Sulfate adsorption in soils of the Paraiba State

Abstract

The aim of this study was to investigate the adsorption of sulfate in surface samples of a Yellow Latosol (Oxisol), and Gray Argisol (Ultisol) and a Red Argisol eutrophic (Ultisol) of the Paraiba State. Samples were collected from three weathered soils in the layer of 0-30 cm depth, in order to have a wide variation of values of physical and chemical characteristics of soils. The S-SO4-2 concentrations of equilibrium solutions used for adjusting the Langmuir isotherm were based on soil values P-rem and corresponded to 0.0; 2.5; 5.0; 7.5; 10.0 and 12.5 mg L-1 of S-SO4-2 for LA and PAC soil; 0.0; 3.5; 7.0; 10.5; 14.0 and 17.5 mg L-1 S-SO4-2 for PVe soil. The hyperbolic model of Langmuir isotherm was adjusted by means of linearization technique obtaining the maximum adsorption of sulfate by soils. The spontaneity of adsorption reaction was obtained from the separation factor and the Gibbs free energy. The Langmuir model was able to satisfactorily describe the adsorption of sulfate to the soils. Among the soils studied, Yellow Latosol was the most adsorbed sulfate. In general, the adsorption capacity of sulfate in these soils was very low. The thermodynamic study indicated that the behavior of the free energy of adsorption was inversely with the concentration of sulfate in equilibrium solution and the reaction of sulfate adsorption was favorable and spontaneous.