CHARACTERIZATION OF ZINC ADSORPTION SITES IN TWO MINERAL SOILS

概要

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The adsorption of Zn, as compared with Mg, on two mineral soils, which differed in their major cation-exchange materials and with and without Ca-saturation, was measured in the presence of free CaCl_2. The adsorption of Zn as well as Mg occurred on cation-exchange sites. The Zn adsorption data conformed to a two-term Langmuir equation. The presence of two kinds of adsorption sites and their numbers and bonding energies were deduced. However, the Langmuir approach was not adopted on the basis of comparison between the total number of the adsorption sites for Zn deduced and the CEC of the soils. As an alternative approach, the selectivity coefficient K^<Zn>_<Ca>, as defined by the equation : K^<Zn>_<Ca>=([Zn]_<soil>/[Ca]_<soil>)・([Ca]_<soln>/[Zn]_<soln>) was calculated for each adsorption equilibrium and plotted against the amount of Zn adsorbed. This K^<Zn>_<Ca>-[Zn]_<soil> plot was used to estimate the capacities of the soil to adsorb Zn with specified affinities. The K^<Zn>_<Ca> value varied between 1 and 1,000,whereas the corresponding K^<Mg>_<Ca> value varied only between 0.5 and 1. The K^<Zn>_<Ca> value was dependent upon the amount of Zn adsorbed, the status of exchangeable cations, and the major cation-exchange materials (montmorillonite vs. allophane-imogolite) in the soils. The importance of surface OH groups in allophane-imogolite as specific adsorption sites for Zn was suggested.
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