Mechanistic insights into the reactivity of Ferrate(VI) with phenolic compounds and the formation of coupling products.

In this paper, the removal of 2-benzylphenol (2-BP), phenol (Ph), chlorophene (CP), and 4-chlorophenol (4-CP) by Fe(VI) have been examined at pH 8.0. The second-order rate constant (k) for substrates degradation at a Fe(VI) concentration of 0.2 mM was in the order of k _CP (353 M ^-1  s ^-1 ) > k _4-CP (131 M ^-1  s ^-1 ) > k _2-BP (102 M ^-1  s ^-1 ) > k _Ph (40 M ^-1  s ^-1 ), indicating that the presence of chlorine and benzyl groups in benzene ring can enhance the reactivity of the phenolic compounds with Fe(VI). Reaction products were identified by a liquid chromatography-quadrupole-time-of-flight-mass spectrometry (LC-Q-TOF-MS), and four reaction mechanisms, including hydroxylation of benzene ring, cleavage of C-C bridge bond, substitution of chlorine atom by hydroxyl group, and the single-electron coupling mechanism were proposed for phenols degradation by Fe(VI). The extracted peak areas of the degradation products showed that the single-electron coupling reaction is the main degradation mechanism in Fe(VI) oxidation processes. In addition to direct attack by Fe(VI), hydroxyl radical, as detected by electron paramagnetic resonance (EPR) spectra, also plays a role in phenols degradation. The •OH initiated reactions and single-electron coupling reactions were further explored by total charges distribution, transition state calculations and potential energy profiles. In addition, Fe(VI) could also work as a highly effective oxidant for substrates removal from real waters.
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