| Along with the water-level-fluctuating belt of the Three Gorges reservoir area taking into shape and the continuous development of industry and agriculture along the Yangtse River, environment pollution, especially for heavy metals contamination of soil in water-level-fluctuating belt, becomes serious gradually. After the course of entering into soil by all sorts of methods, then accumulating themselves continually, finally finding their way into food chain through plant intake or into underground water under certain conditions, heavy metals will pose threat to human beings'health and ecological environment balance. Through taking balance adsorption method, this paper focuses on the prior soil type (purple soil) and heavy metals Cu and Zn in water-level-fluctuating belt of the Three Gorges reservoir area, studying the features of adsorption, desorption and competitive adsorption for Cu and Zn in purple soil of different pH, analyzing adsorption and desorption ability of different purple soil to heavy metals, as well as researching the effect of different captions on available heavy metals'content in purple soil. This study will play a significant role in risk assessment and predication of soil heavy metals pollution, prevention and control of environmental pollution as well as soil resources utilization etc. in reservoir area. The results of this study showed:1) As the concentrations of exogenous Cu and Zn get increased, the content of Cu and Zn adsorbed by the three kinds of purple soils grows with an excellent linear correlationship between the equilibrium concentration and balance concentration (R2>0.91). The adsorption of Cu and Zn by the three kinds of soils well coincide with the Langmuir or Freundlich isotherm. For the initial contention of heavy metal being below 15 mg/L, the content order of Cu adsorbed by purple soil follows as: alkaline purple soil>neutral purple soil>acid purple soil, while it changes as:alkaline purple soil>acid purple soil> neutral purple soil for the initial content above 15 mg/L. And the order for Zn is alkaline purple soil>neutral purple soil>acid purple soil, showing the adsorption ability of alkaline purple soil to Cu and Zn is stronger than that of either neutral purple soil or acid purple soil, which is mainly related to the features of alkaline purple soil-higher pH, organic matter content, CEC and adhesiveness. However, the adsorption abilities of neutral purple soil and acid purple soil was slightly different and the ability of a certain kind soil is stronger to Cu than to Zn. As the initial concentrations of Cu and Zn rise, the pH of soil balance solution decreases, and the reduction is larger for alkaline purple soil than for acid purple soil.2) In different purple soils, showing the significant linear correlation (R2>0.91), the content of desorbed heavy metal grows with the increased adsorbed content, and the desorption ratio of Cu is far less than that of Zn. The order of desorption ability of Cu in purple soil follows neutral purple soil>acid purple soil>alkaline purple soil, while that of Zn being acid purple soil>neutral purple soil>alkaline purple soil. In addition, the desorption ability of Cu and Zn in alkaline purple soil is far lower than that in neutral or acid purple soil. By Kd, the coefficient of adsorptive distribution under the equal ratio, the adsorption ability orders for Cu and Zn in different purple soils are all Cu>Zn whose ratio exceeds more than 10 times especially under the circumstance of low initial concentration.3) There is a competitive adsorption behavior in the purple soil where Cu and Zn coexist in, and the competitive adsorption ability of Cu is stronger than that of Zn. It probably is attributed to hydrolyzing ability (pKcu8.0< pKZn9.0), for hydrolyzed metal compound with hydroxyl is much easier to be adsorbed than metal ion itself. For the same initial concentration ratio of Cu/Zn in the purple soil, the isotherm adsorption results of Cu and Zn still can be described by Langmuir or Freundlich formula. In the environment where Cu and Zn coexisting, the maximum adsorbing content of Cu or Zn is less than that when it resides alone, but the sum of the maximum adsorbing contents of coexisting Cu and Zn is larger than the sum of them in the condition of solidarity. It is reflected that there are some special adsorbing points only available for Cu and Zn in the purple soil. In the range of low concentrations for the Cu and Zn coexisting in the purple soil, it is showed unremarkable competition, however, which gradually becomes fierce as the initial concentration goes up. 4) The content of available Cu and Zn in purple soil can be affected by different cations and their concentrations whose growth causes the pH of purple soil decrease of different degrees. The variation trends of K+ and Na+ are similar and so are Ca2+ and Mg2+. The addition of cations leads to increase of available Cu content in alkaline purple soil and decrease of that either in acid purple soil or in neutral purple soil, and the reduction in the former is less than that in the latter. In addition, although in neutral purple the content of available Cu decreases due to the cation'rise, it will increase as long as the bivalent cation is rich enough. In the purple soil, the concentration of available Zn rises with different content whose order is acid purple soil>neutral purple soil>alkaline purple soil. |
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