Cesium‐Adsorption Method for Measuring Accessible Structural Surface Charge

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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 (σ0) 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 σ0, 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, σ0 represented about 85 and 70% of the accessible cation‐adsorption sites at pH 7 on 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.

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