Surface Complexation Modeling in Variable Charge Soils: Charge Characterization by Potentiometric Titration

Abstract

intrinsic equilibrium constants of 17 representative brazilian oxisols were estimated from potentiometric titration measuring the adsorption of h + and oh − on amphoteric surfaces in suspensions of varying ionic strength. Equilibrium constants were fitted to two surface complexation models: diffuse layer and constant capacitance. The former was fitted by calculating total site concentration from curve fitting estimates and pH-extrapolation of the intrinsic equilibrium constants to the pznpc (hand calculation), considering one and two reactive sites, and by the FITEQL software. The latter was fitted only by FITEQL, with one reactive site. Soil chemical and physical properties were correlated to the intrinsic equilibrium constants. Both surface complexation models satisfactorily fit our experimental data, but for results at low ionic strength, optimization did not converge in fiteQl. data were incorporated in Visual minteQ and they provide a modeling system that can predict protonation-dissociation reactions in the soil surface under changing environmental conditions.

Successive off take of elements by maize grown in pure basalt powder

“…As a matter of comparison, the CEC values of the montmorillonites range between 70 and 120 cmol c kg −1 , while the CEC values for the vermiculites range between 130 and 210 cmol c kg −1 (Essington, 2003). Also, Oxisols from Cerrado present CEC ~9.4 cmol c kg -1 (STD = 3.9), and some are as low as 4.1 cmol c kg -1 at pH 7.0 (Marchi et al, 2015). These soils present variable charge and natural pH is lower than 5, with effective CEC to even lower values.…”

Research Perspectives on the Use of Phosphogypsum in the Brazilian Cerrado

“…Regarding variable-charge soils, adsorption of SO 4 2is related to the increase of positive surface charges as pH decreases [19]. Sulfate is both specifically − adsorption by ligand exchange; ionic exchange of SO 4…”