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1 Introduction
Uranium is a lustrous, silvery-grey, dense (19 g cm
−3
) metal of the actinide series. U
has the atomic number 92 and atomic weight 238.02 g mol
−1
. It is ubiquitously present in nature with an average concentration of less than 4 mg kg
−1
(Nolan and Weber
2015; Tawussi et al. 2017) in rock, soil, water and all biosystems, including plants
and humans. Three natural radioactive isotopes of U are common, with
238
U being
the mostly abundant one (99.27%), followed by
235
U (0.72%) and
234
U (0.0054%).
Radioactive isotopes of U have long half-lives (for
234
U, 244,000 years;
235
U,
710 million years; and
238
U, 4500 million years). They decay into various progenies,
ultimately forming stable lead (Pb) (WHO 2012). Based on the fraction of
235
U, one
classifies natural uranium (0.72%
235
U), enriched uranium (>0.72%
235
U) and
depleted uranium (DU, <0.72% U). DU is used, e.g. in armour-piercing ammunition. There are several instances of DU release into the environment in the Gulf War,
in former Yugoslavia and in Afghanistan, causing pollution of the environment and
contamination of water and food sources (Pinney et al. 2003; Lloyd et al. 2009;
Tasat et al. 2012). Mining of uranium caused (and still causes in less developed
countries) severe contaminations of the environment. Since uranium coexists with
many other substances, also mining of metals like the noble metals Au and Ag
causes release of uranium to the environment through leaching and weathering of
tailing material. The same is true for the NORM industries’ oil, gas and coal exploitation but also use of geothermy. Furthermore, phosphate fertilizers contain considerable amounts of uranium, depending on their origin.
Overuse of groundwater is a concern of many countries worldwide. A recent
study by Coyte et al. (2018) on groundwater quality in different states of India suggests considerably high prevalence of natural U, which exceeds the WHO (World
Health Organization) provisional guideline value of 30 μg L
−1
across India.
Although, primarily, U is geogenic, though, huge water exploitation, decline of
groundwater table and nitrate pollution typically augment the situation of U mobilization within the aquifers (Coyte et al. 2018). To avoid formidable situations
related to U contamination and related human health risks, suitable remediation
technologies must be adopted.
Apart from physical and chemical remediation practices, plant-based remediation is gaining its momentum due to its various advantages. Like any other elements, U translocation to a plants’ body depends on its bioavailability. Several
investigators working with plant-based U remediation practices (phytoremediation)
suggested rhizofiltration as one of the important ways to decontaminate U from the
contaminated environment (Dushenkov et al. 1997; Tomé et al. 2008; Baumann
et al. 2014). Nevertheless, the process includes a number of soil factors, compositions, bioavailability/mobility (Gupta and Walther 2014) and also availability of the
U, chelating agents, robust sorption and precipitation activities by the plants (Lee
and Yang 2010). Addition of chelating substances in soil can alter the U availability
to the plant, but also increase the U concentration in the ambience, which may lead
to reduction in plant growth and phytoremediation efficiency (Lozano et al. 2011;
D. K. Gupta et al.
1 Introduction
Uranium is a lustrous, silvery-grey, dense (19 g cm
−3
) metal of the actinide series. U
has the atomic number 92 and atomic weight 238.02 g mol
−1
. It is ubiquitously present in nature with an average concentration of less than 4 mg kg
−1
(Nolan and Weber
2015; Tawussi et al. 2017) in rock, soil, water and all biosystems, including plants
and humans. Three natural radioactive isotopes of U are common, with
238
U being
the mostly abundant one (99.27%), followed by
235
U (0.72%) and
234
U (0.0054%).
Radioactive isotopes of U have long half-lives (for
234
U, 244,000 years;
235
U,
710 million years; and
238
U, 4500 million years). They decay into various progenies,
ultimately forming stable lead (Pb) (WHO 2012). Based on the fraction of
235
U, one
classifies natural uranium (0.72%
235
U), enriched uranium (>0.72%
235
U) and
depleted uranium (DU, <0.72% U). DU is used, e.g. in armour-piercing ammunition. There are several instances of DU release into the environment in the Gulf War,
in former Yugoslavia and in Afghanistan, causing pollution of the environment and
contamination of water and food sources (Pinney et al. 2003; Lloyd et al. 2009;
Tasat et al. 2012). Mining of uranium caused (and still causes in less developed
countries) severe contaminations of the environment. Since uranium coexists with
many other substances, also mining of metals like the noble metals Au and Ag
causes release of uranium to the environment through leaching and weathering of
tailing material. The same is true for the NORM industries’ oil, gas and coal exploitation but also use of geothermy. Furthermore, phosphate fertilizers contain considerable amounts of uranium, depending on their origin.
Overuse of groundwater is a concern of many countries worldwide. A recent
study by Coyte et al. (2018) on groundwater quality in different states of India suggests considerably high prevalence of natural U, which exceeds the WHO (World
Health Organization) provisional guideline value of 30 μg L
−1
across India.
Although, primarily, U is geogenic, though, huge water exploitation, decline of
groundwater table and nitrate pollution typically augment the situation of U mobilization within the aquifers (Coyte et al. 2018). To avoid formidable situations
related to U contamination and related human health risks, suitable remediation
technologies must be adopted.
Apart from physical and chemical remediation practices, plant-based remediation is gaining its momentum due to its various advantages. Like any other elements, U translocation to a plants’ body depends on its bioavailability. Several
investigators working with plant-based U remediation practices (phytoremediation)
suggested rhizofiltration as one of the important ways to decontaminate U from the
contaminated environment (Dushenkov et al. 1997; Tomé et al. 2008; Baumann
et al. 2014). Nevertheless, the process includes a number of soil factors, compositions, bioavailability/mobility (Gupta and Walther 2014) and also availability of the
U, chelating agents, robust sorption and precipitation activities by the plants (Lee
and Yang 2010). Addition of chelating substances in soil can alter the U availability
to the plant, but also increase the U concentration in the ambience, which may lead
to reduction in plant growth and phytoremediation efficiency (Lozano et al. 2011;
D. K. Gupta et al.