104
disadvantages are associated with the use of bioassays, since they are often timeconsuming, laborious, and with a high cost (Kim et al. 2015).
Predicting soil-to-plant transfer is another important step to assess U bioavailability in terrestrial ecosystems. One of the main sources of U exposure to the population is through the food ingestion. The most common index used to estimate the
radionuclide transport and their accumulation in vegetables is the transfer factor
(TF), defined as the ratio between the concentration of the element in plants and in
the soil. The TF describes the amount of the element that can be absorbed by a plant
from its substrate under equilibrium conditions, assuming that the accumulation is
directly proportional to the concentration of the element in the soil (Sheppard and
Sheppard 1985).
Soil parameters that might affect the U concentration in the mean, its mobility in
the rhizosphere, and its availability to plants are the soil pH, electrical conductivity,
cation exchange capacity, total organic carbon, nutrients, mineralogy, and climate,
in addition to (bio)chemical processes such as complexation, precipitation, cation
exchange, and redox reactions (Ehlken and Kirchner 2002; Shtangeeva 2010;
Sohlenius et al. 2013; Gupta et al. 2014; Zhao et al. 2016).
Overall, processes controlling U activity in soils are dependent of chemical and
physicochemical characteristics of the geologic material exposed to weathering
conditions (Arbuzov et al. 2012). Uranium present in the mining residues may be
complexed by chelating agents produced by microbial activity mainly under reducing conditions, favoring its mobilization and consequently its bioavailability to the
biota (Francis 1990).
Although the uptake of metals by plants can be affected by numerous biogeochemical factors, in about 50% of the investigations focusing on the TF, they were
not considered (Vandenhove et al. 2009), which limits our understanding on the U
uptake by plants, mainly in tropical regions, where the chemical and bioassay methods were less applied to investigate U uptake by terrestrial organisms.
An assessment of the U distribution in the soil-plant system may be also rather
complicated because there is little information related to the rate of U uptake and
storage by different plant species and different parts of the plants (Shtangeeva
2010). Effects of the soil properties and natural differences in the ability of plants to
assimilate U may be some of the reasons why concentrations of this element differ
significantly in various plant species (Galhardi et al. 2017).
Availability of U contributes to the degree of its transfer to plants, but other factors could help in understanding potential risks related to levels of radioactive elements in soils, from molecular to individual and ecological scales. In general, the
biological factors are less well understood than the geological factors controlling
the U availability in soils (Willey 2014). Only complex models considering soil and
biotic characteristics could help in predicting U uptake by terrestrial plants, as TF is
not related to simple bioavailability parameters (Vandenhove et al. 2009).
Besides some researchers investigated the absorption of radionuclides by species
of plants in tropical areas (e.g., Vasconcellos et al. 1987; Santos et al. 1993; Lauria
et al. 1994; Santos et al. 2002; Mazzilli et al. 2012); a systematic investigation
focusing on areas affected by industrial, agricultural, and mining areas was not
J. A. Galhardi et al.
disadvantages are associated with the use of bioassays, since they are often timeconsuming, laborious, and with a high cost (Kim et al. 2015).
Predicting soil-to-plant transfer is another important step to assess U bioavailability in terrestrial ecosystems. One of the main sources of U exposure to the population is through the food ingestion. The most common index used to estimate the
radionuclide transport and their accumulation in vegetables is the transfer factor
(TF), defined as the ratio between the concentration of the element in plants and in
the soil. The TF describes the amount of the element that can be absorbed by a plant
from its substrate under equilibrium conditions, assuming that the accumulation is
directly proportional to the concentration of the element in the soil (Sheppard and
Sheppard 1985).
Soil parameters that might affect the U concentration in the mean, its mobility in
the rhizosphere, and its availability to plants are the soil pH, electrical conductivity,
cation exchange capacity, total organic carbon, nutrients, mineralogy, and climate,
in addition to (bio)chemical processes such as complexation, precipitation, cation
exchange, and redox reactions (Ehlken and Kirchner 2002; Shtangeeva 2010;
Sohlenius et al. 2013; Gupta et al. 2014; Zhao et al. 2016).
Overall, processes controlling U activity in soils are dependent of chemical and
physicochemical characteristics of the geologic material exposed to weathering
conditions (Arbuzov et al. 2012). Uranium present in the mining residues may be
complexed by chelating agents produced by microbial activity mainly under reducing conditions, favoring its mobilization and consequently its bioavailability to the
biota (Francis 1990).
Although the uptake of metals by plants can be affected by numerous biogeochemical factors, in about 50% of the investigations focusing on the TF, they were
not considered (Vandenhove et al. 2009), which limits our understanding on the U
uptake by plants, mainly in tropical regions, where the chemical and bioassay methods were less applied to investigate U uptake by terrestrial organisms.
An assessment of the U distribution in the soil-plant system may be also rather
complicated because there is little information related to the rate of U uptake and
storage by different plant species and different parts of the plants (Shtangeeva
2010). Effects of the soil properties and natural differences in the ability of plants to
assimilate U may be some of the reasons why concentrations of this element differ
significantly in various plant species (Galhardi et al. 2017).
Availability of U contributes to the degree of its transfer to plants, but other factors could help in understanding potential risks related to levels of radioactive elements in soils, from molecular to individual and ecological scales. In general, the
biological factors are less well understood than the geological factors controlling
the U availability in soils (Willey 2014). Only complex models considering soil and
biotic characteristics could help in predicting U uptake by terrestrial plants, as TF is
not related to simple bioavailability parameters (Vandenhove et al. 2009).
Besides some researchers investigated the absorption of radionuclides by species
of plants in tropical areas (e.g., Vasconcellos et al. 1987; Santos et al. 1993; Lauria
et al. 1994; Santos et al. 2002; Mazzilli et al. 2012); a systematic investigation
focusing on areas affected by industrial, agricultural, and mining areas was not
J. A. Galhardi et al.