The Study Of Characteristics On Pb²⁺ Adsorption-desorption By Bulk Soil And Micro-aggregates In Mingshan River Watershed

Four kinds of purple soil of different land-use patterns from Mingshan River were selected as object of the study, including a paddy soil whose parent material is purple soil. Investigated the characteristics of adsorption-desorption of Pb2+ in bulk soil and different size fraction of the micro-aggregates(2-0.25mm,0.25-0.053mm,0.053-0.002mm,<0.002mm),using the methods of equilibrium adsorption and desorption by NH4OAC and EDTA solution, and investigated the effect of organic matter, free oxide iron, calcium carbonate on the characteristics of adsorption-desorption. The results are as follows:(1) The adsorption capacities of Pb2+ by the bulk soil and different size fraction of the micro-aggregates of the four kinds of purple soil follows the order of initial concentration of Pb2+.there were some difference between the bulk soil and four size micro-aggregates, the tea soil:(<0.002mm)>(2-0.25mm)> (0.053-0.002mm)> the bulk soil >(0.25-0.053mm), the paddy soil:(<0.002mm)>(2-0.25mm)> the bulk soil> (0.053-0.002mm)>(0.25-0.053mm), the forest soil:(<0.002mm)> the bulk soil> (2-0.25mm)>(0.25-0.053mm)> (0.053-0.002mm), the dryland soil:(<0.002mm)>(2-0.25mm)> the bulk soil>(0.25-0.053mm)> (0.053-0.002mm), the total adsorption were all high on<0.002mm. Adsorption of Pb2+ by the bulk soil and four size micro-aggregates of the four kinds of purple soil fitted well the Freundlich isotherm model, the result showed the sorption of Pb2+ was multilayer adsorption by the bulk soil and micro-aggregates. The coefficient K of Freundlich isotherm model represents the adsorption capacities and the maximum adsorption amount in this research, It was well correlated with the content of organic matter, free oxide iron, and CEC. The bulk soil and different size fraction of the micro-aggregates of the four kinds of purple soil were the most active to the exchange mechanism, and the percentage of exchange mechanism followed the order of Initial concentration of Pb2+,while the percentage of complexation mechanism were in reverse order, there were special adsorption also.(2) Organic matter, free oxide iron, calcium carbonate were the important factors of adsorption capacities of heavy metal in soil, the adsorption capacities decreased when organic matter, free oxide iron and calcium carbonate iron were removed in the bulk soil and four size micro-aggregates of the four kinds of purple soil, the order was:that removing organic matter> that removing calcium carbonate> that removing free oxide iron, that means organic matter was the most important factor, and calcium carbonate was better important than free oxide iron. Adsorption of Pb2+ by the bulk soil and four size micro-aggregates of the four kinds of purple soil still fitted well the Freundlich isotherm model after removal of soil components, and K increased significantly, the adsorption capacities and the maximum adsorption amount decreased in some extent. After removal of components, the percentage of exchange mechanism increased respectively, but the percentage of complexation mechanism decreased. The order of the percentage of exchange mechanism increased was:that removing organic matter> that removing free oxide iron> that removing free calcium carbonate.(3) The order of maximum adsorption amount among the purple soil of different land-use patterns by the bulk soil was:the forest soil> the dryland soil> the paddy soil >the tea soil. The order of coefficient K of Freundlich isotherm model was:the paddy soil> the forest soil> the dryland soil> the tea soil, so as the order of adsorption capacities and the maximum adsorption amount of four kinds of soil. the four kinds of purple soil were the most active to the exchange mechanism, The order of the percentage of non-adsorption:the tea soil> the dryland soil> the forest soil> the paddy soil, that explained the retention ability of tea soil was significantly greater than others, and the paddy soil at the lowest level.