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Abstract

Goethites consisting of acicular and lathlike crystals and containing between 0 and 14 mole % Co, Cr, Ni, Mn, or Al were synthesized at high pH. Hematites were prepared by heating goethites at 300° and 800°C for 2 hr. The dissolution rates of these iron oxides in 1 M HCl differed greatly (from 0.03 to 21.8 × 10–2 g Fe/g/hr) and increased with increasing temperature. On the basis of unit surface area, Cr-goethites dissolved at about one-tenth the rate of unsubstituted, Ni- and Al-goethites (~3 g Fe/rnVhr), whereas Co- and Mn-goethites dissolved at about twice this rate. The frequency factor for Cr-goethites (~ 10–16–10–14 g Fe/m2/hr) was much larger than for other goethites, which may have been due to the extensive development of etchpits during dissolution; however, these large frequency factors were not sufficient to counteract the effect of the high activation energies (21–33 kcal/mol for Cr-goethites), and the dissolution rate for Cr-goethites was the lowest recorded.

Hematite produced by dehydroxylation of goethite at 300°C is microporous and dissolved much more quickly (3–107 times as fast) than corresponding goethites. Hematites produced at 800°C appeared sintered and massive; they mostly dissolved at similar rates (0.3–2.7 times) to goethites, although Cr-substituted hematite heated to 800°C dissolved much more rapidly (10 times) than the poorly soluble Cr-goethite.

Dissolution curves for both goethite and hematite heated to 300°C showed that Fe and other metals dissolved congruently and therefore were probably uniformly distributed within crystals. The only exceptions were for goethites containing Al and Ni which did not dissolve congruently; these elements were slightly concentrated towards the centers and rims of crystals respectively. For hematite heated to 800°C, Co dissolved much more rapidly than Fe, and Ni much more slowly than Fe, indicating that these elements were not present in the hematite structure. Cr-, Mn-, and Al-hematites (800°C), however, dissolved congruently, indicating that the nearly uniform substitution of these elements was preserved on heating to 800°C.

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