| The rapid development of chemical and medical industries in China results in enormous discharge of industrial wastewater containing toxic refractory organics, which has become a major barrier to environmental health and water ecological security. Chloronitrobenzenes (CINBs) as an important basic raw material, have been widely used for synthesis of dyes, pharmaceuticals and pesticides. These compounds have stable chemical and biorefractory properties, which could be easily accumulated in sediments and soils, and subsequently threaten the human health and social security via food chains. Therefore, relevant environmental behavior and effective control technologies for CINBs have attracted more and more attention in the world. Since anaerobic bioreactor has the disadvantages of long period start-up, easy to acidification and poor performance in refractory wastewater treatment, magnetite enchanced bioelectrode-UASB is developed to treat wastewater containing CINBs using the techniques of electrochemistry, SEM-EDX and GC-MS. Pollutant removal capacity, start up performance and mechanisms of magnetite enchanced bioelectrode-UASB were investigated in this paper:1、 Because of direct electron transfer process playing a key role in anaerobic reduction of persistent toxic organic compounds, multiple sets of separate anaerobic sludge and bioelectrode-anaerobic sludge batch device were established in this paper. The influence of magnetite dosage on anaerobic sludge system as well as bioelectrode-anaerobic sludge system treating p-CINB were discussed. Results showed that all systems have experienced the same p-CINB transformation process. Among them, magnetite enchanced bioelectrode-anaerobic coupling system had the highest dechlorination kinetic constant and the strengthening factor Q’reached 3.73. Analysis of cyclic voltammograms (scan rate:10 mV s-1) revealed the presence of redox peaks in biocathode and when potentials were lower than-1000mV, its current density was over 6.7 times higher than that observed with graphite electrodes that were not inoculated (both in the presence and in the absence of magnetite nano-particles). In comparison, the biological cathode current density (1.6 fold) could be further enhanced by adding magnetite. This finding confirmed that extracellular electron transfer (EET) can be significantly enhanced by adding magnetite.2、In order to explore the influence of magnetite dosage on pollutants removal in bioelectrode-UASB system and anaerobic sludge granulation, magnetite enchanced bioelectrode-UASB reactor was established. Results showed that under the startup stage, performance of pollutants removal improved significantly, COD removal was eventually stabilized at above 99% and the average particle size of granular sludge reached 621.4μm, which were significantly better than that of the control reactor. Meanwhile, in comparison of bioelectrode-UASB reactor, a higher current density and lower oxidation-reduction potential were achieved in magnetite enchanced bioelectrode-UASB reactor. These changes benefited methanogenesis and sludge granulation. Scanning electron microscopy (SEM) images showed a more clear look and dense structure of anaerobic granular sludge in magnetite enchanced bioelectrode-UASB reactor. They were covered with iron on their surface and nano-magnetite inside, confirming that the innovative bioelectrode-UASB reactor performance could be strengthened by adding magnetite.3、On the basis of successful startup in magnetite enchanced bioelectrode-UASB reactor,2,4-dichloro-nitrobenzene (2,4-DClNB) removal performance and reductive dechlorination mechanism were investagated. Results showed that with the influent 2,4-DClNB load rates increasing from 25g m-3 d"1 to 200 g m-3 d-1, the COD, target pollutant removal and pH in combined system were stable relatively, and the removal efficiencies were up to 97% and 100%, respectively. The reaction pathway for microbial reduction of 2,4-DClNBs was followed as 2,4-DClNB' 2,4-DClAn' 4-ClAn (primary product)/2-ClAn (secondary product)'…' mineralization. Results showed that both magnetite enchanced bioelectrode-UASB reactor and bioelectrode-UASB reactor could enhance the final mineralization process 2,4-DClNB, thereby reducing the accumulation of poison intermediates. The electrode biofilm was analysed by SEM, and results showed that both the anode and cathode biofilms were covered with nano-magnetite. Inside the biofilms, there also existed nano-magnetite. This finding could further confirmed that nano-magnetite plays an important role in ehchancing extracellular electron transfer (EET) and pollutants removal in magnetite enchanced bioelectrode-UASB reactor. |
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