Metais pesados, urânio e tório em rochas, solos e plantas do Agreste Meridional de Pernambuco

Abstract

The Southern Agreste of Pernambuco has a lithological diversity and economic importance for the state, through dairy farming, makes it important to study the availability of heavy metals and radionuclides. However, detailed studies involving total contents and environmental availability of heavy metals, uranium and thorium geologically associated with the different materials of origin, have not yet been performed. The objective of this study was to compare cadmium (Cd), chromium (Cr), copper (Cu), iron (Fe), manganese (Mn), nickel (Ni), lead (Pb), zinc (Zn) uranium (U) and thorium (Th) in the soils with the lithological diversity of the Southern Agreste of Pernambuco (PE). In order to do so, samples composed of soils at depths 0-20 and 20-40 cm were collected in different geological units and the total and environmentally available contents of Cd, Cr, Cu, Fe, Mn, Ni, Pb, Zn, U and Th. The contents of these elements were also evaluated in grasses and cacti, as well as the total rock content and petrography. The high levels of thorium in soils are associated to the presence of monazite, mainly in the mineralogical constitution of gneisses, granites and pegmatites. The average environmental contents of heavy metals, uranium and thorium in soils developed on different materials of origin of Southern Agreste of PE followed the order: Fe> Mn> Th> Zn> Cr> Cu> Pb> Ni> U> Cd. higher values of Cd and Ni were observed in soils derived from granites in the municipalities of Águas Belas and Itaíba, respectively. Soils originated from gneiss had the highest levels of Cr. In Itaíba, higher levels of Fe and Mn were detected in the soils developed on amphibolite and Pb soils on granites. Soils originated from gneiss in the municipality of Garanhuns showed the highest concentrations of Zn. The highest levels of Th were observed in the soils originated from orthognaisse, in the municipality of Venturosa. The highest concentration of U was identified in soils originating from the intrusive suite of the municipality of Buíque. The environmental contents of heavy metals, U and Th in soils presented great variability as a consequence of the lithological diversity. In 84% of the soils, there were environmentally available levels of Cu above the quality reference value for the State of PE. Soils originated from amphibolite had Cu and Ni contents higher than the value established by the National Environmental Council. The average content of metals absorbed by grasses and cacti followed the order: Fe> Mn> Zn> Cu> Cr> Th> Ni> Pb Except for Fe, the other micronutrients are considered normal levels for forage grasses. The highest coefficients of biological accumulation were observed for micronutrients, with soil contents considered medium to low (Cu, Zn and Ni). The results of this research demonstrate the importance of knowing the influence of the different geological contexts of Southern Agreste of Pernambuco to understand the dynamics of heavy metals, Th and U in soils, grasses and cacti of the region.