The roles of different Fe-based materials in alleviating the stress of Cr(VI) on anammox activity: Performance and mechanism

Hexavalent chromium Cr(VI) is widely present in industrial wastewater, which can inhibit anammox performance significantly. Iron is a critical element of anammox metabolism and also has the capacity to remove Cr(VI). In this study, batch experiments were conducted to investigate the effects of commercial zero-valent iron (Fe0) and ferroferric oxide (Fe3O4) in the nano-(n) and micron-(m) scales (mFe0, nFe0, mFe3O4, and nFe3O4, respectively) on recovering anammox activity with inhibition of Cr(VI). Their recovery efficiencies on the anammox process were in the order of mFe0, mFe3O4, nFe0, and nFe3O4 at their minimum effective dosages, with specific anammox activities being promoted by 95.2%, 57.0%, 45.0%, and 15.8%, respectively, in comparison to the control. More hydroxyl groups in extracellular polymeric substances (EPS) were observed with mFe0, and the electron transport performances of soluble and loosely-bound EPS were particularly enhanced. By contrast, Fe3O4 stimulated more production of EPS than mFe0, which helped remove Cr(VI) outside the cell membrane. Moreover, the contents of heme c and cytochrome c were improved most with mFe3O4 by 10.9% and 41.3%, respectively, and hydrazine dehydrogenase activity was enhanced by 47.7% with mFe0, at their optimal dosages. In addition, cytochrome c showed a significant positive correlation with recovery performance for all four materials (R2 > 0.82, P < 0.05), indicating its pivotal role in alleviation effectiveness. More FeOOH was found on the two micron-scale materials detected by X-ray diffractometry and Fourier transform infrared spectroscopy, which would help electron transfer between materials and anammox bacteria. In contrast, the fast corrosion and adsorption processes and biological toxicity of nanoscale materials may lead to their poor performance.