The Degradation Of Nitrobenzene In Biological Sponge Iron System

Nitrobenzene（NB）is chemically stable,highly toxic and widely used.It is difficult to treat nitrobenzene wastewater by conventional biological methods.Adding functional carrier to activated sludge system,the interaction between the carrier and the microorganism can enhance the biodegradation.This technology is simple and can be easily implemented for engineering application.The biological sponge iron system is formed by adding sponge iron to activated sludge system.It not only has the technological features and advantages of the Fe⁰/O₂ system and the bioferric process,but also produces iron-oxidizing bacteria.Iron-oxidizing bacteria can promote the formation of H₂O₂ in the process of oxidizing Fe²⁺.The H₂O₂ secreted by iron-oxidizing bacteria and the Fe²⁺dissolved from sponge iron,could theoretically produce Fenton reaction,thus strengthening the degradation of nitrobenzene.However,there are few reports about the Fenton effect and refractory organics of biological sponge iron system.Nitrobenzene was used as the target pollutant.The degradation ability of nitrobenzene was compared with the activated sludge system and the biological sponge iron system in the domestication process by contrast experiments.High-throughput sequencing was performed on the bacterial flora of the biological sponge iron system and the activated sludge system after 5 months of nitrobenzene domestication.The degradation characteristics and the Fenton-like effects of the biological spongy iron system under different conditions were studied.Through comparison of iron oxidase activity and nitrobenzene degradation ability,a high-efficiency nitrobenzene-degrading bacterium with iron oxide function was selected from the domesticated sponge iron system.The growth and degradation characteristics of this strain were studied.Besides,the biodegradation mechanism of nitrobenzene in biological sponge iron system was discussed.The study found that the activated sludge system required 56 d to degrade 300mg/L nitrobenzene.The removal rate of nitrobenzene was more than 98%with initial concentration of nitrobenzene at 300mg/L when the biological sponge iron system domesticated to 28 th day,and the acclimation cycles was shortened by 28 days compared with the activated sludge system.High-throughput sequencing was performed on the bacterial flora of the biological sponge iron system and the activated sludge system after 5 months of nitrobenzene domestication.At the level of phylum,the top ten dominant populations of this two systems were the same,but the proportions were changed,especially the Proteobacteria.Compared with the activated sludge system,the proteobacteria in biological spongy iron system was increased by 15%.At the level of genus,it was found that Thauera had the highest proportion in the biological sponge iron system,accounting for 23.36%of the total bacterial population,followed by Dokdonella,which was 5.61%.Thauera and Dokdonella in biological sponge iron system were improved by 21.99%and 3.01%respectively than the activated sludge system.Thauera has a good ability to degrade aromatic compounds,and Dokdonella can promote the conversion of nitrogen.The existence of a large number of Thauera and Dokdonella detected in the biological sponge iron system provided strong evidence for the efficient degradation of nitrobenzene in this system.The degradation reaction of nitrobenzene by biological sponge iron system followed zero-order kinetics model.From a practical application point of view,the optimum conditions for the degradation of nitrobenzene by the biological sponge iron system were determined as follows:the pH value was 6.4（tap water pH value）,the reaction temperature was 30℃and added 90g/L sponge iron with particle size of 2³mm.Under the optimum conditions,when the initial concentration of nitrobenzene simulated wastewater was 200mg/L,the degradation of nitrobenzene in the biological sponge iron system inoculated with domesticated iron sludge（biological sponge iron system sludge after nitrobenzene domestication）was 31.5min^-1.Besides,the degradation rate of nitrobenzene and TOC were 92.0%and 63.1%respectively,which were 22.3%and 11.4%higher than the superposed values for degradation rates of nitrobenzene in sponge iron system and in iron sludge system alone.The strain Y-9 was identified as Arthrobacte sp.The growth and degradation characteristics of strain Y-9 were studied.It was found that the fastest time point for strain growth was 40 hours.In addition,the suitable growth conditions for strain Y-9 were as follows.Nitrobenzene initial concentration was 200 mg/L,inoculation amount was 10%,pH range was 6⁸,sponge iron dosage was 30g/L,temperature range was 25⁴0℃.In order to further explore the mechanism of biodegradation of nitrobenzene in the biological sponge iron system,the strain Y-9 was enriched into the bacterial sludge under suitable growth conditions.Through investigation of the Fenton-like effect in the sponge iron system,sludge system,and biological sponge iron system,the concentrations of Fe²⁺,H₂O₂ and·OH in the biological sponge system,especially inoculated with iron sludge,were significantly higher than that of in the sponge iron system and the sludge system,which provided stronger Fenton-like effect.The degradation pathway of nitrobenzene in the biological sponge iron system was mainly based on partial reduction pathways.