Role of humic acid in regulating reductive dechlorination: Modulating Fe(II) electronic-structure through antibonding eg* orbital occupancy.

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.