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

Abstract

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/H2O2 system because of improved iron redox cycling. The continuous-flow experiments revealed that the reactor performance increased in the order of: ZVI/H2O2 < pyrite/H2O2 < ZVI/pyrite/H2O2 under dynamic flow conditions. The methylene blue treatment by the ternary ZVI/pyrite/H2O2 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/H2O2 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.