Watershed-scale assessment of environmental background values of soil potential toxic elements from the Caatinga and Atlantic forest ecotone in Brazil

The evaluation of the concentration of a potentially toxic element (PTE) in soils under native vegetation is the base study to obtain the quality reference values (QRVs), and the watershed is the strategic planning unit for decision making. The objective of this study was to determine the natural concentrations of As, Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb and Zn and to establish QRVs for the Verruga river basin. Soils with no or minimal anthropic intervention from the surface layer (0.0-0.2 m) were collected and processed, and PTEs were extracted according to the USEPA 3051A method and determined by ICP‒OES. The quality of the analyses was checked by blank tests and soil samples certified SRM 2709 - San Joaquin Soil. The data set was subjected to exploratory analysis and multivariate statistics. The mean background concentrations of PTEs in soils showed high variability compared to other locations in Brazil and in the world and were (mg kg-1) Fe (24,300) > Mn (211.10) > Cr (40.98) > Zn (28.28) > Cu (10.68) > Ni (9.44) > Pb (4.95) > Co (4.08) > As (3.48) > Cd (0.09). The QRVs for the PTEs were established based on the 75th percentile, where (mg kg-1) Mn (124.59) > Cr (54.51) > Zn (31.66) > Cu (7.89) > Ni (7.20) > Pb (5.98) > As (4.05) > Co (3.40) > Cd (0.10). The chemical attributes and topography variation along the watershed are very heterogeneous and influence the dynamics of the PTEs. This survey will support future research on the impact of human activities on soil contamination in the watershed. This survey will support future research on environmental monitoring and the impacts caused by increased human activities on soil contamination in the Verruga river watershed, in the state of Bahia, Brazil.

Near-infrared spectroscopy for the prediction of rare earth elements in soils from the largest uranium-phosphate deposit in Brazil using PLS, iPLS, and iSPA-PLS models

The largest uranium-phosphate deposit in Brazil also contains considerable levels of rare earth elements (REEs), which allows for the co-mining of these three ores. The most common methods for REE determination are time-consuming and demand complex sample preparation and use of hazardous reagents. Thus, the development of a safer and faster method to predict REEs in soil could aid in the assessment of these elements. We investigated the efficiency of near-infrared (NIR) spectroscopy to predict REEs in the soil of the uranium-phosphate deposit of Itataia, Brazil. We collected 50 composite topsoil samples in a well-distributed sampling grid along the deposit. The NIR measures in the soils ranged from 750 to 2500 nm. Three partial least squares regressions (PLSR) were selected to calibrate the spectra: full-spectrum partial least squares (PLS), interval partial least squares (iPLS), and successive projections algorithms for interval selection in partial least squares (iSPA-PLS). The concentrations of REEs were measured by inductively coupled plasma optical emission spectroscopy (ICP-OES). In addition to raw spectral data, we also used spectral pretreatments to investigate the effects on prediction results: multiplicative scatter correction (MSC), Savitzky-Golay derivatives (SG), and standard normal variate transformation (SNV). Positive results were obtained in PLS for La and ΣLREE using MSC pretreatment and in iSPA-PLS for Nd and Ce using raw data. The accuracy of the measurements was related to the REE concentration in soil; i.e., elements with higher concentrations tended to present more accurate results. The results obtained here aim to contribute to the development of NIR spectroscopy techniques as a tool for mapping the concentrations of REEs in topsoil.

Spatial variability and geochemistry of rare earth elements in soils from the largest uranium-phosphate deposit of Brazil

The Itataia uranium-phosphate deposit is the largest uranium reserve in Brazil. Rare earth elements (REEs) are commonly associated with phosphate deposits; however, there are no studies on the concentrations of REEs in soils of the Itataia deposit region. Thus, the objective of the research was to evaluate the concentration and spatial variability of REEs in topsoils of Itataia phosphate deposit region. In addition, the influence of soil properties on the geochemistry of REEs was investigated. Results showed that relatively high mean concentrations (mg kg^-1) of heavy REEs (Gd 6.01; Tb 1.25; Ho 1.15; Er 4.05; Tm 0.64; Yb 4.61; Lu 0.65) were found in surface soils samples. Soil properties showed weak influence on the geochemical behavior of REEs in soils, except for the clay content. On the other hand, parent material characteristics, such as P and U, had strong influence on REEs concentrations. Spatial distribution patterns of REEs in soils are clearly associated with P and U contents. Therefore, geochemical surveys aiming at the delineation of ore-bearing zones in the region can benefit from our data. The results of this work reinforce the perspective for co-mining of P, U and REEs in this important P-U reserve.