RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

The adsorption of sodium ion onto goethite was studied as a function of both pH and ionic strength (NaClO₄ electrolyte) using ²²Na tracer. The pH ranged from 4.1 to 12.0. The ionic strength was controlled by varying the NaClO₄ concentration and ranged from 0.005 to 0.016 molar. The results indicate that sodium ion is adsorbed onto goethite as an outer-sphere complex at pH values above the point of zero charge. Below the point of zero charge sodium ions are repelled from the double layer by the positively charged goethite surface. This phenomenon of 'cation exclusion' is similar in principle to the 'anion exclusion effect' [1] observed in soils and clays. The pH dependence for sodium ion adsorption is in good agreement with triple layer surface complexation model. The surface complexation constant log (K_Na^{int is 1.2±0.1.}