Adsorption–desorption processes of calcium on Brazilian soils

Abstract

Thermodynamic data on ion-exchange equilibrium were obtained on 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: SH n+ M n+⇌ SM +n H + where SH is the soil surface containing H + ions available for exchanging and M n+ is the cation exchanged. After exchanging, the desorption process, which depends on the natural structure and properties of soil, presents reversibility expressed by: SM ⇌n S −+ M n+ . The thermodynamic data related to the coefficient of distribution, K d, equilibrium constant, K, and Gibbs free energy, Δ G, provide information about adsorbent–adsorbate interactions in the soil surface. K and Δ G 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 h for both soils. The number of moles adsorbed, N f, on red latosol, (30.17±1.51)×10 −5 mol g −1, is five times higher than that found for red–yellow latosol, (6.07±0.30)×10 −5 mol g −1. N f for lead saturation and calcium/lead equilibrium were (7.09±0.35)×10 −5 and (4.59±0.23)×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)×10 −2 dm 3 g −1; K L, (1.04±0.05 and 7.10±0.36)×10 2; K R, (39.20±1.96 and 6.91±0.35)×10 2; −Δ G L, 11.51±0.58 and 16.27±0.81 kJ mol −1; and −Δ G 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.