Identification of Heavy Metals in Crystals of Sand and Silt Fractions of Soils by Scanning Electron Microscopy (SEM EDS/WD-EPMA)

Abstract

Studies of heavy metals are concentrate on clay fractions, but coarser fractions of the soil can constitute significant sources of structural forms of heavy metals. The aim of this study was to evaluate the occurrence of heavy metals in the structure of minerals of the sand and silt fractions of soils from three different parent materials (metamorphic rocks and granite) in southern Brazil using SEM/EDS -Scanning Electron Microscopy with Energy Dispersive Spectroscopy and with WD-EPMA -with Wavelength Dispersive-Electron Probe Microanalysis. We sampled soils from two areas naturally rich in heavy metals, with high mineral deposits (galena -PbS) hosted in carbonate rocks and phyllite/mica schist. The main form of Ba in the sand and silt fractions was as barite (BaSO 4 ). The precipitation of Ba and S from the soil solution occurred on the surface of silicate mineral particles. Due to the proximity of ionic radius of Ba-Pb, there was isomorphic substitution of Ba for Pb in the barite structure. The only primary mineral source of Pb in the coarse soil fractions was trioctahedral mica. Several secondary minerals in the silt and sand are sources of structural Pb: plumbogummite, plumboferrite, magnetoplumbite, and cerussite. There was a strong geochemical association of Pb-Fe-Mn. Zinc was also associated with Fe. The SEM/EDS/WD-EPMA techniques are important analyses to complement standard procedures, such as X-ray diffraction and total chemical digestion, in geochemical studies.

Portable X-ray fluorescence (pXRF) spectrometry applied to the prediction of chemical attributes in Inceptisols under different land uses

“…The five areas are on clayey-skeletal, kaolinitic, subactive, thermic, shallow, Typic Dystrustepts, developed from phyllite (Silva et al, 2018). The dominant minerals present in this rock are feldspars, micas (mainly muscovite), and quartz (Silva et al, 2006;Batista et al, 2018). The managed areas were initially submitted to conventional soil tillage by plowing to 20 cm depth.…”

Human bioaccessibility and absorption by intestinal cells of potentially harmful elements from urban environmental matrices

“…The median concentration for Pb was higher in the 2 mm (405 mg kg -1 ) than in 250 µm (257 mg kg -1 ). Several Pb-bearing minerals can be found in the coarsest fractions of the soil (Batista et al, 2018). A greater distribution of the data, however, was observed by the interquartile range in fraction 250 µm than in the fraction 2 mm (Figure 1a).…”

“…The concentrations of As, Cu, and Zn are relatively high in tailing samples, however no crystalline phases associated with these elements were found, but it can be inferred that these elements are in the form of amorphous oxides or insoluble precipitates, probably due to the pH value of the samples collected in the areas (5.2 to 6.6). In the soil samples, the peaks of quartz with high crystallinity interfere with those minor mineral containing heavy metals (Batista et al, 2018) Although the concentrations in BS and FF were not different (p > 0.05) for the most elements, the bioaccessibility and fractionation analyses were performed on FF, since this fraction has a greater potential risk to humans, mainly when inhaled (Witt III et al, 2014).…”

“…Even though there was no difference from the Wilcoxon test between the BS and FF fraction, the highest concentrations of PHE in the tailings samples were present in the BS fraction ( Table 3). Heavy metals can be associated with Fe and Mn oxides and aluminosilicate minerals and incorporated into the structure of minerals (Batista et al, 2018). Batista et al (2018) were quartz and kaolinite (APPENDIX D), with illite-montmorillonite, gibbsite and calcite being found in smaller amounts in the clay fraction.…”

“…Heavy metals can be associated with Fe and Mn oxides and aluminosilicate minerals and incorporated into the structure of minerals (Batista et al, 2018). Batista et al (2018) were quartz and kaolinite (APPENDIX D), with illite-montmorillonite, gibbsite and calcite being found in smaller amounts in the clay fraction. In the tailing samples, it was possible to observe peaks corresponding to cerussite (PbCO 3 ).…”