The structure and surface reactivity of aluminum- and silicate-bearing ferrihydrite [electronic resource] : a comparison between natural and synthetic analogs
Keywords: Geological and Environmental Sciences.

Ferrihydrite is a highly reactive Fe(III) oxyhydroxide nanomineral (~1-7 nm) that occurs in a variety of natural surface environments, where it is recognized for controlling the fate of pollutant metal(loid)s and nutrients by means of sorption reactions. Despite its importance as an environmental mineral, the crystallographic structure and the nature of the surfaces of this poorly crystalline phase have remained elusive, and have been intensely disputed over the past decades. A new structural model was recently proposed for synthetic ferrihydrite based on synchrotron-based high energy x-ray total scattering and pair distribution function (PDF) analysis, and represents the most comprehensive structural model of ferrihydrite to date. In this research, the newly proposed ferrihydrite structural model was used as a foundation to evaluate the structure and surface reactivity of compositionally complex ferrihydrites that typically occur in natural environments. Aluminum and silicate are among the most common impurities associated with ferrihydrite as a result of their natural abundance as dissolved species in surface aqueous environments. The association of these species with ferrihydrite is however not well understood, and determining their mode of association with ferrihydrite (e.g., incorporation, surface complexation, formation of separate Al or Si phases) is essential to interpret ferrihydrite interaction with metal(loid)s, and its stability with respect to transformation or reductive dissolution reactions in the environment. The goals of this research project were to (1) determine the speciation of Al and Si, and their effects on the ferrihydrite structure in simple, synthetic Al- and Si-ferrihydrite samples, (2) evaluate the changes in the surface reactivity of synthetic Al- and Si-ferrihydrites, (3) evaluate changes in Zn(II) complexation and the geometry/availability of surface reactive sites as a function of ferrihydrite composition, and (4) determine the equivalence between synthetic and naturally occurring Al- and Si-rich ferrihydrite, in terms of their structures and surface reactivities. The structural aspects of natural and synthetic ferrihydrites were evaluated by means of synchrotron-based high energy x-ray total scattering and PDF analysis. Additional synchrotron-based as well as laboratory-based analytical techniques were used on these samples to determine their composition (ICP-AES/OES), particle sizes (TEM), distribution and speciation of Al and Si (NMR, scanning transmission x-ray microscopy (STXM) mapping and spectroscopy), surface area and surface charge (BET N2 adsorption analyses, electrophoretic mobility measurements). Furthermore, their surface structure and composition were evaluated indirectly by means of Zn(II) adsorption experiments, combined with Zn K-edge extended x-ray absorption fine structure (EXAFS) spectroscopy . The characterization of synthetic ferrihydrite samples containing between 5 and 40 mol% Al or Si indicated that the type and amount of the impurity ions, as well as the synthesis rate had an effect on the ferrihydrite structure. In the case of aluminous ferrihydrite, Al was incorporated in the structure by isomorphous substitution up to a maximum content of ~30 mol% Al, and the precipitation rate (rapid - over a few seconds, and slow -- over a period of ~25 minutes) did not affect the amount of Al substitution. A heterogeneous co-precipitation process occurred in both rapidly and slowly precipitated Al-ferrihydrite series, defined by formation of Al hydroxide phases (e.g. gibbsite, amorphous Al hydroxide) in parallel with Al-substituted ferrihydrite, although these Al-rich phases were difficult to detect in the rapidly precipitated samples, likely due to the formation of smaller Al hydroxide domains. In contrast, increasing silicate content up to 40 mol% Si had a more pronounced effect on the structure of ferrihydrite, causing the formation of smaller ferrihydrite particles, which exhibit significant structural strain. Rapid precipitation rates resulted in the formation of increasingly strained, or disordered ferrihydrites, which was likely due to an greater interaction of Fe3+ with dissolved monomeric SiO44- species during co-precipitation. No indication for Si substitution could be found in these samples; instead, silicate was proposed to occur at particle surfaces as complexes, Si polymers or, at the highest Si content, silica surface precipitates. Additional information about the nature of Al- or Si-ferrihydrites surfaces was obtained by evaluating their interaction with aqueous Zn(II). Zn(II) adsorption experiments coupled with Zn(II) K-edge EXAFS spectroscopy agree with the structural data described above, and indicate that Zn(II) speciation at the surface of siliceous ferrihydrite is different in comparison with pure or aluminous ferrihydrite. Fewer next-nearest Fe neighbors were identified for Zn(II) sorbed onto Si-ferrihydrite, which indicates that silicate blocks surface reactive sites. By reevaluating Zn(II) speciation at pure ferrihydrite surfaces, it was also possible to determine an additional reactive sorption site, a six-membered ring of Fe(O, OH)6 octahedra, also known to occur at the hydrated magnetite and maghemite surfaces. Zn(II) was suggested to form hexanuclear tridentate complexes at this site at low surface coverage for pure and aluminous ferrihydrite, at an average Zn-Fe distance of ~3.46 Å. A series of naturally occurring Al- and Si-bearing ferrihydrite samples collected from an acid mine drainage environment (New Idria Mine, CA) was characterized in parallel with the synthetic Al- and Si-ferrihydrite series in order to evaluate their equivalence. Although natural ferrihydrites exhibited additional structural disorder, a good correspondence was observed overall in terms of their short- and intermediate-range structural order compared with synthetic Al- and Si-ferrihydrites. Silicate was suggested to be responsible for significant structural strain in the natural samples, and according to the synthetic series, it is expected to occur as complexes or silica precipitates at the particle surfaces. The cumulative contribution of both Si and Al on the structural parameters of the natural samples could not explain entirely their structural disorder. The additional structural strain observed in the natural samples was attributed to the presence of other components, such as natural organic matter, minor phosphate (or other minor components such as Mn, Zn, etc.) which have been reported to have similar effects as silicate on ferrihydrite particle crystallinity/growth. The presence of natural organic matter at ferrihydrite aggregate surfaces also impacted the surface reactivity of these samples with respect to Zn(II) adsorption by providing additional sorption sites. This study represents a first comparison between natural and synthetic ferrihydrite at this level of detail. X-ray total scattering and PDF analysis proved to be an especially suitable technique for the investigation of compositionally complex nanomaterials, and provided evidence of a good structural equivalence between natural and synthetic ferrihydrite. However, this comparison also pointed out some gaps in our knowledge with respect to the cumulative effects of impurities on ferrihydrite (i.e. simultaneous presence of Al and Si), as well as the role of organic matter on ferrihydrite particle development and reactivity. By adding additional layers of compositional complexity to synthetic ferrihydrites, it may be possible to reproduce synthetic analogs that represent more accurately naturally occurring samples. Such synthetic analogs can be especially useful for conducting reactivity studies and developing better models that predict the behavior and reactivity of environmental ferrihydrites.