Modelling soil surface charge density using mineral composition

The proposed mathematical model consists of a first approach to estimate the soil composition from an experimental titration curve and the quantitative analysis of the soil mineral constituents. Mineral mixtures made using different percentages of kaolinite, illite and quartz were used to simulate a natural porous media and evaluate the proposed mathematical model assuming no interaction among the solid components of mixtures. Three soil samples from Argentine provinces collected from surface horizons (0–15 cm depth, horizon A1) and typified as Typic Haplocryoll, Rhodudult and Typic Hapludoll were titrated by successive additions of small volumes of 0.1 M HCl and/or KOH at ionic strength of 10−3, 10−2 and 10−1 M of KCl. Soils are strongly reflective of the individual point of zero charge (PZC) of the soil components. Iron oxides have high PZC values while silica, clays and soil organic matter have low PZC. In these soils, the difference between isoelectric point (IEP) and PZC was used to evaluate the existence of specific adsorption and the soils charge behaviour was fitted using the developed mathematical model. A theoretical titration curve was calculated using the charge density and area fraction of each soil or mineral mixtures components. The experimental and theoretical titration curves present a regression coefficient, R2, higher than 0.95 and a significant p-level p=0.00 for both, soils and mineral mixtures. This model could also be used to interpret the contaminants mobilization in the natural environments, which is dependent of the surface charges and on the mineralogical components of the soils as well as of the chemical interactions among contaminants and soil constituents.