Construction And Enhancement Mechanism Of Direct Interspecific Electron Transfer During Anaerobic Digestion Of Reactive Red 2

Azo dyes are currently widely used dyes in the printing and dyeing industry.During the process of production and use,hard biodegradable printing and dyeing wastewater with complex components will be produced.If not properly treated,it will cause persistent pollution to the environment.Anaerobic digestion technology has been widely used in the treatment of high-concentration wastewater due to its characteristics of low cost and recycling energy.However,in the traditional treatment process,the electron transfer method based on interspecific hydrogen（or formic acid）as a carrier is relatively fragile and susceptible to environmental conditions,resulting in the increase of the partial pressure of H₂ in the anaerobic system,which limits the efficiency of anaerobic digestion.The latest research has shown that there is a new method of electron transfer in the anaerobic system,namely direct interspecies electron transfer（DIET）,which uses cytochrome or extracellular conductive hyphae（pili）to replace H₂ for electron transfer and has a higher electron transfer efficiency.Nevertheless,the low abundance of electroactive microorganisms in traditional anaerobic digestion systems limits the construction of DIET.In this study,the azo dye reactive red 2（RR2）was used as the substrate.Exogenous substances（i.e.ethanol and nano-ferric oxide loaded activated carbon（NFGAC））were added to the anaerobic system to increase the abundance of electroactive microorganisms and strengthen DIET process.and then achieve the purpose of improving the degradation efficiency of RR2.The specific research content is as follows:1.The ethanol was added into the anaerobic digestion system of wastewater with RR2.The study found that at the end of the fourth cycle,the RR2 removal rates of the blank group,ethanol group and sodium acetate group were 21.6%,91.4%and76.8%,and COD removal rates were 35.1%,82.6%and 68.0%,respectively.Similar to the removal of RR2 and COD,the CH₄ production of the ethanol group was 11.1times and 1.2 times as many as that of the blank group and the sodium acetate group,respectively.The mechanism study showed that the sludge in the ethanol reactor has high activity and enriched with a large amount of Geobacter,which formed DIET with methanogens,solving the problem of slow electron transfer rate.In addition,the analysis of ultraviolet spectroscopy and gas chromatography-mass spectrometry（GC-MS）data showed that RR2 was gradually transformed into hydrocarbons,phenols and alcohols with smaller molecular weight and even to CH₄,CO₂ and H₂O with fracture of the azo bond.2.Nano-Fe₃O₄ modified granular activated carbon（NFGAC）with different impregnated iron solution concentrations was prepared by chemical co-precipitation method,and it was proved to be well loaded by XPS,XRD and SEM and other characterization methods.NGFAC was used for the annoying of azo dye RR2wastewater.The results showed that the addition of NGFAC effectively promote the removal of organic matter and the generation of methane.In the 0.18-NFGAC reactor with RR2 as the sole substrate,the RR2 removal rate and methane production were increased by 117.9%and 167.2%compared with the blank reactor,respectively.Further analysis showed that in the RR2 anaerobic digestion process,the addition of NGFAC effectively improved the activity of the electron transfer system（ETS）.One possible mechanism was that nano-Fe₃O₄ doping on the GAC surface increased the number of functional groups in NGFAC,which was beneficial to improve the electron accepting capacity（EAC）and electron supply capacity（EDC）.The microbial community analysis showed that bacteria（Halomonas,Pseudomonas and Bacillus）related to RR2 removal in the NGFAC reactor were abundantly enriched.In addition,it was proved that the addition of NGFAC promote DIET between Pseudomonas and Methanosarcina.