Abstract
The Gains-Thomas ion exchange selectivity coefficients, KGT 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:KGT 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 (EFe), indicating that the selectivity of Fe2+ to the surface of kaolinite was similar to that of Ca2+. The KGT values of Fe2+ to Ca-montmorillonite were high at low EFe and shifted to near 1.0 with an increase of EFe. In contrast, the KGT values of Fe2+ to Ca-illite were apparently high, near 40, at low EFe and decreased to a range of 2 to 3 with an increase of EFe, being markedly higher than those for kaolinite and montmorillonite. For both Ca-kaolinite and Ca-montmorillonite, the selectivity coefficients KGT values of Mn2+ were constant, near 1.0 with a variation of 0.66–1.51, regardless of EMn. The KGT 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.
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