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Cesium‐Adsorption Method for Measuring Accessible Structural Surface Charge
Author(s) -
Anderson Sharon J.,
Sposito Garrison
Publication year - 1991
Publication title -
soil science society of america journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.836
H-Index - 168
eISSN - 1435-0661
pISSN - 0361-5995
DOI - 10.2136/sssaj1991.03615995005500060011x
Subject(s) - adsorption , chemistry , surface charge , charge density , illite , montmorillonite , caesium , charge (physics) , analytical chemistry (journal) , inorganic chemistry , clay minerals , chromatography , mineralogy , organic chemistry , quantum mechanics , physics
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 (σ 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 σ 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 NH 4 OAc extraction until no Cs was present in the equilibrium supernatant solution. According to this method, σ 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 σ 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.