Field- and Laboratory-Scale Evaluation of Uranium Sequestration: The Role of Sulfur and Iron Species
Abstract
Over the past decade, field and laboratory studies have revealed the critical role of sulfur and iron species in uranium sequestration. Pilot-scale studies of in-situ U(VI) reduction were conducted at a site adjacent to the former S3 ponds (source zone) of the U.S. Department of Energy Oak Ridge Field Research Center, Oak Ridge, TN. The site contains uranium concentrations up to 800 mg/kg in soil and 250 μM (60 mg/L) in groundwater. In field tests, flushing and pH adjustment decreased aqueous U concentrations by more than 1000 fold from 30-40 to ~1 mg/L. Ethanol addition stimulated microbial reduction of U(VI) and decreased U concentrations below the U.S. EPA maximum contaminant level for drinking water (30 ppb). U(VI) reduction was concomitant with iron(III)- and sulfate respiration. Spectroscopic analyses indicated sequential changes in the solid-associated uranium: U(VI) initially bound to mineral-surface and organic-bound complexes containing phosphate and carbonate, or as hydroxide and phosphate precipitates, was reduced mainly to a U(IV)-Fe complex. The U(IV) was stable under anaerobic conditions, but partially remobilized upon exposure to oxygen. In separate experiments, nitrate was injected into a reduced region of the subsurface containing previously immobilized U(IV). The nitrate was reduced to nitrite, ammonium, and nitrogen gas; sulfide levels decreased; and Fe(II) levels increased then deceased. Re-reduction of oxidized sediments released Fe(II) and soluble U(VI), suggesting that the decrease in soluble U during reoxidation was due to U(VI) sorption to Fe(III) oxides. Follow-up laboratory studies established that both biotically-generated hydrogen sulfide and soluble ferrous iron species reduce U(VI). For a sulfate-reducing bacterium isolated from the Oak Ridge site, microbially-generated sulfide could explain the observed rate of U(VI) reduction. Laboratory studies established that soluble Fe(II) was able to reduce soluble U(VI) at rapid rates when conditions were thermodynamically favorable, resulting in formation of U(IV)-Fe precipitate. Finally, U(IV) formed by sulfate- and iron-reducing enrichments was evaluated for re-oxidation. In a sulfate-reducing enrichment, oxygen oxidized and mobilized U(IV), but less mobilization occurred in the Fe(III)-reducing enrichment, and the mobilized U(VI) was ultimately removed from solution. In three-year batch microcosm experiments, uranium remained immobilized in re-oxidized sediment at pH 6.5-7 and DO 5-6 mg/L. Addition of nitrate did not mobilize uranium. The results suggest that long-term sequestration of uranium is feasible and may be achievable through sequential reduction and oxidation.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.H23I..03C
- Keywords:
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- 0414 BIOGEOSCIENCES / Biogeochemical cycles;
- processes;
- and modeling