Competitive Adsorption of Zn and Cd onto Three Typical Soils

In this study, an isothermal batch experiment was conducted to assess the characteristics of adsorption of Zn and Cd in surface samples of three typical soils from Qingdao City, China. Sorption isotherms were characterized using Linear, Langmuir and Freundlich equations. The results indicated that all the sorption isotherms were well described by the Freundlich and Langmuir equations with high coefficients of determination (R²>0.971 and R²>0.924). The distribution coefficient values K_d for Linear equation decreased with the increasing concentration of metal ions in equilibrium solution, which indicated that sorption capacity by soils was influenced by the metal ions concentration in solution. Metal sorption capacity by Lime concretion black soil was much greater than Brown and Fluvo-aquic soils for its high pH value, organic matter and clay contents. The pH values of soil solution decreased after adsorption which was caused by the replacement of H⁺ adsorbed onto soil surfaces with metal ions. The competitive phenomena was not evidence at the lower initial concentrations (<50 mg/l). On the basis of maximum sorption capacity Q and distribution coefficient values K_d for the initial metal concentrations for each soil, the selectivity sequence was obtained and that is Zn>Cd.