The adsorption is the main process responsible for the accumulation of heavy metals in the surface of soil colloids. Detailed knowledge of this phenomenon can contribute to improve the remediation practices for contaminated soil. The free energy (deltaG0) of Ni adsorption was evaluated in surface (0.0-0.2 m) and subsurface (in the maximum expression of B horizon) samples of a clayey Rhodic Acrudox (RA) and a very clayey Eutric Kandiudalf (EK). Ni was added (2, 5, 10, 20, 30, 40, 50, and 70 mg dm-3), in a 1:10 soil:solution ratio. Two background electrolytes (NaCl and CaCl2) and three ionic strengths - IS (1.0; 0.1 and 0.01 mol L-1) were tested. The Ni adsorption reaction was spontaneous, since the deltaG0 values were negative in all concentrations. Values of DG0 decreased with the increasing Ni doses. deltaG0 was higher in the EK presented than in RA, mainly due to its chemical and mineralogical characteristics. In surface samples, deltaG0 was generally higher than in subsurface samples due to their higher organic matter contents. The deltaG0 was higher for solutions with lower IS, regardless of the electrolyte type.
pH; heavy metal; thermodynamics
