Uranium is a key soil and groundwater contaminant at many U.S. Department of Energy sites and is of great environmental concern due to the toxicological properties of the uranyl species. The behavior of uranium and its mobility in the subsurface is affected by various factors such as pore water and groundwater chemical composition, soil mineralogy, and microorganisms that thrive under these conditions. The addition of tripolyphosphate amendments is one of the methods used to decrease the concentration of soluble uranium in contaminated plumes at Hanford Site. The introduction of sodium tripolyphosphate into uranium-bearing saturated porous media results in the formation of uranyl phosphate solid phases (autunite) of general formula {X1-2[(UO2)(PO4)]2-1●nH2O}, where X is a monovalent or divalent cation. The stability of the uranyl phosphate solids in the subsurface is a critical factor that allows for determining the long-term effectiveness of the sodium tripolyphosphate remediation strategy. Physico-chemical factors such as the presence of bicarbonates, plays an important role in uranium mobility, since bicarbonates form stable uranyl-carbonate complexes at circumneutral conditions. Furthermore, the presence of soil bacteria can affect uranium mobility due to the dissolution of uranyl-phosphate minerals by bacterial communities in an effort to obtain phosphorous for their metabolism. The objective of this research is to investigate autunite dissolution under anaerobic conditions in a range of bicarbonate concentrations and in the presence of the bacterial strain Shewanella oneidensis sp. MR1. Batch experiments were conducted in phosphorus-limiting media amended with Ca-autunite and 0, 3 and 10 mM of bicarbonates. Elemental analysis of supernatant (Ca, U and P) was performed by means of ICP-OES and KPA instruments and cell viability was determined for each bicarbonate concentration used in the experiments. Data plotted as a function of time showed that dissolution of autunite is the driving force of the phenomenon, without excluding the possibility of secondary mineral formation due to the large amount of U, Ca and P released to the aqueous phase. The presence of bicarbonates becomes a liability when phosphates are used as sinks for uranium.

Autunite dissolution in the presence of Shewanella oneidensis in different bicarbonate concentrations under anaerobic conditions

254th National Meeting (2017)

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