Desorption of Phosphate from Kaolinite

Initially all the phosphate adsorbed onto kaolinite is isotopically exchangeable. On washing some of the phosphate ceases to be exchangeable, but if the amount that remains labile is plotted against the solution concentration the original isotherm is preserved. Thus, some of the total phosphate goes into a nonexchangeable “fixed” sink. If phosphate is adsorbed in very small increments some of it is converted directly into the fixed form. In the presence of large concentrations of phosphate, all the adsorbed phosphate becomes exchangeable.