The adsorption and reduction of anionic Cr(VI) in groundwater by novel iron carbide loaded on N-doped carbon nanotubes: Effects of Fe-confinement

Confining reactive nanoparticles (e.g., zero-valent iron) in nano-capsules prevents their aggregation and preserves their reactivity. However, confined Fe-based materials are ineffective in removing anionic contaminants, and the mechanisms are unclear. In this study, at different pyrolysis temperatures, novel iron carbide (Fe3C) loaded N-doped carbon nanotubes (MU-CNTs/Fe) were prepared, in which Fe was loaded onto CNTs as unconfined Fe prepared at 700 °C (U7) or confined Fe prepared at 800–900 °C (U8 and U9, respectively). When tested for their ability to remove Cr(VI) oxyanions, unconfined Fe showed higher adsorption, reduction, and reusability than confined Fe. Moreover, the selective adsorption of Cr(VI) using U7 was demonstrated in a binary solution system with coexisting anions and complicated groundwater. The key mechanism of the enhanced Cr(VI) removal by unconfined Fe was its considerably enhanced reductive precipitation mediated by surface adsorbed and dissolved Fe2+, accounting for 40.8 % of the overall Cr(VI) removal. This study proposes a novel strategy for modulating the transformation of nonconfinement to confinement of nanoparticles based on the pyrolysis temperature and provides new insights into the mechanisms of iron confinement on the removal of metal oxyanions from groundwater.