Enhanced Sequestration of Chromium by Mechanochemically Silicified Microscale Zerovalent Iron: Role of the Silicate-Modified Surface

Remediation of chromium-contaminated groundwater remains a significant environmental challenge around the world. Herein, we synthesized silicified microscale zerovalent iron (Si-mZVIbm) using a mechanochemical method and demonstrated that the silicate modification could significantly improve the Cr(VI) removal efficiency (up to 37.5-fold) compared with its un-silicified counterpart. Results of atomic force microscopy, scanning transmission electron microscopy, and positron annihilation measurements revealed that silicate acted as a milling lubricant to boost strain within zerovalent iron particles, inducing more plastic deformation and surface defects. The defect-rich silicified surface accelerates the electron transfer and subsequent in situ generation of Fe(II). More importantly, the surface-modified silicate can act as a ligand to coordinate leached Fe(II) ions, thus strengthening the reduction of Cr(VI) via surface-bound Fe(II) and favoring subsequent co-precipitation of Cr(III) and Fe(III) species on Si-mZVIbm surfaces. During column experiments using real Cr-contaminated groundwater, Si-mZVIbm (4 wt % in sand) was able to reduce the Cr(VI) concentration from 2 to 0.05 mg L–1, the World Health Organization drinking water standard for up to 1720 bed volumes with an empty-bed contact time of 5.1 min. These results demonstrate the potential field applicability of Si-mZVIbm in real contaminated groundwater remediation.