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

[Display omitted] • A low cost and eco-friendly tannin product TE was used as a reductant to improve Fe(Ⅱ)/H 2 O 2 system. • TE successfully broadened the pH range of Fe(Ⅱ)/H 2 O 2 system by enhancing iron cycle and promoting ROS generation. • OH and O 2 − were both identified in Fenton system, and OH played a dominant role in Met degradation. • The complexes formed by TE and iron species showed reaction activity with O 2 −, and converted O 2 − into OH. 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)/H 2 O 2 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 algae-induced black bloom events. The results found that TE could significantly promote Fe(Ⅱ)/H 2 O 2 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 and O 2 − contributed to Met degradation, and OH played a key role in TE/Fe(Ⅱ)/H 2 O 2 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−1min−1) based on the formation model. TE and its complex catalyzed the dismutation of O 2 − to form OH. The degradation rate constant of TE/Fe(Ⅱ)/H 2 O 2 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 H 2 O 2 activation, and it offered an energy-saving and eco-friendly way for organic pollutants treatment. [ABSTRACT FROM AUTHOR]