Anion Adsorption in Ferruginous Tropical Soils

Abstract

AbstractThe difference in pH of a soil suspension prepared with 1N KCl and with water was used to determine net charge of colloids with constant potential type surface. The quantity (pHKCl‐pHH2O) called delta pH had a positive, zero, or negative value corresponding to the net surface charge. Negative and positive adsorption of chloride or nitrate ions were measured in soil suspensions with negative and positive delta pH, respectively. Increasing the nitrate ion concentration increased sulfate adsorption in suspensions with negative delta pH values. Negative adsorption of nitrate and chloride ions was measured when sulfate ions were added to a soil colloidal suspension which was initially net positively charged. This supports the present belief that specifically adsorbed anions render a surface more negative by displacing the zero point of charge to lower pH values. This was substantiated by a measured increase in CEC from an initial value of 26 meq/100 g in a phosphated soil. Each millimole of adsorbed phosphate increased CEC by 0.8 meq.