Research On The Process And Mechanism Of The Removal Of Nitrate Nitrogen By Zero-valent Iron Coupled Microorganisms

Due to the use of nitrogen-containing chemical fertilizers and the discharge of industrial wastewater and domestic sewage,nitrate nitrogen pollution in groundwater is increasingly aggravated.At present,the methods to remove nitrate nitrogen in groundwater mainly include physical remediation technology,bioremediation technology and chemical remediation technology.Physical remediation technology,in essence,only occurs the transfer and concentration of nitrate,which has a high cost Bioremediation technology has high efficiency and low consumption,and can convert nitrate nitrogen into nitrogen.However,heterotrophic denitrifying bacteria need organic matter as carbon source,which may cause secondary pollution of organic matter.Among chemical remediation technologies,zero-valent iron remediation technology has relatively low cost and ideal treatment effect and is easy to implement However,the reduction product is mainly ammonia nitrogen,and the removal efficiency is generally high under acidic conditions,and the limitation of p H is large.How to achieve the directional efficient removal of nitrate nitrogen in the zero-valent iron reduction system has been a hot topic in the current research.In this study,the liquid phase reduction method was used to prepare nano-zero-valent iron nano-ferric bimetallic composite material supported nano-ferric composite material,and the nitrate nitrogen removal and denitrification effect of the material were explored through batch experiments and column experiments,and then the microbial system Fe/CSC coupling microbial system was explored The nitrate nitrogen removal effect and directional nitrogen conversion effect of FeS combined with Fe/CSC coupled microbial system are mainly studied as follows1.Nano-sized zero-valent iron was prepared by liquid phase reduction method of sodium borohydride with ferric chloride as iron source.Meanwhile,Fe/Cu and Fe/Pd materials were obtained by introducing copper and palladium on this basis.Under the conditions of p H 3,dosage of 6 g/L and nitrate nitrogen concentration of 20 mg/L,the nitrate nitrogen removal efficiency and nitrogen conversion efficiency of zero-valent nano-iron were 50.43% and12.52%,respectively.In the bimetallic system,when the Fe/Cu loading ratio is 5%,the nitrate nitrogen removal efficiency and nitrogen conversion efficiency are 77.25% and 21.51%,respectively.When the Fe/Pd loading ratio was 0.1%,the maximum nitrate nitrogen removal efficiency and nitrogen conversion efficiency were 80.27% and 26.46%,respectively.The bimetallic system is better than nano zero-valent iron material,and the denitrification performance of Pd loaded is better than that of Cu loaded.The nitrate nitrogen removal efficiency and nitrogen conversion efficiency of zero-valent iron materials can be improved by loading copper and palladium metals.2.In order to improve the material utilization rate and improve the nitrate nitrogen removal efficiency and nitrogen conversion rate,this study supported grapefruit peel carbon diatomite with nanometer zero-valent iron materials by in-situ synthesis method,and prepared different carrier supported nanometer zero-valent iron composites,coconut shell carbon(CSC)was used as the carrier in the Fe/C mass ratio of 1: 2,Fe/CSC,Fe/Cu/CSC and Fe/Pd/CSC nitrate nitrogen removal efficiency are 82.03%,71.51% and 70.59%,respectively,and nitrogen conversion efficiency is 34.10%,36.57% and 49.23%,respectively.Fe/CSC has the highest nitrate nitrogen removal efficiency,and Fe/Pd/CSC has the highest nitrogen conversion efficiency.The nitratenitrogen removal rate and nitrogen conversion rate of the zero-valent iron material were improved greatly,while the nitrate-nitrogen removal rate of the bimetallic material was reduced but the nitrogen conversion rate was improved greatly.In addition,it can be applied to the removal of nitrate nitrogen in a wider p H range.The denitrification performance of the material decreased with the increase of p H and increased with the increase of dosage.With the increase of the initial concentration of nitrate nitrogen,the nitrate nitrogen removal efficiency and nitrogen conversion efficiency of the material will decrease,but the nitrate nitrogen removal amount and nitrogen generation amount will increase.3.In order to investigate the denitrification performance of zero-valent iron material under the actual continuous reaction process conditions,the material was filled in the filter column and the continuous flow experiment was carried out.In column experiment,the pillars of the load type nano composite material filling iron can quickly remove nitrate within 1 h,run 10 h failure,And with the longer run,the column experiment of nitrogen conversion efficiency will become poor In addition,the column experiment materials nitrate removal efficiency and utilization rate of nitrogen transformation is superior to the batch experiment,experiment showed that column can improve material utilization.4.The nitrogen conversion of pure zero-valent iron materials and supported zero-valent iron materials still need to be improved.In this study,microbial and nano zero-valent iron composite materials were coupled for nitrogen removal.Under the conditions of neutral p H and2:1 C/N ratio,the denitrification efficiency of Fe/CSC coupled microbial systems is better than that of Fe/Cu/CSC and Fe/Pd/CSC coupled microbial systems.The removal efficiency of nitrate nitrogen by microorganisms at 20,50 and 100 mg/L was about 19%.The removal efficiency of nitrate nitrogen by Fe/CSC coupled microbial system at 20,50 and 100 mg/L was 100%,60%and 40%,respectively.Fe/CSC coupling microbial system can greatly improve the denitrification effect,and the nitrate nitrogen removed is converted into nitrogen.Fe/CSC has a synergistic effect with microorganisms.Under the condition of no carbon source,the denitrification rate of microorganisms is slow,and the denitrification effect can basically achieve the same effect eventually,but it will cause the accumulation of nitrite nitrogen and the generation of ammonia nitrogen.In the coupling process of Fe/CSC materials,Fe/CSC materials played a major role in nitrogen removal in the early stage,and microorganisms played a major role in the late stage,and the final denitrification effect was better than that of Fe/CSC materials.5.FeS can not only reduce nitrate,but also be used as electron donor by autotrophic denitrifying bacteria for denitrification.The introduction of FeS can promote the microbial denitrification of Fe/CSC and Fe/CSC coupled materials.Under neutral conditions,the removal efficiency of 20,50 and 100 mg/L nitrate nitrogen by Fe/CSC material is 78%,57%,43%,respectively,and the nitrogen conversion efficiency is 49%,39%,28%,respectively.The effect of FeS dosage and nitrate concentration on denitrification was not significant.In terms of nitrate nitrogen removal efficiency,the effect of FeS on nitrate nitrogen removal is related to Fe content.In terms of nitrogen conversion efficiency,Fe/CSC denitrification promoted by FeS is related to the p H of the solution.In the presence of carbon source,because the dominant strain is heterotrophic denitrification bacteria,the utilization rate of FES is very low,and the promotion of microbial denitrification is not significant.When the ratio of carbon to nitrogen is2:1,the removal efficiency of 20,50 and 100 mg/L nitrate nitrogen by FeS combined with Fe/CSC coupled microbial system is 100%,70% and 50%,respectively.The removed nitrate nitrogen is converted into nitrogen.The denitrification efficiency of this system is better than that of other systems.No carbon source will affect the denitrification rate in the process of FeS combined with Fe/CSC material coupling microbial system,but the final complete removal of nitrate nitrogen requires the same time,and will result in the generation of ammonia nitrogen.Compared with the Fe/CSC material coupling microbial system,the introduction of FeS can shorten the time of complete removal of nitrate nitrogen and reduce the accumulation of nitrite nitrogen.In conclusion,the nanometer zero-valent iron material has a certain removal efficiency for nitrate nitrogen,but the nitrogen conversion efficiency is low.The nitrate nitrogen removal rate and nitrogen conversion efficiency can be improved by supporting metal and improving the carrier to prepare nano-sized zero-valent iron composite material.The micro-organism coupling nano-sized zero-valent iron composite material can completely remove 20 mg/L nitrate nitrogen,and the removed nitrate nitrogen can be converted into nitrogen.The introduction of iron sulfide can promote the denitrification of nano-zero-valent iron composites and microbial coupling nano-zero-valent iron composites,and the coupling system finally constructed shows good denitrification performance.