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.