Multicomponent reactive transport modeling of uranium bioremediation field experiments

摘要

A reaction network integrating abiotic and microbially mediated reactions has been developed to simulate biostimulationfield experiments at a former Uranium Mill Tailings Remedial Action (UMTRA) site in Rifle, Colorado. The reaction net-work was calibrated using data from the 2002 field experiment, after which it was applied without additional calibrationto field experiments performed in 2003 and 2007. The robustness of the model specification is significant in that (1) the2003 biostimulation field experiment was performed with 3 times higher acetate concentrations than the previous biostimu-lation in the same field plot (i.e., the 2002 experiment), and (2) the 2007 field experiment was performed in a new unperturbedplot on the same site. The biogeochemical reactive transport simulations accounted for four terminal electron-accepting pro-cesses (TEAPs), two distinct functional microbial populations, two pools of bioavailable Fe(III) minerals (iron oxides andphyllosilicate iron), uranium aqueous and surface complexation, mineral precipitation and dissolution. The conceptual modelfor bioavailable iron reflects recent laboratory studies with sediments from the UMTRA site that demonstrated that the bulk(90%) of initial Fe(III) bioreduction is associated with phyllosilicate rather than oxide forms of iron. The uranium reactionnetwork includes a U(VI) surface complexation model based on laboratory studies with Rifle site sediments and aqueous com-plexation reactions that include ternary complexes (e.g., calcium—uranyl—carbonate). The bioreduced U(IV), Fe(II), and sul-fide components produced during the experiments are strongly associated with the solid phases and may play an importantrole in long-term uranium immobilization.