Neal Andrew L, Amonette James E, Peyton Brent M, Geesey Gill G
Department of Microbiology, Montana State University, Bozeman, Montana 59717-3520, USA.
Environ Sci Technol. 2004 Jun 1;38(11):3019-27. doi: 10.1021/es030648m.
Modeling uranium (U) transport in subsurface environments requires a thorough knowledge of mechanisms likely to restrict its mobility, such as surface complexation, precipitation, and colloid formation. In closed systems, sulfate-reducing bacteria (SRB) such as Desulfovibrio spp. demonstrably affect U immobilization by enzymatic reduction of U(VI) species (primarily the uranyl ion, UO2(2+), and its complexes) to U(IV). However, our understanding of such interactions under chronic U(VI) exposure in dynamic systems is limited. As a first step to understanding such interactions, we performed bioreactor experiments under continuous flow to study the effect of a biofilm of the sulfate-reducing bacterium Desulfovibrio desulfuricans attached to specular hematite (alpha-Fe2O3) surfaces on surface-associated U(VI) complexation, transformation, and mobility. Employing real-time microscopic observation and X-ray photoelectron spectroscopy (XPS), we show that the characteristics of the U(VI) complex(es) formed at the hematite surface are influenced by the composition of the bulk aqueous phase flowing across the surface and bythe presence of surface-associated SRB. The XPS data further suggest higher levels of U associated with hematite surfaces colonized by SRB than with bacteria-free surfaces. Microscopic observations indicate that at least a portion of the U(VI) that accumulates in the presence of the SRB is exterior to the cells, possibly associated with the extracellular biofilm matrix. The U4f7/2 core-region spectrum and U5f2 valence-band spectrum provide preliminary evidence that the SRB-colonized hematite surface accumulates both U(VI) and U(IV) phases, whereas only the U(VI) phase(s) accumulates on uncolonized hematite surfaces. The results suggest that mineral surfaces exposed to a continuously replenished supply of U(VI)-containing aqueous phase will accumulate U phases that may be more representative of those that exist in U-contaminated aquifers than those which accumulate in closed experimental systems. These phases should be considered in models attempting to predict U transport through subsurface environments.
模拟铀（U）在地下环境中的迁移需要全面了解可能限制其迁移率的机制，如表面络合、沉淀和胶体形成。在封闭系统中，诸如脱硫弧菌属等硫酸盐还原菌（SRB）通过将U（VI）物种（主要是铀酰离子UO₂²⁺及其络合物）酶促还原为U（IV），显著影响U的固定化。然而，我们对动态系统中慢性U（VI）暴露下此类相互作用的理解有限。作为理解此类相互作用的第一步，我们在连续流动条件下进行了生物反应器实验，以研究附着在镜面赤铁矿（α-Fe₂O₃）表面的脱硫脱硫弧菌硫酸盐还原菌生物膜对表面相关U（VI）络合、转化和迁移率的影响。通过实时显微镜观察和X射线光电子能谱（XPS），我们表明在赤铁矿表面形成的U（VI）络合物的特征受流经表面的本体水相组成以及表面相关SRB的存在影响。XPS数据进一步表明，与无细菌表面相比，被SRB定殖的赤铁矿表面的U含量更高。显微镜观察表明，在SRB存在下积累的至少一部分U（VI）位于细胞外部，可能与细胞外生物膜基质相关。U4f₇/₂核心区域光谱和U5f₂价带光谱提供了初步证据，表明被SRB定殖的赤铁矿表面同时积累了U（VI）和U（IV）相，而在未被定殖的赤铁矿表面仅积累了U（VI）相。结果表明，暴露于持续补充含U（VI）水相的矿物表面将积累U相，这些U相比在封闭实验系统中积累的U相可能更能代表存在于受U污染含水层中的U相。在试图预测U通过地下环境迁移的模型中应考虑这些相。
