Adsorption and desorption of indifferent ions in variable charge subsoils: The possible effect of particle interactions on the counter-ion charge density

The relationship between soil solution ionic strength (IS) and the surface or counter-ion charge density (CICD) is well described by the double layer theory. However, the magnitude of the counter-ion charge may be affected by phenomena that are likely to occur in variable charge subsoils, and research to investigate the extent of indifferent ion sorption as a function of IS is certainly needed. The objective was to study adsorption and desorption of ions in batch and column experiments, and to propose a mechanism that describes best the experimental observations in subsoils from the southeastern USA. Results showed that the cation and anion of an electrolyte were simultaneously a sorbed in approximately equivalent amounts with no net release of other ions into the soil solution. Ions were adsorbed in the Stern and diffuse layers of oppositely charged colloids since the subsoil was treated with dilute solutions. They were immediately displaced when the subsoil was washed with distilled water (dFH(2)O), confirming the reversible nature of this phenomenon. Three different magnitudes of the CICD were observed at similar IS values. The surface charge density on negatively and positively charged soil particles may not be equal to the CICD in dilute soil solutions, and other mechanisms may operate to balance the surface particle charges. It is possible that a portion of the surface charge may be balanced as a result of the interactions among oppositely charged soil particles when their diffuse layers extend and overlap, or compress and separate in response to IS changes.