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

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 (Cu²⁺) and cadmium (Cd²⁺) ions under different land use types were studied using the bath method and a locally-made dynamic adsorption device. The study discussed Cu²⁺ and Cd²⁺ 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 Cu²⁺ and Cd²⁺ adsorption capacities in the three soil conditions increased with increasing equilibrium concentration. The adsorption capacities of Cu²⁺ and Cd²⁺ in farmland soils were higher than in woodland and grassland soils. The desorption of Cu2+ and Cd2+ 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 Cu²⁺ and Cd²⁺ 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 Cu²⁺ were 147.2 mg·kg^-1, 118.5 mg·kg^-1 and 61.9 mg·kg^-1, and those of Cd²⁺ 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 Cu²⁺ and Cd²⁺ processes in different soil conditions. While the Cu²⁺ adsorption process was fitted using the Langmuir equation, that of Cd2+ was fitted using the Freunlich equation. Both Cu²⁺ and Cd²⁺ desorption in farmland soil were higher than those in woodland and grassland soils. The dynamic adsorptions of Cu²⁺ and Cd²⁺ 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 Cu²⁺ and Cd²⁺ 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.