Lead and zinc in the structure of organic and mineral soil components

Abstract

SUMMARYIn addition to the more reactive forms, metals can occur in the structure of minerals, and the sum of all these forms defines their total contents in different soil fractions. The isomorphic substitution of heavy metals for example alters the dimensions of the unit cell and mineral size. This study proposed a method of chemical fractionation of heavy metals, using more powerful extraction methods, to remove the organic and different mineral phases completely. Soil samples were taken from eight soil profiles (0-10, 10-20 and 20-40 cm) in a Pb mining and metallurgy area in Adrianópolis, Paraná, Brazil. The Pb and Zn concentrations were determined in the following fractions (complete phase removal in each sequential extraction): exchangeable; carbonates; organic matter; amorphous and crystalline Fe oxides; Al oxide, amorphous aluminosilicates and kaolinite; and residual fractions. The complete removal of organic matter and mineral phases in sequential extractions resulted in low participation of residual forms of Pb and Zn in the total concentrations of these metals in the soils: there was lower association of metals with primary and 2:1 minerals and refractory oxides. The powerful methods used here allow an identification of the complete metal-mineral associations, such as the occurrence of Pb and Zn in the structure of the minerals. The higher incidence of Zn than Pb in the structure of Fe oxides, due to isomorphic substitution, was attributed to a smaller difference between the ionic radius of Zn 2+ and Fe 3+ .Index terms: isomorphic substitution, sequential analysis, iron oxide, kaolinite.(

Clay Mineralogy of Subtropical Soils under Long-Term Organic Fertilization in No-Tillage Systems

“…The Zn content was highest in the surface layer in the treatments with highest residue rates, indicating the potential of these residues to increase this metal in the soil (Mattias et al, 2010) and its interaction with Fe oxides, mainly those with low crystallinity. According to Kummer et al (2013), the affinity of metals such as Zn for isomorphic substitution in the Fe oxide structure is high, due to the similarity of the ionic radii of Zn 2+ and Fe 3+ .…”

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

“…Chemical extraction also does not have sufficient selectivity for dissolution of only one mineral phase, which makes the speciation of heavy metals in the soil fractions very imprecise (Duarte et al, 2012;Kummer et al, 2013). The analysis of isolated crystals is the only way to accurately identify the mineral source of structural forms of heavy metals in soil fractions (Batista et al, 2017).…”

“…As the ionic radius of Mg 2+ (0.72 nm) and Ni 2+ (0.73 nm) is quite similar, isomorphic substitution of these two cations in minerals (octahedral coordination), such as thekerolite-pimelite series (Brindley et al, 1979), is very common. The greater the difference between the ionic radius of the elements involved, the more difficult isomorphic substitution will be (Kummer et al, 2013).…”

“…Successive transformations due to compressive efforts led to metamorphism of the rocks and the flow of hydrothermal fluids rich in heavy metals (Faleiros et al, 2007). Lead ores, containing galena (PbS), are often associated with minerals rich in Zn, Cu, As, and Ag in the area studied (Kummer et al, 2013).…”

“…These analyses have been used in geochemical studies (Ryan et al, 2008;Kummer et al, 2013) but are not enough to ensure identification of all mineral sources of structural forms of heavy metals in soil. In chemical extractions, the contribution of individual mineral sources of heavy metals is lost.…”