Effects of Al substitution on goethite formation

Goethites containing up to 32.7 mole % Al were synthesized and characterized through diffractometry, spectrometry, DSC and chemical techniques. Results indicate that crystal growth was favoured or hindered depending on the percentage of aluminum incorporated into the structure. Goethites to which 5-10 mole % Al were initially added crystallized best, as shown by the highest crystallite dimensions calculated from X-ray and Mössbauer analyses, and by the lowest surface areas values. Over such substitution degree goethite crystallization was hindered and for sample synthesized in presence of 40 mole % Al the separation of a discrete amount of gibbsite and bayerite occurred. Both crystallinity and Al-substitution degree influenced the goethite dehydroxylation endothermic peak temperature playing a competitive action in the peak shift. In fact, the increase in the thermal stability induced by the presence of Al3+ ions into the structure was counteracted by the decrease due to the particle size reduction. The stabilizing effect of aluminum was confirmed by the oxalate treatment which removed from the Al-free sample a Fe amount higher than that from the sample containing about 20 mole % Al, although both had similar crystallinity.