Nature of mixed iron and aluminium gels as affected by Fe/Al molar ratio, pH and citrate

Universita di Napoli Federico II, Dipartimento di Scienze Chimico-Agrarie, Portici, Italy

The nature and mineralogy of mixed Al and Fe(III) gels (initial Fe/Al molar ratios (Ri) of 1.0 and 2.5) formed at pH values ranging from 4.0 to 10.0, both in the presence and absence of citric acid (citrate/Fe+Al molar ratio (Rcit) of 0.01 and 0.1) and aged for a long period at room temperature and at 50 degrees C have been studied. The complexes showed considerable differences in the mineralogy of the precipitation products. The samples formed at Ri = 1.0 contained ferrihydrite at pH 4.0, ferrihydrite+gibbsite at pH 5.0-7.0, and hematite+Al(OH) ₃ polymorphs+ferrihydrite at pH 9.0-10.0. The samples formed at Ri = 2.5 had greater quantities of poorly crystallized ferrihydrite. Large amounts of Fe+Al (25-82%) were solubilized from the samples aged for 60 days at 50 degrees C by ammonium oxalate. The addition of increasing concentrations of citrate to the gel suspensions containing equimolar amounts of Fe and Al strongly inhibited the formation of Al(OH) ₃ polymorphs both at pH 5.0 or 8.5 by promoting the formation of short-range ordered materials. Citrate added initially to Fe-Al solutions (R (super *) cit = 0.1) completely inhibited formation of crystals even after 135 days at 50 degrees C.

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