Submicron Magnetite-enhanced Tribromophenol Removal In Sequencing Batch Reactor

Halogenated phenol,with environmental persistence,high biotoxicity and bioaccumulation,has attracted wide attention from the world as a kind of priority pollutants.In the treatment of halogenated phenol wastewater,aerobic dehalogenation process hardly occurs with the effect on the electron absorption of the benzene ring by the halogen atoms,the process of anaerobic dehalogenation is widely used through the reduction of halogenated compounds by obtaining electrons,and the degradation rate depends largely on the electron transfer rate among the microorganisms.In recent years,magnetite has been used to promote the direct interspecies electron transfer(DIET)of microbes because of its excellent conductivity.Moreover,magnetite shows the advantage of enhancement in biological treatment of halogenated wastewater.However,magnetite will be utilized by iron reducing bacteria as their growth and metabolic energy in anaerobic environment,dissolving ferrous ions and resulting in the loss of magnetite with effluent,thus weakening the long-term mediating effect.In this study,the submicron magnetite was added to the anaerobic-aerobic reactor for the first time to enhance the treatment of 2,4,6-tribromophenol(TBP)wastewater.The optimum Fe3O4/AS complex(WFe3O4:WAS=0.2,0.3?0.5?m)was added to the continuous operating sequence batch activated sludge reactor(SBR)which was started and operated with the hydraulic retention time of anaerobic 12h-aerobic 12h.The results showed that compared with the control group(AS system),the Fe3O4/AS system has better ability of TBP removal and stronger impact load resistance.When the influent concentration of TBP was 30mg/L,the average removal efficiencies of TBP in Fe3O4/AS system were 91.09%,14.06%higher than that in AS system.In the effluent of Fe3O4/AS system after anaerobic reaction,the main production was bromophenol(BP)and the content of BP was more than those of AS system,which provided a good reaction condition for the complete mineralization of dehalogenated products.Moreover,the Fe3O4/AS system has a better hydrolytic acidification effect,the acetic acid produced could be fully utilized by the methanogenic bacteria,without accumulation of propionic acid and butyric acid.Compared with the AS system,the activity of dehydrogenase and the content of Heme C in the sludge are 5-folds and 1.7-folds respectively,the abundances of the iron reductive bacteria(Geobacter),iron oxidative bacteria(Thiobacillus)and magnetite biosynthetic related bactiteria(Magnetospirillum)greatly improved,despite the microbial community composition rarely changed.The microbes in the Fe3O4/AS system produced more hydrogen peroxide which could react with dissolved Fe(?)in the system to form Fenton like reaction to generate hydroxyl radicals with strong oxidation,thus removing organic substances efficiently.Overall,the reasons for better TBP and COD removal efficiency of Fe3O4/AS system are as follows.During the anaerobic phase,the supplementation of magnetite promotes DIET,thereby accelerating the reductive dehalogenification of TBP.Fe3O4/AS system enhances the hydrolysis and acidification of organic compounds and the magnetite is dissolved.During the aerobic phase,the microorganisms degrade and mineralize the dehalogenation products further.the Fenton like reaction occurs by the reaction between hydrogen peroxide and Fe(?)in Fe3O4/AS system,which enhances the oxidative degradation.Magnetite was reduced and dissolved by eugonic iron reducing bacteria during the anaerobic phase and the Fe(?)concentration was up to 42.3mg/L,but the Fe(?)concentration of the effluent after aerobic phase was always maintained below 10mg/L.After 80 days of reactors operation,large amounts of iron were remained in the Fe3O4/AS system.It is found that magnetite was still the main form of iron oxide by XRD analysis,which confirmed that the dissolved Fe(?)was oxidized back to the Fe3O4/AS system through the aerobic phase of the reaction in-situ oxidation effect.The existence of magnetite biosynthetic related bacteria(Magnetospirillum)brought in sustainable biointensification effects for the reaction.