Highly reactive green rust (GR) nanoparticles are believed to play an important role in the geochemistry of water saturated sediments (e.g. hydromorphic soils) and engineered systems where zero-valent iron is used for decontaminating polluted sites (e.g. permeable reactive barriers). The presence of structural Fe2+ within GR and its high specific surface area make it an effective reductant for many inorganic (e.g. Cr, U, Se) and organic substances (e.g. tetrachloroethene (TCE)). These reduction processes can lead to breakdown of organic molecules or the formation of insoluble reduced inorganic phases (e.g., UO2(s)), thus reducing the bioavailability of these toxic compounds. Understanding the formation and geochemical stability of GR is key to assessing its potential role in natural sediments and engineered environments. However, characterizing GR is difficult due to the rapid oxidation (seconds - minutes) of structural Fe2+ in the presence of air. Thus, to obtain detailed information about the mechanism and kinetics of GR formation, stabilisation and oxidative breakdown, novel synchrotron-based methods have been developed which combine in situ and time-resolved X-ray diffraction/scattering (XRD/SAXS) analysis with controlled anaerobic chemical synthesis. This system allowed the simultaneous quantification of several chemical parameters in the aqueous solution (i.e., pH, Eh) with detailed analysis of the changes in the solid phase crystal structure. In conjunction with this X-ray Absorption Spectroscopy (XAS) was used to characterise the speciation of trace elements (i.e. U, Zn and Se) associated with GR as it crystallised and/or transformed. The formation of green rust (Fe2+/Fe3+ > 1.2) from solution occurs via a 3 stage process. The first stage is the nucleation and growth of ferric hydroxysulfate (schwertmannite) nanoparticles (~5 nm). With increasing pH the schwertmannite transforms into nanogoethite particles (< 50 nm). This process is catalyzed by adsorbed Fe2+ ions on the surface of schwertmannite which facilitates the transformation reaction. Green rust formation occurs when the surface adsorbed Fe2+ hydrolyses and reacts with the goethite above pH 7. With further increases in pH the GR remains stable up to pH 10. Experiments conducted at lower Fe2+/Fe3+ ratios (0.5-1) show that green rust is formed at pH 7-9, but is metastable with respect to magnetite above pH 7. Oxidation reactions reveal that the green rust breaks down to a mixture of Lepidocrocite (L) and goethite (G) with the L/G ratio and the rate of breakdown increasing with pH. Speciation analysis indicates that U6+ and Se6+ are reduced to U4+ and Se4+/Se0 during GR formation and that during the oxidation process the reduced trace elements are fully or partially reoxidised, yet a significant amount remains sorbed to the FeOOH particles. This study suggests that green rust nanoparticles could be utilised to sequester key inorganic contaminants under a range of natural pH conditions (neutral - alkaline). Also, following oxidation, a significant proportion of some trace elements are retained within the oxidised minerals which could significantly reduce their long-term mobility.