranium Immobilization in an Iron-Rich Rhizosphere of a Native Wetland Plant from the Savannah River Site under Reducing Conditions
Citation:
Chang, H., S. Buettner, J. Seaman, P. Jaffé, P. Koster van Groos, D. Li, A. Peacock, K. Scheckel, AND D. Kaplan. Uranium Immobilization in an Iron-Rich Rhizosphere of a Native Wetland Plant from the Savannah River Site under Reducing Conditions. J. Schnoor (ed.), ENVIRONMENTAL SCIENCE & TECHNOLOGY. ACS Publications, Washington, DC, 48(16):9270-9278, (2014).
Impact/Purpose:
The biogeochemistry of U in wetlands is expected to be profoundly different than in uplands because of the presence of sharp geochemical gradients (e.g., pH, DO), elevated organic carbon concentrations and microbial activity, and the transient nature of hydraulic regimes (e.g., flooding event and drought conditions).1-4 These conditions, caused by the interface between relatively anoxic groundwater and oxic surface water, greatly affect the mobility of the U as well as other redox sensitive metals, for example, Fe. Generally, the oxidized form, U(VI), is more mobile than the reduced form, U(IV), because of the higher solubility of the former.5 Thus, remediation of U contaminated sites has focused on the reduction of U(VI) to U(IV) through various mechanisms.6-9 However, reduced U can be readily reoxidized to U(VI) as wetlands undergo various natural events, for example, seasonal drying or receive large amounts of oxygenated rain.10,11 Therefore, maintaining the stability of reduced U in wetland environments is challenging as a long-term U remediation strategy. Currently, there is a knowledge gap in our understanding of how Fe redox cycling in wetland rhizosphere influences U immobilization. Thus, the objectives of this research were (1) to confirm the role of plant roots in oxidizing Fe(II) to bioavailable Fe(III) for sustaining microbial Fe-reducing activity, which is a key for biological U(VI) reduction; (2) to evaluate whether plant-induced biogeochemical changes can stimulate the Fe/U reducing microbial activity in rhizosphere; and (3) to investigate the stability of the reduced U in a wetland system. The approach was to transplant naturally occurring wetland plants into a greenhouse microcosm. The plants were then grown under reducing conditions that were favorable for iron plaque formation, and then U(VI) was introduced using steady-state flow conditions.
Description:
The hypothesis of this study was that iron plaque formed on the roots of wetland plants and their rhizospheres create environmental conditions favorable for iron reducing bacteria that promote the in situ immobilization of uranium. Greenhouse microcosm studies were conducted using native plants (Sparganium americanum) from a wetland located on the Savannah River Site, Aiken, SC. After iron plaques were established during a 73-day period by using an anoxic Fe(II)-rich nutrient solution, a U(VI) amended nutrient solution was added to the system for an additional two months. Compared to plant-free control microcosms, microcosms containing iron plaques successfully stimulated the growth of targeted iron reducing bacteria, Geobacter spp. Their population continuously increased after the introduction of the U(VI) nutrient solution. The reduction of some of the U(VI) to U(IV) by iron reducing bacteria was deduced based on the observations that the aqueous Fe(II) concentrations increased while the U(VI) concentrations decreased. The Fe(II) produced by the iron reducing bacteria was assumed to be reoxidized by the oxygen released from the roots. Advanced spectroscopic analyses revealed that a significant fraction of the U(VI) had been reduced to U(IV) and they were commonly deposited in association with phosphorus on the iron plaque.
