An autocatalytic Fe(III)/H2O2 Fenton-like process triggered by tetracycline: The overlooked effect of quinone intermediates

The conventional Fe(III)/H2O2 system is of low efficiency on the removal of most organic pollutants. However, we discovered that the Fe(III)/H2O2 system was robust on remediating tetracycline (TC) in water (0.109 min−1 for the pseudo-first kinetic constant), and the reaction was even slightly faster than that in the conventional Fe(II)/H2O2 system (0.105 min−1). The addition of TC could also initiate the degradation of other refractory pollutants in the Fe(III)/H2O2 system. Complexation between Fe(III) and TC with the stoichiometric ratio of 1:1 was observed. Electron transfer from TC to Fe(III) gave rise to the slight production of Fe(II), which served as the initiator of the following Fenton reaction. •OH was ascertained as the main reactive species through quenching experiments, EPR tests and the probe method, while O2•−/HO2• was also a subsidiary contributor. Hydroquinone intermediates were proved to be generated through the hydroxylation of the benzene ring of the TC molecule by •OH and thus facilitate the reductive transformation of Fe(III) to Fe(II). This study analyzes the relationship between molecular structural features of TC and the catalytic efficiency of ferric Fenton-like reactions. We expect to shed light on new mechanistic insights on autocatalysis in the Fenton-like catalytic processes.