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Role of humic acid in regulating reductive dechlorination: Modulating Fe(II) electronic-structure through antibonding
Qian-Qian Jia1, Hua-Feng Li2, Liandong Zhu3
1School of Environmental and Chemical Engineering, Jiangsu University of Science and Technology, Zhenjiang 212003, PR China; School of Civil Engineering, Wuhan University, No. 8, East Lake South Road, Wuhan, PR China.
None:
Humic acid (HA), a redox-active soil and groundwater compound rich in functional groups (e.g., carboxyl, phenolic hydroxyl, and quinyl group), can complex with Fe(II) to form HA-Fe(II). While HA significantly influences contaminant behavior, the mechanisms it regulates Fe(II) structure-reactivity relationships remain unclear. This study demonstrates that HA content critically controls the Fe(II)-mediated reductive dechlorination of carbon tetrachloride (CT), exhibiting a dual regulatory effect: dechlorination is initially suppressed at lower HA concentrations but enhanced at higher concentrations. Macroscopically, HA mediates the exposure of Fe(OH)2 (100) facets via a concentration-dependent threshold mechanism, directly modulating the electron-donating capacity of surface Fe(II) sites. HA content tunes Fe(II)'s electron-donating capacity at the electronic-structure level by synergistically altering Fe-O bond lengths, octahedral distortion, and the internal electric field. This regulation stems from HA's role as a weak-field ligand, which modifies the electronic occupancy of Fe(II) complexes' antibonding eg* orbitals, thereby impacting electron transfer efficiency. HA content modulates the Schikorr reaction, providing insights into complex iron-based reduction processes. Through multi-scale analysis, this work elucidates the structure-activity relationship between natural organic matter and iron minerals in pollutant degradation, offering a theoretical foundation for designing environmental remediation materials.
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