2.8 Nuclear Weapons Trial
Nuclear weapons trials are also a source of HM pollution. Uranium and plutonium,
the main components of nuclear weapons, present relatively strong radioactivity.
After a nuclear weapons trial, these elements enter into the soil, and their high
radioactive properties are harmful for all the living organisms.
3 Types of Toxic Metals
About 45 HMs occur in the periodic table of elements, and the toxicity of these
elements varies from one to another. Therefore, only several metal elements with
high toxicity are a concern, including heavy metals such as copper (Cu), cadmium
(Cd), chromium (Cr), zinc (Zn), mercury (Hg), lead (Pb), nickel (Ni), and metalloids
such as arsenic (As) and selenium (Se). Actually, copper and zinc, and even ferrum
(iron) (Fe) and manganese (Mn), are necessary for plant growth, being involved in
the activation of plant enzymes and formation of carbohydrates, for example.
Similarly, selenium is critical in forming seleno-proteins for humans and animals,
such as glutathione peroxidase (GPx) and thio-redoxin reductases (TrxR) (Barcelo
and Poschenrieder 2011; Kaur et al. 2014), which benefit DNA synthesis, thyroid
hormone metabolism, reproduction, and protection from oxidative damage and
infection. However, some HMs such as Cd, As, Hg, and Pb are very toxic even at
low concentrations and are not essential for any system.
When the concentration of HMs in soil is more than the crop needs and is outside
the tolerable range, the crop demonstrates toxicity symptoms, and then HM contamination occurs. HM contamination is the main threat causing severe effects on arable
lands worldwide (Lin et al. 2012), possibly by making the lands barren for agricultural production. The uptake of HMs by plant crops depends on their chemical form
in the soil and their bioavailability to the plant (Richard et al. 2000; Rajakaruna and
Boyd 2008). The absorption of HMs by plants is influenced by soil physiochemical
properties, including concentrations of HMs and other trace elements in the soil, clay
content, soil temperature and moisture, soil pH, redox potential, cation-exchange
capacity (CEC), soil organic matter (SOM), and aeration (Neilson and Rajakaruna
2012; Gall and Rajakaruna 2013). Among these, pH and SOM are the two most
important factors contributing to the bioavailability of HMs (Guo et al. 2011).
Generally, most HMs becomes more bioavailable under acidic pH ranges, and
other soil conditions (Kumar et al. 1995) including HM concentration. However,
the ability to endure HMs varies from plant to plant. A high concentration of HMs in
the soil promotes the growth of some plants whereas HMs are toxic even at low
concentration levels for other plant varieties.
The bioaccumulation of HMs in soil can occur in many ways: HMs enter the
human body by the food web, such as soil–plant–human or soil–plant–animal–
human pathways. HMs can react with a variety of substances in the human body,
86
M. K. Awasthi et al.
Nuclear weapons trials are also a source of HM pollution. Uranium and plutonium,
the main components of nuclear weapons, present relatively strong radioactivity.
After a nuclear weapons trial, these elements enter into the soil, and their high
radioactive properties are harmful for all the living organisms.
3 Types of Toxic Metals
About 45 HMs occur in the periodic table of elements, and the toxicity of these
elements varies from one to another. Therefore, only several metal elements with
high toxicity are a concern, including heavy metals such as copper (Cu), cadmium
(Cd), chromium (Cr), zinc (Zn), mercury (Hg), lead (Pb), nickel (Ni), and metalloids
such as arsenic (As) and selenium (Se). Actually, copper and zinc, and even ferrum
(iron) (Fe) and manganese (Mn), are necessary for plant growth, being involved in
the activation of plant enzymes and formation of carbohydrates, for example.
Similarly, selenium is critical in forming seleno-proteins for humans and animals,
such as glutathione peroxidase (GPx) and thio-redoxin reductases (TrxR) (Barcelo
and Poschenrieder 2011; Kaur et al. 2014), which benefit DNA synthesis, thyroid
hormone metabolism, reproduction, and protection from oxidative damage and
infection. However, some HMs such as Cd, As, Hg, and Pb are very toxic even at
low concentrations and are not essential for any system.
When the concentration of HMs in soil is more than the crop needs and is outside
the tolerable range, the crop demonstrates toxicity symptoms, and then HM contamination occurs. HM contamination is the main threat causing severe effects on arable
lands worldwide (Lin et al. 2012), possibly by making the lands barren for agricultural production. The uptake of HMs by plant crops depends on their chemical form
in the soil and their bioavailability to the plant (Richard et al. 2000; Rajakaruna and
Boyd 2008). The absorption of HMs by plants is influenced by soil physiochemical
properties, including concentrations of HMs and other trace elements in the soil, clay
content, soil temperature and moisture, soil pH, redox potential, cation-exchange
capacity (CEC), soil organic matter (SOM), and aeration (Neilson and Rajakaruna
2012; Gall and Rajakaruna 2013). Among these, pH and SOM are the two most
important factors contributing to the bioavailability of HMs (Guo et al. 2011).
Generally, most HMs becomes more bioavailable under acidic pH ranges, and
other soil conditions (Kumar et al. 1995) including HM concentration. However,
the ability to endure HMs varies from plant to plant. A high concentration of HMs in
the soil promotes the growth of some plants whereas HMs are toxic even at low
concentration levels for other plant varieties.
The bioaccumulation of HMs in soil can occur in many ways: HMs enter the
human body by the food web, such as soil–plant–human or soil–plant–animal–
human pathways. HMs can react with a variety of substances in the human body,
86
M. K. Awasthi et al.