Department of Environmental & Earth System Science, Stanford University, Stanford, California 94305, United States.
Environ Sci Technol. 2011 Jul 1;45(13):5572-9. doi: 10.1021/es200351m. Epub 2011 Jun 7.
The speciation and partitioning of arsenic (As) in surface and subsurface environments are controlled, in part, by redox processes. Within soils and sediments, redox gradients resulting from mass transfer limitations lead to competitive reduction-oxidation reactions that drive the fate of As. Accordingly, the objective of this study was to determine the fate and redox cycling of As at the interface of birnessite (a strong oxidant in soil with a nominal formula of MnO(x), where x ≈ 2) and dissimilatory As(V)-reducing bacteria (strong reductant). Here, we investigate As reduction-oxidation dynamics in a diffusively controlled system using a Donnan reactor where birnessite and Shewanella sp. ANA-3 are isolated by a semipermeable membrane through which As migrates. Arsenic(III) injected into the reaction cell containing birnessite is rapidly oxidized to As(V). Arsenic(V) diffusing into the Shewanella chamber is then reduced to As(III), which subsequently diffuses back to the birnessite chamber, undergoing oxidation, and establishing a continuous cycling of As. However, we observe a rapid decline in the rate of As(III) oxidation owing to passivation of the birnessite surface. Modeling and experimental results show that high [Mn(II)] combined with increasing [CO(3)(2-)] from microbial respiration leads to the precipitation of rhodochrosite, which eventually passivates the Mn oxide surface, inhibiting further As(III) oxidation. Our results show that despite the initial capacity of birnessite to rapidly oxidize As(III), the synergistic effect of intense As(V) reduction by microorganisms and the buildup of reactive metabolites capable of passivating reactive mineral surfaces-here, birnessite-will produce (bio)geochemical conditions outside of those based on thermodynamic predictions.
砷（As）在地表和地下环境中的形态和分配部分受氧化还原过程控制。在土壤和沉积物中，由于物质转移限制导致的氧化还原梯度引起了竞争的还原-氧化反应，从而驱动了 As 的命运。因此，本研究的目的是确定在锰矿（土壤中的强氧化剂，其标称分子式为 MnO(x)，其中 x ≈ 2）和异化砷（V）还原细菌（强还原剂）界面处 As 的命运和氧化还原循环。在这里，我们在扩散控制体系中使用 Donnan 反应器研究 As 氧化还原动力学，其中锰矿和希瓦氏菌 ANA-3 通过半透膜分离，As 可以通过半透膜迁移。将注入含有锰矿的反应池中的砷（III）迅速氧化为砷（V）。扩散到希瓦氏菌腔室的砷（V）随后被还原为砷（III），随后扩散回锰矿腔室，经历氧化，并建立 As 的连续循环。然而，我们观察到 As(III)氧化速率的迅速下降，这是由于锰矿表面的钝化。模型和实验结果表明，高[Mn(II)]与微生物呼吸产生的[CO(3)(2-)]结合导致菱锰矿的沉淀，最终钝化 Mn 氧化物表面，抑制进一步的 As(III)氧化。我们的结果表明，尽管锰矿最初具有快速氧化 As(III)的能力，但微生物强烈还原 As(V)和反应性代谢物的积累的协同效应能够钝化反应性矿物表面（此处为锰矿），从而产生超出基于热力学预测的（生物）地球化学条件。
