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Iron mineral dynamics in redox-affected soils and sediments: Pushing the frontier toward in-situ studies

Periodic Reporting for period 2 - IRMIDYN (Iron mineral dynamics in redox-affected soils and sediments: Pushing the frontier toward in-situ studies)

Reporting period: 2020-05-01 to 2021-10-31

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. To date, iron mineral recrystallization and transformation processes and their influence on other element cycles have mostly been studied in strongly simplified laboratory systems. In-situ studies in soils and sediments are 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 do not know if and how fast the same processes as observed in simplified laboratory systems actually occur in nature. The aim of this project is to develop novel methods for investigating important iron mineral recrystallization and transformation processes in natural environments, and to apply these methods to study selected mineral transformation processes and their impact on other elements in redox-affected soils and sediments.
During the first year, suitable field sites were selected and sampled for chemical, physical, and mineralogical characterization. The selected field sites include contrasting rice paddy soils in Thailand, tidal flat and salt marsh sites in northern Germany, and iron-organic-rich wetland soils in Iceland. Some large soil samples were collected for method development in the laboratory. In the second year, first methodological tests using mesh-bags with iron minerals were conducted at the selected field sites. In the laboratory, we successfully established a Mössbauer spectroscopy facility with He cryostat for the analysis of 57Fe-labelled iron minerals mixed into soil, as well as a confocal Raman microscope for mineralogical studies with micrometer spatial resolution. Suitable laboratory protocols were developed and tested for the synthesis of iron minerals including ferrihydrite, lepidocrocite, goethite, vivianite, jarosite, siderite, and mackinawite. Some minerals were synthesized without and with ion substitutions in the crystal lattice (e.g. Mg/Mn-vivianite, Al-jarosite). For specific experiments, iron minerals were synthesized from isotopically enriched metallic iron, which requires modified procedures. A mesh-bag technique was tested in soil microcosms and applied in a study on ferrihydrite transformation in flooded rice paddy soils. For field experiments, an insertion device was developed facilitating the installation and retrieval of mineral mesh-bags in the field. Model studies on Fe(II)-catalyzed transformation of ferrihydrite and jarosite as affected by structural impurities were completed. Also, the structural substitution of iron by manganese or magnesium in vivianite was investigated, and the products will be used in further mineral transformation studies. Since vivianite is a redox-active iron phosphate mineral, we studies the kinetics of mercury reduction by vivianite in slurry experiments.
The major outcome of this project will be 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 will 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 will be compared with results from simplified laboratory systems to better understand environmental factors influencing iron mineral transformations in nature.
Field work in the Elbe tidal flats in Germany (August 2019).