Adsorption-desorption processes of calcium on Brazilian soils

Thermodynamic data on ion-exchange equilibrium were obtained oil the heterogeneous cationic red or red-yellow latosol soil/aqueous solution interface. The equilibrium with the attached acidic functional groups can be represented by a general equation: SHn + Mn+ reversible arrow SM + nH(+) where SH is the soil surface containing H+ ions available for exchanging and Mn+ is the cation exchanged. After exchanging, the desorption process, which depends on the natural structure and properties of soil, presents reversibility expressed by: SM reversible arrow nS(-) + Mn+. The thermodynamic data related to the coefficient of distribution, K-d, equilibrium constant, K, and Gibbs free energy, DeltaG, provide information about adsorbent-adsorbate interactions in the soil surface. K and DeltaG values were calculated by the linearization of the Langmuir equation, to give a constant K-L (Langmuir constant), with the values being compared with an absorption method, K-R (adsorption constant). The equilibrium time for surface saturation was established as 2 It for both soils. The number of moles adsorbed, N-f, on red latosol, (30.17 +/- 1.51) x 10(-5) mol g(-1), is five times higher than that found for red-yellow latosol, (6.07 +/- 0.30) x 10(-5) mol g(-1). N-f for lead saturation and calcium/lead equilibrium were (7.09 +/- 0.35) x 10(-5) and (4.59 +/- 0.23) x 10(-5) mol g(-1), respectively. The desorption process on red latosol soil occurred to the extent of 37.5% with nitric acid, after the soil surface was saturated with calcium as the index ion. The adsorption of the index cation caused a decrease in the adsorption curve of lead on the surface. The values obtained for red latosol and red-yellow soil for exchanging with calcium are: K-d, (3.35 +/- 0.17 and 0.77 +/- 0.04) x 10(-2) dm(3) g(-1); K-L, (1.04 +/- 0.05 and 7.10 +/- 0.36) x 10(2); K-R, (39.20 +/- 1.96 and 6.91 +/- 0.35) x 10(2); -DeltaG(L), 11.51 +/- 0.58 and 16.27 +/- 0.81 kJ mol(-1); and - DeltaG(R), 20.50 +/- 1.03 and 16.20 +/- 0.81 kJ mol(-1), respectively. Based on Gibbs free energy values, a spontaneous exchange process for cations on soils is easily detected. (C) 2002 Elsevier Science B.V. All rights reserved.