SODIUM ADSORPTION ONTO GOETHITE AS A FUNCTION OF PH AND IONIC-STRENGTH

The adsorption of sodium ion onto goethite was studied as a function o f both pH and ionic strength (NaClO4 electrolyte) using Na-22 tracer, The pH ranged from 4.1 to 12.0. The ionic strength was controlled by v arying the NaClO4 concentration and ranged from 0.005 to 0.016 molar T he results indicate that sodium ion is adsorbed onto goethite as an ou ter-sphere complex at pH values above the point of zero charge. Below the point of zero charge sodium ions are repelled from the double laye r by the positively charged goethite surface. This phenomenon of 'cati on exclusion' is similar in principle to the 'anion exclusion effect' [1] observed in soils and clays. The pH dependence for sodium ion adso rption is in good agreement with triple layer surface complexation mod el. The surface complexation constant log (K-Na(int)) is 1.2 +/- 0.1.