Differential changes of trichloroethylene degradation efficiency and carbon isotope fractionation during oxygenation of different Fe( <scp>ii</scp> ) species

Hydroxyl radicals (˙OH) generated from Fe(ii) oxidation by O₂ play a key role in chlorinated hydrocarbon (CHC) degradation, yet the effects of different Fe(ii) species on degradation kinetics and carbon isotope fractionation remain unclear. Using trichloroethylene (TCE) as a model CHC, this study demonstrates that Fe(ii) species influence TCE degradation <i>via</i> variations in ˙OH production rates, efficiencies, and quenching capacities. After 24 h, TCE degradation efficiencies followed the order: Fe(ii) in rNAu-2 (72.4%) > Fe(ii)-citrate (52.6%) > FeCO₃ (46.1%) > Fe(ii)-Al₂O₃ (27.6%) > dissolved Fe(ii) (7.9%). This outcome can be attributed to two aspects: (1) the rate and efficiency of ˙OH production were highest for Fe(ii)-citrate, moderate for Fe(ii) adsorbed on Al₂O₃ and mineral structural Fe(ii), and lowest for inorganic Fe(ii)_dis. (2) Fe(ii)-citrate had the highest quenching efficiency, followed by inorganic Fe(ii)_dis and adsorbed Fe(ii), with mineral structural Fe(ii) being lowest. Despite these differences, carbon isotope enrichment factors (<i>ε</i> ¹³C) were relatively consistent across systems (average <i>ε</i> ¹³C = -1.6‰ ± 0.6‰), distinct from most other reported degradation mechanisms. These results advance our understanding of ˙OH-mediated TCE degradation and aid in differentiating ˙OH pathways from other natural attenuation processes.