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Effect of organic matter and calcium carbonate on behaviors of cadmium adsorption–desorption on/from purple paddy soils

Zhao, XiuLan, Jiang, Tao, Du, Bin
Chemosphere 2014 v.99 pp. 41-48
adsorption, cadmium, calcium carbonate, desorption, electrostatic interactions, energy, enthalpy, entropy, equations, hydrogen bonding, organic matter, paddy soils, soil organic matter, sorption isotherms
Batch experiments and sequential extraction analysis were employed to investigate the effects of soil organic matter and CaCO3 on the adsorption and desorption of cadmium (Cd2+) onto and from two purple paddy soils, an acidic purple paddy soil (APPS) and a calcareous purple paddy soil (CPPS). The Cd2+ adsorption isotherms on both soils could be well-described by the Langmuir and Freundlich equations. CPPS had a higher capacity and a stronger affinity for Cd2+ adsorption compared with APPS. The adsorption process of Cd2+ on APPS was dominated by electrostatic attractions, whereas the adsorption mechanism varied depending on the Cd2+ concentrations in equilibrium solutions on CPPS. At low equilibrium concentrations, the adsorption process was primarily specific adsorption, but nonspecific adsorption dominated at high equilibrium concentrations. Removal of organic matter decreased the amount of Cd2+ adsorption on both of the soils, slightly affected the Cd2+ desorption rate and exchangeable Cd (EXC-Cd) in APPS and increased the desorption rate and EXC-Cd in CPPS, suggesting that the effect of organic matter on Cd2+ adsorption–desorption depends on the soils. CPPS and APPS containing CaCO3 exhibited higher adsorption amounts but lower desorption rates and lower proportions of EXC-Cd than those of their corresponding soils without CaCO3, demonstrating that CaCO3 played an important role in Cd2+ specific adsorption on soil. The changes in the thermodynamic parameters, including free energy (ΔG0), enthalpy (ΔH0) and entropy (ΔS0), as evaluated by the Van’t Hoff equations, indicated that the adsorption was a spontaneous and endothermic process with the primary interaction forces of dipole interactions and hydrogen bonds on APPS, whereas both physical and chemical interactions dominated the adsorption on CPPS.