Soil manganese: Speciation, transformation, and reactivity

Manganese is an essential micronutrient for plants which may cause phytotoxic effects at elevated solution concentrations. In addition, manganese(III,IV)-(oxyhydr)oxides are important scavengers and oxidants for trace metal(loid)s in soils. Despite its ecological relevance, solid-phase speciation and transformation reactions of manganese in soils are still poorly understood. The proposed research on the speciation, transformation, and reactivity of soil manganese will explore the relative abundance and variability of different manganese pools in soils, validate existing sequential extraction procedures used to target soil manganese, and explore the reductive transformation of manganese oxides in sub-/anoxic soil environments and its effect on heavy metal solubility. To achieve these objectives, we will use synchrotron-based X-ray absorption spectroscopy to study the speciation of manganese in major soil types of Europe and validate sequential extraction procedures as simple and cost-effective means to investigate the soil manganese cycle. Additionally, we will conduct batch experiments in combination with X-ray diffraction and vibrational spectroscopies to explore the effect of dissolved manganese(II) on the reductive transformation of heavy metal-loaded manganese oxides in the presence of aqueous iron(II), a process which is hypothesized to strongly impact the mobility of heavy metals in soils subjected to periodic water logging. In summary, the planned project will provide fundamental insights into the speciation of soil manganese, the reliable quantification of soil manganese pools using sequential extractions, and important geochemical controls of the soil manganese cycle.