Magnetite Particle-mediated Methanogenesis From Sulfate Organic Wastewater And Its Microbiological Mechanism

As a renewable clean energy source,methane can be produced from wastewater through anaerobic digestion,and it is one of the main ways to achieve resourcezation of wastewater.However,many types of wastewater,for example,sulfate-containing organic wastewater,exhibit low efficiency in the process of methane production,and even stagnated during fermantation.In recent years,the discovery of direct interspecies electron transfer(DIET)has provided a new perspective for the treatment of these types of organic wastewater.The direct interspecies electron transfer efficiency of the methanogenesis pathway would be enhanced by adding conductive materials,thereby enhanced the methane production efficiency of anaerobic digestion system.In this study,in the anaerobic digestion system,the DIET pathway of methane production was constructed by adding conductive materials in order to promote the capacity of methane production in wastewater.Firstly,nano-magnetite(Fe₃O₄)was added to the sulfate-containing wastewater methanogenesis system with different types of carbon source,and the effect of Fe₃O₄on the methane production and sulfate reduction process was explored.Fe₃O₄ could significantly increase the substrate degradation rate and the maximum methane production rate of different types of sulfate organic wastewater(the maximum methane production rate could be increased by up to 465%),and shorten the lag phase of the methane production reaction(up to 4.85 d of lag phase could be reduced).The sludge morphology,XPS,XRD and other characterization methods of the fermented sludge confirmed that Fe₃O₄ could generate Fe S and Fe S₂ with sulfate ions to reduce the sulfide through chemical reaction pathway,thus reduced the toxicity inhibitory effect of sulfide on sulfate-reducing bacteria and methane-producing archaea.Secondly,microbial community analysis showed that the syntrophic acetate oxidation-methanogenesis pathway might be constructed by Mesotoga and hydrogen-consuming sulfate-reducing bacteria or hydrogenotrophic methanogen in the CK group,which could be an emergency stragtegy for microorganisms in each system to cope with the inhibition of sulfide toxicity under high sulfide concentration.Moreover,the added Fe₃O₄ could significantly enrich Methanosaeta(the maximum increase in relative abundance could reach 30.29%),and it could effectively avoid the enrichment of Thermovirga,a bacteria that can adapt to the high-salt environment and has the sulfate-reducing ability under the condition of extremely high sulfate concentration in each system.Moreover,Fe₃O₄ could also establish the potential DIET methanogenesis between Syntrophomonas with Methanosaeta and Methanobacterium in the butyrate system to improve the efficiency of methane production.In addition,in the continuous flow mode,the sulfate-containing ethanol wastewater system was amended with nano Fe₃O₄,the influence of Fe₃O₄ on the methane production and sulfate reduction process was evaluated,and the conversion process of Fe₃O₄ was also explored.The addition of Fe₃O₄ can significantly improve the methane production performance of ethanol wastewater without sulfates,and demonstrates a higher organic load resistance to impact.When the sulfate start to increase gradually,Fe₃O₄ can significantly reduce the generation of hydrogen sulfide and the concentration of total sulfide in the system,but it did not exhibit a good methanogenesis promotion effect.XPS,XRD combined with 2D-COS analysis indicated that Fe₃O₄ would generate Fe S and Fe S₂ in the process of continuous reaction with sulfide ions,and finally mainly exist in the system in the form of Fe₃O₄ sulfur equivalent Fe₃S₄(Greigite).The results of microbial community analysis showed that,under sulate-containing conditions,Desulfovibrio dominates the degradation of ethanol in these two reactors(the relative abundance is as high as 20.23?59.52%),and Methanothrix is the most dominating methanogen(the relative abundance is as high as43.97?88.87%).In the reactor amended with Fe₃O₄,Methanothrix and Geobacter have the potential to construct the DIET methanogenesis pathway,and in the other reactor without Fe₃O₄,Methanothrix may build a syntrophic relationship with the sulfur-oxidizing bacteria Sulfurovum to promote the efficiency of methanogenesis,so these two reactors showed roughly the same methanogenesis performance.It can be seen from the above that the addition of conductive materials to the wastewater methanogenesis system can enhance the efficiency of methane production to varying degrees.The research results of this paper can provide theoretical and technical support for the construction of high-efficiency methanogenesis system and the long-term operation of the system.