Unravelling zinc and lead distributions in dolomitic and metapelitic soils of the Brazilian Central Plateau: insight from physical fractionation, optical microscopy and X-ray microfluorescence

Abstract

Large zinc and lead concentrations occur in strongly weathered soils of Cambisol–Ferralsol toposequences in the Paracatu-Vazante area (Central Plateau, Brazil). Weathering of the mineralized dolomite parent material of the Cambisols is hypothesized to be the geogenic source of zinc (Zn) and lead (Pb), with dissemination downslope into the Ferralsols. This leads to different metal distribution patterns in the two soils. We studied Zn and Pb distributions in selected A and B horizons of two typical profiles to examine this hypothesis and assess the contribution of sesquioxides to the retention of these metals. Physical separation into 200–2000, 50–200, 20–50 and < 20-μm size fractions in water without chemical dispersants was carried out before (F1) and after (F2) ultrasonification. The fractions were analysed for total and extractable Zn and Pb concentrations and studied by X-ray diffraction and optical microscopy. Microscale Zn and Pb distribution maps were obtained by using micro-X-ray fluorescence on thin sections. For the Cambisol, the composition, morphology and large Zn and Pb concentrations of coarse-sized F2 fractions were consistent with a geogenic metal origin. In both soils, < 20-μm fractions contained the largest amounts of Zn and Pb. In the Cambisol, this < 20-μm fraction included poorly crystalline Mn-rich material, encouraging strong Pb sorption. The Ferralsol < 20-μm fractions contained more Al- and Fe-oxide-rich microaggregates, which also enhanced strong metal retention. Large sesquioxide contents in these and similar tropical soils reduce metal mobilities. This limits the risk of toxicity when such soils, with metal contents exceeding guidelines, are used for agriculture.