Enhancement of tribromophenol removal in a sequencing batch reactor via submicron magnetite

Conductive magnetite (Fe3O4) has been applied into some anaerobic bioprocesses to accelerate direct interspecies electron transfer (DIET), however, Fe3O4 is usually dissolved by iron-reducing bacteria under anaerobic conditions, resulting in the loss of magnetite. Therefore, submicron magnetite particles were added to the sequencing batch reactor (SBR) to build a Fe3O4/SBR system, which could alleviate magnetite dissolution and simultaneously remove tribromophenol (TBP) effectively. The average removal efficiencies of chemical oxygen demand (COD) and TBP in Fe3O4/SBR system were 81% and 91%, respectively, which were 51% and 18% higher than those of the control group without Fe3O4 (SBR system). The enhanced TBP biodegradation was likely related to potential DIET, which was supported by the scanning electron microscopy (SEM) analysis, the increase of dehydrogenase and heme c (fivefold and 1.7-fold), and the enrichment of iron-redoxing bacteria (Geobacter and Thiobacillus). Furthermore, magnetite mainly remained intact in structure as indicated by X-ray diffraction (XRD), which might be ascribed to in situ iron redox cycle and magnetite biosynthesis via Magnetospirillum. Notably, the content of hydrogen peroxide (H2O2) and hydroxyl radical (OH) in Fe3O4/SBR system was 4-5 times higher than that of SBR system. These findings could provide insights into the development of cost-effective strategy for the removal of refractory organic pollutants.