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

Abstract

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 via 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 (ε¹³C) were relatively consistent across systems (average ε¹³C = −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.