Unveiling the dual roles of chromium in mediating vanadium sequestration by ferrihydrite transformation in groundwater: Insights from field evidence to molecular mechanisms.

Vanadium (V) contamination in mining-impacted groundwater poses significant environmental risks, yet the processes by which Fe minerals stabilize mobile V remain unclear. The metastable Fe minerals possibly evolve into permanent V sinks. Here, we conducted three-season groundwater sampling from a vanadium-titanium magnetite tailings pond. Ferrihydrite (Fh) transformation experiments, spectroscopic characterization, electron microscopy, and density functional theory calculations were performed to unravel the sequestration pathway of V and the regulatory role of coexisting chromium (Cr). Field evidence showed that V was predominantly immobilized in groundwater suspended solids, with non-extractable V accounting for 83% of total V, far exceeding the extractable and aqueous V. These results indicated that suspended Fe minerals served as a major sink for groundwater V. Laboratory experiments further demonstrated that Fh transformation progressively transferred initially adsorbed V into non-extractable forms during hematite formation. 94.53% of V was stabilized as the non-extractable form in the V-Cr coexisting system after 10 days, while only 36.55% of Cr was incorporated into the non-extractable fraction. Molecular evidence revealed that V first formed inner-sphere Fe-O-V complexes on Fh surfaces and was subsequently incorporated into hematite through isomorphic substitution, accompanied by partial reduction of V(V) to V(IV). Cr played a dual role by competing for Fh surface sites and retarding V incorporation kinetics, while promoting reductive stabilization of V via enhanced structural Fe(II). Our findings identify Fh transformation as a key natural attenuation pathway for V in Fe-rich groundwater. They also provide mechanistic insights into oxyanion sequestration under multi-metal contamination.