Ca²⁺-Fe²⁺ and Ca²⁺-Mn²⁺ exchange selectivity of kaolinite, montmorillonite, and illite

The Gains-Thomas ion exchange selectivity coefficients, K_GT of Fe²⁺ and Mn²⁺ on three types of Ca²⁺-saturated clay minerals (kaolinite, montmorillonite, and illite) were determined in an oxygen-free condition in order to understand better the behaviors of these ions in anaerobic soils. The average ion exchange selectivity coefficients: K _GT of Fe²⁺ to Ca-kaolinite was 1.13, with a variation of 0.72-1.46, regardless of the charge fractions of Fe²⁺ adsorbed on the surface sites (E_Fe), indicating that the selectivity of Fe ²⁺ to the surface of kaolinite was similar to that of Ca ²⁺. The K_GT values of Fe²⁺ to Ca-montmorillonite were high at low E_Fe and shifted to near 1.0 with an increase of E_Fe. In contrast, the K_GT values of Fe ²⁺ to Ca-illite were apparently high, near 40, at low E_Fe and decreased to a range of 2 to 3 with an increase of E_Fe, being markedly higher than those for kaolinite and montmorillonite. For both Ca-kaolinite and Ca-montmorillonite, the selectivity coefficients K _GT values of Mn²⁺ were constant, near 1.0 with a variation of 0.66-1.51, regardless of E_Mn. The K_GT values of Mn²⁺ in Ca-illite were approximated 1.4 with a range of 0.52-2.41. These results suggest that the solution systems composed of Mn-clay minerals and Ca-clay minerals can be regarded as thermodynamically ideal behavior. This confirms that Mn²⁺ can be adsorbed uniformly on the surface of the clay minerals with selectivity similar to that of alkaline-earth ions.