POWRE: The Kinetics of Microbially-Mediated Reduction of Fe(III), As(V), and S(VI): A Laboratory Study of Non-Steady-State Conditions in Anoxic Reservoir Sediments

砷在淡水水环境中的滞留和流动是一个非常令人担忧的问题，因为它对植物、动物和人类健康都有毒性影响。在过去，砷被称为环境问题的“四大”金属之一(其他三种分别是铅、镉和汞)。砷通过吸附在矿物表面，在高氧化程度下以金属砷的形式沉淀，在还原条件下以硫化物的形式保留在水生沉积物中。由于砷酸盐As(V)的流动性小于As(III)，因此，砷的还原为亚砷酸盐是增加砷流动性的主要机制。对砷迁移的预测依赖于热力学平衡假设。这是一个很大的问题，因为人们经常观察到亚砷酸盐和亚砷酸盐的分布不平衡。缓慢的反应动力学是造成这种不平衡的合理原因，但人们对反应速度和反应机理了解很少。这项拟议的实验室研究将研究高砷含量的缺氧沉积物中砷的还原。它将检验以下假设：1)As(V)通过微生物呼吸发生还原；2)As(V)在铁相还原溶解过程中从氢氧化铁中吸附出来，但可能被重新吸收到其他底物；3)微生物呼吸中电子受体的顺序使用遵循热力学预测；As(III)在溶液中的形态与硫化物密切相关，As(III)(溶液中沉淀或络合)的去向取决于硫酸盐的总浓度以及砷和硫酸盐的相对还原速率。这项技术通常用于研究营养限制对微生物生长和种群间竞争的影响；然而，在本方案中，恒化器将用于模拟水、营养物质和溶质通过沉淀物包的连续流动。这项技术的优点是，沉积核心固有的细菌联合体可以根据其沿着PE梯度递减的不同电子受体的顺序使用，在恒化器的连接的血管中空间排列自己。因此，这种设置将密切模拟微生物种群随深度的变化，就像在厌氧沉积物中发现的那样。此外，它还提供了在微生物种群确定的PE条件下对每个相连的微观世界中发生的化学过程进行采样和观察的机会。类似的流动反应器已被用于研究无机体系中的反应动力学，在无机体系中，入口溶液的化学经常被操纵以研究反应器对变化和非稳态条件的响应。类似的技术将被应用于恒化器系统，以观察溶液化学(pH、营养物质以及Fe、As和S的总浓度)的变化对微生物介导的Fe(III)、As(V)和S(VI)还原动力学的影响。

9806121StillingsArsenic retention and mobility in freshwater aquatic environments is a great concern because of its toxic effects on plants, animals and human health. In the past, arsenic has been referred to as one of the "Big Four" metals of environmental concern (the others being lead, cadmium and mercury). Arsenic is retained in aquatic sediments by adsorption at mineral surfaces, by precipitation as metal arsenate's at high oxidation levels, and as arsenic sulfides under reduced conditions. Because arsenate, As(V), is less mobile than arsenite, As(III), the reduction of arsenate to arsenite is a primary mechanism for increasing arsenic mobility.Predictions of arsenic mobility rely on assumptions of thermodynamic equilibria. This is a large problem because the distribution of arsenate and arsenite are often observed to be out of equilibrium. Slow reaction kinetics are a logical reason for the disequilibria, yet reaction rates and mechanisms are poorly understood.This proposed laboratory study will investigate arsenic reduction in anoxic sediments with a high arsenic content. It will test the following hypotheses: 1) As(V) reduction occurs through microbial respiration; 2) adsorbed As(V) is released from ferric oxyhydroxides during reductive dissolution of the iron phase, yet may be reabsorbed to other substrates; 3) the sequential use of electron acceptors for microbial respiation follows thermodynamic predictions; and 4) the appearance of As(III) in solution is closely tied to sulfide, and the fate of As(III) (precipitated or complexed in solution) is determined by the total concentration of sulfate, and the relative rates of arsenate and sulfate reduction.A multistage chemostat will be employed to investigate reduction rates of Fe(III), As(V), and S(VI) in sediment cores. This technique is often used in studies on the effects of nutrient limitation on microbial growth and competition between populations; however, in this proposal, the chemostat will be used to simulate the continuous flux of water, nutrients, and solutes through a sediment package. The advantage to this technique is that consortium of bacteria, indigenous to the sediment core, can spatially arrange itself among the linked vessels of the chemostat, according to its sequential use of various electron acceptors along a decreasing PE gradient. Thus this setup will closely mimic changes in the microbial population with depth, as found in an anaerobic sediment. Also, it provides an opportunity to sample and observe chemical processes occurring within each linked microcosm, at pe conditions determined by the microbial population.Similar flow-through reactors have been used to study reaction kinetics in inorganic systems where the chemistry of the inlet solution is often manipulated to study the reactor response to changing and non-steady-state conditions. Similar techniques will be applied to the chemostat system to observe the effect of a change in solution chemistry (pH, nutrient, and total concentrations of Fe, As, and S) on microbially-mediated kinetics of Fe(III), As(V), and S(VI) reduction.