Nanoscale zero-valent iron particles for the remediation of uranium contaminated groundwater

The increased development and use of engineered nanomaterials has the potential to offer great benefits to society through their exploitation within numerous products developed by diverse industries. Some applications of nanomaterials have the potential to afford environmental benefits and of particular interest is the use of nanoscale particles of zerovalent iron for the in-situ cleanup of contaminated land and groundwater. The current presentation describes a comparative and site specific study for the application of zero-valent iron nanoparticles, zero-valent iron-nickel nanoparticles and magnetite nanoparticles for the remediation of uranium contaminated groundwater taken from the Lişava valley, Banat, Romania. Nanoparticles were introduced to the Lişava groundwater under surface (bench top) and deep (anoxic) aquifer oxygen conditions and synthetic solutions containing uranyl-only and uranyl-carbonate were also tested as simple-system analogues. The batch systems were analysed over a 28 day reaction period during which the liquid samples were tested for changes in Eh/pH/DO and dissolved metal concentrations were determined using ICP-AES and ICP-MS. Extracted solid samples were also tested using Raman spectroscopy, XPS and XRD to determine the mechanism of uranium sorption and nanoparticle corrosion product evolution. The results provide clear evidence that both zero-valent iron nanoparticles and zero-valent iron-nickel nanoparticles are highly effective for the rapid removal of uranium from the Lişava groundwater despite the presence of appreciable concentration of complexing agents (namely carbonate) within the water. In comparison, the nanoscale magnetite particles were recorded to have limited ability to remove uranium from the Lişava groundwater, which is attributed to the presence of Fe0 within both the zero-valent iron nanoparticles and the zero-valent iron-nickel nanoparticles, providing an additional and active source of electrons for aqueous reaction and associated contaminant removal. Despite high uranium removal recorded in the early stages of the reaction period (timescales less than approximately 1 week) for both the zero-valent iron nanoparticles and the zero-valent iron-nickel nanoparticles, for longer timescales, significant uranium re-release was recorded for systems under surface aquifer oxygen conditions. In comparison, minimal uranium re-release was recorded for all systems containing deep aquifer oxygen conditions. Results demonstrate that for waters containing appreciable concentrations of complexing agents, namely dissolved carbonate, this re-release of uranium following a period of “apparent remediation” is driven by the ingress of atmospheric oxygen and other associated gases (including CO2) back into the experimental solutions, facilitating the reformation of thermodynamically stable uranyl carbonate complexes. The results presented in the current work demonstrate that, in principle, iron nanoparticles can be applied as a highly efficient tool for the remediation of uranium contaminated groundwater. However, with complexing agents such as carbonate ubiquitous in the natural environment, clear evidence is presented that further work is required in order to develop suitable nanoparticle physico-chemical modifications and/or deployment strategies to prevent uranium remobilisation during the in-situ treatment of uranium contaminated groundwater in the vadose (oxygenated) zone.