Reductive Dissolution Kinetics of Aluminum-Substituted Maghemite.

Maghemite (γ-Fe₂O₃), the ferrimagnetic form of Fe₂O₃, frequently occurs in soils of the tropics and subtropics. In some soils, mainly those originated from mafic rocks, around 50% of the iron oxides can be maghemite.  In natural environments, chemical reduction is by far the most important dissolution mechanism of iron oxides; therefore, the kinetics for reductive dissolution (citrate-bicarbonate-dithionite) of synthetic, aluminum-substituted maghemites (γ-Fe_2-xAl_xO₃), where the isomorphic substitution (IS) values varied from 0.0 to 14.3 mol % Al, in controlled temperature (20°C ± 1) were studied.  At set times (5, 10, 20, 40, 60, 120, 240, 720, 1440, 3180 and 7200 min) the reaction was stopped, the sample was centrifuged, and Fe and Al were analyzed by ICP. The resulting data were modeled using both the Kabai and Avrami-Erofejev equations.  The overall dissolution rate of Fe and Al were very similar during the experiment indicating congruent dissolution; however, initially larger amounts of Fe were observed than Al.  Both the Kabai and the Avrami-Erofejev equations yielded a linear adjustment for the maghemites.  Dissolution rates considering the specific surface area (SSA), as k2 (k2=k/ASE) were the most efficient for describing the influence of IS on mineral dissolution and demonstrated that the maghemites became more resistant to dissolution with increasing Al substitution. Observations from transmission electron microscopy (TEM) did not indicate any preferential dissolution of the maghemites, in spite of observing specific sites of dissolution in the maghemite with 14.3 mol% Al.