Effect of chloride ions on adsorption of cadmium from a landfill leachate

Abstract

This study examines possible formation of neutral and negative complex ions between trace quantities of cadmium (Cd) and chloride ions (Cl−). Experiments were first performed for the adsorption of Cd, from 2 to 10 mg/L, onto kaolinite in a soil suspension from a landfill leachate. A companion set of experiments was performed involving Cd, also from 2 to 10 mg/L, onto kaolinite in a soil suspension from a pure solution. By comparing the results from these two sets of experiments, it was apparent that there were constituents in the leachate which interfered with the adsorption of cadmium onto kaolinite. With the use of thermodynamic equilibrium calculations, it is seen that the chloride ions could account for some of the interference. Further experiments involving adsorption of Cd, from 5 to 80 μg/L, in the presence of Cl−, at 1000 mg/L, indicate that there is less adsorption of Cd than in an experiment involving [Formula: see text] at pH values from 4 to 5. This result is in agreement with the thermodynamic equilibrium calculations. Additional experimental results of adsorption of Cd at 80 μg/L for pH values of from 2 to 9 indicate that the apparent Cd – Cl complex formation interferes with the precipitation of Cd as Cd(OH)2 and is in agreement with those obtained from predicted thermodynamic equilibrium. The study demonstrates that if accurate and precise predictions of Cd transport are required, detailed aqueous hydrogeochemistry studies may be important and necessary. Key words: adsorption, cadmium, leachate, speciation, cadmium–chloride complexes, thermodynamic equilibrium.