Cong Fan, Wenwu Pan, Limiao Cai, Yan Yang, Qian Yao, Xiaohu Jin, Xiaofei Li, Chuling Guo, Fenglan Lu, Bing Zhang, Zhi Dang
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, and mineral transformation and Cr immobilization were partially decoupled: low FA (12 mg/L) facilitated lepidocrocite-to-goethite conversion via molecular bridging, whereas high FA (86 mg/L) significantly inhibited mineral recrystallization. Notably, Cr immobilization pathways varied substantially with FA dosage: at low FA level, Cr immobilization was primarily associated with mineral transformation; whereas in high FA systems, the dominant pathway shifted 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 demonstrate that efficient Cr immobilization can be attained with reduced dependence on mineral transformation, where FA-Cr(III) complexation and Cr(III)-Fe(III) interfacial passivation dominate. Synchronous redistribution of FA at the solid-liquid interface further reinforced Cr sequestration. This work clarifies the decoupled mechanism between mineral transformation and Cr immobilization, providing a mechanistic basis for optimizing Cr immobilization strategies in organic-rich contaminated environments.