Formation and characterization of OH–Al–humate–montmorillonite complexes

We studied the formation of OH–Al–humate–montmorillonite complexes as affected by the nature of the humic acid and the sequence of addition of montmorillonite, humic acid and hydrolytic species of Al. Complexes were prepared at pH 7.0 by different addition sequences of montmorillonite, 3 or 6 mmol Al and 5, 10 or 20 mg of humic acid per g of clay. d-spacings of samples ranged from 14.50 to 19.00 Å vs. 11.70 Å of montmorillonite. Interlayering of humic acids was promoted when preformed OH–Al–humate complexes were added to montmorillonite and partially impeded when the complexes were synthesized by neutralizing Al ions in the presence of montmorillonite. Humic acids were not interlayered in the absence of OH–Al-species, indicating that the hydrolytic products of Al play a vital role in the formation of organo–mineral complexes in natural environment.