Rev Bras Cienc Solo 2018;42:e0170342 Article
Division - Soil Use and Management | Commission - Soil and water management and conservation
Geochemistry and Spatial Variability
of Rare Earth Elements in Soils under
Different Geological and Climate
Patterns of the Brazilian Northeast
Cinthia Maria Cordeiro Atanázio Cruz Silva(1), Ronny Sobreira Barbosa(1),
Clístenes Williams Araújo do Nascimento(2), Yuri Jacques Agra Bezerra da Silva(1)* and
Ygor Jacques Agra Bezerra da Silva(2)
(1)
Universidade Federal do Piauí, Campus Professora Cinobelina Elvas, Curso de Bacharelado em Engenharia
Agronômica, Bom Jesus, Piauí, Brasil.
(2)
Universidade Federal de Pernambuco, Departamento de Agronomia, Recife, Pernambuco, Brasil.
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
* Corresponding author: do Norte (RN), Brazil, were in the followed order (mg kg-1): Ce (40.4) > La (18.9) > Nd (15.8) >
E-mail:
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) >
Received: October 16, 2017 Lu (0.2). The parent material was the main factor that governed the geochemistry of the REEs in
Approved: April 3, 2018 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
How to cite: Silva CMCAC,
Barbosa RS, Nascimento soils with lower contents of REEs in the state. In addition, soils developed from the same parent
CWA, Silva YJAB, Silva YJAB. material and under different climatic conditions showed the same geochemical signatures for
Geochemistry and spatial
variability of rare earth elements REEs in soils. These results confirm the small effect of climate on REE geochemistry in soils of
in soils under different geological RN and lead to the conclusion that the geochemical signature of REEs in these soils reflects
and climate patterns of the
Brazilian Northeast. Rev Bras the composition of the underlying parent material. The lack of significant correlation between
Cienc Solo. 2018;42:e0170342. (La/Yb)N ratio and the Chemical Alteration Index also confirms the low influence of climate on
https://doi.org/10.1590/18069657rbcs20170342
soil REE geochemistry. Among the major elements, Fe and Si had a greater influence on soil
Copyright: This is an open-access REE geochemistry. Higher REEs were seen in areas with more Fe and less Si. These REE levels
article distributed under the
terms of the Creative Commons were clearly controlled by the type of parent material. The Nd, Sm, Tb, Dy, Ho, Yb, and Er levels
Attribution License, which permits showed strong spatial dependence; this dependence was moderate for the Pr, La, Ce, Eu, Gd,
unrestricted use, distribution,
and reproduction in any medium, and Lu levels. Spatial variability maps of REEs are particularly important to identify areas under
provided that the original author environmental impact. Our results represent the most detailed study of the surface geochemistry
and source are credited.
of REEs in Brazilian soils and contribute to the scarce data available on these elements in Brazil.
Keywords: natural contents, lanthanides, geostatistics, factor analysis, soil quality.
https://doi.org/10.1590/18069657rbcs20170342 1
, Silva et al. Geochemistry and spatial variability of rare earth elements in soils...
INTRODUCTION
Rare earth elements (REEs) are a group of fifteen chemical elements in the lanthanide
series. These elements are divided into light rare earth elements (LREEs; La to Eu) and
heavy rare earth elements (HREEs; Gd to Lu) based on the atomic number (Tyler, 2004;
Hu et al., 2006a; Sadeghi et al., 2013; Davranche et al., 2016). Not all are technically “rare” -
indeed, cerium is the 25th most abundant element in the Earth’s crust, with contents similar
to Cu and Zn (Tyler, 2004). Rare earth elements can be found in more than 270 primary
and secondary minerals (Chakhmouradian and Wall, 2012; Jordens et al., 2013). They are
mainly found in Fe and Al phosphates, carbonates, silicates, and oxides. Parent material
and climate directly influence soil REE geochemistry (Zhang et al., 2001; Cidu et al., 2013;
Silva et al., 2017). The intensity of weathering controls the transformation of minerals
that act as sources of REEs in soils and several other environmental compartments.
The growth of the agricultural and industrial sectors has increased the demand for REEs
in the production of technological devices (Strauch et al., 2008; Long et al., 2010; USEPA,
2012). Due to disposal of these materials at the end of their useful life, the accumulation
of REEs in the soil is an environmental concern (Wang and Liang, 2016). Knowledge of
the natural levels of REEs in soils is the first step in monitoring potentially contaminated
areas. In addition, determination of these values deserves special attention because of
the wide utility of these elements as tracers of soil erosion (Zhu et al., 2011; Wen et al.,
2014), pedogenetic processes (Berger et al., 2014; Silva et al., 2017), and geochemical
cycles (Viers et al., 2009).
Geochemical associations between major elements and REEs in different climatic conditions
are important for understanding the behavior of REEs in soils (Laveuf et al., 2012).
Some authors have observed a high correlation between REEs and Fe in tropical soils
(Silva et al., 2016; Alfaro et al., 2018). This is logical because the process of weathering
and crystallization of Fe oxides can release REEs.
Spatial variability of REEs represents the scale of change in geology and helps identify REE
hotspots and their sources (Wang and Liang, 2016). This approach assists in observing
the influence of climate and parent material on REE distribution. Spatial distribution of soil
properties is often described (Aquino et al., 2015; Azevedo et al., 2015; Camargo et al.,
2015; Shukla et al., 2016; Moraes et al., 2017). However, the spatial variability of REEs has
rarely been shown. Spatial variability maps are particularly important for identifying areas
subject to environmental impact - an essential step in establishing future environmental
policies that affect human health and environmental protection. In this context, we aim to
determine the natural levels (background) of REEs in soils derived from different parent
materials and in climatic conditions ranging from humid to semi-arid; and to evaluate the
influence of major elements and soil properties on the geochemistry of REEs. This study
fills a gap in the scarce data on surface geochemistry of REEs in Brazilian soils.
MATERIALS AND METHODS
Study area and sample preparation
The study area covers the state of Rio Grande do Norte, Brazil, whose total area is
52,796.79 km². The sampling sites were selected based on the exploratory map of
soil recognition (Brasil, 1968) and the geological framework of the state adapted from
Medeiros et al. (2010). Soil and climate were considered for this sampling, and the soil
samples included the most representative geomorphological, pedological, and geological
compartments of the state.
Rio Grande do Norte can be divided into two major climatic environments. The semi-arid
region covers a large part of the state’s territory, with mean annual rainfall between
500-750 mm. The wetland is the second most dominant environment - it is located in the
Rev Bras Cienc Solo 2018;42:e0170342 2
, Silva et al. Geochemistry and spatial variability of rare earth elements in soils...
eastern portion of the state, with mean annual rainfall of 750-1,500 mm. The sub-humid
region and semi-humid regions have mean annual rainfall of 800-1,200 and 600-800 mm,
respectively. Mean annual air temperatures ranged from 26 to 27 °C.
The geology of the area is mostly pre-Cambrian (crystalline basement rocks), with
Cretaceous units and Cenozoic sedimentary material (sediments of the Barreiras Group)
(Figure 1) (Medeiros et al., 2010).
Different soil classes and source materials were represented by 104 composite soil
samples (Figure 1). Each sample was formed from five simple samples. The samples
were obtained with a stainless steel core sampling device from a layer of 0.00-0.20 m in
places with minimal anthropogenic influence (geochemical background). Many definitions
of geochemical background content are discussed in the literature (Tack et al., 1997;
Matschullat et al., 2000; Reimann and Garrett, 2005; Dung et al., 2013). In the present
study, geochemical background content is defined as the element content that has little to
no influence from human activities and thus reflects natural processes (Matschullat et al.,
2000). The soil samples were air dried, homogenized, and passed through a 2-mm sieve.
A 5 cm3 portion of soil was taken from each sample, macerated in an agate mortar, and
passed through a stainless-steel sieve with 0.15 mm mesh openings (ABNT n° 100).
Physical and chemical characterization of the soil
Particle size analysis was performed according to Gee and Or (2002). Chemical characterization
was performed according to Donagema et al. (2011). The pH was determined in water
(1:2.5); Ca2+, Mg2+, and Al3+ were extracted with KCl 1.0 mol L-1 and titrated. Exchangeable
K and Na were extracted with Mehlich-1 and measured by flame photometry. Potential
acidity (H+Al) was determined by extraction with calcium acetate (0.5 mol L-1) and titration.
Organic carbon (OC) was determined via a modified Walkley-Black method (Silva et al.,
1999). The values of the sum of bases (SB) and total cation exchange capacity (CEC) were
9450000
9400000
9350000
9300000
N
W E
9250000
s 0 15 30 60
1:500,000 km
550000 600000 650000 700000 750000 800000 850000 900000
Archean metamorphic rocks Sedimentary rocks
Neoproterozoic metamorphic rocks Pendência sandstone
Paleoproterozoic metamorphic rocks Açu sandstone
Igneous/magmatic rocks Jandaíra limestone
Serra basalt Sampling site
Figure 1. Simplified geological map of the state of Rio Grande do Norte, Brazil.
Rev Bras Cienc Solo 2018;42:e0170342 3
Division - Soil Use and Management | Commission - Soil and water management and conservation
Geochemistry and Spatial Variability
of Rare Earth Elements in Soils under
Different Geological and Climate
Patterns of the Brazilian Northeast
Cinthia Maria Cordeiro Atanázio Cruz Silva(1), Ronny Sobreira Barbosa(1),
Clístenes Williams Araújo do Nascimento(2), Yuri Jacques Agra Bezerra da Silva(1)* and
Ygor Jacques Agra Bezerra da Silva(2)
(1)
Universidade Federal do Piauí, Campus Professora Cinobelina Elvas, Curso de Bacharelado em Engenharia
Agronômica, Bom Jesus, Piauí, Brasil.
(2)
Universidade Federal de Pernambuco, Departamento de Agronomia, Recife, Pernambuco, Brasil.
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
* Corresponding author: do Norte (RN), Brazil, were in the followed order (mg kg-1): Ce (40.4) > La (18.9) > Nd (15.8) >
E-mail:
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) >
Received: October 16, 2017 Lu (0.2). The parent material was the main factor that governed the geochemistry of the REEs in
Approved: April 3, 2018 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
How to cite: Silva CMCAC,
Barbosa RS, Nascimento soils with lower contents of REEs in the state. In addition, soils developed from the same parent
CWA, Silva YJAB, Silva YJAB. material and under different climatic conditions showed the same geochemical signatures for
Geochemistry and spatial
variability of rare earth elements REEs in soils. These results confirm the small effect of climate on REE geochemistry in soils of
in soils under different geological RN and lead to the conclusion that the geochemical signature of REEs in these soils reflects
and climate patterns of the
Brazilian Northeast. Rev Bras the composition of the underlying parent material. The lack of significant correlation between
Cienc Solo. 2018;42:e0170342. (La/Yb)N ratio and the Chemical Alteration Index also confirms the low influence of climate on
https://doi.org/10.1590/18069657rbcs20170342
soil REE geochemistry. Among the major elements, Fe and Si had a greater influence on soil
Copyright: This is an open-access REE geochemistry. Higher REEs were seen in areas with more Fe and less Si. These REE levels
article distributed under the
terms of the Creative Commons were clearly controlled by the type of parent material. The Nd, Sm, Tb, Dy, Ho, Yb, and Er levels
Attribution License, which permits showed strong spatial dependence; this dependence was moderate for the Pr, La, Ce, Eu, Gd,
unrestricted use, distribution,
and reproduction in any medium, and Lu levels. Spatial variability maps of REEs are particularly important to identify areas under
provided that the original author environmental impact. Our results represent the most detailed study of the surface geochemistry
and source are credited.
of REEs in Brazilian soils and contribute to the scarce data available on these elements in Brazil.
Keywords: natural contents, lanthanides, geostatistics, factor analysis, soil quality.
https://doi.org/10.1590/18069657rbcs20170342 1
, Silva et al. Geochemistry and spatial variability of rare earth elements in soils...
INTRODUCTION
Rare earth elements (REEs) are a group of fifteen chemical elements in the lanthanide
series. These elements are divided into light rare earth elements (LREEs; La to Eu) and
heavy rare earth elements (HREEs; Gd to Lu) based on the atomic number (Tyler, 2004;
Hu et al., 2006a; Sadeghi et al., 2013; Davranche et al., 2016). Not all are technically “rare” -
indeed, cerium is the 25th most abundant element in the Earth’s crust, with contents similar
to Cu and Zn (Tyler, 2004). Rare earth elements can be found in more than 270 primary
and secondary minerals (Chakhmouradian and Wall, 2012; Jordens et al., 2013). They are
mainly found in Fe and Al phosphates, carbonates, silicates, and oxides. Parent material
and climate directly influence soil REE geochemistry (Zhang et al., 2001; Cidu et al., 2013;
Silva et al., 2017). The intensity of weathering controls the transformation of minerals
that act as sources of REEs in soils and several other environmental compartments.
The growth of the agricultural and industrial sectors has increased the demand for REEs
in the production of technological devices (Strauch et al., 2008; Long et al., 2010; USEPA,
2012). Due to disposal of these materials at the end of their useful life, the accumulation
of REEs in the soil is an environmental concern (Wang and Liang, 2016). Knowledge of
the natural levels of REEs in soils is the first step in monitoring potentially contaminated
areas. In addition, determination of these values deserves special attention because of
the wide utility of these elements as tracers of soil erosion (Zhu et al., 2011; Wen et al.,
2014), pedogenetic processes (Berger et al., 2014; Silva et al., 2017), and geochemical
cycles (Viers et al., 2009).
Geochemical associations between major elements and REEs in different climatic conditions
are important for understanding the behavior of REEs in soils (Laveuf et al., 2012).
Some authors have observed a high correlation between REEs and Fe in tropical soils
(Silva et al., 2016; Alfaro et al., 2018). This is logical because the process of weathering
and crystallization of Fe oxides can release REEs.
Spatial variability of REEs represents the scale of change in geology and helps identify REE
hotspots and their sources (Wang and Liang, 2016). This approach assists in observing
the influence of climate and parent material on REE distribution. Spatial distribution of soil
properties is often described (Aquino et al., 2015; Azevedo et al., 2015; Camargo et al.,
2015; Shukla et al., 2016; Moraes et al., 2017). However, the spatial variability of REEs has
rarely been shown. Spatial variability maps are particularly important for identifying areas
subject to environmental impact - an essential step in establishing future environmental
policies that affect human health and environmental protection. In this context, we aim to
determine the natural levels (background) of REEs in soils derived from different parent
materials and in climatic conditions ranging from humid to semi-arid; and to evaluate the
influence of major elements and soil properties on the geochemistry of REEs. This study
fills a gap in the scarce data on surface geochemistry of REEs in Brazilian soils.
MATERIALS AND METHODS
Study area and sample preparation
The study area covers the state of Rio Grande do Norte, Brazil, whose total area is
52,796.79 km². The sampling sites were selected based on the exploratory map of
soil recognition (Brasil, 1968) and the geological framework of the state adapted from
Medeiros et al. (2010). Soil and climate were considered for this sampling, and the soil
samples included the most representative geomorphological, pedological, and geological
compartments of the state.
Rio Grande do Norte can be divided into two major climatic environments. The semi-arid
region covers a large part of the state’s territory, with mean annual rainfall between
500-750 mm. The wetland is the second most dominant environment - it is located in the
Rev Bras Cienc Solo 2018;42:e0170342 2
, Silva et al. Geochemistry and spatial variability of rare earth elements in soils...
eastern portion of the state, with mean annual rainfall of 750-1,500 mm. The sub-humid
region and semi-humid regions have mean annual rainfall of 800-1,200 and 600-800 mm,
respectively. Mean annual air temperatures ranged from 26 to 27 °C.
The geology of the area is mostly pre-Cambrian (crystalline basement rocks), with
Cretaceous units and Cenozoic sedimentary material (sediments of the Barreiras Group)
(Figure 1) (Medeiros et al., 2010).
Different soil classes and source materials were represented by 104 composite soil
samples (Figure 1). Each sample was formed from five simple samples. The samples
were obtained with a stainless steel core sampling device from a layer of 0.00-0.20 m in
places with minimal anthropogenic influence (geochemical background). Many definitions
of geochemical background content are discussed in the literature (Tack et al., 1997;
Matschullat et al., 2000; Reimann and Garrett, 2005; Dung et al., 2013). In the present
study, geochemical background content is defined as the element content that has little to
no influence from human activities and thus reflects natural processes (Matschullat et al.,
2000). The soil samples were air dried, homogenized, and passed through a 2-mm sieve.
A 5 cm3 portion of soil was taken from each sample, macerated in an agate mortar, and
passed through a stainless-steel sieve with 0.15 mm mesh openings (ABNT n° 100).
Physical and chemical characterization of the soil
Particle size analysis was performed according to Gee and Or (2002). Chemical characterization
was performed according to Donagema et al. (2011). The pH was determined in water
(1:2.5); Ca2+, Mg2+, and Al3+ were extracted with KCl 1.0 mol L-1 and titrated. Exchangeable
K and Na were extracted with Mehlich-1 and measured by flame photometry. Potential
acidity (H+Al) was determined by extraction with calcium acetate (0.5 mol L-1) and titration.
Organic carbon (OC) was determined via a modified Walkley-Black method (Silva et al.,
1999). The values of the sum of bases (SB) and total cation exchange capacity (CEC) were
9450000
9400000
9350000
9300000
N
W E
9250000
s 0 15 30 60
1:500,000 km
550000 600000 650000 700000 750000 800000 850000 900000
Archean metamorphic rocks Sedimentary rocks
Neoproterozoic metamorphic rocks Pendência sandstone
Paleoproterozoic metamorphic rocks Açu sandstone
Igneous/magmatic rocks Jandaíra limestone
Serra basalt Sampling site
Figure 1. Simplified geological map of the state of Rio Grande do Norte, Brazil.
Rev Bras Cienc Solo 2018;42:e0170342 3
