The transformation of Fe(III) oxides catalysed by Fe²⁺ and the fate of arsenate during transformation and reduction of Fe(III) oxides

Iron oxides are ubiquitous in soils and sediments and have a profound influence
on the water chemistry of aquifers and subsurface waters. For example, iron oxides are considered the most important adsorbents for trace metals and arsenic in sandy aquifers because of their great abundance and strong binding affinity. Upon reduction of the iron oxides, the adsorbed species may be released. Furthermore, iron oxides may undergo structural transformations when entering an anoxic environment which may also potentially mobilize the adsorbed species. The transformation of iron oxides was investigated using the isotopic exchange between aqueous Fe(II) and iron oxides in experiments with ⁵⁵Fe-labelled iron oxides. ⁵⁵Fe was incorporated congruently into a ferrihydrite, two lepidocrocites (#1 and #2), synthesized at 10 °C and 25 °C, respectively, a goethite and a hematite. The transformation of the iron oxides was induced by submerging the iron oxides in Fe²⁺ solutions (0-1.0 mM) with a pH of 6.5. In the presence of aqueous Fe²⁺, an immediate and very rapid release of ⁵⁵Fe was observed from ferrihydrite, the two lepidocrocites and goethite, whereas in the absence of Fe²⁺ no release of ⁵⁵Fe was observed. Hematite did not release any ⁵⁵Fe, even at the higher Fe²⁺ concentration. The release rate is mainly controlled by the characteristics of the iron oxides, whereas the concentration of Fe²⁺ has only a minor influence. Within days, ferrihydrite transformed completely into new and more stable phases such as lepidocrocite and goethite at the lower Fe²⁺ concentrations and into magnetite at the higher Fe²⁺ concentration. No transformation of the other oxides was observed, except for a minor fraction of lepidocrocite that at the higher Fe²⁺ concentration transformed into magnetite. For ferrihydrite and 5 nm sized lepidocrocite crystals complete isotopic equilibration with aqueous Fe(II) was attained implying a total disintegration of the original iron oxides. Lepidocrocite #2 and goethite, having larger particles, did not reach isotopic equilibrium within the time frame of the experiment; however, the continuous slow release of ⁵⁵Fe suggests that isotopic equilibrium will ultimately be attained. The fate of trace amounts of arsenate coprecipitated with ferrihydrite, lepidocrocite and goethite was studied during reductive dissolution and phase transformation of the iron oxides using ⁵⁵Fe and ⁷³As labelled iron oxides. The As/Fe molar ratio of the iron oxides ranged from 0 to 0.005 for ferrihydrite and lepidocrocite and from 0 to 0.001 for goethite. All the arsenate remained associated with the surface of ferrihydrite and lepidocrocite whereas only 30% of the arsenate was desorbable from goethite. The rate of reductive dissolution in 10 mM ascorbic acid at pH 3 was unaffected by the presence of arsenate for all of the iron oxides. Arsenate was not reduced to arsenite by the ascorbate and was released incongruently with Fe²⁺ for all the
iron oxides during the reductive dissolution of the iron oxides. For ferrihydrite and goethite, the arsenate remained adsorbed to the surface and was only released once the surface area became too small to adsorb all the arsenate. In contrast, arsenate preferentially desorbes from the surface of lepidocrocite.
During the Fe²⁺ catalysed transformation of ferrihydrite and lepidocrocite, arsenate became bound more strongly to the product phases and was not released to the solution. Arsenate appeared to be preferentially sorbed to the surface of ferrihydrite and was only incorporated into the crystalline phase at the end of the transformation process. The transformation rate and the transformation products were unaffected by the presence of arsenate at As/Fe molar ratios less than 0.005. The results presented here imply a recrystallization of solid Fe(III) phases induced by the catalytic action of aqueous Fe(II). Accordingly iron oxides should be considered as dynamic phases that change composition when exposed to variable redox conditions. These results necessitate a re-evaluation of current models for the release of trace metals under reducing conditions, the sequestration of heavy metals by iron oxides and the significance of stable iron isotope signatures. Furthermore, the results show that it may be difficult to predict the release of arsenate during reductive dissolution of iron oxides in natural sediments and that the transformation of the least stable iron oxides into more crystalline Fe(III) oxide phases may be an important trapping mechanism for arsenic in natural sediments. The results are of importance for the understanding of the behaviour of arsenic in aquifers and on sand filters in water works and for the disposal of arsenic containing chemical waste.