Distribution Characteristic, Chemical Behaviors And The Ecological Risk Assessment Of Heavy Metals In Xiangjiang River

This paper has investigated heavy metals (Cu,Pb,Zn,Cd,Cr,As,Hg) in the water and sediments of Xiangjiang River in HuNan, studied their contents and spatial distribution. The collective distributing trend and their geo-chemical characteristic of the elements were analyzed. The adsorption characteristics and desorption of Cd in surface sediments were studied. The effects of the value of pH, initial concentration of Cd and some substance have been discussed,Based on the distribution and enrichment of heavy metals in surface sediments of Xiangjiang River, the method of potential ecological risk index presented by Lars Hakanson was used to assess the heavy metals' ecological risk. The results indicate that:1. The totle contents of the heavy metals such as Cd,Pb,Cu and Cr in the downriver of Xiangjiang River was much higher in low water season than in high water season, by contrast, The totle contents of Zn and As were the counterparts; The totle contents of the heavy metals in the surface water is in order, in summer, Zn>As> Pb>Cu>Cr>Cd ; in winter, Zn>Cr>Pb>Cu>As>Cd.2. Because of the complicated water circumstance and the influence of varied pollution source, it is difficult for the spatial distribution to form a certain feature.3. The results showed that the contents of Cu,Zn appeared gradually increased firstly and then come back to the former standard from upriver to downriver. the contents of Cd and As appeared gradually increased from upriver to downriver. while the contents of Pb,Hg and Cr appeared no apparent rule. Among all these heavy metals, the enrichment indice of Zn was first highest, and Cd was the second highest, the other heavy metale were low .4. The assessment result of the heavy metals' ecological risk indicated that Cd was high potential ecological risk ,and Hg was middle potential ecological risk.other heavy (Pb,As,Cu,Zn) metals belong to low potential ecological risk. The order of polluting degree of these heavy metals in surface sediments of Xiangjiang River was Cd>Hg> Pb >As>Cu >Zn. Cd and Hg were the primary elements of pollution.5. The value of pH and the concentrations of the irons are the two main factors to the adsorption and desorption ability of heavy metals. The adsorption capacity increases gradually along with the value of pH, The adsorption capacity of Cd assumes a linear relation with initial concentration of absorbent basically, The adsorption capacity increases rapidly under lower initial concentration of absorbent, but then becomes lower along with initial concentration adding.6. The adsorption isotherms of Cd in sediments are calculated using Langmuir adsorption equations, the correlation coefficient is relatively high, The adsorption isotherms conform to Langmuir type.7. With the addition of glycin and L-glutamic acid, the adsorption of Cd in sediments was restrained. The addition of tartaric acid is disadvantage to the desorption of Cd in sediments. citric, malic and oxalic acid can promote the desorption of Cd to some extent, and the ability of promotion is in order: malic acid >oxalic acid >citric acid.8. With the addition of urea, the value of pH value increased with the higher concentration of urea, five or six days later , the value of pH reach the summit, then descended; when in the low concentration of urea, the desorption of As,Pb and Hg in sediments increased with the higher concentration of urea, and reached the summit when the concentration of urea from 5mmol/L to 10mmol/L,then decreased with the higher concentration of urea.The desorption of Cd,Cu,Zn and Cr decreased with the concentration of urea increasing.9. With the addition of Na₂CO₃, the desorption of Hg did not have too much change, but the desorption of Cr increased, and reached the summit when the concentration of Na₂CO₃ is 40mmol/L, then decreased, but the desorption of As,Cd,Pb,Zn and Cu increased with the higher concentration of Na₂CO₃ To sum up,Na₂CO₃ can promote the desorption of heavy metals. 10. Disturbance can promote the desorption of heavy metals in the sediments, the nearer of distance from the disturbance center, the more heavy metals would be desorbed, and denser of heavy metals in water would be.