Mechanism of zerovalent iron-mediated homolog- and site-selective dechlorination of short-chain chlorinated paraffins

Short-chain chlorinated paraffins (SCCPs) have raised global concerns owing to their toxicity and bioaccumulation as persistent organic pollutants. Although zerovalent iron (ZVI) is promising in SCCP dechlorinating, the selectivity mechanisms are scarcely investigated, which hinders the understandings on the environmental and toxicological profiles of the dechlorinated products. This study investigated the homolog- and site-selective dechlorination of C 12 -SCCPs through kinetic experiments and density functional theory (DFT) analysis. The dechlorination efficiency change with ZVI dosage (10–40 g L −1 ) was nonmonotonic, with the efficiency peaking at 20 g L −1 , possibly due to the higher Fe 2+ abundance on ZVI surface. The dechlorination priority followed C 12 Cl 10  > C 12 Cl 9  > C 12 Cl 8  > C 12 Cl 6  > C 12 Cl 7  > C 12 Cl 5 , which was driven by both SCCP adsorption and reactivity. The accumulation of C 12 Cl 7 as the dechlorinated products of C 12 Cl 8-10 led to the anomalous phenomenon of C 12 Cl 6  > C 12 Cl 7 . In particular, higher chlorinated homologs showed stronger affinity to ZVI; meanwhile, DFT calculations demonstrated that chlorine substitution patterns altered the molecular softness and steric accessibility. The enhanced reactivity of homologs with even chlorine numbers was explained by the symmetric lowest unoccupied molecular orbital (LUMO). Intramolecular crowding shifted the dechlorination hotspots from peripheral C(3)/C(10) to central C(6)/C(7) in C 12 Cl 9–10 , where the reduced steric hindrance maximized the reactivity. The number of chlorine atoms and the substitution positions jointly determined the LUMO distribution of SCCPs and the spatial effect to regulate their properties. Overall, these findings elucidate the molecular-scale determinants of SCCP dechlorination selectivity and provide a mechanistic framework for optimizing ZVI-based remediation.