Model of cation adsorption on allophanic andisols I. Theory and algorithm

Abstract

A new model of cation adsorption on allophanic Andisols was proposed. This model assumes that cations are adsorbed in the internal space of the hollow spherical ball of allophane either in the form of a diffuse ion or through the formation of surface complexes with SO-groups arranged on the inner surface swarm. The distinctive characteristics of this model is that the mean-field approximation was not adopted, but the Coulomb potential among charged particles was directly evaluated by using the primitive restricted model (Valeau and Cohen 1980: J. Chern. Phys., 72, 5935-5941) and the grand canonical Monte Carlo (GCMC) simulation. This report introduces the theory and the algorithm of the GCMC simulation.

Model of cation adsorption on allophanic andisols II. Description of mono-component adsorption

“…These results confirm the original concept introduced in Fig. 1 in the preceding paper (Nakahara 2000). In the case of Na+ adsorption, a similar tendency was observed, but the change from the dominance of diffuse ion for n Na < 1.0 to the dominance of surface complexes for nNa> 80 is not as clear as that for divalent cations.…”

“…These results show that when a large number of Mg2+ cations are adsorbed, most of the Mg2+ cations form surface complexes, but one or two diffuse cations are still present under these conditions, suggesting that a minor correction had to be made in Fig. I in the previous report (Nakahara 2000). The reason for the stability of one or two diffuse Mg2+ can be explained in two ways.…”

“…(5) The change in the total electrostatic energy U e1ec is calculated by using Eqs. 4 and 5 described in the previous report (Nakahara 2000). (6) If the energy change is negative, accept the new configuration and return to Step (3).…”

“…In the preceding paper (Nakahara 2000), a new model of cation adsorption on allophane was introduced. This report shows the results of the application of this model to N a+, K+, Mg2+, and Ca2+ adsorption on an allophanic Andisol.…”

A geometrical model of bound water permittivity based on weighted averages: the allophane analogue

“…This approach may serve as a basis to model other more complex porous systems such as soil pores or water adsorbed to non-planar soil particles. Additionally the 3 in estimates obtained may help to improve current cation adsorption models on allophanic Andisols, where the effect of water molecules is simply considered by assuming a uniform dielectric constant of bulk water 3 in Z78.5 (Nakahara, 2000).…”