| First we discussed a new method of DMT(Donnan membrane technique) that is used to measure the heavy metal activities to measure the activities of Copper(Cu) and Cadmium (Cd) in Cu-Cd-Ca-EDTA solution. At the same time we calculated the activities of Cu and Cd in Cu-Cd-Ca-EDTA solution by MINTEQ2 to compare the results that was measured and calculated. Second the usage of DMT to measure heavy metal activities in soils were studied. We measured the free Cd,Zinc (Zn),Lead (Pb) and Cu concentrations in soil solutions. And analyzed the relationship between the dissolved heavy metals concentrations and free heavy metals concentrations with the properties of soils by multiple regression. At the same time we simulated the heavy metal activities in soil solutions by WinHumicV. At last we discussed the adsorption of Cd and the effect of As or V on the adsorption of Cd in soils. We used the Freundlich equation to simulate the data of Cd adsorption in soils. Our object was to study the adsorption character of Cd in soils and the factors that influence Cd adsorption and the effect of Arsenate (As) or Vanadium (V) on the adsorption of Cd.In the case of determination of free Cu and Cd concentration by DMT we found that in the Cu-Cd-Ca-EDTA solution studied 50% of the total Cu concentration was free Cu and 98% of the total Cd concentration was free Cd, the pCu(-logCu mol/L)is 5.45, the pCd(-logCu mol/L)is 5.18. In the case of calculation by MINTEQ2 it was shown that 50% of the total Cu concentration was free Cu and 98% of the total Cd concentration was free Cd, the pCu(-logCu mol/L)is 5.40, the pCd(-logCu mol/L)is 5.10.These results showed that the method gived a fair indication of the free metal ion concentration.The solubilities of Pb and Cu were smaller than Cd and Zn in soils. Compared to Pb and Cu, the soils had much lower affinity for Cd and Zn. The metals solubilities decreased with increasing pH. It was shown that the solubility of Zn was controlled by the level of soil contamination (total Zn) and pH, the logKd (Kd ～ partition coefficient)increased one unit as the pH increased 0.2 to 0.5 unit. The solubility of Pb was controlled by the level of soil contamination (total Pb) and pH. Cd,Zn,Pb and Cu exitd as free cation forms and organo-metal complexes mostly in soil solutions, the inorgano-metal complexes can be neglected. Cu was largely in the form of organo-metal complexes, the free ion form and organo-metal complexes form of Pb were almost same in soils solutions, whereas Cd and Zn occurred substantially in the free cation forms. The activities of Cu and Zn were controlled by the level of soil contamination (total Cu and Zn) ,pH and the dissolved organic matter. Comparing the results measured with the prediction of WinHumicV showd that: the agreement between the model predicted and the measured of Cd and Zn was good, but model overestimated the free Pb and Cu concentrations in soil solutions.It was shown that the adsorption data of the four soils studied could be fitted satisfactorily using the Freundlich equation. The Cd adsorption capacity of the soils was correlated with the type of the soil. The adsorption capacity of Beijing drab soil (constant surface charge soil) was greater than the other three red soils (variable charge soils). The adsorption capacity of the three red soils was controlled by pH and CEC. High pH and CEC soils adsorbed greater quantities Cd than did low pH and CEC soils. The presence of As or V both enhanced the adsorption of Cd by soils, but they did not change the adsorption capacity order of the four soils. |
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