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Characteristics and Boundary Line between Adsorption and Precipition of Cadmium in Soil

Fei Yuhong Chao Shutang Zhang Guanghui Nie Zhenlong (Institute of Hydrogeology and Engineering Geology,CAGS,Shijiazhuang)  
Based on a comparison between the experimental results and previous test results, the authors consider that the adsorption forms of soil adsorbing cadmium from soil solution are Freundlich's forms in unsaturated zone under the condition of pH = 6.5~11.5 and that the remnant speciation is the main form of cadmium existing in the unsaturated soil,formulae have been set up for the relationship between unit time adsorption content of cadmium from soil solution and soil solution concentration of cadmium, and between adsorption content and the concentration in each process of adsorbing cadmium by soil from soil solution; It is also held that the adsorption dominantly controls soil solution concentration of cadmium under the acid condition, the precipitation controls it under the alkaline condition, and the adsorption together with the precipitation control cadmium concentration under pH=6.~8.5 . In general, the relationship between the dissolution and the precipitation of CdCO 3-CO 2-pH or Cd(OH) 2-CO 2-pH controls the concentration of cadmium in soil, but the relationship between the adsorption and pH of soil solution controlls the concentration under the condition that the soil solution concentration of cadmium is lower than its critical concentration of the precipitation. The boundary line between the adsorption and the precipitation, which dominantly control the concentration of soil solution, is preliminarily established, which is at about 10 -8 (pH=8.5) ~10 -4 mol/L (pH=4.5), based on the condition of soil pH and p CO 2 in China and on the research results of this paper.
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