Dechlorination Helps Defluorination: Insights into the Defluorination Mechanism of Florfenicol by S‑nZVI and DFT Calculations on the Reaction Pathways

Defluorination
is essential to eliminate the antibiotic
resistance
and detrimental effects of florfenicol (C12H14Cl2FNO4S, FF), which is achievable by sulfidated
nanoscale zerovalent iron (S-nZVI), yet a comprehensive understanding
of the mechanism is lacking. Herein, we used experimental data and
density functional theory calculations to demonstrate four dechlorination-promoted
defluorination pathways of FF, depending on S-nZVI or not. FF was
defluorinated in a rapid and then slow but continuous manner, accompanying
a consecutive dechlorination to deschloro (dFF) and dideschloro FF
(ddFF). Unexpectedly, the predominant defluorination occurs by spontaneous
hydrolysis of ddFF to form the hydrolyzed byproduct (HO-ddFF), i.e.,
independent of S-nZVI, which is initiated by intramolecular attack
from carbonyl O to alkyl F and is thus limited for FF and dFF owing
to the diminished nucleophilicity by electron-withdrawing Cl. The
removal of Cl also makes the reductive defluorination of ddFF by S-nZVI
amenable. The other two minor but more rapid defluorination pathways
occur in synergy with the dechlorination of FF and dFF, which are
mediated by the reactive carbanion intermediates and generate HO-dFF
and HO-ddFF, respectively. The reliability of these dechlorination-facilitated
defluorination pathways was verified by the consistency of theoretical
calculations with experimental data, providing valuable insights into
the degradation of fluorinated contaminants.