Brodie, Eoin

Firestone, Mary

Hazen, Terry

Herman, Don

Larsen, Joern

Tokunaga, Tetsu

Wan, Jiamin

Zheng, Zuoping

eScholarship - University of California

Lawrence Berkeley National LaboratoryRecent WorkTitleMesoscale Coupled Transport and Biogeochemical Effects on Reduction of U(VI) and NO3- as Co-contaminants in Natural Sediments and SoilsPermalinkhttps://escholarship.org/uc/item/8gc26752AuthorsWan, JiaminTokunaga, TetsuHazen, Terryet al.Publication Date2003-12-01eScholarship.org Powered by the California Digital LibraryUniversity of CaliforniaProgram Element 1: Biotransformation   20Mesoscale Coupled Transport and Biogeochemical Effects  on Reduction of U(VI) and NO3– as Co-contaminants in Natural  Sediments and Soils Jiamin Wan1 (PI), Tetsu Tokunaga1, Terry Hazen1, Mary Firestone1,2, Zuoping Zheng1, Eoin Brodie2,  Joern Larsen1, Don Herman2 1Lawrence Berkeley National Laboratory, Berkeley, CA; 2University of California, Berkeley, CA Since fluids in subsurface environments are not well mixed, even at relatively small scales, the sub-surface can be categorized into advective and diffusion-limited domains. Information is lacking on local, diffusion-limited processes that arise from small-scale variations in permeability, preferential flow paths, and soil/sediment structure. The primary objective of this research is to study coupled transport and biogeochemistry of U(VI) and NO3–.The redox reactions of these co-occurring contaminants are in-vestigated in realistically heterogeneous model systems. We emphasize direct measurements within dif-fusion-limited domains at the mesoscale (about 10–4 to 10–1 m). It is within these diffusion-limited do-mains that large gradients in microbial activity, chemical potentials, reaction rates, and transport rates can coexist. Thus, the dynamics at this mesoscale can control redox-dependent biotransformations in na-ture. Diffusion-limited U biogeochemical transformations. Measurements include determinations of U oxidation state (micro-XANES spectroscopy), soluble U, redox potential profiles, microbial activity, and microbial communities. Strong retardation of the U(VI) diffusion front in initially reducing sedi-ments has been found to persist even after nearly two years without resupply of organic carbon. In oxi-dizing soils, U(VI) diffusion was relatively rapid under both acidic and slightly alkaline conditions be-cause of relatively weak sorption. The strength of U(VI) sorption can be orders of magnitude less at high concentrations typical of initial waste sources.  U(VI) sorption in soils containing calcium carbonate. Calcite is a common component in slightly alkaline soils and sediments. However, the impact of calcite on U(VI) sorption in whole soil systems (as opposed to model systems comprised of single mineral phases) has not previously been evaluated. We found substantial suppression of U(VI) sorption at neutral to slightly alkaline pH, and showed that the depressed level of U(VI) sorption is fairly well accounted for by the stability of aqueous Ca2UO2(CO3)3, identified by Bernhard et al. [1]. Long-term stability of in situ microbially reduced U(IV). Area 2 sediments containing 200 ppm U, initially primarily as U(VI), were packed into columns and infused with lactate solutions to acceler-ate reduction. Supporting measurements included analyses of U concentrations in effluents, in situ de-termination of sediment U(VI):U(IV), redox potential profiles, pore water chemical analyses, microbial activity, and microbial community structure. Nearly complete U reduction was achieved by about 90 days, with effluent U concentrations decreased from initial values in excess of 1,000 ppb down to less than 5 ppb. However, continued organic carbon infusion and monitoring (over 500 days) revealed sub-stantial U remobilization, with later effluent U concentrations exceeding 300 to 500 ppb. Net U reoxida-tion within the soil columns was supported by micro-XANES analyses. The remobilization of U was at-tributed to in-situ production of biogenic carbonate which shifts equilibrium towards stable U(VI) car-bonate species. A shift in bacterial community structure was observed in the re-oxidation stage (relative to the earlier reducing stage). Thus, in some cases, reduction-based U immobilization can be transient and unreliable. This result also demonstrates the need for experiments that address long-term stability of in situ strategies for remediating actinide and metal contaminants. 1. G. Bernhard, G. Geipel, T. Reich, V. Brendler, S. Amayri, H. Nitsche. 2001. Radiochim. Acta  89, 511.  

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Mesoscale Coupled Transport and Biogeochemical Effects on Reduction of U(VI) and NO3- as Co-contaminants in Natural Sediments and Soils

https://escholarship.org/uc/item/8gc26752

Mesoscale Coupled Transport and Biogeochemical Effects on Reduction of U(VI) and NO3- as Co-contaminants in Natural Sediments and Soils

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