The effects of mineralogy and chemistry in ionic retention, displacement, and transport in variable charge subsoils

Some highly weathered soils from southeastern United States and other tropical and subtropical areas around the world, are considered variable charge soils because of their unique characteristics that are different from the permanent charge soils. The weathering toward a “no or little net charge state” in variable charge subsoils causes changes in the clay mineral contents and their surface chemistry. The study of these systems has lagged behind the study of permanent charge soils primary because of the lack of economic and environmental interest in such sods, and became a passionate research topic only in the recent years.; The clay fraction mineralogy in the majority of the variable charge subsoils in the collection was dominated by the quartet kaolinite, gibbsite, goethite, and hematite. The subsoils had acidic pH and extremely low EC values of the soil solution. In such conditions, kaolinite and Al or Fe oxides have opposite net surface charge and the subsoils are mixed charge colloidal system. Salt adsorption in these subsoils appears to be neither a simple stochiometric exchange reaction, because both, cations and anions of the added salt, disappear from the subsoil solution, nor an exchange reaction between the ions of the added electrolyte and H+ and OH− adsorbed on outer spheres and diffuse layers on the subsoil colloids. The magnitude is higher in subsoils with appreciable AEC and equivalent CEC, where kaolinite and active Al and Fe oxides dominate the clay fraction. Salt adsorption was found to be a reversible phenomenon, namely the adsorbed salt is “displaced” immediately after the subsoil is leached with a more diluted solution or distilled water. The NO₃− depletion from the soil solution at lower electrolyte concentrations is believed to occur because of the interactions between Al or Fe oxides and silicate colloids present in this subsoil. A three step model for indifferent ion adsorption is proposed. The first step is the reestablishment of a portion of the effective charge that vanishes as a result of the overlapping of oppositely charged diffuse layers on different colloids, termed “the mutually neutralized charge”, $s$_M. The “newly developed charge”, $s$_N, is developed on variable charge surfaces during the second step, in response to ionic strength increase in subsoil solution. The reestablishment of these two components of the effective charge causes “salt adsorption”, manifested in depletion of the added salt from the subsoil solution. The last step, “ionic exchange”, with the corresponding effective charge component $s$_EX, begins when the salt adsorption capacity of the subsoil is satisfied. Literature references and data collected in this study were used to validate the model. The relative contributions of each effective charge components and especially, the large changes in the magnitude of $s$_M and $s$_N in response to changes in soil solution ionic strength must be taken into account in adsorption studies involving acidic variable charge subsoils. The separation of “temporarily effective” ($s$_M + $s$_N) from “permanent effective” ($s$_EX) portions of the effective charge is very crucial when displacement and transport of ions like NO₃− are considered, in order to understand that they are only temporarily adsorbed and can be immediately leached out by rain.