EFFECT OF AGING ON THE NATURE AND INTERLAYERING OF MIXED HYDROXY AL-FE-MONTMORILLONITE COMPLEXES

Mixed hydroxy Al-Fe-montmorillonite complexes (5 mmol of Al+Fe per g o f clay) were prepared at pH 5.0 by mixing different amounts of Al(NO3) (3) and Fe(NO3)(3) to give initial Fe/Al molar ratios (R) of 0, 0.1, 0 .25, 0.5, 1.0, 4.0, 10.0 or infinity. The effect of ageing on the inte rlayering, mineralogy, chemical composition, CEC and external surface area of the complexes was studied. The degree of interlayering of fres hly prepared and aged complexes (up to 120 days at 50 degrees C) was a lways greater in the samples with R ranging from 0.5 to 4. Gibbsite an d hematite (with traces of goethite) formed in the samples with R less than or equal to 1 and R infinity, respectively. No crystalline Fe ox ide was detected by XRD analyses in the complexes with R ranging from 0.1 to 10, whereas no crystalline Al and Fe oxides were formed in the complexes at R = 4-10. However, some evidence suggests the possible fo rmation of a microcrystalline Fe oxide/oxyhydroxide undetectable by XR D analyses in the complexes richer in Fe. The CEC and the surface area of the freshly prepared mixed hydroxy Al-Fe-montmorillonite complexes increased with increasing R. Large quantities of Al+Fe were solubiliz ed by oxalate in the complexes with R greater than or equal to 0.5, ev en after ageing for 1:0 days at 50 degrees C, indicating that interlay ering of Al-Fe species at certain Fe/Al molar ratios was particularly stable.