142
The use of extractants with varying concentrations was reported to influence
speciation results, increasing extraction/exchangeable capacity as concentration
increased (Rout et al. 2015). Desorption capacity of CaCl 2 , MgCl 2, and NaCl for
uranium was reported to be proportional to ionic radius (Ca
2+
> Mg
2+
> Na
2+
) (Rout
et al. 2015). Solid–liquid ratio in extraction procedure also had some influence,
slightly increasing the amount of uranium extracted with increasing volume of
NH 4 OAc used until reaching saturation (Guillén et al. 2018). This effect depended
on soil type.
Other nonconventional extraction procedures to assess uranium bioavailability
were also reported in the literature for abandoned uranium mines, as the use of soft
cola drinks (Lottermoser et al. 2011) or simulated physiological fluids (Foulkes
et al. 2017). Lottermoser et al. (2011) compared uranium extracted with different
types of cola soft drink, CaCl 2 , and DTPA (diethylenetriaminepentaacetic acid),
obtaining similar results with classic cola and DTPA and reporting differences
between different cola types. Foulkes et al. (2017) assessed the bioavailable fraction
via simulated digestive fluids (saliva, bile, gastric, and duodenal fluids), reporting an
amount between 3% and 7% of bioavailable uranium in waste solids and 5–36% in
soils. This information is relevant when soil ingestion is considered, for instance,
along with foodstuff.
Uranium chemical species present in soil and soil solution can be also theoretically estimated using thermodynamic and geochemical databases, such as J-Chess
and PHREEQ (Laurette et al. 2012b; Mihalík et al. 2012; Hormann and Fischer
2013; Neiva et al. 2016; Boghi et al. 2018). These databases can be used to develop
models for estimating K d values (ratio between concentration in liquid and solid
fractions) for different radionuclides. Regarding uranium, they consider exchange
and complexation on clay silicate surfaces and complexation on hydrous ferric
oxides and organic matter (Hormann and Fischer 2013; Boghi et al. 2018)
3.2 Influence of Soil Properties
The presence of ions in soil and soil solution can influence plant uranium uptake
ability. Carbonate ions can increase uranium solubility via complexation reactions,
occurring at pH 4 and higher, and also affect uranium sorption in ferric hydroxides
(Li and Kaplan 2012; Boghi et al. 2018). On the contrary, complexation with phosphates reduces uranium bioavailability as insoluble uranyl phosphate is created
(Laurette et al. 2012b). Uranium Kd values in soil increase with CO 2 pressure and
are pH dependent, with maximum values in the range 5–7 (Hormann and Fischer
2013). Uranium concentration in soil can also be correlated with other metals, such
as As, Fe, or Mn (Canha et al. 2010; Neiva et al. 2016), although this correlations
should be considered as site dependent.
Organic matter in soil can also influence uranium uptake by plants. In organic
matter-rich soils, correlation between uranium concentration in soil and percentage
of total organic carbon was reported (Regenspurg et al. 2010). Organic acid exudation
J. Guillén and F. M. Gómez-Polo
The use of extractants with varying concentrations was reported to influence
speciation results, increasing extraction/exchangeable capacity as concentration
increased (Rout et al. 2015). Desorption capacity of CaCl 2 , MgCl 2, and NaCl for
uranium was reported to be proportional to ionic radius (Ca
2+
> Mg
2+
> Na
2+
) (Rout
et al. 2015). Solid–liquid ratio in extraction procedure also had some influence,
slightly increasing the amount of uranium extracted with increasing volume of
NH 4 OAc used until reaching saturation (Guillén et al. 2018). This effect depended
on soil type.
Other nonconventional extraction procedures to assess uranium bioavailability
were also reported in the literature for abandoned uranium mines, as the use of soft
cola drinks (Lottermoser et al. 2011) or simulated physiological fluids (Foulkes
et al. 2017). Lottermoser et al. (2011) compared uranium extracted with different
types of cola soft drink, CaCl 2 , and DTPA (diethylenetriaminepentaacetic acid),
obtaining similar results with classic cola and DTPA and reporting differences
between different cola types. Foulkes et al. (2017) assessed the bioavailable fraction
via simulated digestive fluids (saliva, bile, gastric, and duodenal fluids), reporting an
amount between 3% and 7% of bioavailable uranium in waste solids and 5–36% in
soils. This information is relevant when soil ingestion is considered, for instance,
along with foodstuff.
Uranium chemical species present in soil and soil solution can be also theoretically estimated using thermodynamic and geochemical databases, such as J-Chess
and PHREEQ (Laurette et al. 2012b; Mihalík et al. 2012; Hormann and Fischer
2013; Neiva et al. 2016; Boghi et al. 2018). These databases can be used to develop
models for estimating K d values (ratio between concentration in liquid and solid
fractions) for different radionuclides. Regarding uranium, they consider exchange
and complexation on clay silicate surfaces and complexation on hydrous ferric
oxides and organic matter (Hormann and Fischer 2013; Boghi et al. 2018)
3.2 Influence of Soil Properties
The presence of ions in soil and soil solution can influence plant uranium uptake
ability. Carbonate ions can increase uranium solubility via complexation reactions,
occurring at pH 4 and higher, and also affect uranium sorption in ferric hydroxides
(Li and Kaplan 2012; Boghi et al. 2018). On the contrary, complexation with phosphates reduces uranium bioavailability as insoluble uranyl phosphate is created
(Laurette et al. 2012b). Uranium Kd values in soil increase with CO 2 pressure and
are pH dependent, with maximum values in the range 5–7 (Hormann and Fischer
2013). Uranium concentration in soil can also be correlated with other metals, such
as As, Fe, or Mn (Canha et al. 2010; Neiva et al. 2016), although this correlations
should be considered as site dependent.
Organic matter in soil can also influence uranium uptake by plants. In organic
matter-rich soils, correlation between uranium concentration in soil and percentage
of total organic carbon was reported (Regenspurg et al. 2010). Organic acid exudation
J. Guillén and F. M. Gómez-Polo