Human health risk assessment and geochemical mobility of rare earth elements in Amazon soils
Graphical abstract
Introduction
Rare earth elements (REEs), which encompass the lanthanides, yttrium and scandium, have similar atomic radii and are predominantly trivalent (REE3+), although Ce4+ and Eu2+ may occur in some environments (Aide and Aide, 2012). Having similar chemical characteristics, REEs may substitute for each other in crystal lattices, which leads to multiple occurrences of REEs in the same ore deposits and other rock types within the Earth's crust. REEs abundance within rocks is often close to 0.01% to 0.02%, and these elements can be found in a diversity of minerals such as carbonates, oxides, phosphates and silicates (Loell et al., 2011; Ramos et al., 2016). The only exception is Promethium (Pm), which is radioactive and rapidly decays (half-life is 2.62 years) (Khan et al., 2017).
Use of REEs in technology, medicine, and agriculture has increased in the last decades (Pagano et al., 2015). The rapidly expanding use of REEs has resulted in poor containment and disposal of mine waste, extraction by-products, and pose-use waste (Xinde et al., 2000). The REEs exhibit a patterns consistent with other emergent contaminants that can threaten human and ecosystem health (Mihajlovic et al., 2014). Several health complications can result upon human exposure to REEs that include respiratory problems and neurological damage (Meryem et al., 2016; Pagano et al., 2019), restriction in protein synthesis (Gonzalez et al., 2014), and oxidative stress and tissue damage to liver, lungs and kidneys (Pagano et al., 2015).
The possible impacts of REEs on ecosystems are relatively unknown, especially in tropical systems. In Brazil, for example, studies involving REEs have been mainly concentrated outside of the topical zones and only total concentration in soils have been examined (Pereira et al., 2019; Sá Paye et al., 2016; Silva et al., 2016). Recently, however, high levels of REEs were reported for soils of the Amazon in Brazil (Ferreira et al., 2021a). Total concentration, however, are not representative of risk to either humans or ecosystems (Li et al., 2014). Rather, the propensity for exposure and transfer into the living organisms needs to be assessed to understand human and ecosystems health threats (Khadhar et al., 2020).
Gaining and understanding of how REEs bind to different soil solid phases may provide information about their transport, availability and ecosystem threats associated (Mihajlovic et al., 2014). Classically, sequential extraction has been used to examine the phases hosting the elements within soils, which can then be used to project possible bioavailable fractions. (Mihajlovic et al., 2014; Wang and Liang, 2015). On the other hand, the sequential extraction methods are not sufficient for defining phase association or the propensity for uptake within humans.
In recent decades, the concept of bioaccessibility has been used to define elemental concentrations available for human uptake (Wang et al., 2017). Ingestion of soil is as a major route of exposure to many soil contaminants (Oomen et al., 2002). For this reason, the bioaccessible fraction is normally measured through in vitro essays that mimic the effects of gastrointestinal human tract parameters (Drexler and Brattin, 2007; Juhasz et al., 2007). Different in vitro protocols have been used in assess the bioaccessibility of elements and their health risks (Li et al., 2014).
The use of in vitro tests to simulate the gastrointestinal tract is lengthy and laborious (Mingot et al., 2011). In order to streamline and minimize difficulties related to gastro-intestinal protocols, simplified (single step) extractions have been reported as a possible alternative (Rao et al., 2010) that can provide a fast and relative low-cost assessment (Oomen et al., 2002; Pelfrêne et al., 2020). Previous reports involving trace elements in Brazilian and European soils (Rodrigues et al., 2010a, Rodrigues et al., 2018) and REEs in soils of European and Asian countries (Rao et al., 2010) have presented promising results about the use of unbuffered, mild extractions and diluted acids as simplified protocols to determine bioaccessibility. However, the characteristics of soils used in these studies are considerably different in comparison to the characteristics of tropical soils, such as those in the Amazon region, especially in terms of pH values and rock-derived nutrient concentrations, which may change the efficiency of these potential extractors to assess the REEs bioaccessibility in Amazon soils.
Here, we present the first assessment of the reactivity, bioaccessibility, and health human risk of REEs (Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) in tropical soils of Brazilian Amazon. We also evaluated the efficiency of two low-cost simple (single-step) protocols, using 0.01 mol L−1 CaCl2 (unbuffered solution) and 0.43 mol L−1 HNO3 (diluted acid solution), to determine the REEs bioaccessibility. Our study presents a clear understanding of the reactivity of REEs in Amazon soils, including data about REEs concentrations in different geochemical pools and the concentrations available for human absorption.
Section snippets
Study area and geological settings
The Amazonas state is the largest state of Brazil, having an area of 1,550,000 km2 and bordering Colombia, Peru and Venezuela. The population of Amazonas state is around 4.2 million, with more than 52% of the state population living in the capital (IBGE, 2019). The average temperature in Amazonas state is 26 °C, the average humidity is 80%, and the precipitation levels range from 2200 mm to 3200 mm (Alvares et al., 2013). Two climate types are presented in Amazon State: Tropical rainforest
Soil parameters
The pHKCl values ranged from 3.1 to 5.7, and pHw ranged from 3.7 to 6.8 (Table S5). The sum of bases (SB) was considered low (< 20 cmolc/dm−3) in most samples, and it ranged from 23.6 cmolc/dm−3 to 0.1 cmolc/dm−3. The cation exchange capacities (CECe) were less than 5.97 cmolc/dm−3. The cation exchange capacities at pH 7 (CECpH7) ranged from 2.8 cmolc/dm−3 to 8 cmolc/dm−3. TOC was less than 1.2% in all soil samples. The PRem values ranged from 4.4 to 50 mg L−1. Particle-size analysis
Soil properties
The acidity and the low base cation content observed are common features of Amazon soils due to the strong weathering processes (Horbe et al., 2007; Lima et al., 2006; Mafra et al., 2002). The Amazon soils have been under strong weathering conditions (wet tropical climate) over at least 45 million of years, resulting in the destruction of primary minerals and removal of silica necessary for generation or preservation of 2:1 secondary minerals, which explains the nutrient poverty of these soils (
Conclusions
The sequential extraction procedure showed that the REEs content in Amazon soils are distributed in ascending order: organic matter fraction (~1%) < exchangeable fraction (~4.3%), Fe/Mn oxides (~4.3%) < residual phase (~90.1%). Thus, the bioavailable fractions (exchangeable phase, organic matter phase and oxalate extractable phase) represent less than 20% of the total amount of REEs in these soils.
Similarly, the concentrations obtained by single-step extraction using 0.43 mol L−1 HNO3 showed
Credit authorship contribution statement
Matheus da Silva Ferreira: Conception or design of the work, Data collection, Laboratory procedures, Data analysis and interpretation, Drafting the article, Critical revision of the article, Final approval of the version to be published. Maurício Paulo Ferreira Fontes: Conception or design of the work, Data analysis and interpretation, Drafting the article, Critical revision of the article, Final approval of the version to be published. Maria Tereza Weitzel Dias Carneiro Lima: Conception or
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – CAPES (Brasil) – through the project Programa Nacional de Cooperação Acadêmica - PROCAD 2013 - Finance Code 001.
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