Iron Speciation in Insoluble Dust from High-Latitude Snow: An X-ray Absorption Spectroscopy Study

Shiwei Liu,Cunde Xiao,Wei Xu,Giannantonio Cibin,Yingcai Zhu,Alessandro Puri,Augusto Marcelli,Zhiheng Du,Valter Maggi,Giovanni Baccolo

doi:10.3390/condmat3040047

Abstract

Iron is thought to limit the biomass of phytoplankton populations in extensive regions of the ocean, which are referred to as high-nutrient low-chlorophyll (HNLC) regions. Iron speciation in soils is still poorly understood. We have investigated inorganic and organic standard substances, diluted mixtures of common Fe minerals in insoluble dust in snow from the Laohugou No.12 glacier, and sand (including soil and moraine) samples that were collected from western China. The speciation of iron (Fe) in insoluble dust and sand was determined by X-ray absorption near-edge structure (XANES) spectroscopy. A linear fit combination (LCF) analysis of the experimental spectra compared to a large set of reference compounds showed that all spectra can be fitted by only four species: Fe2O3, Fe3O4, biotite, and ferrous oxalate dihydrate (FOD). A significant altitude effect was detected for snow. The proportion of Fe2O3 in snow decreases gradually, and vice versa for FOD. As for Fe3O4 and biotite, the altitude effect was also detected, but separate regions should be considered to be deduced by topography. The Fe species in moraines and soils were also analyzed to identify the source of moraines and the heterogeneity of soils, and were compared with snow.

Highlights

Iron (Fe) contributes 5.1 mass percent to the earth’s crust, and is a major component of many soil-forming parent materials
The standard substances in which iron is in the Fe (II) state include FeSO4, FeCl2, FeS2, and FeS, and those in which iron is in the Fe (III) state are FeCl3, FePO4, Fe2O3, Fe(SO4)3, and Fe(NO4)3, including the Fe3O4
This study investigates the speciation of Fe in insoluble dust and sand samples from western China by means of X-ray absorption spectroscopy techniques and in particular by X-ray absorption near-edge structure (XANES) spectroscopy

Summary

Introduction

Iron (Fe) contributes 5.1 mass percent to the earth’s crust, and is a major component of many soil-forming parent materials. As a consequence of the considerable importance of Fe-containing minerals in the earth sciences, their identification and quantification in geologic materials and soils has been a major target of research, and numerous methods for the assessment of Fe minerals in soils have been developed These methods include X-ray diffraction [7], wet chemical fractionation procedures, such as selective extraction [8,9,10], differential thermal analysis [11], Mössbauer spectroscopy [7], and colorimetry [5,6]. Most of these methods are either applicable only to crystalline phases (XRD) or provide only operationally defined results (dissolution methods). None of these methods provides information on the micro-morphological arrangement of different Fe species in natural soil structures, e.g., aggregates

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Conclusion