Cesium-Adsorption Method for Measuring Accessible Structural Surface Charge

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 <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 σ₀ 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.