Department of Civil and Environmental Engineering, The Pennsylvania State University , University Park, Pennsylvania 16802-1408, United States.
Environ Sci Technol. 2014;48(5):2750-8. doi: 10.1021/es404885e. Epub 2014 Feb 21.
Iron-bearing phyllosilicate minerals help establish the hydrogeological and geochemical conditions of redox transition zones because of their small size, limited hydraulic conductivity, and redox buffering capacity. The bioreduction of soluble U(VI) to sparingly soluble U(IV) can promote the reduction of clay-Fe(III) through valence cycling. The reductive precipitation of U(VI) to uraninite was previously reported to occur only after a substantial percentage of clay-Fe(III) had been reduced. Using improved analytical techniques, we show that concomitant bioreduction of both U(VI) and clay-Fe(III) by Shewanella putrefaciens CN32 can occur. Soluble electron shuttles were previously shown to enhance both the rate and extent of clay-Fe(III) bioreduction. Using extended incubation periods, we show that electron shuttles enhance only the rate of reduction (overcoming a kinetic limitation) and not the final extent of reduction (a thermodynamic limitation). The first 20% of clay-Fe(III) in nontronite NAu-2 was relatively "easy" (i.e., rapid) to bioreduce; the next 15% of clay-Fe(III) was "harder" (i.e., kinetically limited) to bioreduce, and the remaining 65% of clay-Fe(III) was effectively biologically unreducible. In abiotic experiments with NAu-2 and biogenic uraninite, 16.4% of clay-Fe(III) was reduced in the presence of excess uraninite. In abiotic experiments with NAu-2 and reduced anthraquinone 2,6-disulfonate (AH2DS), 18.5-19.1% of clay-Fe(III) was reduced in the presence of excess and variable concentrations of AH2DS. A thermodynamic model based on published values of the nonstandard state reduction potentials at pH 7.0 (E'H) showed that the abiotic reactions between NAu-2 and uraninite had reached an apparent equilibrium. This model also showed that the abiotic reactions between NAu-2 and AH2DS had reached an apparent equilibrium. The final extent of clay-Fe(III) reduction correlated well with the standard state reduction potential at pH 7.0 (E°'H) of all of the reductants used in these experiments (AH2DS, CN32, dithionite, and uraninite).
含铁的层状硅酸盐矿物由于其粒径小、水力传导率低以及氧化还原缓冲能力有限，有助于确定氧化还原过渡带的水文地质和地球化学条件。可溶性 U(VI)通过价态循环被生物还原为不溶性 U(IV)，可以促进粘土-Fe(III)的还原。先前有报道称，只有在大量粘土-Fe(III)被还原后，U(VI)才会被还原为铀矿。利用改进的分析技术，我们表明 Shewanella putrefaciens CN32 可以同时还原 U(VI)和粘土-Fe(III)。先前已经表明，可溶性电子穿梭体可以同时提高粘土-Fe(III)的还原速率和程度。通过延长孵育时间，我们发现电子穿梭体仅提高还原速率（克服动力学限制），而不提高还原程度（热力学限制）。非钠蒙脱石 NAu-2 中的前 20%的粘土-Fe(III)相对容易（即快速）被生物还原；接下来的 15%的粘土-Fe(III)较难（即动力学限制）被还原，其余 65%的粘土-Fe(III)实际上无法被生物还原。在含有过剩铀矿的非钠蒙脱石 NAu-2 和生物成因的铀矿实验中，有 16.4%的粘土-Fe(III)被还原。在含有过剩和可变浓度的蒽醌 2,6-二磺酸钠（AH2DS）的非钠蒙脱石 NAu-2 实验中，有 18.5-19.1%的粘土-Fe(III)被还原。基于 pH 7.0 时非标准状态还原电位（E'H）的发表值，一个热力学模型表明，NAu-2 与铀矿之间的非生物反应已达到表观平衡。该模型还表明，NAu-2 与 AH2DS 之间的非生物反应已达到表观平衡。所有实验中还原剂的标准状态还原电位（E°'H）与最终的粘土-Fe(III)还原程度密切相关（蒽醌 2,6-二磺酸钠、CN32、连二亚硫酸钠和铀矿）。
