Coupling X-ray Absorption and Raman Spectroscopies to Characterize Iron Species in a Karst Pedosedimentary Record

Beatrice Giannetta,Claudio Zaccone,Gino Mariotto,Danilo Oliveira De Souza,Giuliana Aquilanti,Michele Cassetta

doi:10.3390/soilsystems6010024

Beatrice Giannetta, Claudio Zaccone + Show 4 more

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https://doi.org/10.3390/soilsystems6010024

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Abstract

The identification of iron (Fe) forms throughout a sediment sequence was investigated by X-ray Absorption Near Edge Spectroscopy (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) at the Fe K-edge, paired with Raman micro-spectroscopy. The contribution of different organic and inorganic Fe-bearing compounds was quantified by Linear Combination Fitting (LCF) carried out on both XANES and EXAFS spectra. Fe-XANES showed that the Fe(II)/Fe(III) ratio of different Fe-bearing minerals in sediments can be quantified with reasonable accuracy. The main Fe species detected were ferrihydrite, goethite, hematite, clay minerals (smectite, illite, nontronite), and Fe(III)-organic matter (Fe(III)-OM). A more accurate quantification of ferrihydrite was possible with LCF conducted on Fe-EXAFS spectra. With the exception of hematite, the concentration of these mineral species does not have a clear trend with depth, probably because water infiltration caused continuous Fe reduction and oxidation cycles in these sediments. From an analytical perspective, Fe oxide compounds can be difficult to identify or distinguish unless multiple techniques are used. X-ray diffraction (XRD; previous work) and Raman spectroscopy turn out to be not particularly useful in identifying ferrihydrite, while they are best suited for a broad mineralogical analysis that requires integrative spectral studies for an accurate Fe speciation. In detail, XANES and EXAFS allowed for the detection of Fe-bearing clay minerals and a more refined identification of Fe species, including Fe(III)-OM. Thermal analysis was useful to confirm some mineralogical components observed using both XRD (data previously published) and Raman spectroscopy (e.g., goethite, todorokite). In conclusion, this study underlines how a multi-technique approach is required to investigate peculiar environments such as karst pedosequences.

Highlights

Terra Rossa or Red Mediterranean soils developed on limestones are generally different from soils formed on other parent rocks [1]; they are typically well-structured and show a high percentage of iron (Fe) oxides strongly associated with clay minerals [2].While the main forming processes in this soil type have been widely investigated, very limited data exist about the composition of sediments and/or paleosols filling deep karstic caves and, more in detail, on the genesis and evolution of Fe species in these peculiar environments [3–5]
The pedosediment record under investigation represents a peculiar stratigraphic series showing complex features mainly related to the Fe dynamic and caused by redox cycles
Fe K-edge X-ray Absorption Near Edge Spectroscopy (XANES) can fairly well estimate the relative contribution of different mineral classes and Fe-organic matter (Fe-OM) compounds with different oxidation states of the Fe atom

Summary

Introduction

Terra Rossa or Red Mediterranean soils developed on limestones are generally different from soils formed on other parent rocks [1]; they are typically well-structured and show a high percentage of iron (Fe) oxides strongly associated with clay minerals [2].While the main forming processes in this soil type have been widely investigated, very limited data exist about the composition of sediments and/or paleosols filling deep karstic caves and, more in detail, on the genesis and evolution of Fe species in these peculiar environments [3–5]. Most of the widely utilized geochemical proxies are built upon the environmental behaviour of Fe (e.g., by calculating its enrichment relative to aluminum, the Fe/Al ratio), and highly reactive Fe (i.e., Fe minerals considered highly reactive towards biotic and abiotic reduction in anoxic conditions). Both the type of Fe (oxyhydr)oxides and their degree of crystallinity are indicators of the pedogenetic environment, and they can mirror the conditions under that they formed [13]. The quantification of Fe-bearing minerals in complex matrices such as sediments remains an analytical challenge, and has been mainly approached using sequential extractions [14]

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