Two simple isotherms describing cation adsorption on Andisols

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Abstract
I proposed two simple isotherms with two adjustable parameters to describe cation adsorption on Andisols. Monovalent and bivalent cation adsorption on humic Andisols dominated by AI-humus complexes was described by qM=A [(Mn+)j(H+)sn]exp( − BqM). This equation is the limiting form of the original triple layer model under the following conditions that 1) the amount of adsorbed cation is negligible as compared with the total amount of ionizable surface hydroxyls, and 2) the diffuse ion swarm disappears and Mn+ is adsorbed only through the formation of outer-sphere surface complexes. On the other hand, cation adsorption on allophanic Andisols was described by the Freundlich-type equation, qM=A[(Mn+)j(H+)sn]B. The disparity in the applicable isotherms suggests that different mechanisms operate in the adsorption phenomena in humic and allophanic Andisols. I discussed the relations of these two isotherms to the adsorption models developed in interfacial chemistry and the problems encountered in the application of these models to adsorption phenomena in soils.