Redistribution of Zn during transformation of ferrihydrite: Effects of initial Zn concentration

For an understanding of metal transport at the Earth's surface, ferrihydrite (Fe4–5(OH,O)12) transformation experiments were conducted with and without the presence of Zn, and the redistribution of Zn between solid and solution during the transformation was examined. Batch experiments were carried out at 0, 20, 200 and 750 μM Zn, initial pH 7.5, and a temperature of 70 °C for 1–3504 h at a constant ferrihydrite concentration (2 mM Fe). Sequential extraction was applied to the resultant samples to measure the concentrations of Zn and Fe at three different extraction stages. Solid phases were examined by X-ray diffraction and X-ray absorption fine structure analysis, and solutions by inductively coupled plasma optical emission spectrometry. Ferrihydrite is transformed to hematite (Fe2O3) and goethite (FeO(OH)) at both 0 and 20 μM Zn while the transformation rate is slightly slower at 20 μM Zn. Almost all initial Zn is likely to be accommodated in the hematite and goethite structures at the completion of the transformation at 20 μM Zn. At 200 μM Zn, most of ferrihydrite is transformed directly to hematite, while at 750 μM Zn, ferrihydrite is transformed to hematite through Zn-bearing maghemite (Fe2O3 or Fe2.67O4), an intermediate, metastable phase. The transformation rate is significantly slower at 200 μM Zn than at 20 μM Zn, and further slower at 750 μM Zn. Zinc-bearing maghemite and hematite can contain Zn with ~0.4 and ~0.07 of Zn/Fe mole ratios, respectively, in their structures. Most of the initial Zn at 750 μM Zn (Zn/Fe of ~0.2 in a system) is likely to be present as dissolved Zn at the completion of the transformation since Zn-bearing maghemite is decomposed with progress in transformation and most of Zn is released into solution with less Zn in the hematite structure. Because of the accommodation capacity of Zn in hematite, only a small amount of the initial Zn is likely to remain in solution at 200 μM Zn at the end of the transformation. The present study clearly indicates that Zn redistribution is significantly affected by the factors: the differences in transformation process and rate, the presence or absence of intermediate phase(s), and metal accommodation capacities of the minerals, which must be taken into account for metal transport in open, natural systems. These factors can be controlled by initial Zn concentration for the present study.