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).