| At present, combined pollution is universal phenomenon. With the development of industry and agriculture, more and more heavy metals enter into environment and coexist, its interaction produce ecological effect on environment. In the past, a great deal of literatures on heavy metals usually take single heavy metal as subjective of study, but it can not correctly reflect environmental pollution phenomenon, because the ecological effect caused by many pollutants which were coexist and interact on each other in the same environment have obviously difference from the ecological effect caused by single pollutant, and the evaluating criterion based on the ecological effect caused by single pollutant also can not reflect the requirement of environmental quality. Therefore, combined pollution had become an important developing direction of environment science. So the study on combined pollution for heavy metals had obtained much more attention. Cadmium, as a dangerous heavy metal, can be accumulated through food chains. It will lead to "itai-itai", if people ingest it in a long time. So cadmium pollution has always been one of the focuses environmental problems that scientists concerned. Zinc, which is a nutrient element and is also a toxic element, has similar chemistry nature compared with cadmium. They are always coexist in the nature, through soil-plant system and food chains, they can enter into human body and cause damage to human being. As a result of human activities, such as mining, waste treatment, waste water irrigation and sludge application etc, more and more heavy metals like Cd, Zn etc enter into soil environment and coexist. The effect of combined pollution caused by Cd and Zn have much difference from the effects caused by single Cd or Zn due to the interaction between Cd and Zn. The interaction will have obviously effect on the processes of biological concentration and toxicity of different biology level, so it is very important to confirm the interaction among heavy metals. The interaction among heavy metals was not only influenced by concentration of heavy metals, but also influenced by environmental conditions. And the influence of environment conditions was usually intricate, sometimes, the results was even opposed to each other. And among environmental conditions, the pH value of soil has great effect on the interaction. Because acid rain can transport excessive hydrogen ion into the soil, then cause decrease of pH value and cation saturation, accelerate acidify of soil, therefore, it is significant to study the ecological effects of combined pollution for heavy metals under intimidation of acid rain in the regions where acid rain often take place. According to situation of chongqing city which is the region where the purple soil was wildly distributed and acid rain had caused serious damage, the purple soil with different pH value was chosen to test, adopting pot experiment, thermodynamic adsorption-desorption experiment and forms analysis experiment, we systematically studied and compared the interaction and bioavailability of exotic Cd and Zn with different dose, and revealedthe laws and affecting factors that Cd and Zn transfer and transformation in soil-plant system, further analysis mechanisms of interaction and the form transformation of Cd and Zn in purple soil under intimidation of acid rain. The results are expected to provide rational basis theory for controlling combined pollution of heavy metals of Cd and Zn in purple soil under intimidation of acid rain in Chongqing city.In this paper, the purple soil with different pH value, which belong to neutral and acidity, was chosen as the test soil, and the lettuce as the test plant According to different pH value of acid rain, the purple soil with different pH value and different combinatorial treatments in concentration, the pot experiment included 36 treatments, and 3 replications each. Through the pot experiment, we can study the effect of interaction and form change of Cd and Zn on lettuce biomass affected by acid ra...
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