Soil samples from three inactive mines, corresponding to different Arsenic-bearing mineralization types, were collected and studied. The aim was to determine the influence of mine wastes mineralogy/geochemistry and texture in As mobility and to compare results from sequential chemical extraction and microscopic techniques (optical and electron) at a grain scale. Arsenic in soils is found mainly associated to the residual fraction, indicating that mechanical As dispersion is mainly responsible for As soil pollution. The use of objective microscopic techniques (i.e., Scanning Electron Microscopy-Energy Dispersive Spectroscopy -SEM-EDS-, High Resolution Transmission Electron Microscopy -HR-TEM) has pointed out that the selected sequential extraction method overestimates the role of Mn amorphous oxy-hydroxides and organic matter in As retention while underestimating the mechanism of As adsorption onto clay particle surfaces.
Sequential extraction procedures (SEPs) are widely used in environmental studies to infer the chemical and/or mineralogical forms of pollutants of concern in soils and sediments. Although there is no general agreement among the scientific community, these methods have shown some limitations, especially those with a lack of objectivity in their interpretation. In this work, a soil sampling campaign was carried out in an area affected by an abandoned Sb mine. Samples (0–15 cm) were carefully prepared and analyzed by an SEP. They were also studied by conventional mineralogical methods (optical and electronic microscopy, both scanning and transmission, with a coupled energy dispersive X-ray (EDX) spectrometer). When comparing the results obtained from both techniques, some discrepancies are highlighted, with As, Cu, Pb, Sb and Zn as elements of concern. For Sb, Cu, Pb and As the predominant fraction (excluding the residual one) is that associated with organic matter (from 8.54 for Sb to 18.90% for Cu). The fractions of pollutants linked to Fe and/or Mn oxides are quantitatively important for As, Pb and Zn (6.46%, 12.05% and 7.43%, respectively) and almost negligible for Cu and Sb. On the contrary, analyses carried out by EDX at a grain scale pointed out that no detectable quantities of the elements of concern were present on the surface of the organic particles. Sb and Pb were always detectable in Fe oxides (up to 1.84 and 5.76%, respectively). Regarding the role of the clayey fraction, the only disagreement between the employed SEP and the microanalysis is in relation to As. Arsenic bound to clay minerals was found to be an order of magnitude lower than As associated with Fe oxides (0.56% and 6.46%, respectively); in contrast, EDX microanalyses showed similar As contents in both groups. Given the objectiveness of EDX microanalysis, these differences should be considered inaccuracies in the interpretation of the sequential extraction results.
An evaluation of the pollution, distribution, and mobility of arsenic and heavy metals in spoil heaps and the soils surrounding the abandoned Carmina Pb–Zn mine (Asturias, Northern Spain), is carried out. Fractionation of As was performed by means of an As-specific sequential extraction method while heavy metals fractionations were assessed using the Bureau Community of Reference (BCR) protocol. As appeared predominantly associated with amorphous iron oxyhydroxides. Among the heavy metals, Pb and Zn showed high mobility since significant amounts were extracted in the non-residual fractions, whereas Cr, Cu and Ni showed very low mobility indicating its lithogenic origin. Results show that the extractability of heavy metals in soils is influenced mainly by the presence of Fe and Mn oxides as well as by pH and Eh. Multiple pollution indices were used to assess the soil pollution extent: Enrichment factor (EF), geoaccumulation index (Igeo), ecological risk index (Er) and potential ecological risk index (PERI). All of them showed that Pb was the key factor to cause the pollution and ecological risk in the studied area with significant contribution of Cu, Zn and As. Remarkably, the sites at higher risk coincided with those with higher mobility of As and heavy metals. This study provides an integrative approach as a powerful tool to evaluate the pollution status and potential threat to the local environment in abandoned mining areas which results useful for management decisions in these areas.
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