QUANTITATIVELY DESCRIBING AND PREDICTING ELECTRON BALANCE BETWEEN COMPETITIVE IRON AND URANIUM REDUCTION

摘要

Mining and nuclear enrichment activities throughout the last century have created a legacy of uranium and co-contaminants in the environment. In many cases, uranium, a contaminant of concern due to its toxic effects and radioactivity, has migrated from waste disposal sites to surrounding soils and sediments where its fate is often controlled by redox processes. In its oxidized form, U(VI), uranium exists as the relatively mobile uranyl ion (UO_2~(2+)). Conversely, under reducing conditions, uranium resides in a lower oxidation state, U(IV) and commonly forms relatively insoluble solid phases such as uraninite, (UO2).The process of U(VI) reduction and concurrent immobilization is often facilitated by dissimilatory metal-reducing bacteria (DMRB) that couple the oxidation of carbon or H2 with the reduction of oxidized metals. In soils and sediments subject to anaerobic conditions, competing terminal electron acceptors (TEA), such as nitrate and ferric iron (the latter in the form of Fe(III) (hydr)oxides), can diminish both the rate and extent of microbial U(VI) reduction. Ferrihydrite has the potential to compete with uranium as an electron acceptor in microbial respiration due to the overlapping redox couples of iron and uranium. Wielinga, et al. (2000) for example, observed a 52% decrease in uranyl reduction by Shewanella alga, strain BrY, in the presence of ferrihydrite, compared to only uranyl in solution.