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

9806121由于砷对植物、动物和人类健康的毒性作用，砷在淡水水生环境中的保留和流动性受到极大关注。 过去，砷被认为是环境问题的“四大”金属之一（其他是铅，镉和汞）。 砷通过矿物表面的吸附、在高氧化水平下以金属砷酸盐的形式沉淀以及在还原条件下以砷硫化物的形式保留在水生沉积物中。 由于砷酸盐As（V）比亚砷酸盐As（III）的移动的少，所以砷酸盐还原为亚砷酸盐是增加砷移动性的主要机制。 这是一个大问题，因为经常观察到砷酸盐和亚砷酸盐的分布是不平衡的。 缓慢的反应动力学是一个合乎逻辑的原因不平衡，但反应速率和机制知之甚少。本实验室研究将探讨砷还原缺氧沉积物中的高砷含量。 本研究将验证以下假设：1）As（V）的还原是通过微生物呼吸作用进行的; 2）吸附的As（V）在铁相还原溶解过程中从羟基氧化铁中释放出来，但可能被重吸收到其他基质中; 3）电子受体在微生物呼吸作用中的顺序使用符合热力学预测; As（III）在溶液中的出现与硫化物密切相关，As（III）在溶液中的去向与硫化物有关。（在溶液中沉淀或络合）由硫酸盐的总浓度确定，采用多级恒化器研究沉积物柱样中Fe（III）、As（V）和S（VI）的还原速率。 这种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，在本提案中，恒化器将用于模拟水、营养物和溶质通过沉积物包的连续通量。 该技术的优点是，沉积物芯中固有的细菌的聚生体可以根据其沿沿着逐渐减小的PE梯度顺序使用各种电子受体而在恒化器的连接的容器之间空间地排列自身。 因此，这种设置将密切模拟微生物种群随深度的变化，如在厌氧沉积物中发现的那样。 此外，它提供了一个机会，样品和观察发生在每个连接的微观世界中的化学过程，在pe条件下由微生物种群决定。类似的流通反应器已被用于研究反应动力学在无机系统中的入口溶液的化学经常被操纵，以研究反应器响应变化和非稳态条件。 类似的技术将应用于恒化器系统，以观察溶液化学（pH值，营养物和Fe，As和S的总浓度）的变化对Fe（III），As（V）和S（VI）还原的微生物介导动力学的影响。

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.