| In this paper, the soil samples of three typical sewage-irrigation area (Xiaodian,Jinyuan and Qingxu) in Taiyuan were regarded as the main study areas. The fractions of Cd in different depth of sewage-irrigation soils in Taiyuan were studied by Tessier sequential extraction procedures. The purpose of the study was to research the contents and distrbution of Cd in sewage-irrigation soils. The correlation and regression between the different fractions of Cd with total, available fraction of Cd were discussed by correlation analysis and regression analysis. Batch techniques were used to study the adsorption mechanism and characteristics of Cd in sewage-irrigation soils in Taiyuan. The adsorption data were fitted using adsorption isotherm model and kinetic equations. The main conclusions were as follows:The content of Cd in sewage-irrigation soils in Taiyuan higher than the Taiyuan soil environmental background value, but lower than the national secondary standard (GB15618-1995), and the content of Cd in surface soil (0-20 cm) is highest.The sequence of dieffrent fractions content of Cd in sewage-irrigation soils was Fe-Mn oxides-bound fraction>organic-bound fraction> carbonate-bound fraction>residual fraction>exchangeable fraction.The sequence of exchangeable fraction,carbonate-bound fraction,Fe-Mn oxides-bound fraction content of Cd in surface soils were Xiaodian>Jinyuan >Qingxu. The sequence of residual fraction content of Cd in surface soils was Qingxu>Jinyuan>Xiaodian.The content of non-residual Cd was higher than its in other depth of soils in surface soils. The content of residual Cd was lower in surface soils than its in other depth of soils.The bioavailability index and mobility factor were used in evaluation of bioavailability of Cd. Medium environmental contamination risks was found in sewage-irrigation soils(10%<RAC≤30%).The total of Cd had a positive correlation with non-residual fraction, and had a significantly negative correlation with residual fraction. There was a significantly positive correlation between available Cd with exchangeable Cd. The optimal regression equation wasCd(Ava)=0.016+1.067Exc+0.038Carb-0.048 Fe-MnOx-0.121OM-0.183 Res.According to the results, at 25℃,Elovich model was the best model regardless of the initial concentration of Cd. At 40 ℃, adsorption kinetics equation could be accurately described by Elovich model and double constant model.The results showed that adsorption of Cd displayed nonlinear curves. At 25 ℃ and 40 ℃, adsorption isotherms of the soils could be accurately described by Temkin and Freundlich model respectively under low concentration series of Cd2+(0.02～10mg·L-1).Freundlich model was the best model under high concentration series(10-100mg·L-1), Freundlich model could be fit to the adsorption isotherm of Cd under all concentration series(0.02-100mg·L-1).For the Cd,the hydrogen bonds force was the major force in soils.The Cd adsorption process was a spontaneous and endothermic reaction,the high temperature was favorable to the Cd adsorption.In conclusion, the study provided basic data for the migration of Cd in soils, and provide a scientific basis for environmental management and ecological estoration of soil. The study is an important basic research work of farmland soil environment control. In addition,the study for practical guiding significance to understand the transformation mechanism,avoid Cd contaminated soil environment and from soil enter the food chain. |
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