Methylene blue treatment with zero-valent iron/pyrite/H2O2 system under static and continuous flow conditions: Reaction mechanism and toxicity evaluation.

[Display omitted] • The ZVI/pyrite/H 2 O 2 system was effective in treating MB under dynamic flow conditions. • MB degradation initially occurred with the solution and surface-bound *OH radicals. • Methylene blue intermediates co-precipitated as spherical particles even under acidic conditions. • Fenton degradation decreased the toxicity of methylene blue intermediates considerably. Laboratory batch and continuous flow studies, coupled with surface and toxicity analysis, were performed to evaluate methylene blue treatment by heterogeneous Fenton process using zero-valent iron (ZVI) and pyrite as co-catalyst. The use of pyrite in batch reactors significantly enhanced methylene blue treatment by the ZVI/H 2 O 2 system because of improved iron redox cycling. The continuous-flow experiments revealed that the reactor performance increased in the order of: ZVI/H 2 O 2 < pyrite/H 2 O 2 < ZVI/pyrite/H 2 O 2 under dynamic flow conditions. The methylene blue treatment by the ternary ZVI/pyrite/H 2 O 2 system was described by an initial degradation of methylene blue with *OH radicals, followed by the adsorption and/or co-precipitation of degradation intermediates with some spherical particles. The surface analysis showed that these spherical particles formed, even at pH less than 4. The genotoxicity and cytotoxicity tests performed on mouse embryonic fibroblast (3 T3-L1) and human embryonic kidney (HEK293) cell lines showed that the Fenton treatment of methylene blue using the ZVI/pyrite/H 2 O 2 system resulted in the formation of degradation species with much lower toxicity levels relative to methylene blue. Moreover, the Fenton degradation species of methylene blue significantly enhanced the metabolic activity of several bacterial strains, including E. coli ATCC 8739 and P. aeruginosa PAO1. [ABSTRACT FROM AUTHOR]