Highly efficient removal of Cr(VI) from water with nanoparticulated zerovalent iron: Understanding the Fe(III)–Cr(III) passive outer layer structure

• An outstanding Cr(VI) removal was achieved with these nZVI in only 30 min.
• The passive outer layer formed after treatment prevents further Cr(VI) removal.
• Cr(III) is the only chromium species at the external layer after reaction.
• A structure of hydroxychromites → magnetite → Fe0 developed towards the core of the nZVI.

Nanoscale zerovalent iron (nZVI) particles were successfully employed for Cr(VI) removal from aqueous solutions at pH 3. It was found that the capacity of the system increases with increasing nZVI dosage. Starting at 300 μM, a complete Cr(VI) conversion was achieved in 30 min with a Fe:Cr(VI) molar ratio (MR) of 3, and 45% conversion with MR = 1 over the same period of time. The material exhibited an enhanced reactivity in comparison with other previously tested similar materials.The proposed mechanism involves an initial reduction of Cr(VI) to Cr(III) by reaction with Fe0 or Fe(II) on the particle surface or in solution (secondary pathway), followed by an arrest on Cr(VI) removal attributed to the passivation of the surface of the nanoparticles. Passivation was confirmed by Raman and X-ray photoelectron spectroscopies (XPS). Furthermore, XPS analysis demonstrated that Cr(III) is the only Cr species present in the external layer of the nanoparticles after the reaction. Raman analysis and XPS measurements performed after mild sputtering showed that nZVI exposed to Cr(VI) presented a structure, from outside to inside, of hydroxychromites → magnetite → Fe0.