Differential changes of trichloroethylene degradation efficiency and carbon isotope fractionation during oxygenation of different Fe(ii) 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 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 (ε 13C) were relatively consistent across systems (average ε 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.