Sierra Nevada Research Institute, University of California Merced, 5200 North Lake Road, Merced, California 95343, United States.
Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States.
Environ Sci Technol. 2020 Jul 7;54(13):8010-8021. doi: 10.1021/acs.est.0c01019. Epub 2020 Jun 12.
The long-term stability of U(IV) solid phases in anaerobic aquifers depends upon their reactivity in the presence of oxidizing chemical species and microbial catalysts. We performed flow-through column experiments under anaerobic conditions to investigate the mechanisms and dissolution rates of biogenic, noncrystalline UO(s) by chemical oxidants (nitrate and/or nitrite) or by , a widespread, denitrifying, chemolithoautotrophic model bacterium. Dissolution rates of UO(s) with dissolved nitrite were approximately 5 to 10 times greater than with nitrate alone. In the presence of wild-type and nitrate, UO(s) dissolution rates were similar to those of abiotic experiments with nitrite (from 1.15 × 10 to 4.94 × 10 mol m s). Experiments with a mutant strain defective in U(IV) oxidation supported microbially mediated U(IV) oxidation. X-ray absorption spectroscopy (XAS) analysis of post-reaction solids showed the presence of mononuclear U(VI) species rather than a solid U(VI) phase. At steady-state U release, kinetic and spectroscopic results suggest detachment of oxidized U(VI) from the UO(s) surface as the rate-determining step rather than electron transfer or ion diffusion. Under anaerobic conditions, production of nitrite by nitrate-reducing microorganisms and enzymatically catalyzed, nitrate-dependent U(IV) oxidation are likely dual processes by which reduced U solids may be oxidized and mobilized in the aqueous phase.
在厌氧含水层中，U(IV)固相的长期稳定性取决于它们在氧化化学物质和微生物催化剂存在下的反应性。我们在厌氧条件下进行了流动柱实验，以研究生物成因、非晶 UO(s)在化学氧化剂（硝酸盐和/或亚硝酸盐）或一种广泛存在的、反硝化、化能自养模式细菌的作用下的溶解机制和溶解速率。UO(s)在溶解亚硝酸盐存在下的溶解速率比单独使用硝酸盐时大约快 5 到 10 倍。在野生型 和硝酸盐存在的情况下，UO(s)的溶解速率与含有亚硝酸盐的非生物实验相似（从 1.15×10 到 4.94×10 mol m s）。具有 U(IV)氧化缺陷的 突变体菌株的实验支持微生物介导的 U(IV)氧化。反应后固体的 X 射线吸收光谱（XAS）分析表明存在单核 U(VI)物种，而不是固态 U(VI)相。在稳定状态下 U 的释放时，动力学和光谱结果表明，氧化 U(VI)从 UO(s)表面的脱离是速率决定步骤，而不是电子转移或离子扩散。在厌氧条件下，硝酸盐还原微生物产生的亚硝酸盐和酶催化的、依赖硝酸盐的 U(IV)氧化可能是还原 U 固体在水相中的氧化和迁移的双重过程。
