The Effect Of Iron-Bearing Minerals On Denitrification In Urban River

The emphasis of ammonium-removal nitrogen often leads to too much attention given to the nitrification and thus low removal efficency of total nitrogen in river remediation.Denitrification is an important way to reduce nitrogen load.As an active redox element with high content in the crust,the effect of iron on denitrification can not be ignored.In this research,the effects of iron-bearing minerals on denitrification process was investigated,and the corresponding microbial mechanism as well as the response of related functional microbial community were analyzed.On this basis,the enhanced denitrification of urban river was done.This research can provide theoretical evidence and technical support for enriching nitrogen-cycle theory and solving the problem of low total nitrogen removal effiency in rivers.The main results are as follows:?1?In the study of the effects of different iron-bearing minerals on denitrification process,it was found that iron-bearing minerals could promote denitrification process.Compared with the control?TN removal:60.71±2.24%?,pyrite has the highest total nitrogen removal efficiency?90.35±3.07%?followed by magnetite and siderite?88.16±2.35%and 82.15±3.17%?.Hematite can inhibit the reduction of NO₃^--N at the initial stage of operation,and the highest TN removal rate was 78.16±3.57%.The TN removal rate of iron powder?81.78±4.14%?was close to that of siderite.It was presumed that the increase of TN removal was due to the Fe?II?-mediated autotrophic denitrification process in the reaction system.In addition,NH₄⁺-N was obviously produced in the iron powder group,and it was speculated that dissimilation nitrate reduction to ammonium?DNRA?might occur.High-throughput sequencing results indicated Proteobacteria,Chloroflexi,Bacteroidetes and Euryarchaeota were the predominant phyla of bacteria.The relative abundance of?-Proteobacteria in magnetite group was the highest?52.20%?,while the relative abundance of?-Proteobacteria in iron powder group was the highest?15.49%?.Iron-bearing minerals were beneficial to increase the diversity of nirK-denitrifying bacteria.Moreover,the relative abundance of Thauera,Ferruginibacter and Cupriavidus participating in Fe?II?-mediated autotrophic denitrification were higher in iron-bearing minerals than those in control.The results showed that iron-bearing minerals promoted the denitrification efficient to some extent.In addition,pyrite increased the relative abundance of thiobacillus significantly.?2?In the study of effects of C/N ratio on siderite and pyrite participation in denitrification process.As for the C/N ratio of 8,both the TN removal of siderite and pyrite systems had the best performance?90.11±2.48%and 93.14±2.01%,respectively?,and no accumulation of NO₂^--N was observed.In contrast,the TN removal separately decreased to 88.58±2.37%and 90.18±1.75%for the C/N ratio of12.This observation may be mainly due to the occurrence of the DNRA coupled to Fe?II?oxidation process.Additionally,as for the C/N ratio of 4,the denitrification activity of the system was weak at the initial stage,and then increased with the TN removal of 80.89±1.65%%?85.17±1.25%,respectively.The subsequent increase of TN removal may be attributed to Fe?II?-mediated autotrophic denitrification process.In general,the amount of Fe?II?dissolved on the surface of iron ore increased with the increase of C/N ratio.High-throughput sequencing results indicated the relative abundance of Proteobacteria and Chloroflexi in siderite group increased,and the Alpha diversity of nirS-denitrifying bacteria also enhanced under high C/N ratio.However,the relative abundance of Acidovorax,Cupriavidus and Dechloromonas with the function of Fe?II?oxidation coupled NO₃^--N reduction at low C/N ratio was significantly higher,which could promote the process of autotrophic denitrification.For pyrite group,the relative abundance of Proteobacteria and Bacteroidetes increased under high C/N ratio,and the relative abundance of nirK-and nirS-denitrification dominant bacteria was higher.The relative abundance of Thiobacillus,Dechloromonas and Zoogloea involved in the oxidation of Fe?II?was higher under low C/N ratio,indicating that pyrite could promote the growth of these autotrophic denitrifying bacteria under low C/N ratio.?3?In the study of enhancing denitrification in urban rivers,modified siderite and iron-encapsulated materials could significantly improve the denitrification performance of the reaction system with aeration.Compared with aeration control group,the removal rates of NO₃^--N increased from 37.25±1.58%to 65.18±4.85%,62.35±2.36%,respectively.The removal rate of TN increased from 55.46±3.36%to 74.02±5.25%and71.55±4.32%,respectively.In addition,The transformation between Fe?II?and Fe?III?was in progress,and the total iron level in overlying water tended to decrease.High throughput sequencing showed that under enhanced control of modified siderite and iron-encapsulated materials,the relative abundance of Proteobacteria increased significantly from 24.41%to 29.85%and 41.13%,respectively,and the Alpha diversity of microflora was improved.Moreover,the structure of denitrifying functional bacteria changed a lot.The relative abundance of Mesorhizobium was higher in modified siderite group,while the relative abundance of Bradyrhizobium,Pseudomonas and Thauera were higher in iron-encapsulated materials group.The relative abundance of Cupriavidus and Crenothrix which had the function of Fe?II?-mediated autotrophic denitrification increased significantly in the two groups.