Structure and Reactivity of Natural and Synthetic Ferrihydrites and Natural ZnS Nanoparticles


a. Characterization of natural ferrihydrite nanoparticles from an acid mine drainage environment and comparison with synthetic ferrihydrites.

A series of naturally occurring ferrihydrites sampled from an acid mine drainage environment at the New Idria Mercury Mine, CA – the second largest mercury mine in North America – were characterized and compared with synthetic 2-line ferrihydrite using synchrotron-based high energy x-ray total scattering and pair distribution function analysis, synchrotron-based Scanning Transmission X-ray Microscopy (STXM), Transmission Electron Microscopy (TEM), BET-N2 surface area measurements, and selective chemical extractions in order to place constraints on their structural and physical properties as a function of composition. Overall, the short- and intermediate-range ordering of the natural samples is comparable to that of synthetic ferrihydrite. However, with increasing Al, Si, and organic matter contents, a decrease in particle size and an increase in structural disorder were observed. Silica is suspected to have a pronounced effect on the crystallinity of ferrihydrite as a result of its inhibitory effect on Fe polymerization and particle growth, and it is likely complexed at the surfaces of ferrihydrite nanoparticles. Aluminum, on the other hand may substitute for Fe3+ in natural ferrihydrites, although we found evidence that Al is also present in physically separate phases such as gibbsite. Organic matter is pervasive and intimately associated with ferrihydrite aggregates, and its presence during ferrihydrite precipitation may have contributed to additional structural disorder. The increase in impurity content affects not only the particle size and structural order of ferrihydrite but may also have a significant effect on its surface reactivity.

b. Effects of Al content and precipitation rate on the structure of 2-line ferrihydrite

The association of Al with ferrihydrite (Fh) may have a considerable effect on the composition, structure, and surface properties of Fh nanoparticles, and thus impact its reactivity and interaction with pollutant species. Aluminous Fh is abundant in natural environments, but the mode of association of Al with this nanomineral is not yet fully understood. Al3+ speciation may vary from true chemical substitution for Fe3+, to adsorption or surface precipitation, and/or to formation of a mixture of two (or more) individual nanoscale phases. The conditions of formation (i.e. slow vs. rapid precipitation) may also affect the nature of Fh nanoparticles in terms of their crystallinity, phase purity, and Al speciation. In this study we used a variety of laboratory (TEM, NMR, ICP-AES) and synchrotron-based techniques (X-ray total scattering and PDF analysis, scanning transmission x-ray microscopy, Al K-edge XANES spectroscopy) to characterize two synthetic Al-bearing Fh series formed at different precipitation rates in the presence of 5 to 40 mol % Al. We found that Al is dominantly octahedrally coordinated in the synthetic Fh samples and that up to 20-30 mol % Al can be incorporated into the Fh structure, regardless of the synthesis method we used. Formation of separate aluminous phases (e.g., gibbsite) was most significant at Al concentrations above 30 mol % Al in slowly precipitated samples. However, small amounts (< 6% of total Al) of Al-hydroxide phases were also detected by NMR spectroscopy in samples with lower Al content (as low as 15 mol % Al), particularly in the Fh series that was precipitated slowly. Furthermore, it appears that the amount of Al incorporated in Fh is not affected by the synthesis methods we used and is more likely controlled by the accumulated strain caused by Al substitution in the Fh lattice.

c. Characterization of natural ZnS particles in the Seine River, Paris Basin, France

Zinc is one of the most widespread trace metals (TMs) in Earth surface environments and is the most concentrated TM in the downstram section of the Seine River (France) due to significant anthropogenic input from the Paris conurbation. In order to better identify the sources and cycling processes of Zn in this River basin, we investigated seasonal and spatial variations of Zn speciation in suspended particulate matter (SPM) in the oxic water column of the Seine River from upstream to downstream of Paris using synchrotron-based Extend X-ray Absorption Fine Structure (EXAFS) spectroscopy at the Zn K-edge. First-neighbor contributions to the EXAFS were analyzed in SPM samples dried and stored under a dry nitrogen atmosphere or under an ambient oxygenated atmosphere. We found a sulfur first coordination environment around Zn (in the form of amorphous zinc sulfide) in the raw SPM samples stored under dry nitrogen vs. an oxygen first coordination environment around Zn in the samples stored in an oxygenated atmosphere. These findings are supported by scanning electron microscopic and energy dispersive x-ray spectrometry observations. Linear combination fitting, principal component analysis, and target transformation analysis of the EXAFS data for SPM samples, using a large set of EXAFS spectra of model compounds, indicate dramatic changes in the Zn speciation from uptream to downstream of Paris, with amorphous ZnS particles becoming dominant dowstream.  In contrast, Zn species associated with calcite (either adsorbed or incorporated in the structure) are dominant upstream. Other Zn species representing about half of the Zn pool in the SPM consist of Zn-sorbed on iron oxyhydroxides (ferrihydrite and goethite) and, to a lesser extent, Zn-Al layered double hydroxides and Zn incorporated in dioctahedral layers of clay minerals. Our results highlight the importance of preserving the oxidation state in TM speciation studies when sampling suspended matter, even in an oxic water column. They also demonstrate that systematic use of such careful sampling procedures may reveal the presence of reduced solid phases that might have been overlooked in previous studies of type B metal-bearing samples in oxic surface water columns.