Source: Pedersen, 1981; Rulkens, 2003. Khalil, 2011; Lopez et al., 2020;
Prado et al., 2014
Sources and Characteristics of Heavy metals in SS
Heavy metals are metallic elements with a comparatively high density in
comparison to water. Heavy metals such as chromium (Cr), cadmium (Cd),
mercury (Hg), lead (Pb), nickel (Ni), and thallium (Tl) can be harmful
in mixed or elemental form. Because heavy metal contamination is
colorless and odorless, it is difficult to detect. It does not directly
harm the environment in a short time. However, when it surpasses the
environmental tolerance or when environmental conditions change, heavy
metals in the soil may be activated, causing major ecological harm
(Emenike et al., 2018).
With the rise of the global economy in recent years, both the type and
concentration of heavy metals in soil produced by human activities have
continuously grown, resulting in environmental degradation. (Zojaji et
al., 2014). Heavy metals are extremely dangerous to both the environment
and organisms. To understand the current situation and the impact of
heavy metal contamination in the world, in the present study we would
first understand the characteristics of heavy metals and then analyze
the sources of heavy metals. If the air and water are contaminated, the
pollution problem can most likely be remedied by dilution and
self-purification when the pollution sources are turned off. However,
using dilution or self-purification procedures to reduce heavy metal
pollution and improve soils is challenging. Some heavy
metal-contaminated soils are estimated to take one or two hundred years
to repair (Emenike et al., 2018). Previously, soil contamination was
mostly caused by a single heavy metal. However, in recent years, a
greater number of instances have been discovered to be caused by a range
of heavy metals (Sarkar et al., 2021). The heavy metal burden is
significant in every sector in India. According to two ICMR
investigations conducted in 1993 and 1996, canned food goods contain
metals such as lead, aluminium, tin, and zinc. After a year of storage,
the tin content of canned food goods maintained at room temperature
increased from 27 mg/kg to 542 mg/kg - from 7 to 15 times higher than
when these products were canned. According to a study, vegetables grown
in Dhapa-Bamtala, Calcutta, contain hazardous metals. This region
produces one-fourth of the veggies sold in the city’s markets. Every
kilogramme of Dhapa-grown cauliflower has 44.1mg of lead and 3.3mg of
cadmium (Patra et al., 2001). These heavy metals are mostly found in
industrial sewage. Heavy metals are vital to Indian manufacturing. Soon
after independence, India entered the second part of the twentieth
century with a surge in the creation of heavy industries, which required
a huge number of heavy metals. Mining began on a vast scale, and mine
wastes, sewers, and spewing chimneys poured large amounts of metals into
river channels and the environment. Heavy metals are required in the
production process as catalysts or as additives. The Indian subcontinent
is rich in minerals, and practically every state has its own coal or
metal reserves with substantial mining. India is the world’s fifth
largest coal producer. Mining releases harmful heavy metals such as
chromium, cadmium, lead, and mercury. Some of the ”hot sites” of metal
contamination are Raniganj in West Bengal, Jharia in Bihar, and
Singrauli in Madhya Pradesh. Lead, zinc, nickel, chromium, copper, iron,
manganese, and tin are among the various metals extracted in India.
Cadmium, a strong carcinogen, accounts for approximately 20% of the
cadmium released from zinc mining and smelting processes (Anuj &
Banerjee 2012). Thermal power stations are another cause of heavy metal
contamination. India currently has 80 of these. A 2,000 megawatt (mw)
thermal power plant consumes eight million tonnes of coal each year and
produces 1,600 tonnes of lead, 800 tonnes of zinc, 80 tonnes of cadmium,
and 40 tonnes of uranium (Anbazhagan, 2018). Chlorine is used for a
variety of reasons in the Indian chemical sector, which primarily
produces fertilizers and pesticides. Gujarat and Maharashtra feature
some of the most densely packed chemical industry clusters in the
country (Mahanta, & Bhattacharyya 2011). Tanneries also emit
significant amounts of chromium into the environment. In India, there
are around 2,500 tanneries. According to, total wastewater discharge
ranged from 80,000 to 1,00,000 cubic metres per day (Dotaniya 2017).
70% of the entire amount of lead generated in the world is utilized in
the production of lead batteries. In storage batteries, cadmium and
nickel are also employed. Currently, a significant amount of discarded
batteries are purchased in India by tiny operators who are ill-equipped
to handle these pollutants (Ayyanar, & Thatikonda, 2020). The Indian
information and technology and computer hardware industries are growing
at a rate of 40% per year. Even after subtracting the heavy metal
composition of batteries, many other heavy metals, such as lead, tin,
copper, cadmium, and nickel, are employed in the IT industry
(Chabukdhara, & Nema, 2013). All of this lead will eventually end up in
the environment. It is not simply the environment that suffers. Every
day, its workers are exposed to hundreds of toxins, putting them prone
to diseases caused by long-term toxin exposure. Several studies have
found that the rate of miscarriage among female laborer working with
toxic materials in the United States, for example, is much higher than
normal rates (Rim, 2017).
Apart from the above sources, heavy metals occur naturally in the soil
environment as a result of pedogenetic processes of weathering parent
materials at trace (1000mg/kg) and hazardous levels. The potential paths
or sources of heavy metals are depicted below (Figure 1).