141
thermodynamic databases. The former approach assesses the degree of association/binding of radionuclides to soil particles based on successive application of
reagents with increasing extraction/replacement power or designed to attack a specific geochemical phase. There is no unified sequential extraction procedure, and
it is mainly defined operationally by different authors. However, there are some
connections, as many sequential extraction procedures are based on that proposed
by Tessier et al. (1979), which considered the following fractions in soil: (1) water
soluble, extracted with double-distilled H 2 O; (2) exchangeable fraction, extracted
with 1 M ammonium acetate; (3) carbonated fraction, extracted with 1 M sodium
acetate; (4) easily reducible fraction or bound to Fe and Mn oxides and hydroxides, extracted with 0.1 M hydroxylamine hydrochloride; (5) moderately reducible
fraction, extracted with 0.2 M ammonium oxalate; (6) organic/hydrogen-sulfide
fraction, extracted with H 2 O 2 30% and ammonium acetate; (7) acid fraction,
extracted with HNO 3 40%; and (8) residual fraction. This speciation scheme is
widely used, especially for anthropogenic radionuclides in contaminated areas
(Fawaris and Johanson 1995; Riise et al. 1990; Schultz et al. 1998; Rauret et al.
1999), and naturally occurring radionuclides in areas potentially affected by
NORM industries (Blanco et al. 2004; Guillén et al. 2018; Pérez-Moreno et al.
2018). Other schemes are based on agricultural sequential extraction procedures
based on the use of acids in several concentrations (Pavlotskaya 1974) and the
extraction of organic matter (Cook et al. 1984). Due to the great number of speciation schemes, sometimes it is difficult to compare the results provided by two different procedures applied to the same soil (Blanco et al. 2004), given that they are
not specific for a single geochemical fraction (Schultz et al. 1998). Other authors
also compared different speciation schemes, reporting partial correlations between
Tessier and Pavlotskaya methods (Guillén et al. 2018) or suggesting that Schultz
method would perform better for uranium exchangeable fraction than Rauret
method, acting as a better potential indicator to evaluate plant uptake in soils
(Vandenhove et al. 2014). One of the major difficulties in assessing speciation
schemes is the practically lack of certified reference materials, although some
attempts of validation for naturally occurring radionuclides were carried out
(Pérez-Moreno et al. 2018).
Bioavailable fraction is usually considered as the fraction of radionuclides that
can be up taken or transferred to plants. Water-soluble and exchangeable fractions
are usually considered as readily available for plants. Regarding exchangeable fraction, NH 4 OAc is one of the most used reagents, although MgCl 2 , CaCl 2 , EDTA,
DTPA, NH 4 NO 3 , KCl, or NaNO 3 might be used (Kennedy et al. 1997; Komosa
2002; Rigol et al. 2002). Other authors also considered reducible, oxidizable, and
carbonated fractions to be mobile in soils (Rauret et al. 1999; Laurette et al. 2012a;
Pérez-Moreno et al. 2018). As uranium is naturally occurring, it forms constituent
part of minerals present in soil. Therefore, it is mainly associated with residual and
other fractions strongly bounded to soil particles (Skipperud et al. 2013; Rout et al.
2015; Guillén et al. 2018). This association occurred in natural soils (Guillén et al.
2018) and in those affected by uranium tailings (Skipperud et al. 2013). However, if
soils were artificially contaminated with uranium, it was mainly attached to
exchangeable and carbonated fraction (Vandenhove et al. 2014).
Factors Influencing the Soil to Plant Transfer of Uranium
thermodynamic databases. The former approach assesses the degree of association/binding of radionuclides to soil particles based on successive application of
reagents with increasing extraction/replacement power or designed to attack a specific geochemical phase. There is no unified sequential extraction procedure, and
it is mainly defined operationally by different authors. However, there are some
connections, as many sequential extraction procedures are based on that proposed
by Tessier et al. (1979), which considered the following fractions in soil: (1) water
soluble, extracted with double-distilled H 2 O; (2) exchangeable fraction, extracted
with 1 M ammonium acetate; (3) carbonated fraction, extracted with 1 M sodium
acetate; (4) easily reducible fraction or bound to Fe and Mn oxides and hydroxides, extracted with 0.1 M hydroxylamine hydrochloride; (5) moderately reducible
fraction, extracted with 0.2 M ammonium oxalate; (6) organic/hydrogen-sulfide
fraction, extracted with H 2 O 2 30% and ammonium acetate; (7) acid fraction,
extracted with HNO 3 40%; and (8) residual fraction. This speciation scheme is
widely used, especially for anthropogenic radionuclides in contaminated areas
(Fawaris and Johanson 1995; Riise et al. 1990; Schultz et al. 1998; Rauret et al.
1999), and naturally occurring radionuclides in areas potentially affected by
NORM industries (Blanco et al. 2004; Guillén et al. 2018; Pérez-Moreno et al.
2018). Other schemes are based on agricultural sequential extraction procedures
based on the use of acids in several concentrations (Pavlotskaya 1974) and the
extraction of organic matter (Cook et al. 1984). Due to the great number of speciation schemes, sometimes it is difficult to compare the results provided by two different procedures applied to the same soil (Blanco et al. 2004), given that they are
not specific for a single geochemical fraction (Schultz et al. 1998). Other authors
also compared different speciation schemes, reporting partial correlations between
Tessier and Pavlotskaya methods (Guillén et al. 2018) or suggesting that Schultz
method would perform better for uranium exchangeable fraction than Rauret
method, acting as a better potential indicator to evaluate plant uptake in soils
(Vandenhove et al. 2014). One of the major difficulties in assessing speciation
schemes is the practically lack of certified reference materials, although some
attempts of validation for naturally occurring radionuclides were carried out
(Pérez-Moreno et al. 2018).
Bioavailable fraction is usually considered as the fraction of radionuclides that
can be up taken or transferred to plants. Water-soluble and exchangeable fractions
are usually considered as readily available for plants. Regarding exchangeable fraction, NH 4 OAc is one of the most used reagents, although MgCl 2 , CaCl 2 , EDTA,
DTPA, NH 4 NO 3 , KCl, or NaNO 3 might be used (Kennedy et al. 1997; Komosa
2002; Rigol et al. 2002). Other authors also considered reducible, oxidizable, and
carbonated fractions to be mobile in soils (Rauret et al. 1999; Laurette et al. 2012a;
Pérez-Moreno et al. 2018). As uranium is naturally occurring, it forms constituent
part of minerals present in soil. Therefore, it is mainly associated with residual and
other fractions strongly bounded to soil particles (Skipperud et al. 2013; Rout et al.
2015; Guillén et al. 2018). This association occurred in natural soils (Guillén et al.
2018) and in those affected by uranium tailings (Skipperud et al. 2013). However, if
soils were artificially contaminated with uranium, it was mainly attached to
exchangeable and carbonated fraction (Vandenhove et al. 2014).
Factors Influencing the Soil to Plant Transfer of Uranium