Collaborative Research: The role of layered Fe(II)-Al(III)-hydroxides in the biogeochemical cycling of iron and trace metals in riparian environments

技术描述。水地球化学环境中铁的生物地球化学循环与碳、氮、磷和硫的循环密切相关，并对这些系统中微量金属和类金属的溶解度和形态产生强烈影响。河岸土壤是旱地生境与水生环境的交界面，在旱地生态系统与水生生态系统之间的养分和污染物转移中起着关键作用。我们观察到在地球化学条件下，水Fe（II）与Al-oxide和粘土矿物底物反应形成层状的Fe(II)-Al(III)- oh -矿物。我们假设这些以前未被认识到的铁（II）相在河岸环境中铁和微量金属的生物地球化学循环中起着关键作用。这些次生矿物形成速度很快（在模型系统中以小时为时间尺度，在湿地土壤中以几天为时间尺度），因此预计将成为铁（III）氧化物还原溶解过程中释放的铁（II）的主要汇。此外，由于颗粒尺寸小，层状结构和高铁（II）含量，这些铁（II）矿物可能对氧化还原活性污染物（如Cr(VI)）具有高活性，并可能通过吸附和共沉淀反应控制二价金属（如Ni（II）和Zn(II)）的保留。我们假设在铁（II）与含al底物的初始反应中形成的铁(II)-铝(III)-氢氧化物相是亚稳过渡相，随着时间的推移，它将老化成更结晶的铁吸附产物，对痕量金属（样金属）的反应性降低。我们将研究这些新Fe（II）相形成的热力学、动力学和力学方面，并在几秒到几年的时间跨度内表征它们的结构和反应性。一套最先进的光谱技术，包括Q-XAS，批量XAS和穆斯堡尔分析，将用于解决这些问题。该项目将填补我们在还原和河岸环境中铁循环知识的主要空白，并将提高我们对这些动态系统中污染物命运和运输的理解。更广泛的意义和重要性。大约4-6%的地球陆地表面间歇性或永久性地被淹没。土壤淹水导致土壤孔隙水的化学性质发生剧烈变化，这主要是由微生物活动驱动的，因为土壤微生物被迫从使用氧气转向使用替代的电子受体来呼吸有机碳。在微生物呼吸中使用氧化铁（III）矿物会导致这些矿物的还原性溶解，从而导致溶解铁（II）在水中的高浓度积聚，并释放与氧化铁（III）矿物相关的有毒金属（样质）杂质。该研究解决了在注水过程中释放到溶液中的铁（II）和类金属污染物的命运。我们已经确定了一个以前未知的沉淀机制，可能重新分配释放铁（II）和痕量金属回到固相。沉淀过程由溶解的Fe（II）与含al土壤矿物反应激活，导致次生Fe(II)-Al(III)-氢氧化物矿物的沉淀。这些新的铁（II）相的沉淀在典型的淹水土壤条件下迅速而广泛地发生，因此可能是控制这些系统中释放铁（II）命运的重要过程。铁（II）矿物的形成将有毒金属从溶液中去除，金属被纳入结构或吸附到新相的表面。本文提出的研究将表征洪水土壤中控制这些Fe（II）相形成和反应性的主要地球化学参数，并提供定量的热力学数据，以便预测它们在自然系统中的发生。该项目的结果将填补我们对河岸系统中控制土壤和水质的地球化学过程的理解的主要空白，河岸系统包括极地沼泽和沼泽、热带沼泽、沿海和淡水湿地、稻田和洪泛平原土壤等各种环境。这项工作对于评估前湿地的恢复（通过重新建立河岸条件）作为洪水控制和恢复生物多样性的管理选择具有重要意义，在这些地点（重新）动员以前积累的污染物是一个问题。我们的工作也有望对涉及生物刺激金属还原微生物种群以固定地下污染物的修复策略具有重要意义。

Technical description.The biogeochemical cycling of iron in aqueous geochemical environments is intimately linked to the cycling of carbon, nitrogen, phosphorus and sulfur, and strongly impacts the solubility and speciation of trace metals and metalloids in these systems. The research proposed here focuses on coupled Fe and trace metal cycling in riparian soils, which are located at the interface between dryland habitats and aquatic environments and play a key role in the transfer of nutrients and contaminants between upland and aquatic ecosystems. We observe the formation of layered Fe(II)-Al(III)-hydroxide minerals during reaction of aqueous Fe(II) with Al-oxide and clay mineral substrates under geochemical conditions common to submerged soils. We hypothesize that these previously unrecognized Fe(II) phases play a critical role in the biogeochemical cycling of Fe and trace metals in riparian environments. These secondary minerals form fast (on a time scale of hours in model systems, and within several days in experiments performed with wetland soil) and are therefore expected to be a major sink for Fe(II) released during reductive dissolution of Fe(III)-oxides. In addition, owing to small particle size, layered structure, and high Fe(II) content, these Fe(II) minerals are likely to be highly reactive towards redox-active contaminants such as Cr(VI) and may control retention of divalent metals such as Ni(II) and Zn(II) through adsorption and coprecipitation reactions. We hypothesize that the Fe(II)-Al(III)-hydroxide phases formed during initial reaction of Fe(II) with Al-bearing substrates are metastable transitional phases which over time will age into more crystalline Fe sorption products with reduced reactivity towards trace metal(loid)s. We will study the thermodynamic, kinetic and mechanistic aspects involved in the formation of these novel Fe(II) phases, and to characterize their structure and reactivity over a time span ranging from seconds to years. A suite of state-of-the-art spectroscopic techniques, including Q-XAS, bulk XAS, and Mossbauer analyses, will be used to address these issues. This project will fill a major gap in our knowledge of Fe cycling in reducing and riparian environments, and will improve our understanding of contaminant fate and transport in these dynamic systems.Broader significance and importance.About 4-6% of the Earth's land surface is intermittently or permanently submerged. Soil flooding causes drastic changes in the chemistry of soil pore waters, driven mostly by microbial activity as soil microbes are forced to switch from using oxygen to alternative electron acceptors for respiration of organic carbon. Use of Fe(III)-oxide minerals in microbial respiration causes reductive dissolution of these minerals, which leads to the build-up of high aqueous concentrations of dissolved Fe(II) and release of toxic metal(loid) impurities associated with the Fe(III)-oxide minerals. The research addresses the fate of Fe(II) and metalloid pollutants released to solution during flooding. We have identified a previously unknown precipitation mechanism which may repartition released Fe(II) and trace metals back to the solid phase. The precipitation process is activated by reaction of dissolved Fe(II) with Al-bearing soil minerals causing precipitation of secondary Fe(II)-Al(III)-hydroxide minerals. Precipitation of these new Fe(II) phases occurs rapidly and extensively under conditions typical of flooded soils, and is therefore likely to be an important process governing the fate of released Fe(II) in these systems. Formation of the Fe(II) minerals removes toxic metals from solution as well as metals are incorporated into the structure or adsorb onto the surface of the new phases. The research proposed here will characterize the main geochemical parameters controlling the formation and reactivity of these Fe(II) phases in flooded soils, and provide quantitative thermodynamic data allowing for prediction of their occurrence in natural systems. The results of this project will fill a major gap in our understanding of the geochemical processes controlling soil and water quality in riparian systems, which includes environments as diverse as polar bogs and fens, tropical swamps, coastal and freshwater wetlands, paddy rice fields, and floodplain soils. The work is of importance in assessing the restoration of former wetlands (through re-establishment of riparian conditions) as a management option for floodwater control and restoration of biodiversity at sites where (re)mobilization of previously accumulated pollutants is a concern. Our work is also expected to be of major significance to remediation strategies involving biostimulation of metal-reducing microbial populations to immobilize subsurface contaminants.