The Adsorption Effect Of Wetland Soil On Copper And Lead Under The Action Of Different Molecular Weight DOM

The pollution of heavy metals in soil in my country is becoming more and more serious,and the pollution of heavy metals copper（Cu）and lead（Pb）is more prominent.Their existence in the natural environment and their toxic effects have attracted more and more attention.Dissolved organic matter（DOM）plays a very important role in regulating the biological activity of heavy metals in wetland soil due to its extensive and extremely high reactivity.Its structure is complex,and its influence on the biological activity of heavy metals in wetland soil is the result of the comprehensive effects of various components.Therefore,from the point of view of DOM molecular weight distribution,this article focuses on the heavy metals Cu and Pb,and using the ultrafiltration continuous classification method to divide the Carex DOM into 5different molecular weight components（F0,unfractionated NOM;F1,>10 k Da;F2,5～10 k Da;F3,1～5 k Da;F4,<1 k Da）.Based on the analysis of the elemental composition of each component of DOM,the content of acidic functional groups and the basic properties of ultraviolet spectroscopy,the effects of different molecular weight DOM components on the adsorption-desorption of Cu and Pb in wetland soil were compared.Combining the nature of DOM and the binding constant（log K）of each component with Cu and Pb,we will explore the influence of DOM with different molecular weights on the concentration of Cu and Pb in wetland soil liquid phase and the change of soil solid phase morphology,and further deepen the understanding of the impact of DOM on the environmental behavior of heavy metals in soils.The main findings are as follows:（1）The contents of C,H and N elements in different molecular weight DOMs are positively correlated with the molecular weight,while the O element is the opposite.The content of total acidic groups,carboxyl groups and phenolic hydroxyl groups all decrease with the increase of molecular weight.The high molecular weight DOM component has low oxidation degree,weak polarity,and contains more hydrophobic components.Its degree of humification,degree of condensation,degree of substitution of substituents on the aromatic ring,and electron transfer capacity are all higher than those of low molecular weight components.The proportion of initial humified organic matter and the ratio of humified substances to non-humified substances increase as the molecular weight of DOM increases.Deeply humified organic matter is mainly distributed in smaller molecular weight components.The results of principal component analysis show that the molecular weight of DOM is closely related to the degree of humification,aromaticity,and degree of condensation.（2）The addition of DOM with different molecular weights can promote the apparent adsorption capacity of wetland soil for Cu and Pb.The order of Cu adsorption by wetland soil is F1>F2>F3>F0>F4>CK,the order of adsorption amount for Pb is F1>F3>F2>F4>F0>CK;and the order of desorption of Cu is CK>F4>F0>F3>F2>F1,the order of desorption of Pb is CK>F0>F4>F2>F3>F1.By exploring the relationship between the binding constant（log K）of DOM and heavy metals Cu and Pb and the chemical indicators of DOM,it can be seen that the molecular weight of DOM cannot be used to predict the adsorption behavior of Cu and Pb on DOM.The adsorption mechanisms of heavy metals Cu and Pb on DOM mainly include complexation,hydrophobic interaction andπ-πinteraction..（3）DOM can reduce the ability of high-concentration(120 mg·L^-1)Cu and Pb to migrate to the wetland soil liquid phase,while the adsorption and fixation degree of low-concentration(40 mg·L^-1)Cu and Pb has decreased.The higher the p H in the system,the stronger the holding and fixing effect of soil solids on Cu and Pb,and the lower the migration ability of Cu and Pb to the liquid phase.Different molecular weights of DOM have activating effect on Cu and passivation effect on Pb.As the wetland soil progresses from low pollution to high pollution,the solid-liquid distribution coefficients of components F1 and F2 to wetland soil Cu and Pb increase,while components F3,F4 and F0 are opposite.