Magnetite and zero-valent iron nanoparticles for the remediation of uranium contaminated environmental water

摘要

The current work presents a comparative and site specific study for the application of zero-valent iron nanoparticles (nano-Fe°) and magnetite nanoparticles (nano-Fe_3O_4) for the removal of U from carbonate-rich environmental water taken from the Lisava valley, Banat, Romania. Nanoparticles were introduced to the Lisava water under surface and deep aquifer oxygen conditions, with a U~(VI)-only solution studied as a simple system comparator. Thebatch systems were analysed over an 84 day reaction period, during which the liquid and nanoparticulate solids were periodically sampled to determine chemical evolution of the solutions and particulates. Results indicated that U was removed by all nano-Fe° systems to <10 μg L~(-1) (>98% removal) within 2 h of reaction, below EPA and WHO specified drinking water regulations. Similar U concentrations were maintained until approximately 48 h. X-ray photoelectron spectroscopy analysis of the nanoparticulate solids confirmed partial chemical reduction of U~(VI) to U~(VI) concurrent with Fe oxidation. In contrast, nano-Fe_3O_4 failed to achieve >20% U removal from the Lisava water. Whilst the outer surface of both the nano-Fe° and nano-Fe_3O_4 was initially near-stoichiometric magnetite, the greater performance exhibited by nano-Fe° is attributed to the presence of a Fe~0 core for enhanced aqueous reactivity, sufficient to achieve near-total removal of aqueous U despite any competing reactions within the carbonate-rich Lisava water. Over extended reaction periods (>1 week) the chemically simple U~(VI)-only solution treated using nano-Fe° exhibited near-complete and maintained U removal. In contrast, appreciable U re-release was recorded for the Lisava water solutions treated using nano-Fe~0. This behaviour is attributed to the high stability of U in the presence of ligands (predominantly carbonate) within the Lisava water, inducing preferential re-release to the aqueous phase during nano-Fe° corrosion. The current study therefore provides clear evidence for the removal and immobilisation of U from environmental waters using Fe-based nanoparticles. As a contrast to previous experimental studies reporting impressive figures for U removal and retention from simple aqueous systems, the present work demonstrates both nanomaterials as ineffective on timescales >1 week. Consequently further research is required to develop nanomaterials that exhibit greater reactivity and extended retention of inorganic contaminants in chemically complex environmental waters.