102
mining and milling, U processing, phosphate, coal and metallic mining, and inappropriate waste disposal (Gavrilescu et al. 2009).
The geochemistry of U in tropical soils depends on soil types and their properties
(Ribeiro et al. 2018). Several factors influence the solubility of U in tropical soils, such
as pH, oxidation-reduction potential, concentration of complexing anions, soil porosity, particle size distribution, organic matter content, sorption properties, and others.
Uranium retention in soils can occur by adsorption, chemisorption, ion exchange, or a
combination of several mechanisms (Allard et al. 1982; Vandenhove et al. 2007).
Uranium is characterized as a lithophile element and can exist in the trivalent
(U
+3
), tetravalent (U
+4
), pentavalent (U
+5
), and hexavalent (U
+6
) states. The U
+4
and
U
+6
states are the most important in mineralogy (Romberger 1984). Uranium is a
reactive metal, and, as a result, it combines with other elements in the soil to form
U compounds. In soils, U is usually found in the oxidized form. Abiotic and biological processes are responsible for chemical reactions that alter the oxidation states of
U (Yamaguchi et al. 2009). Because most of the tropical soils exhibit pH between
4.0 and 7.5, the hydrolyzed form of U (U
+6
) predominates in soils. Under acid
(pH < 5) and oxidizing conditions, typical characteristics of tropical soils, U exists
predominantly as uranyl (UO 2
2+
). Under conditions close to neutrality, the common
U forms are soluble complexes, such as UO 2 OH
+
, (UO 2 ) 2 (OH 2 )
2+
e UO 2 (HPO 4 ) 2
2−
.
At higher pH, several carbonated complexes predominate (i.e., UO 2 (CO 3 ) 2
2−
,
UO 2 (CO 3 ) 3
4−
) (Langmuir 1978; Gavrilescu et al. 2009).
Mineral colloids and organic complexes also influence the geochemistry of U in
soils (Chen et al. 2018; Santos-Frances et al. 2018). Studies have shown that both
clay minerals and humic acids can affect the solubility and mobility of U in soils
(Bednar et al. 2007; Mibus et al. 2007; Wang et al. 2014), depending on the mobility
of the humic acids and mineral colloids and their capacity to retain U. For example,
uranyl has a high affinity to be complexed with fulvic and humic acids (Kim 1986).
Some of the minerals acting as important sorbents of U in soils are feldspars, kaolinite, goethite, and montmorillonite (Catalano and Brown 2005; Kabata-Pendias
2011). The adsorption of uranyl by clay minerals as well as by oxides and organic
matter is an important process controlling the geochemistry of U in solutions, since
it might reduce U anomalies in surface waters (Rose 1994). In solid phase, the
physicochemical characteristics of the soils and sediments control the bioavailable
fraction of U (Galhardi et al. 2017); thus, understanding U geochemistry is an
important tool to predict the risks of this element to terrestrial organisms.
3.1 Bioavailability of U in Soils and Soil-to-Plant Transfer
Mechanisms
The fate of U in the natural environment is of great importance. Worldwide, the
contaminant bioavailability has been considered for the assessment and remediation of contaminated soils. There is an increasing concern about the significant persistency of metals and radionuclides in soils and their potentially high
J. A. Galhardi et al.
mining and milling, U processing, phosphate, coal and metallic mining, and inappropriate waste disposal (Gavrilescu et al. 2009).
The geochemistry of U in tropical soils depends on soil types and their properties
(Ribeiro et al. 2018). Several factors influence the solubility of U in tropical soils, such
as pH, oxidation-reduction potential, concentration of complexing anions, soil porosity, particle size distribution, organic matter content, sorption properties, and others.
Uranium retention in soils can occur by adsorption, chemisorption, ion exchange, or a
combination of several mechanisms (Allard et al. 1982; Vandenhove et al. 2007).
Uranium is characterized as a lithophile element and can exist in the trivalent
(U
+3
), tetravalent (U
+4
), pentavalent (U
+5
), and hexavalent (U
+6
) states. The U
+4
and
U
+6
states are the most important in mineralogy (Romberger 1984). Uranium is a
reactive metal, and, as a result, it combines with other elements in the soil to form
U compounds. In soils, U is usually found in the oxidized form. Abiotic and biological processes are responsible for chemical reactions that alter the oxidation states of
U (Yamaguchi et al. 2009). Because most of the tropical soils exhibit pH between
4.0 and 7.5, the hydrolyzed form of U (U
+6
) predominates in soils. Under acid
(pH < 5) and oxidizing conditions, typical characteristics of tropical soils, U exists
predominantly as uranyl (UO 2
2+
). Under conditions close to neutrality, the common
U forms are soluble complexes, such as UO 2 OH
+
, (UO 2 ) 2 (OH 2 )
2+
e UO 2 (HPO 4 ) 2
2−
.
At higher pH, several carbonated complexes predominate (i.e., UO 2 (CO 3 ) 2
2−
,
UO 2 (CO 3 ) 3
4−
) (Langmuir 1978; Gavrilescu et al. 2009).
Mineral colloids and organic complexes also influence the geochemistry of U in
soils (Chen et al. 2018; Santos-Frances et al. 2018). Studies have shown that both
clay minerals and humic acids can affect the solubility and mobility of U in soils
(Bednar et al. 2007; Mibus et al. 2007; Wang et al. 2014), depending on the mobility
of the humic acids and mineral colloids and their capacity to retain U. For example,
uranyl has a high affinity to be complexed with fulvic and humic acids (Kim 1986).
Some of the minerals acting as important sorbents of U in soils are feldspars, kaolinite, goethite, and montmorillonite (Catalano and Brown 2005; Kabata-Pendias
2011). The adsorption of uranyl by clay minerals as well as by oxides and organic
matter is an important process controlling the geochemistry of U in solutions, since
it might reduce U anomalies in surface waters (Rose 1994). In solid phase, the
physicochemical characteristics of the soils and sediments control the bioavailable
fraction of U (Galhardi et al. 2017); thus, understanding U geochemistry is an
important tool to predict the risks of this element to terrestrial organisms.
3.1 Bioavailability of U in Soils and Soil-to-Plant Transfer
Mechanisms
The fate of U in the natural environment is of great importance. Worldwide, the
contaminant bioavailability has been considered for the assessment and remediation of contaminated soils. There is an increasing concern about the significant persistency of metals and radionuclides in soils and their potentially high
J. A. Galhardi et al.