Effect Of Drying-wetting Alternation On The Structural Characteristics Of Wetland Sediment-derived DOM And The Binding Behavior With Heavy Metals

In recent years,the study of dissolved organic matter(DOM)is still on the surface,and there is still a lack of in-depth understanding of the composition and structure of DOM and the microscopic mechanism of the interaction between DOM and heavy metals in sediment needs further study.In this paper,we analyzed the structural characteristics of DOM in wetland sediments under drying-wetting alternation through indoor wet and dry simulation experiments,combined with spectroscopy,mass spectrometry,and chemometrics,and revealed the influence of DOM composition on heavy metal distribution patterns,and explored the mechanism of DOM and heavy metal binding under drying-wetting alternation.The research results provide references and theoretical guidance for treating and remediating heavy metal pollution in wetland sediments.Three main aspects of the research were carried out.(1)Characterization of elemental stoichiometry,spectral characteristics,and molecular structure of DOM in wetland sediments under alternating wet and dry conditions.The DOC: DON of wetland sediment DOM increased by 32.36,and DOC: DOP and DON:DOP decreased by 116.42 and 5.64 after three cycles of drying-wetting alternation,respectively,and no significant difference in the control(p >0.05).In addition,C1% increased by 13%,C2% and C3% decreased by3%,10%,respectively.The degree of humification,hydrophobicity,aromaticity and molecular weight of wetland sediment DOM increased significantly after drying-wetting alternation,protein-like fractions were decomposed,and humic-like fractions were accumulated.CHO-like compounds were the main compounds in sediment DOM before and after drying-wetting alternation,and the composition of sediment DOM was more complex after drying-wetting alternation,with more aromatic groups and less phenolic substances.Microbial activities during wet and dry alternation strongly induce differences in the compositionand structural characteristics of wetland sediment DOM,which affect the interaction of wetland sediment elements and constrain the ecological balance of wetlands.(2)Correlation analysis of wetland sediment DOM composition and heavy metal fugacity patterns under the influence of drying-wetting alternation.The results of redundancy analysis(RDA)showed that the different compositions of sediment DOM had heterogeneous effects on the distribution of different heavy metal forms during drying-wetting alternation.The morphological distribution of Cr(Cr)was mainly influenced by the elemental stoichiometric composition of DOM,while the morphological distribution of Zn(Zn),Cu(Cu)and Cd(Cd)was influenced by the fluorescence component as well as the aromatic and hydrophobic components of DOM,thus the complexity of DOM composition under dry and wet alternation complicated the migration transformation of heavy metals.The ratio of secondary phase to primary phase(RSP)of Cr,Cu and Cd,except Zn,underwent different degrees of decrease after alternating wet and dry incubation,with the largest decrease of 26.7% for Cd.The risk assessment code(RAC)indicated that the risk level of Cd decreased after drying-wetting alternation incubation.Compared with continuous flooding and continuous drying environments,the drying-wetting alternation was more helpful to reducing the level of heavy metal contamination.(3)Study on the binding of DOM and heavy metals in wetland sediments during drying-wetting alternation.The binding capacity,binding sites and binding order of different heavy metals and DOM are heterogeneous.There are differences in the stability and superiority of the binding behavior with heavy metals resulting from structural changes of wetland sediment DOM during drying-wetting alternation.The binding capacity and binding sites between Cu(Ⅱ)and DOM increased and the binding affinity increased after drying-wetting alternation,but the stability constant decreased by 1.21 and the binding stability decreased.The binding sites between DOM and Cd(Ⅱ)decreased and the binding affinity decreased after drying-wetting alternation,and the binding behavior of Cd(Ⅱ)and DOM was not obvious.In addition,protein-like components,aliphatic and carboxyl groups in DOM preferentially bind to Cu(Ⅱ)and Cd(Ⅱ)after drying-wetting alternation.Under drying-wetting alternation,Cd and DOM are more likely to migrate and transform to release heavy metals under the action of water flow,and the environmental hazard of Cd increases,which intensifies the heavy metal pollution of wetland soil and lake water.In summary,the elemental stoichiometry,spectral characteristics,and molecular composition of DOM in wetland sediments under the influence of alternating wet and dry conditions exhibit a high degree of complexity,thus complicating the influence on heavy metal distribution patterns,which further increases the binding capacity and binding sites of DOM and heavy metals in wetland sediments,decreases the binding stability,and affects their binding priorities.