Chemical Behavior of Lanthanum in the Presence of Soils Components: Adsorption and Humate Complexes

The inorganic and organic components of soils from a forest and from a semiarid region were separated and characterized by different techniques. The kinetic adsorption behavior of lanthanum ions by the inorganic components of the soils indicated that the systems behave according to the kinetic model of pseudo-second order, so 3 h of contact was sufficient to achieve equilibrium. The sorption isotherm data, qe vs. Ce, were best adjusted to the Langmuir model for the soil of the forest; this means that a monolayer is formed and that the energy of adsorption is the same in all adsorption sites so the material is homogeneous. The data for the soil of the semiarid area were adjusted to the Freundlich model; this indicates that the material is heterogeneous and multilayers are formed. The competition between the lanthanum sorption by the inorganic components and the formation of a complex with humic acids was shown by a decrease of qe. The method of Schubert was used to determine the stoichiometric coefficient of the complex La:(HA)(n), which was 1: 1 for both soils, whereas the log beta(app)(La),(HA)(n) was 5.1 +/- 0.2 for the soil of the forest and 7 +/- 1 for the semiarid soil.