From delivery to lock-in: Ascorbate-functionalized magnetite achieves reaction-triggered self-immobilization for sustainable Cr(VI) remediation in soil and groundwater.

Although numerous studies have aimed to enhance the reactivity and transportability of Fe-based materials for soil and groundwater remediation, there is a limited understanding of the transport behavior of injected materials and their post-reaction fate in subsurface environments. In this study, dispersible magnetite nanoparticles were synthesized via ascorbic acid functionalization (AA@Fe3O4) and applied to Cr(VI) removal in batch and column experiments. While Fe3O4 exhibited rapid aggregation and limited transport, AA@Fe3O4 showed improved dispersion in water and enhanced mobility in the soil column. Despite the unfavorable electrostatic interaction with anionic Cr(VI), AA@Fe3O4 demonstrated higher removal capacity (27.66 mg g-1) than Fe3O4 (17.47 mg g-1). The enhanced performance was attributed to the redox-active role of surface-bound ascorbate, which promoted Fe(III)/Fe(II) redox cycling and reductive dissolution of the Fe3O4 surface, thereby contributing to sustained Cr(VI) reduction. Following Cr(VI) reduction, AA@Fe3O4 transformed into aggregated clusters and film-like interparticle networks. The reduced Cr(III) species were preferentially associated with Fe-rich domains and immobilized within the aggregated matrix, likely through Fe-Cr oxyhydroxide formation and interaction with oxidized AA derivatives. These findings demonstrate that AA functionalization mitigates the conventional trade-off between reactivity and mobility, enabling mobile delivery, enhanced Cr(VI) reduction, and subsequent Cr immobilization through reaction-induced aggregation.