The Transformation And Fate Of Nitrogen And Fe In Ferrous-dependant Denitrification Process

Nitrate is a crucial nitrogen pollutant in surface water,and its transformation and fate are directly related to the environmental behavior of nitrogen in water.Fe,an important element widely distributed and abundant in water environments,plays an important role in the microbially mediated Fe-N cycles.In anoxic habitats,NO₃^–acts as the most preferable electron acceptor and is reduced to gas products（N₂O or N₂）via ferrous-dependant denitrification.At the same time,the form of Fe is also transformed to produce different types of minerals.In order to clarify the effect of ferrous-dependant denitrification on N transformation in water environment under changing conditions,and the impact of important factors on the N and Fe transformation,laboratory experiments of overlying-sediment incubation and pure culture of Paracoccus denitrificans were performed in this study.The main results were as follows:（1）After the Fe（Ⅱ）and nitrate were added into the overlying water-sediment simulation experiments,the decreases of Fe（Ⅱ）and nitrate concentrations were accompanied by increases in nitrite and N₂O production.The contribution of Fe（Ⅱ）oxidation to the nitrate reduction accounted for 27.7%at the end of the incubation,and the rate of Fe（Ⅱ）decrease was significantly correlated（P<0.05）with the production of N₂O.The abundance of nitrate-dependent Fe（Ⅱ）-oxidizing bacteria in the sediment ranged from1.1×10⁵ cell g^-1 wet sediment to 1.4×10⁶ cell g^-1 wet sediment,and increased with the increase of Fe（Ⅱ）input concentration.The abundance of NDFOB was significantly correlated（P<0.01）with the concentrations of NO₂^–and Fe（ⅡI）,and a positive relation（P<0.05）between NDFOB and accumulative N₂O emission were also found.The production of N₂O was mainly mediated by biological role in the sediment.Results of metagenomics sequencing of bacteria in the sediment showed that NDFOB included Pseudogulbenkiania sp.MAI-1,Pseudogulbenkiania ferrooxidans,Acidithiobacillus ferrooxidans,Pseudomonas stutzeri,Rhodocyclaceae bacterium Paddy-1,Paracoccus denitrificans and Citrobacter freundii.（2）The driving effect of Paracoccus denitrificans to ferrous-dependant denitrification was showed in pure cultivation experiment.Compared with the groups in the absence of Paracoccus denitrificans,the reduction rates of NO₃^-in the groups added with bacterial suspension increased,accompanied by Fe（Ⅱ）oxidation,and the accumulation of denitrification product N₂O was significantly higher（P<0.05）than that in the control group without bacterial suspension.The formed Fe mineral obtained from the pure culture system in the presence of bacterial suspension was detected to beβ-Fe OOH using M?ssbauer spectrum.（3）The transformation of N in ferrous-dependant denitrification of pure culture system was afftected by changing Fe/N molar ratios.In general,the reduction rate of NO₃^-decreased with increases in Fe/N ratios,and the accumulative NO₂^-was higher at an initial Fe/N molar ratios of 1-5.Fe/N ratios could alter the proportion between N₂ and N₂O yields,and N₂O emissions were observed to be promoted by high Fe/N ratios;when the Fe/N ratios within the scope of 1-6,the emission of N₂ showed a tendency to increase with increases in Fe/N ratios,and the hightest N₂ production rate was found at an initial Fe/N ratios of 6.（4）The Fe/N molar ratios played a crucial role on the fate and behavior of Fe in ferrous-dependant denitrification of pure culture system.The dominant Fe oxyhydroxides were detected to be vivianite（Fe₃（PO₄）₂）·8H₂O),goethite（α-Fe OOH）and ferrihydrite（Fe₅HO₈·4H₂O）in different Fe/N series,while a marginal fraction of magnetite（Fe₃O₄）was also observed.Results of field emission scanning electron microscopy（FESEM）showed that the coverage of Fe contents in the cell surface tended to increase with increases in Fe/N ratios during the culture,and a significant inhibit for cell metabolism were observed when the Fe/N higher than 8:1.The contents of P-Na₂S₂O₄ adsorpted by Fe minerals in all Fe/N series were measured to be the most abundant fraction.A significant（P<0.05）positive relationship between total phosphorus（P_T）and reducible oxides(Fe_ox2)was obsetved.