Biodegradation Characteristics Of Dissolved Organic Matter In Water Environment Of The Middle Route Of South-to-North Water Diversion Project

The Middle Route of the South-to-North Water Project alleviates the water shortage in northern China,but it also brings risks to the water quality safety in the downstream,especially the Dissolved organic matter(DOM)in the water environment,which leads to the CODMn and other water quality indexes rising.Therefore,clarifying the causes of DOM rise and its migration and transformation rule in the main channel of the Middle Route of the South-toNorth Water Diversion Project can provide references for the safeguard of water quality and the prevention and control of potential risks.In this paper,uv-vis spectrum,fluorescence spectrum and fourier transform ion cyclotron resonance mass spectrometry(FT-ICR MS)are used to analyze the chemical structure and substance composition characteristics of DOM from different sources in the main channel,and to identify the direct source of DOM in main channel.Through biodegradation experiments,the removal and transformation effects of microbial activities on DOM from different sources are explored,and indirect sources of DOM from main channels are identified.Finally,the influence mechanism of irradiation on DOM biodegradation and the key substances of the interaction between photodegradation and biodegradation are discussed.The main results are as follows:(1)The origin of DOM determines its initial chemical structure and substance composition.Spectroscopic analysis of the photochemical characteristics of DOM from different sources showed that the humus content and aromatics of extracellular organic matter(E-DOM)and water dissolved organic matter(W-DOM)released by algae activity and metabolism were higher than those of intracellular organic matter(I-DOM)released by algal cell breakdown and organic matter from sediment(S-DOM).FT-ICR MS is used to analyze the molecular structure and composition of DOM.It is found that W-DOM and EDOM were mainly composed of unsaturated lignin,while I-DOM and S-DOM were mainly composed of lipids and proteins.Through the analysis of DOM element composition,it is found that E-DOM and W-DOM are rich in N and S elements respectively,and the relative abundance of CHO components in each sample is about 50%～80%.Finally,through the analysis of the correlation and differences between DOM from different sources,it can be seen that the similarity between W-DOM and E-DOM is more than 60%.The results indicate that E-DOM metabolized by algae during the growth process is the direct source of river DOM with the characteristics of refractory organic matter(RDOC).(2)The chemical structure and composition of DOM affect the removal rate,migration and transformation of DOC in the process of biodegradation in river.In the biodegradation process,the DOC removal rate of I-DOM and S-DOM was significantly higher than that of E-DOM and W-DOM.Spectroscopic analysis of the photochemical characteristics of DOM from different sources showed that in the biodegradation process,the concentration of colored dissolved organic matter(CDOM)in I-DOM and S-DOM first decreased and then increased.This is because the use of CDOM in the biodegradation process produced fluorescent and aromatic macromolecular humus substances.Although the biodegradation efficiency of E-DOM and W-DOM is low,the concentration of CDOM is still significantly reduced,and the macromolecular substances are decomposed into small molecular substances,while the aromatics remain basically unchanged.FTICR MS was used to analyze the DOM molecular level,and it was found that the biodegradation of I-DOM and S-DOM would generate lignin rich in water,indicating that the newly generated substances in the biodegradation process of IDOM and S-DOM were indirect sources of organic matter in river channels.(3)The effects of irradiation on the biodegradation efficiency of DOM from different sources are different.Compared with biodegradation,in the process of photo-biodegradation,photodegradation reduces the biodegradation rate of WDOM,I-DOM and S-DOM samples mainly by competing with biodegradation for lignin.However,in E-DOM samples,photodegradation consumes lignin that is less labiled to biodegradable,and at the same time,part of the biodegradable tannin supplement system is generated.As a result,the biodegradation rate of EDOM is basically unchanged after irradiation.Lignin,as a key substance in the interaction between photodegradation and biodegradation,becomes stable over time and may accumulate in rivers,leading to a series of water quality problems.Therefore,in river water quality management,it is important to monitor E-DOM released by algae,especially the evolution and turnover of lignin components.