The oxidation of ferrous (Fe(II)) to ferric iron (Fe(III)) in waters containing phosphate, silicate, calcium (Ca) and trace elements results in short-range-ordered (SRO) Fe(III)-precipitates that control the cycling of major and trace elements at anoxic-oxic boundaries in natural and engineered systems. These precipitates largely vary in composition and degree of Fe(III) polymerization. Their structural diversity and resulting differences in colloidal and chemical behavior however are still poorly understood. The proposed project takes advantage of state-of-the-art synchrotron X-ray absorption spectroscopy (XAS) and analytical electron microscopy (AEM) for the study of SRO Fe(III)-precipitates formed under widely varying chemical conditions. It thereby aims at achieving a new level of understanding regarding the formation, structure and stability of SRO Fe(III)-precipitates and their role in the biogeochemical cycling of major and trace elements. Such mechanistic knowledge is increasingly important for the quantitative description of nutrient and contaminant behaviour in aquatic systems and improved water resource use and water treatment. Specifically, the proposed research will focus on the following four research questions:1) How do the kinetics of Fe oxidation, precipitation, and aggregation/coagulation affect the types, structure and nanometer- to micrometer-scale spatial arrangement of SRO Fe(III)-precipitates?2) How does the composition, structure, and morphology of Fe(III)-Ca-phosphates and -arsenates formed by Fe(II) oxidation in presence of phosphate or/and arsenate vary as a function of solution chemistry?3) How does the composition, structure and morphology of different SRO Fe(III)-precipitates change during aging and how do these changes affect their solubility?4) How does Fe oxidation, precipitation and aging affect the co-oxidation and co-transformation of As and what are the implications for As removal in water treatment and for biogeochemical As cycling?To address these questions, Fe(II) (and As(III)) oxidation experiments will be designed that capture the complex interplay of key parameters (concentrations of Fe(II), phosphate, silicate, Ca, As (III) or As(V); solution pH; O2 partial pressure; ionic strength) while being controlled enough to allow for a quantitative description and modeling of the observed processes. XAS will provide molecular-level insight into precipitate structure from the perspective of local element coordination, and AEM into particle composition, structure, and morphology from the nanometer- to the micrometer scale.