Effect Of Magnetite Nanoparticles On Methanogenesis By Anaerobic Digestion

As a synthesis technique to combine environmental protection, energy recovery and ecological cycle, anaerobic digestion attracts extensive attention worldwide and widely used in the treatment of wastewater, sludge, municipal solid waste and anaerobic fermentation of crop straw,livestock waste. Among the microorganisms involved in anaerobic digestion process, for the characteristics of high demanding to anaerobic conditions,lower redox potential, narrow optimum p H range and sensitive to toxic and hazardous substances, methanogens become the main reasons to constrain the anaerobic digestion efficiency.As one of the metal involved in the metabolic acticity of methanogens, the introuduction of different state and valence Fe to improving the metabolic activity and system efficiency by e has been a hot topic in the field. While problems such as toxicity caused by some complicated compounds formed by high dosage, high consumption rate of Fe ion and anion inhibition limit the application of adding Fe ions. In order to solve this problem, this study puts forward that introduce the Fe3O4 NPs into the anaerobic digestion system, and through two stage experiments to study the effect of Fe3O4 NPs on anaerobic methanogenesis and analyz the promoting/inhibiting effect on anaerobic methanogenesis, which could provide new ideas and theoretical basis to improve gas production by anaerobic degrade wastewater,sluge, solid waste,crop straw and manure.In the first stage, study use simulated wastewater of sodium acetate as the substrate, and investigate the effect of different concentration and size Fe 3O4 NPs on anaerobic methanogenesis from macro perspective such as CODCr degradation, system environmental paramrters(p H/ORP), cumulative gas/methane yield and methane producing rate by static sequencing batch experiment device.The results show that except size of 80~100nm, low particle size Fe3O4NPs(0 ~ 80 nm) can speed up the process of anaerobic degradation of CODCr, and the promoting effect of 40~60 nm Fe3O4 NPs on substrate degradation is most obvious.The influences of different size Fe3O4 NPs on the system environmental parameters(p H/ORP) are analogous:Fe3O4NPs could increase the p H and promote the formation of weak alkaline environment(7.0~7.6) by enhance the degradation rate of CODCr. Meanwhile, as mixed valence iron oxide, Fe3O4 NPs makes ORP decrease to ~340 m V rapidly after the slight increase, and form anaerobic environment which is more conducive to the growth of anaerobic methanogens. The effect of Fe3O4 NPs on cumulative gas/methane yield showed “promotion at lower and inhibition at higher” with the increase of particle size and concentration of Fe3O4 NPs. and add the appropriate concentration(100 ~ 400 mg/L) can be obviously im proved the methane production rate, and keep it at high level in a relatively long period of time. Comprehensive comparative investigate the effect of Fe3O4 NPs on macro indicators of anaerobic methanogenesis process, the study selected 40~60 nm, 200 mg/L(0.117 g/g VSS) as the optimal batching.In the second stage, study use simulated wastewater of sodium acetate as the substrate, and analyze the effect of Fe3O4 NPs on anaerobic methanogenesis from micro perspective such as morphology of Fe, structure change of Fe3O4 NPs,ecological paramrters(p H/ORP), cumulative gas/methane yield and methane producing rate. key enzymes activity(F420、Co M) and colony diversity and abundance through a semi-continuous flow experiment device under the condition of optimal batching.The results show that the Fe3O4 NPs can slowly release Fe2+/Fe3+ into the system, and can increase the concentration of Fe2+ from 1.75mg/L to 3.6 mg/L by the synergism of microorganisms. Combined wtih the dilution experiment, study found Fe3O4 NPs as the source of iron could maintain Fe2+ content and improve the system stability. In addition, compared with the result of experimental group of Fe2+(4 mg/L), study found Fe3O4 NPs mainly as a kind of electron transfer carrier to improve the efficiency of anaerobic digestion.By the contents of differernt morphology Fe in the anaerobic sludge, study found Fe3O4 NPs could obviously increase the total iron and the biological effectiveness iron content in the sludge, which be increased from 5.06% to 12.4% by adsorption and complexation reaction.In addition, the research investigated the influence of Fe3O4 NPs on metabolism activity of anaerobic methanogensis through the content changes of key enzyme(F420, Co M).The results show that the content of F420 and Co M been increased from 0.523 umol/g VSS and 5.352 umol/g VSS upto 0.956 umol/g VSS and 9.267 umol/g VSS, respectively.At the same time combined with the growth of soluble microbial metabolites content, study found Fe3O4 NPs can obviously increase the metabolism activity of anaerobic methanogensis. The results of high-throughput sequencing shows that Fe3O4 NPs can improve the microorganism flora diversity in the anaerobic sludge, increasing OTU number from 43 in the control group to 47. Meanwhile Fe3O4 NPs can cause the changes of microbial dominant species and the methanogens abundance, further strengthen dominant strains position of Methanolinea, and increase the contents of Methanoregula, Methanosaet and Methanosarcina,but decrease the content Methanobacterium.Combine the promoting effect of Fe2+(4 mg/L) on anaerobic methanogengesis with the changes of colony abundance, we conclude that the main way of Fe3O4 NPs promote methanogensis is Fe3O4 NPs act as electron transfer carrier to improve the efficiency of electron transfer and change the acetic acid degradation pathway;the second way is promote methanogenesis by release Fe2+.