Denitrification Characteristics Of Sediments And Transport Behavior Of Ammonia Nitrogen In The Yarlung Zangbo River

In river systems,nitrogen pollution has always been an environmental issue concerned by domestic and foreign scholars,and ammonia nitrogen（NH₄⁺-N）is main nitrogenous pollutant in the rivers in China.The Yarlung Zangbo River is a plateau river with unique water quality and ecological characteristics.With the development of tourism and hydropower stations,the ecological environment of this plateau river is under increasing pressure.In this study,the middle and lower reaches of the Yarlung Zangbo River were taken as the research area to analyze the denitrification characteristics and the community structure of denitrifying bacteria in the sediments,and identify the key environmental factors affecting the denitrification process.On this basis,the adsorption and desorption behaviors of NH₄⁺-N in the sediments in the Yarlung Zangbo River were determined,and the responses of NH₄⁺-N transport to the environmental factors were analyzed.（1）The denitrification rates ranged from 2.70 to 31.83μmol N/（m²·h）in the sediments in the middle and lower reaches of the Yarlung Zangbo River,which were significantly higher than anammox rates,suggesting that denitrification was main process to remove nitrogen.The diversity of bacterial communities as well as nir S type denitrifying bacteria were significantly different in the sediments of different sections of the main stream of the Yarlung Zangbo River.The discharge of urban sewage increased the content of NH₄⁺-N in the river water,increased the relative abundance of Bacteroidetes,Firmicutes and Proteobacteria,and reduced the diversity of bacteria and nir S type bacteriain the sediments.NH₄⁺-N concentration,ORP and temperature in water and NO₂^--N concentration in sediments were key environmental factors affecting the community structure of nir S type denitrifying bacteria.（2）The adsorption process of NH₄⁺-N in the surface sediments in the Yarlung Zangbo River included rapid adsorption,slow adsorption and equilibrium stage,which followed pseudo-second order model or particle internal diffusion model.Significant desorption hysteresis was observed for NH₄⁺-N in the sediments.The adsorption thermodynamics of NH₄⁺-N were influenced by its initial concentration in water,the adsorption isotherm curves at higher concentrations（>5mg/L）followed Langmuir（R²≥0.97）and Freundlich models（R²≥0.91）,and the desorption isotherm data were fitted to Freundlich model（R²>0.93）.However,the adsorption isotherm data at lower concentrations（≤5mg/L）followed Henry Model（R²≥0.90）.The critical mass concentrations of adsorption-desorption for NH₄⁺-N were much higher than that in overlying water,indicating that the sediments in the Yarlung Zangbo River may play the role of"source"for some time to come.（3）Through the single-factor influence study,it was found that the temperature,DO,p H,conductivity,disturbance in the water,and the particle size and organic matter content in the sediments influenced the adsorption and desorption behaviors of NH₄⁺-N,in which,p H,disturbance,the particle size and organic matter content of sediments are key factors.By multiple factors of orthogonal test,the adsorption isotherms of NH₄⁺-N followed the Langmuir model,and the desorption isotherm curves followed the Freundlich model,and the predicted maximum adsorption capacity of NH₄⁺-N was 5205.41 mg/kg.In water-surface sediment system,the particle size of sediment posed the strongest effect on the adsorption behavior of NH₄⁺-N,while the disturbance had the greatest influence on the desorption behavior of NH₄⁺-N.