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

铁在水相地球化学环境中的生物地球化学循环与碳、氮、磷和硫的循环密切相关，并强烈影响这些系统中痕量金属和类金属的溶解度和形态。河岸土壤位于旱地生境和水生环境的交界处，在旱地和水生生态系统之间的营养物质和污染物的转移中起着关键作用。我们观察到在淹没土壤常见的地球化学条件下，Fe(II)水溶液与氧化铝和粘土矿物基质反应时形成层状Fe(II)-Al(III)-氢氧化物矿物。我们推测，这些以前未被认识的Fe(II)相在河岸环境中Fe和痕量金属的生物地球化学循环中起着关键作用。这些次生矿物的形成速度很快(在模型系统中为数小时的时间尺度，在湿地土壤中的实验中为数天内)，因此有望成为Fe(III)-氧化物还原溶解过程中释放的Fe(II)的主要汇。此外，由于这些Fe(II)矿物颗粒小、层状结构和Fe(II)含量高，对氧化还原活性污染物(如Cr(VI))具有很高的反应性，并可能通过吸附和共沉淀反应控制Ni(II)和Zn(II)等二价金属的保留。我们假设，在Fe(II)与含铝基质的初始反应中形成的Fe(II)-Al(III)-氢氧化物相是亚稳态过渡相，随着时间的推移，这些相将老化为更结晶化的Fe吸附产物，与痕量金属(Loid)S的反应活性降低。我们将研究这些新的Fe(II)相形成的热力学、动力学和机制，并表征它们的结构和反应活性，时间跨度从几秒到几年不等。将使用一套最先进的光谱技术，包括Q-XAS、BULK XAS和穆斯堡尔分析来解决这些问题。这个项目将填补我们在还原和河岸环境中铁循环知识的一个主要空白，并将提高我们对这些动态系统中污染物的命运和迁移的理解。更重要的意义和重要性。地球表面大约4%-6%的土地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成分发生巨大变化，这主要是由微生物活动推动的，因为土壤微生物被迫从利用氧气转向替代电子受体来进行有机碳的呼吸。在微生物呼吸中使用Fe(III)-氧化物矿物会导致这些矿物的还原溶解，从而导致高浓度的溶解Fe(II)，并释放与Fe(III)-氧化物矿物相关的有毒金属(Loid)杂质。这项研究涉及在洪水期间释放到溶液中的铁(II)和金属污染物的去向。我们已经确定了一种以前未知的沉淀机制，它可能重新分配释放的Fe(II)并将痕量金属返回到固相中。溶解的Fe(II)与含铝土壤矿物发生反应，形成次生Fe(II)-Al(III)-氢氧化物矿物，激活了沉淀过程。在典型的淹水土壤条件下，这些新的Fe(II)相的沉淀迅速而广泛地发生，因此很可能是决定这些系统中释放的Fe(II)的去向的重要过程。Fe(II)矿物的形成将有毒金属从溶液中去除，金属被结合到新相的结构中或吸附到新相的表面上。本文提出的研究将表征控制这些Fe(II)相在淹水土壤中的形成和反应的主要地球化学参数，并提供定量的热力学数据，以便能够预测它们在自然系统中的存在。该项目的成果将填补我们对控制河岸系统土壤和水质的地球化学过程的理解的一个重大空白，河岸系统包括各种环境，如极地沼泽和沼泽、热带沼泽、沿海和淡水湿地、水稻田和泛滥平原土壤。这项工作对于评估恢复以前的湿地(通过重建河岸条件)作为洪水控制和恢复(重新)先前积累的污染物令人担忧的地点的生物多样性的管理选择具有重要意义。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.