Geochemistry and Spatial Variability of Rare Earth Elements in Soils under Different Geological and Climate Patterns of the Brazilian Northeast

Abstract

Growth in the agricultural and industrial sectors has increased the demand for rare earth elements (REEs) in the production of technological devices and fertilizers. Thus, the accumulation of these elements in the soil has become an environmental concern. Here, we aim to determine the natural contents of REEs in soils derived from different parent materials and under climatic conditions ranging from humid to semi-arid. We then evaluate the influence of major elements and soil properties on the geochemistry of REEs. The contents of REEs were determined using inductively coupled plasma optical emission spectroscopy. Major elements were determined by X-ray fluorescence spectrometry. The mean content of REEs in soils from Rio Grande do Norte (RN), Brazil, were in the followed order (mg kg -1 ): Ce (40.4) > La (18.9) > Nd (15.8) > Pr (7.3) > Sm (3.0) > Gd (2.6) > Dy (1.0) > Er (0.7) > Yb (0.6) > Eu (0.5) = Tb (0.5) > Ho (0.3) > Lu (0.2). The parent material was the main factor that governed the geochemistry of the REEs in soils of RN. Higher levels of REEs were observed in soils derived from igneous and metamorphic rocks. In contrast, sedimentary rocks -except for the region formed from limestone -generated soils with lower contents of REEs in the state. In addition, soils developed from the same parent material and under different climatic conditions showed the same geochemical signatures for REEs in soils. These results confirm the small effect of climate on REE geochemistry in soils of RN and lead to the conclusion that the geochemical signature of REEs in these soils reflects the composition of the underlying parent material. The lack of significant correlation between (La/Yb) N ratio and the Chemical Alteration Index also confirms the low influence of climate on soil REE geochemistry. Among the major elements, Fe and Si had a greater influence on soil REE geochemistry. Higher REEs were seen in areas with more Fe and less Si. These REE levels were clearly controlled by the type of parent material. The Nd, Sm, Tb, Dy, Ho, Yb, and Er levels showed strong spatial dependence; this dependence was moderate for the Pr, La, Ce, Eu, Gd, and Lu levels. Spatial variability maps of REEs are particularly important to identify areas under environmental impact. Our results represent the most detailed study of the surface geochemistry of REEs in Brazilian soils and contribute to the scarce data available on these elements in Brazil.

Quality reference values for rare earth elements in soils from one of the last agricultural frontiers in Brazil

“…The lowest levels were observed in Entisols Fluvents, which reflect their composition known to be enriched in quartz sediments. These results are in agreement with those shown by Silva et al (2018a) and Alfaro et al (2018), who observed that even under humid tropical patterns, the parent material played an important role in the geochemistry of the REEs. Soils derived from igneous rocks tend to have the highest concentrations of the REEs (Hu et al, 2006).…”

“…The average background concentrations of REEs in tropical soils of Gurguéia watershed followed the order (mg kg -1 ): Ce (14.01) > Nd (6.19) > La (5.52) > Pr (2.51) > Sm (1.45) > Gd (0.93) > Dy (0.63) > Er (0.42) > Yb (0.39) > Tb (0.28) > Eu (0.26) > Lu (0.20) (Table 1). These values were much lower than those found in soils from Brazil (Silva et al, 2016;Paye et al, 2016;Silva et al, 2018a), China (Wei et al, 1991), Japan (Yoshida et al, 1998), Cuba (Alfaro et al, 2018) and Europe (Sadeghi et al, 2013) (Table 1). The lowest concentrations of LREEs and HREEs in the soils from Gurguéia watershed are governed by parent material, with a predominance of soils originating from quartz sandstones.…”

“…The proportions observed for Ce and LREEs in this study were similar to those found by Wang and Liang (2016) in soils from China. In Brazil, several authors also showed Ce as the most abundant LREE, with proportions equivalent to 45 % (Silva et al, 2016), 54 % (Paye et al, 2016), and 47 % (Silva et al, 2018a). The sum of REEs in the soils of Gurguéia watershed ranged from 4.23 mg kg -1 to 116 mg kg -1 , with an average concentration of about 32.94 mg kg -1 .…”

“…Recovery rates were satisfactory and ranged from 88 % to 105 %; thus, this digestion method does not extract the contents strongly bounded to silicate minerals. Several authors have used this method to extract REEs from soils (BBodSchV, 1999; Rao et al, 2010;Loell et al, 2011;Mercurio et al, 2014;Mihajlovic et al, 2014;Censi et al, 2014;Alfaro et al, 2018;Silva et al, 2018a;Mihajlovic et al, 2019). Major elements were measured by wavelength dispersive X-ray fluorescence (WDXRF, S8 TIGER ECO-WDXRF).…”

“…Therefore, establishing QRVs for REEs under different scales is critical for the proper management of the soils. In Brazil, despite recent efforts to determine the background concentrations of heavy metals in soils (Silva et al, 2016;Paye et al, 2016), the levels of QRVs for REEs in Brazilian soils are not established (Silva et al, 2018a), which can hinder the monitoring of REEs in the soils by environmental agencies.…”