不同土地利用方式土壤对铜、镉离子的吸附解吸特征

Characteristics of adsorption and desorption of copper and cadmium ions in soils under different land use types

采用一次平衡法对Cu2+、Cd2+在城市及城郊农田、林地、草地3种土地利用方式土壤中的吸附解吸过程进行比较研究,结果表明:Cu2+、Cd2+在3种土地利用方式土壤中的吸附量均随平衡液浓度的增加而增大,Cu2+、Cd2+在农田土壤上的吸附量均高于林地和草地土壤。分别用Langmuir和Freunlich两种等温吸附方程对吸附过程进行拟合,3种土壤对Cu2+的吸附过程运用Langmuir方程拟合效果好,而对Cd2+的吸附过程运用Freunlich方程拟合效果更好。Cu2+在3种土壤的解吸量大小顺序为农田>林地>草地,Cd2+在3种土壤的解吸量大小顺序为农田>草地>林地。两种离子在3种土壤中的动态吸附是个快速反应的过程,随时间延长,吸附反应趋于平衡。运用双常数函数方程和Elovich方程能较好地拟合重金属在土壤上的吸附动力学过程。Cu2+、Cd2+的吸附与土壤黏粒含量、有机质含量、CEC和pH均有关。

Heavy metal adsorption and desorption behaviors significantly influence the transformation, migration and fate of heavy metals in soils and ecological environment, which in turn affects the quality of agricultural products and human environment. Although much research has been conducted on soil heavy metal adsorption and desorption, less work has been done relating to soil heavy metal adsorption and desorption characteristics under different land use types. Soils under different land use types could have significant differences in pH, organic matter content, clay content and microbe species. This in turn not only affects the physical and chemical behaviors of heavy metals, but also restricts the morphology, migration, bioavailability and toxicity of these metals. Soil heavy metal adsorption is a dynamic equilibrium process. Soil properties, heavy metal types and environmental factors influence the adsorption and desorption capacity of soil heavy metals. This paper selected three different land use types （woodland, grassland and farmland） in Wuhu City for investigation. The adsorption and desorption processes of soil copper （Cu^2＋） and cadmium （Cd^2＋） ions under different land use types were studied using the bath method and a locally-made dynamic adsorption device. The study discussed Cu^2＋ and Cd^2＋ adsorption kinetics in soils under three land use types. It laid the theoretical basis for further studies on land use and soil heavy metal adsorption and desorption mechanisms. The results showed that Cu^2＋ and Cd^2＋ adsorption capacities in the three soil conditions increased with increasing equilibrium concentration. The adsorption capacities of Cu^2＋ and Cd^2＋ in farmland soils were higher than in woodland and grassland soils. The desorption of Cu^2＋ and Cd^2＋ in soil under different land use types increased with increasing adsorption of the ions. A positive linear correlation was noted between ion adsorption and desorption in different soil conditions. The absorbed Cu^2＋ and Cd^2＋ by soil was incompletely desorbed due to fixation of part heavy metals ions in soils. The amount of fixed exogenous heavy metal ions reflected the soil fixing capacity of the heavy metal ions. The fixing capacities of three soil conditions for the two heavy metal ions were non-identical. The fixed amounts of Cu^2＋ were 147.2 mg.kg^-l, 118.5 mg.kg^-1 and 61.9 mg.kg^-1, and those of Cd^2＋ were 20.12 mg.kg^-1, 33.33 mg.kg^-1 and 9.97 mg.kg^-1 in woodland, grassland and farmland conditions, respectively. The Langmuir equation and Freunlich adsorption equations were used to fit the Cu^2＋ and Cd^2＋ processes in different soil conditions. While the Cu^2＋ adsorption process was fitted using the Langrnuir equation, that of Cd^2＋ was fitted using the Freunlich equation. Both Cu^2＋ and Cd^2＋ desorption in farmland soil were higher than those in woodland and grassland soils. The dynamic adsorptions of Cu^2＋ and Cd^2＋ in the three conditions showed a rapid reaction process. Also the adsorption reaction tended to balance out with time. The adsorption kinetics of heavy metals in the soils fitted well with the double constant equation and the Elovich equation. The adsorption of Cu^2＋ and Cd^2＋ in the three soils was related with soil clay content, organic matter content, pH and CEC. This was driven by surface soil adsorption force, properties of hydroxyl and carboxyl groups, soil surface charges, soil solution exchangeable ion concentrations, etc.