| The sewage irrigation development in Beijing region can be divided in 4 periods: beginning, increasing rapidly, stabilization, decreasing. The location of sewage water irrigating farmland had a west-northwest-north-south-southeast shift progress. The initial area was only more than 666.7hm2 in the west suburb, then it was increased to its maximum 80,000hm2 in the late of 1980s, and now it sharply reduced to about 2.7,000 hm2 as for the following main reasons:First,Beijing's land-use construction has changed greatly, the sewage irrigation farmland was occupied. Secondly,In order to protect emvironment, the farmland quondamly irrigated with sewage water has used groudwater in recently 5 years. Finally, the planting systems was also transferred, the crop farmlands change into vegetable fields.The sandy loamy soil were from the experimental plots of Beijing Academy Agriculture and Forestry Sciences and the clay loam soil were from Beijing Dixing Beiyechang region, the adsorption capacity of the two soils onto Cadmium increased with increasing Cadmium concentration, but the adsorption rate decreased gradually. For this experiments, the influencing factors of adsorption included the organic matter content of soil, the clay content of soil, temperature and the soil original Cadmium content. Adding the organic matter to the soil could enhance the soil adsorption capability, the same to the clay content. When the Cadmium concentration was in a relatively range, the less of the soil original Cadmium content, the more of the soil adsorption capability. In addition, the Cadmium adsorption processes of the sandy loamy soil was endothermic, however the clay loamy soil's was exothermic.At 25℃ and 5℃, when the Cadmium concentration ranged from 0.110 mg L-1, Henry formula could describe the Cadmuim adsorption processes well, when the Cadmium concentration ranged from 50900mg L-1, Langmuir formula could do it well. When the Cadmium concentration ranged from 0.1900mg L-1, Langmuir formula could also work well, what's more, the Cadmium maximum adsorption capatility could be calculated, it is 5000mg kg-1.The soil, which was from the Beijing Shijingshan Yamenkou region, Cadmium releasing was a dynamic reversible process. 04h was a fast release process, 624h was a relatively slow one. The second release process Cadmium concentration's peak value was also smaller than the first one. The first release kinetic process could be described by Elvoich equation. The modified Freundlich equation was suit to the second release process. Additionally, the acid-base environment could effect the soil Cadmium release intensively. |
