Periodic Reporting for period 4 - IRMIDYN (Iron mineral dynamics in redox-affected soils and sediments: Pushing the frontier toward in-situ studies)
Période du rapport: 2023-05-01 au 2024-10-31
Iron is the fourth most abundant element in the Earth’s crust (following oxygen, silicon, and aluminum), and therefore, iron is a major constituent of all soils and sediments. During rock weathering at the Earth’s surface, iron forms secondary minerals with very small particle sizes and sometimes weak crystallinity. Since the element iron can be oxidized or reduced, it is also of fundamental importance in natural environments which are periodically anaerobic due to high water contents or flooding. Such environments include wetlands, river floodplains, estuaries, tidal flats, irrigated rice paddy soils, groundwater aquifers, and lake or marine shelf sediments. In recent decades, it has been established that biogeochemical redox processes can directly or indirectly lead to a complete recrystallization and/or transformation of iron minerals, which has important implications for understanding modern and past biogeochemical cycles of carbon, phosphorus, nitrogen, sulfur, and essential or potentially toxic trace elements. In these element cycles, iron minerals are important as sorbents and/or host phases for other elements, as electron acceptors or donors in biogeochemical redox reactions, and as catalysts of surface reactions with inorganic or organic compounds. Recently, the importance of poorly-crystalline iron oxyhydroxide minerals for organic carbon stabilization in soils and sediments has been recognized and is now receiving much attention because of its possible impact on global climate and soil fertility. In addition to natural environments, iron mineral transformation processes also play important roles in engineered systems (e.g. wastewater treatment, groundwater remediation, geological storage of nuclear waste), corrosion sciences, archaeology and cultural heritage sciences, and research on paleoclimate and the evolution of early Earth and Mars. Previous to the IRMIDYN project, iron mineral recrystallization and transformation processes and their influence on other element cycles had mostly been studied in strongly simplified laboratory systems. In-situ studies in soils and sediments were lacking because it is very challenging to detect poorly-crystalline minerals in small quantities in a soil matrix containing many other more crystalline minerals. Therefore, we developed new experimental approaches to study iron mineral transformations in-situ in soils and sediments in the field, making use of 57Fe as a stable isotope tracer combined with 57Fe Mössbauer spectroscopy providing information of Fe redox states and mineral structure. This new approach was tested in the lab and applied in field studies in the German Wadden Sea, rice paddy soils in Thailand, and organic-rich wetland soils in Iceland. These studies resulted in novel insights in iron mineral transformation processes in soils and sediments and consequences for nutrient and contaminant behavior. Our project including the new experimental approach developed was explained in a short documentary film available here: https://www.youtube.com/watch?v=XFWnumTda5s(s’ouvre dans une nouvelle fenêtre).
We have developed novel methods to study a wide range of iron mineral transformation processes in soils and sediments under natural field conditions. Previously, iron mineral transformations had been studied mostly in simplified laboratory systems, which offer good control on environmental conditions but lack the complexity of soil systems in many respects. Therefore, it is important to also study iron mineral transformation processes under natural field conditions. Our new methods using minerals enriched in the stable iron isotope 57Fe, combined with Mössbauer spectroscopy which is sensitive only to this isotope, we are now able to follow speciation changes of Fe in soils and sediments, even if the minerals are minor and nano-crystalline phases mixed into a complex soil matrix. Our results demonstrated that contact to other soil components and the balance between microbial Fe reduction and Fe(II)-catalyzed transformation processes have great influence on the transformation rates and pathways of iron minerals. We also showed that other minerals can serve as growth templates and thereby influence transformation products, not only in model suspensions but also under field conditions. Another major finding was that green rust is a much more common transformation product formed from ferrihydrite than previously thought, especially when microbial Fe reduction is dominant. We also adopted our new methodology to investigate for the first time the formation, stability and transformations of less studied iron mineral transformations such as vivianite, siderite, jarosite, and mackinawite. We are convinced that our new experimental approach using 57Fe Mössbauer spectroscopy will also offer new avenues in other fields of science where iron mineral transformation processes play important roles, such as engineered systems (e.g. wastewater treatment, groundwater remediation, geological storage of nuclear waste), corrosion sciences, archaeology and cultural heritage sciences, and research on paleoclimate and the evolution of early Earth and Mars.
The major outcome of this project is a novel approach for investigating iron mineral transformation processes in-situ in soils, sediments, or other complex environments, using isotopically (57Fe) labelled iron minerals in combination with Mössbauer spectroscopy and other analytical tools. This new approach was be used to study selected iron mineral transformation processes in rice paddy soils, coastal sediments, and iron-rich organic wetlands, both in microcosm experiments and in the field. The results of these studies were compared with results from laboratory systems to better understand environmental factors influencing iron mineral transformations in nature.