Desorption of Phosphate from Goethite

The reversíbilíty of the adsorption of phosphate on goethite was measured by dlluting suspensions of goethite on which phosphate was adsorbed with large volumes oí phosphate-frce solution at the same ioníe strength. The effect on the reversibility of various adsorptíon and desorption conditions was studied, particularly pH, temperature, ionic strength and time of reaction. The apparent irreversibílity of the adsorption reaction seems to be due to a slow adsorption reaction of part oí the phosphate after a very rapid initiai adsorption. with a similar behaviour when the phosphate is desorbed. When the total adsorption plus desorption time is long enough. both adsorption and desorption poínts tend to He on a single curve which corresponds to the isotherm calculated according to the Stern doublelayer theory.