Abstract: | Conventional K: Al exchange isotherms for montmorillonite showed that Al3+ was strongly preferred to K+ in o-oin solutions. The exchange coefficient, K', calculated using the isotopically exchangeable K, was greater than unity and did not vary with the Al-saturation or with the initial pH of the AlCl3 solutions. Isotherms for vermiculite, illite, and soils in o·oin solutions also showed Al3+-preference but unlike those for montmorillonite were not asymptotic to qAl/qo= 1, qAl being the amount of adsorbed Al and qo the total adsorbed (Al + K), indicating that some of the isotopically exchangeable K could not easily be exchanged by Al3+ ions; this difficultly exchangeable K (DEK) was estimated for each exchanger. K' values for vermiculite, illite, and soils were less than unity and did not vary with Al-saturation or initial pH if the isotopically exchangeable K was corrected for DEK. This showed that K+ was adsorbed more strongly than Al3+. Strengths of K+ adsorption referred to Al3+ as the counter-cation were in order: soils > vermiculite, illite > montmorillonite. |