Zero-valent iron nanoparticles (nano-Fe0) have proved effective at the remediation of pollution in environmental settings. However, the Royal Society has advised the UK government to prohibit the release of these particles, due to concerns over the safety of the particles in relation to their reactive timeframe, and their final state post-utilisation. The following work aimed to address these concerns, by performing experiments that studied the interaction of nano-Fe0 with common groundwater anions and contaminants. As determined by X-ray diffraction and X-ray photoelectron spectroscopy analysis, ageing of nano-FeD in both simple and more complex geochemical solutions produced the iron (hydr)oxide forms of magnetite/maghemite (Fe304/y-Fe2O3), lepidocrocite (y-FeOOH) and goethite (a-FeOOH); biogenic nanoparticulate forms of all detected iron compounds have been identified in nature. In chemically simple solutions, goethite was inferred to be the final state of the corrosion serie.s. However, after four months aqueous exposure, this was not the sole form of iron. This indicated that the transformation process take place over a matter of months or years, rather than days. Further work suggested that more crystalline nano-FeD, e.g. produced by hydrogen reduction would have a considerably extended reactive longevity in ~he environment as compared to the more reactive nanoFeD produced using borohydride reduction. Using inductively coupled plasma-mass and - optical emission spectroscopy, increasing corrosion was shown to have a negative impact on nano-FeD reactivity; metal uptake rates decreased with increased oxidation. Metal contaminants when not incorporated into the iron nanopartic1e structure, i.e. when simply reduced or complexed on the surface, are very susceptible to remobilisation. This process was ascribed principally to the loss of the particle's zero-valent core rather than surface stoichiometry or geochemical conditions; hence, these require electron transfer to remain associated with the nanoparticle surface. Anions at concentrations found in natural waters have a significant influence on the transformation of zero-valent iron nanopartic1es, with chloride, sulphate, and most significantly, bicarbonate accelerating the corrosion of nano-FeD compared to an anion free solution. Furthermore, the anions sulphate and chloride accelerated desorption of adsorbed metals from the nanoparticle surface. Conversely, nitrate was found to encapsulate any associated metals in a magnetite/maghemite shell, which could be utilised for the long-term capture of metal contaminants. The complex influence of anions on the corrosion and reactivity of nano-Fe0 was corroborated by results from a field trial with uranium-contaminated natural waters. To conclude, this PhD used numerous analysis techniques to determine the fate and reactivity of nano-FeD when exposed to an aqueous environment. This work clearly demonstrates that solution composition has a significant impact on the performance of nano-FeD, and emphasises the importance of performing pilot studies in analogous conditions before the particles are deployed on site.