Enhanced degradation of methionine by eco-friendly and low-cost tannin extract modified Fenton oxidation: Promotion of the Fe(III)/Fe(II) cycle

In this study, a low-cost natural biomass extracted from higher plants - Tannin - was used to improve the Fenton system by promoting iron cycling due to the abundant phenolic hydroxyl groups in the molecular structure. Tannin extract (TE) of black wattle was combined with Fe(II)/H2O2 system at room temperature and under pH 3 to 8. Methionine (Met) was chosen as a target pollutant, as its critical role in the global sulfur cycle and algaeinduced black bloom events. The results found that TE could significantly promote Fe(II)/H2O2 system, Met degradation increased from 44 % to 86 % under pH 6.0, and the mineralization increased from 14.86 % to 38.96 %. Both quenching experiments and electron paramagnetic resonance (EPR) analysis confirmed that (OH)-O-center dot and O-2(center dot-) contributed to Met degradation, and (OH)-O-center dot played a key role in TE/Fe(II)/H2O2 system. The reducibility of TE at different pH values was evaluated, the highest Fe(III) reducing rate of TE was obtained at pH of 4.5 (k = 22.48 M(-1)min(-1)) based on the formation model. TE and its complex catalyzed the dismutation of O-2(center dot-) to form (OH)-O-center dot. The degradation rate constant of TE/Fe(II)/H2O2 system was 3.63 min(-1), and the intermediates of Met degradation was proved to be non-toxic. In conclusion, TE could facilitate the iron recycle in Fenton system, consequently to strengthen H2O2 activation, and it offered an energy-saving and eco-friendly way for organic pollutants treatment.