101
Considerable amount of information about guideline values for the protection of
the environment concerning the U contamination exists. However, the values
obtained for a given species in each area might not be extrapolated for the wide
variety of conditions observed in the whole natural aquatic ecosystems. Since many
environmental conditions might influence the U speciation and consequently its
bioavailability and toxicity, every generalization including a wide number of species might be taken very carefully, and more investigations for the tropical conditions must be performed.
3 Geochemistry of U in Tropical Soils
Tropical regions are located between the Tropics of Cancer and Capricorn and are
characterized by high annual temperature and rainfall, which results in high weathered soils. The warm humid climates cause soil acidification and can affect the U
distribution in soils. Besides tropical soils being considered vulnerable to radioactive contamination (Wasserman et al. 2008), studies related to the geochemistry of
U in tropical areas are still scarce (Kritsananuwat et al. 2015; Ribeiro et al. 2018)
mainly due to the complexity of the soils (Wasserman et al. 2008).
Overall, the mineralogy of U is well documented in the literature (e.g., Romberger
1984; Bernard 2003; Kabata-Pendias 2011). Uranium can be found in all rock types,
commonly in small concentrations. The average U concentration in the Earth’s crust
is 2.7 mg kg
−1
(Taylor 1996). The mean concentration of U in rocks decreases in the
following order: granitic rock (3 mg kg
−1
) > basic rock (1 mg kg
−1
) > ultrabasic rock
(0.001 mg kg
−1
). Uranium content in sedimentary rocks varies according to the
redox conditions during their formation (Turekian and Wedepohl 1961). Compared
to the mantle, U is concentrated in the Earth’s crust, and its content ranges from
0.013 mg kg
−1
for the undepleted mantle (Bjorlykke 1974) to 0.032 mg kg
−1
for the
present mantle (Burt and Sheridan 1980).
Uranium is present in the pedosphere as a result of three main pathways: as part
of the Earth’s layer (primordial radionuclides), originated from the cosmic ray interactions, and as a consequence of anthropogenic sources (USEPA 2006). The average concentration of U in soil ranges from 0.79 to 11 mg kg
−1
(Kabata-Pendias
2011). Total U content in soils depends on the composition of the underlying bedrock and the intensity of the weathering, which can sometimes result in a higher
content of U in the geologic material. For example, Cunha et al. (2018) investigated
the geochemistry of U in soils from the largest uranium-phosphate deposit in Brazil
and observed concentrations higher than 900 mg kg
−1
.
Nevertheless, U concentrations in soils are not only influenced by their parent
material but also by abiotic and biological processes (Ribeiro et al. 2018).
Consequently, the geochemistry of U in soils and the human external exposures to
radiation is not homogeneous, varying according to the geological context, climatic
conditions, and other environmental characteristics (UNSCEAR 2000). The main
anthropogenic sources of U in the soils are the fuel fabrication, fuel reprocessing, U
Biogeochemistry of Uranium in Tropical Environments
Considerable amount of information about guideline values for the protection of
the environment concerning the U contamination exists. However, the values
obtained for a given species in each area might not be extrapolated for the wide
variety of conditions observed in the whole natural aquatic ecosystems. Since many
environmental conditions might influence the U speciation and consequently its
bioavailability and toxicity, every generalization including a wide number of species might be taken very carefully, and more investigations for the tropical conditions must be performed.
3 Geochemistry of U in Tropical Soils
Tropical regions are located between the Tropics of Cancer and Capricorn and are
characterized by high annual temperature and rainfall, which results in high weathered soils. The warm humid climates cause soil acidification and can affect the U
distribution in soils. Besides tropical soils being considered vulnerable to radioactive contamination (Wasserman et al. 2008), studies related to the geochemistry of
U in tropical areas are still scarce (Kritsananuwat et al. 2015; Ribeiro et al. 2018)
mainly due to the complexity of the soils (Wasserman et al. 2008).
Overall, the mineralogy of U is well documented in the literature (e.g., Romberger
1984; Bernard 2003; Kabata-Pendias 2011). Uranium can be found in all rock types,
commonly in small concentrations. The average U concentration in the Earth’s crust
is 2.7 mg kg
−1
(Taylor 1996). The mean concentration of U in rocks decreases in the
following order: granitic rock (3 mg kg
−1
) > basic rock (1 mg kg
−1
) > ultrabasic rock
(0.001 mg kg
−1
). Uranium content in sedimentary rocks varies according to the
redox conditions during their formation (Turekian and Wedepohl 1961). Compared
to the mantle, U is concentrated in the Earth’s crust, and its content ranges from
0.013 mg kg
−1
for the undepleted mantle (Bjorlykke 1974) to 0.032 mg kg
−1
for the
present mantle (Burt and Sheridan 1980).
Uranium is present in the pedosphere as a result of three main pathways: as part
of the Earth’s layer (primordial radionuclides), originated from the cosmic ray interactions, and as a consequence of anthropogenic sources (USEPA 2006). The average concentration of U in soil ranges from 0.79 to 11 mg kg
−1
(Kabata-Pendias
2011). Total U content in soils depends on the composition of the underlying bedrock and the intensity of the weathering, which can sometimes result in a higher
content of U in the geologic material. For example, Cunha et al. (2018) investigated
the geochemistry of U in soils from the largest uranium-phosphate deposit in Brazil
and observed concentrations higher than 900 mg kg
−1
.
Nevertheless, U concentrations in soils are not only influenced by their parent
material but also by abiotic and biological processes (Ribeiro et al. 2018).
Consequently, the geochemistry of U in soils and the human external exposures to
radiation is not homogeneous, varying according to the geological context, climatic
conditions, and other environmental characteristics (UNSCEAR 2000). The main
anthropogenic sources of U in the soils are the fuel fabrication, fuel reprocessing, U
Biogeochemistry of Uranium in Tropical Environments