Geoquímica de elementos terras raras em solos da Bacia Cabo, Nordeste do Brasil

Abstract

Geochemistry studies the distribution of chemical elements on Earth. The rare earth elements (REE) are Group 3B of the periodic table consisting of the lanthanides, Y and Sc, which are similar as to their properties (chemical, magnetic and flourescência). The REE are strategic due to the specificity of their applications. The continuous expansion of the agricultural and industrial sectors makes it necessary to monitor the environmental quality of soils. Therefore, know the natural levels of REE are essential to assess human impacts on the environment, and its extension. Some studies of determination of REE at the national level were developed. However, few are aimed at determining the natural levels of REE under well-defined geopedological conditions and quantifying additions of REE in agricultural soils. Thus, this study provides new information about the effect of a volcano-sedimentary lithosequence Cretaceous of northeastern Brazil in pedogenesis, mineralogy and geochemistry of REE in soils; and provides information on the effect of sugarcane crops, since 1930, with intense use of agricultural inputs, in the edition of REE in soils originated from rhyolite in the same basin. REE concentrations were determined by ICP-OES, after total extraction (USEPA 3052). In general, high recovery percentages were obtained for the REE. The REE results were analyzed using patterns of fractionation, anomalies and geochemical signature. Petrographic and MEV-EDS analyzes were performed on soil parental materials (ignimbrito, rhyolite, conglomerate, traquiandesite and basalt). Soil properties were analyzed to assist in the interpretation of the behavior of soils REE. The results showed that despite the strong chemical weathering, developed distinct lithologies soils of the Cape Basin showed different geochemical and mineralogical signatures of REE in soils. The concentration of REE in the lithosequence decreased in the following order: rhyolite> traquiandesite> ignimbrito> basalt. The total content of REE in soils was reduced according to their respective source materials in the order: traquiandesitoito> basalt> rhyolite> conglomerate> ignimbrito. Minerals feldspar, zircon, ilmenite and biotite are the main source of crystalline phases REE in the Cabo Basin soils, as well as apatite in ignimbrite and traquiandesito. These results evidenced the conservative properties of the ETR in the Cabo Basin. In the assessment of the anthropogenic impact on agricultural soil originated rhyolite, it was observed that almost all REE showed enrichment. The greatest enrichments in the agricultural soil were the elements Sm, Pr, Nd, Gd, La, Yb and Lu. The HREE present strong positive correlation with phosphorus. The higher REE content in the Ap horizon of the agricultural soil, as well as the lack of REE enrichment in the soil of the preserved area, evidences that the entrance of REE in the Cabo Basin is occurring due to the agricultural activity of the region. The results of this work provide unpublished data of natural levels of REE by pedogenetic horizon, using a lithosequence of soils in the Cabo Basin, as well as a study of the evaluation of REE additions through agricultural inputs in agricultural soil, Northeast Brazil. These results demonstrate the complex interactive control of the parental material composition and soil properties in geochemical REE in soils of a volcano-sedimentary lithosequence Cretaceous of Northeastern Brazil and are important to predict and prevent future impacts of REE accumulation by agricultural activity in this region, evidencing the urgent need to develop specific legislation in Brazil regarding the limits of REE in soils.