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Cesium-Adsorption Method for Measuring Accessible Structural Surface Charge

Dep. of Crop and Soil Sciences, Michigan State Univ., East Lansing, MI 48824-1325

Garrison Sposito

Dep. of Soil Science, Univ. of California, Berkeley, CA 94720

*Corresponding author.

ABSTRACT

Measurements of the separate contributions of structural and pH-dependent charge are essential to understanding and predicting surface-chemical reactions in soils and other heterogeneous adsorbents that contain a mixture of surface functional groups. An ion-adsorption method was developed to measure the accessible structural charge density (₀) in adsorbents that comprise both structural-charge and variable-charge surface functional groups. The method is based on the preference of siloxane surface sites for Cs⁺ over Li⁺ and on the much lower selectivity of ionizable surface groups for Cs⁺. To estimate ₀, CsCl-saturated adsorbents were reacted with 0.05 mol CsCl kg^–1 at pH 7, washed with ethanol to reduce the entrained CsCl concentration, dried at 65 °C to promote the formation of inner-sphere surface complexes between Cs⁺ and permanent-charge sites, and then extracted with 0.01 mol LiCl kg^–1 to displace Cs⁺ from variable-charge adsorption sites, which exhibit little or no preference for Cs⁺ over Li⁺. Finally, Cs⁺ was displaced from permanent-charge sites by repeated NH₄OAc extraction until no Cs was present in the equilibrium supernatant solution. According to this method, ₀ represented about 85 and 70% of the accessible cation-adsorption sites at pH 7 on <2-µm Wyoming montmorillonite and <106-µm Silver Hill illite, respectively; 15% and 30%, respectively, were variable-charge sites. These results were consistent with published reports of the amount of pH-dependent cation adsorption on montmorillonite and with model calculations of the relative contributions of edge and structural charge in montmorillonite and illite. When the method was used to measure Cs-accessible ₀ in four soils, the percentage of structural charge ranged from 1% in an Oxisol to 65% in an Alfisol. The method is appropriate for adsorbents that contain montmorillonite, illite, and repotassified (nonexpandable) vermiculite. It may be suitable for adsorbents that contain clay-sized, but probably not coarse-grained, expandable vermiculite.

Contribution from the Dep. of Soil and Environmental Sciences, Univ. of California, Riverside, and Dep. of Crop and Soil Sciences, Michigan State Univ.

Received for publication December 10, 1990.

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