Effect Of Dissolved Organic Matter On Heavy Metal Migration In Sediment Suspension Under The Coexistence Of Phosphate

The harm of heavy metals to lake ecological environment is closely related to their migration.The migration of heavy metals in lake water and sediments has been widely studied,while the migration behavior of heavy metals in sediment suspensions is relatively scarce.In addition,some lakes are affected by both heavy metal pollution and eutrophication.Lake eutrophication is usually attributed to high concentration of phosphate and accompanied by high concentration of dissolved organic matter（DOM）,both of which can exert great influence on the migration of heavy metals in lakes.Therefore,in this study,the influence of phosphate and DOM on the migration behavior of typical cationic heavy metal(Cu²⁺)in the sediment suspension of eutrophic lake polluted by heavy metals was explored.Firstly,the influence mechanism of DOM on Cu²⁺adsorption of sediment suspended particles was analyzed through sediment suspension simulation test.Then,by constructing an aqueous solution system of phosphate,DOM and Cu²⁺coexisting,the influence mechanism of phosphate on the complexation of Cu²⁺and DOM,and the inhibition mechanism of DOM on the precipitation reaction of Cu²⁺and phosphate were studied by using a variety of spectroscopic techniques.Finally,based on the above studies,Cu²⁺adsorption tests were carried out through indoor simulated eutrophic lake sediment suspension to explore the influence and mechanism of DOM on Cu²⁺adsorption of sediment suspended particles under the condition of phosphate coexistence.This study aims to improve the effects of phosphate and DOM on the migration of heavy metals in eutrophic lake sediment suspensions and the related mechanisms,and to provide theoretical reference for the protection and treatment of eutrophic lake polluted by heavy metals.The main conclusions of this thesis were as follows:（1）DOM promoted the adsorption of Cu²⁺by sediment suspended particles,but the adsorbed Cu²⁺is easy to be released again in the process of resuspension.Under the action of DOM or fulvic acid（FA）,the mass fraction of adsorbed Cu²⁺increased from 71.51%（control group）to 85.69%and 75.31%,respectively,after 48 h of adsorption.However,the adsorbed Cu²⁺is easy to be released again in the process of sediment particle resuspension.By analyzing the ultraviolet-visible spectrum（UV-Vis）of DOM and Cu²⁺concentration during the adsorption reaction,it was found that Cu²⁺and DOM had a complexation and then dissociation process.Sediment suspended particles can adsorb both Cu²⁺and DOM-Cu²⁺complexes.The results of fluorescent excitation-emission matrix（EEM）combined with parallel factor analysis（PARAFAC）showed that protein-like component of DOM promoted Cu²⁺adsorption of sediment suspended particles more strongly than humic-like component.Then,synchronous fluorescence spectroscopy（SFS）combined with two-dimensional correlation analysis（2D-COS）indicate that the tyrosine-like components of protein-like components played a priority role in promoting adsorption process.（2）Phosphate can inhibit the complexation between Cu²⁺and DOM and influence the complexation mechanism.Cu²⁺concentration and the amount of DOM-bound Cu²⁺in supernatant was reduced with the increase of phosphate in their coexistence solution.Furthermore,the results of EEM-PARAFAC analysis showed that when Cu²⁺concentration increased from 0 to 5.0 mg·L^-1,the fluorescence intensity of tyrosine,humic-like component and tryptophan in the treatment group containing 50μmol·L^-1 phosphate decreased by 36.46%,57.34%and 74.70%,respectively,compared with that in the control group without phosphate.Therefore,phosphate inhibited Cu²⁺quenching effect on fluorescence components in DOM.Fourier transform infrared correlation spectroscopy（2D-FTIR-COS）showed that when no phosphate was added the organic ligands bound to Cu²⁺in the following order:C-O vibration of carbohydrates>N-H stretching of amide（Ⅰ）>OH deformation of phenolic hydroxyl group.And under the influence of 50μmol·L^-1 phosphate,the combination of the corresponding order for OH deformation of phenolic hydroxyl group>N-H stretching of amide（Ⅰ）>C-O vibration of carbohydrates.Therefore,phosphate alters the binding order of Cu²⁺to the organic ligands on DOM.（3）DOM can limit the precipitation reaction of phosphate with Cu²⁺in aqueous solution,and different components of DOM play different roles in different degrees.When 50μmol·L^-1phosphate reacts with 50μmol·L^-1 Cu²⁺in solution,the results of flame atomic absorption spectrometry showed that 51.61%of the Cu²⁺were precipitated.Under the interference of FA with DOC concentration of 5 mg·L^-1,only 29.75%of Cu²⁺was precipitated.The results of EEM-PARAFAC analysis showed that both humic-like fluorescent component and the protein-like fluorescence component can limit the precipitation reaction between phosphate and Cu²⁺,and the humic-like fluorescent component played a greater role than the protein-like fluorescence component.The results of 2D UV-Vis show that functional groups with peaks at short wavelengths were primarily responsible for restricting the precipitation of Cu²⁺with phosphate.（4）The dual action of phosphate and DOM can improve the adsorption capacity of Cu²⁺on sediment suspended particles and stabilize Cu²⁺on the surface of sediment particles.By simulating the sediment suspension in the laboratory,the mass fraction of adsorbed Cu²⁺increased from 71.51%（control group）to 94.61%after adsorption for 48 h under the action of phosphate and sediment DOM.In contrast,when phosphate and FA interact together,the mass fraction of Cu²⁺adsorbed is relatively low,which is 87.63%.Combined with EEM-PARAFAC analysis,it was found that phosphate and DOM,mainly composed of protein-like components,could better promote the adsorption of Cu²⁺by sediment suspended particles.Different from DOM alone,the release amount of Cu²⁺in the process of resuspension was lower than that in the control group without phosphate and DOM,phosphate can promote the stabilization of adsorbed Cu²⁺on the surface of suspended particles in sediments.In conclusion,DOM can still promote the migration of Cu²⁺in aqueous solution under the condition of phosphate coexistence,but it can also promote the adsorption of Cu²⁺in sediment suspension particles.DOM dominated by protein-like fluorescence components is more conducive to the stabilization of Cu²⁺in sediment suspension than DOM dominated by humic acid-like fluorescence components.