Release Mechanism And Risk Assessment Of Heavy Metal Cadmium In Surface Sediments Of Xihe Rive

Heavy metal pollution in river surface sediments seriously affects the river ecosystem and even brings great harm to people’s health.Taking Xihe River in Shenyang,China as the research object,the physicochemical properties,total amount and chemical form of the sediments in some representative locations in the basin were analyzed,and the risk of cadmium pollution was assessed,and then the particle size and water conditions（pH,organic acids,inorganic anions,and inorganic cations）on the chemical composition of heavy metal cadmium in sediments and explore the possible mechanism of cadmium migration.（1）Most of the sediments at each sampling point in the Xihe River Basin in Shenyang are near neutral,with an average pH of 7.09,little change in carbonate content,and large difference in organic matter content.（2）The heavy metal Cd in the sediments of the Xihe River Basin is mainly composed of acid-soluble components,with an average proportion of 45.06%and an average content of 4.88 mg/kg,Compared with 2006 and 2009,the cadmium content is significantly lower.In the study area,the Dapan village section（6.90 mg/kg）had the heaviest cadmium pollution,and the Zhangyi village section（2.69 mg/kg）had the least cadmium pollution,but both were higher than the background value of soil cadmium in Shenyang（0.16 mg/kg）;Potential risk index method and risk assessment code analysis found that the Cd pollution in Xihe sediments was heavier,and the potential ecological risk level was higher.（3）There is a high correlation between the Cd content,F1 composition and pH in the sediment,the correlation coefficients are-0.542 and-0.477,respectively.and there is a significant positive correlation between Cd-F1（0.887**）,other than There are differences in the correlation between the various speciation.（4）The sediment in Xihe District is mainly medium sand and coarse sand,and the proportion of particle size>250μm accounts for 78.02%.The heavy metal cadmium with different particle sizes is in a state of heavy pollution and high risk.With the decrease of particle size,the content of heavy metal cadmium in the surface sediments of the Xihe River increases,and the Speciation distribution order of heavy metals is acid-soluble state（F1）>residue state（F2）>reducible state（F3）>oxidizable state（F4）,in which the acid-soluble state accounts for the largest proportion of the total content and is the main Speciation.（5）The content of heavy metal cadmium in Dapan sediment was 6.90 mg/kg,and the content of cadmium after water extraction was 6.30 mg/kg.The decrease of the content was mainly related to the acid-soluble component.The extraction of sediment by oxidant（H₂O₂）and reducing agent（ascorbic acid）can promote the release of heavy metal cadmium,so that the content of acid-soluble and reducible cadmium can be significantly reduced,but H₂O₂ can increase the distribution ratio of oxidizable cadmium,However,ascorbic acid can increase the distribution ratio of reducible cadmium.In addition,through the linear and nonlinear simulations of the kinetics,it is found that the exponential equation can well describe the release process of the heavy metal cadmium in these two cases.（6）pH between 2 and 10 can promote the release of heavy metal cadmium in the sediment,which has a significant effect on the flow components（F1 and F2）of the sediment,but has little effect on the residual components.When Ph value increased from 2to 6,the total amount of cadmium in the sediment increased and remained constant at Ph values of 8 and 10.organic acids mainly affected the concentration of acid-soluble cadmium in the sediments.content,the migration order is EDTA>CA>TA>OA>CK.（7）Four cations can promote the release of heavy metal cadmium in sediments,among which Fe（III）and Al（III）solutions are acidic,which is the main factor leading to the release of cadmium in the sediments,while Mg（II）and CA（II）have little effect on the release of cadmium;the four anions(SO₄^2-、Cl^-、CO₃^2-、NO₃^-.)have little effect on the release of heavy metal cadmium in sediments,and will not be significant Changes in the chemical species of Cd in the sediment as a percentage of the total.