Decoupling effect of fulvic acid on Fe(II)-mediated transformation of Cr(VI)-loaded lepidocrocite: Enhanced Cr immobilization independent of mineral transformation.

Hexavalent chromium (Cr(VI)) immobilization on iron oxides is commonly considered to rely on Fe(II)-mediated mineral transformation. Fulvic acid (FA) is ubiquitous in natural environments and interferes with interfacial reactions, yet its regulatory linkage between mineral evolution and Cr immobilization remains poorly understood. This study explored Fe(II)-mediated transformation of Cr(VI)-loaded lepidocrocite (LepCr) under anoxic conditions using batch experiments, spectroscopic characterization, and sequential extraction. Results confirmed that Cr(VI) was completely reduced to Cr(III) with no secondary release in all Fe(II)-amended systems. FA displayed a concentration-dependent effect on mineral conversion, which was partially decoupled from Cr immobilization. Lw FA (12 mg/L) facilitated lepidocrocite-to-goethite conversion via molecular bridging, whereas high FA (86 mg/L) significantly inhibited mineral recrystallization. Notably, as FA concentrations increased from 12 to 86 mg/L, the dominant Cr immobilization pathway shifted from mineral-transformation-associated processes to interfacial FA-Cr(III) complexation. Despite suppressed mineral transformation, high FA greatly elevated the proportion of non-extractable Cr from 25% to 54%, strengthening Cr immobilization. These results demonstrated that efficient Cr immobilization could be attained with reduced dependence on mineral transformation, where FA-Cr(III) complexation and Cr(III)-Fe(III) interfacial passivation dominated. Synchronous redistribution of FA at the solid-liquid interface further reinforced Cr sequestration. This work clarified the decoupled mechanism between mineral transformation and Cr immobilization, providing a mechanistic basis for optimizing Cr immobilization strategies in organic-rich contaminated environments.