Adsorption of copper, zinc, and cadmium on clay fraction of two acid soils: surface complexation modeling

The adsorption of copper (Cu), zinc (Zn), and cadmium (Cd) as a function of pH was studied on the clay fraction of two acid soils (Typic Rhodoxeralf and Aquic Palexeralf), in two ionic strengths and two different metal concentrations. Specific surface area was determined by BET-N2 and EGME. The charge development on the clay surface was studied in the pH range approximately 3.5 to approximately 10.0 by potentiometric titration under continuous flow of argon. The least square optimization program FITEQL32, for Constant Capacitance (CCM) and the Double-Layer Model (DLM) was used to fit the titration and adsorption data, In both models surface site density was fixed at 2.31 sites/nm2, while for CCM capacitance density was set at C = 1.06. The Palexeralf soil exhibited higher specific surface area by both methods and is adsorbing all three metals in lower pH values than the Rhodoxeralf. Both clays exhibited PZSE values higher than montmorillonite and kaolinite but in close approximation to the second, possibly due to higher content of iron oxides. The adsorption order for both clays was Cu > Zn > Cd in any case, thus more Cu is adsorbed at a certain pH than Zn and even more than Cd, while the increase of metal concentration or ionic strength shifts the adsorption curve towards higher pH values. CCM is describing the titration data satisfactorily by using EGME, while by using BET, alteration of Ns and C values is needed. Adsorption data are described by CCM by emphasizing the formation of monodentate surface complex. Bidentate complex in most of the cases was of no importance in describing the data despite the evident of its presence in recent spectroscopic studies for Cu and Cd on goethite. When BET surface area was applied better fit was achieved and bidentate complex became important in some cases for the optimization procedure. Double-layer model is exhibiting worse fit in any case.