Preventing Uranium(VI) redissolution in water after treatment with zerovalent iron nanoparticles by passivation with chromium(VI)

Abstract

It has been proved that nanoscale zerovalent iron (nZVI) is efficient for a fast removal of U(VI) in the absence of dissolved oxygen. However, the reoxygenation of the system leads to the re-release of U(VI) to the solution. In this work, U(VI) removal by nZVI was studied and its redissolution was prevented by addition of Cr(VI) that produces the passivation of the nZVI surface. Starting at 0.25 mM of U(VI) under anoxic conditions, nZVI was added in a Fe(total):U(VI) molar ratio of 4 achieving a fast and complete U(VI) removal. After 0.5 h reaction, 0.33 mM Cr(VI) was added to the U(VI) system and reoxygenation of the system was allowed for 24 h. No re-release of U was observed, in contrast with the experiments performed in the absence of Cr(VI), where 16 % of U was redissolved after reoxygenation. Raman spectroscopy and X-ray diffraction analyses performed on the solids obtained after the experiments demonstrated the presence of magnetite, maghemite and Fe(0) in all the samples. X-ray photoelectron and energy disperse spectroscopies indicated that a larger amount of U was present in the nanoparticles coming from the experiment where Cr(VI) was added, compared with the experiments in the absence of Cr(VI). The proposed mechanism involves the reduction of U(VI) to U(IV) and its adsorption on the nanoparticles, followed by surface passivation produced by reduction of Cr(VI) to Cr(III), which prevents the reaction between oxygen and the adsorbed uranium.