Adsorption and Desorption of Sulfate and Charge Relationships in Allophanic Clays

Sulfate adsorption by a synthetic and a natural allophane increased with increasing solution concentration. Initially both the materials had net positive charges. With the progressive adsorption of sulfate the positive charges were neutralized, followed by increases in the negative charges. Sulfate adsorption released OH ‐ into the solution and linear relationships were obtained between the two parameters. Very little Si was released on sulfate adsorption. From the relationships between the various parameters studied it is proposed that on a net positive surface sulfate is adsorbed as a bidentate forming a six‐membered ring, displacing either two aquo or hydroxyl ligands. On a neutral or a negative surface sulfate is adsorbed as a monodentate displacing one aquo or one hydroxyl ligand and this makes the surface more negative. The desorption studies showed that aquo and hydroxyl ligands from the solution exchanged with the sulfate on the surface and, depending on the surface charge, a 1:1 or 1:2 stoichiometry apparently exists between the moles of sulfate desorbed and the ligands adsorbed. Irrespective of the initial concentration of sulfate on the clay surface the amount left on the surface after desorption was about the same.