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compounds (Hashemi et al., 2018; Barak et al., 2020). Thus, there is a need to establish the management of and industrial wastewaters based on strict environmental protocols, with the objective of eliminating these contaminants from the environment as much as possible. As a result, through the use of recent technologies, research has been carried out to provide intelligent solutions not only from an environmental point of view but also from an economic standpoint. Therefore, several technologies based on physical, chemical, and biological methods are investigated (Hashemi et al., 2018).

This chapter describes some of the contaminants present in industrial wastewaters, their environmental impacts, and their risks to human health. In addition, the efforts made to evaluate and define the best ways to control their dissemination in the environment are presented, as well as the treatment processes used and studied for eliminating contaminants.

Toxic Heavy Metals

      Heavy metals are a classic group of metallic elements that cannot be overlooked as industrial wastewater contaminants due to their toxicity. According to the World Health Organization (WHO), some of these pollutants (e.g. arsenic, lead, mercury, and cadmium) are included in the top 10 issues of primary concern (air pollution, arsenic, asbestos, benzene, cadmium, dioxin and dioxin‐like substances, inadequate or excess fluoride, lead, mercury, and highly hazardous pesticides) because they are widely spread worldwide, they are highly hazardous, and negatively affect human health and natural ecosystems (WHO, 2020). Moreover, such substances are significantly dense and persistent and have shown toxicity even at part per billion (ppb) levels (Yadav et al., 2019). Compared to other contaminants, heavy metals can be neither degraded nor destroyed and can easily accumulate along the food chain. This phenomenon, known as bioaccumulation, increases the concern about suitable methods to treat industrial wastewaters.

      Heavy metals are natural constituents of the Earth's crust, and heavy metal contamination in the environment results from natural and anthropogenic sources. Human activities are the leading causes of pollution. First, these metals are found in the form of chemical compounds or elemental state in rocks as ore deposits, which are explored through mining techniques. Environmental contamination by these compounds is not only a result of the business of mining – the largest contributor to heavy metal pollution – but may also be caused by improperly treated industrial wastewaters that reach natural ecosystems. The most common heavy metals found in industrial wastewaters are lead, cadmium, mercury, arsenic, copper, zinc, nickel, and chromium, which come from the paint, mining, metal manufacturing (electroplating, smelting, finishing, etc.), and textile industries (Akpor et al., 2014). From natural sources, the main contributions come from volcanic activity, soil erosion, and run‐off. These hazardous waters deteriorate aquatic systems and terrestrial environments and retard biota growth (i.e., seed germination, chlorophyll production, and enzyme activity), with reported cases of carcinogenic, teratogenic, mutagenic, and neurotoxic effects.

      There are multiple criteria to define an element as a heavy metal; however, one of the most frequently adopted classifications is based on the element's density. Some authors define heavy metals as metals with a density greater than 4 g/cm³, while others say that the density needs to be greater than 7 g/cm³, or five times denser than water. Clearly, there is no consistency in the literature (Duffus, 2002).

Regardless of the definition, the term heavy metals is considered inadequate and obsolete, since the majority of the population misuse and misunderstand it and the term sometimes leads to confused policies and regulations. In this chapter and most discussions, such as the WHO's list, when people refer to heavy metals, they relate heaviness to toxicity and are actually referring exclusively to toxic metallic elements. However, in appropriate concentrations, some of these elements are vital in our daily diets, such as calcium, iron, magnesium, and zinc. Their roles are very diverse. Calcium is the primary constituent of human bones and teeth, providing structure and hardness, but it also is necessary for muscles, metabolism, and nerve responses, together with magnesium and zinc. While iron and magnesium are essential for blood production, especially hemoglobin and myoglobin, zinc supports the human immune system and male reproductive activity, and magnesium also aids in muscle recovery and sleep. Signs of illness (diarrhea, anemia, liver and kidney failure, bloody urine, vomiting) have been reported when inadequate zinc concentrations are present, which can lead to a misdiagnosis of lead poisoning (Duruibe et al., 2007). Since the main concern is not the vital concentrations of heavy metals in humans, it was decided to name this section of the chapter “Toxic Heavy Metals” to avoid any room for misinterpretation.

      That being said, toxic heavy metals are a subgroup with harmful effects leading to poisoning and bio‐intoxication. Some of them are dangerous at extremely low levels. For instance, the general effects of cadmium exposure in humans have been reported as renal and gastrointestinal symptoms since the body tries metabolize and eliminate the substance. However, fumes and dust can cause high levels of cadmium exposure, which compromises the respiratory tract and causes coughing and chest pain because the lungs may fill with watery fluids; the result is a high risk of death. On the other hand, long‐term, low‐dose exposure can be associated with osteoporosis, myocardial dysfunctions, proteinuria, and higher blood pressure (Hallenbeck, 1986; Duruibe et al., 2007). This metal is commonly associated with high‐tech and power‐generation components, such as batteries, alloys, solar cells, pigments, and neutron absorbers in nuclear reactors. It is also used in the automotive industry due to its corrosion resistance. Since it inhibits organism growth, like many heavy metals, it is found in phosphate fertilizers as well (Sharma et al., 2015).

      Arsenic is a harmful metalloid widely distributed around the world through natural and human activities. In reported long‐term, low‐dose exposure, victims suffer from skin lesions, neuropathy, diabetes, cardiovascular diseases, and cancer of the skin and internal organs (WHO 2019). Arsenic can be found in drinking water, industrial processes, food, and various tobacco products. In the industrial sector, wastewaters from pesticide, glass, and pharmaceutical manufacturing may contain the metalloid, but microelectronics and optical industries also handle arsenic compounds. Arsenic has been historically used in the agricultural sector as a pesticide, an insecticide, a defoliant, and a desiccant. However, due to its high biotoxicity, the United States Environmental Protection Agency (EPA) has banned its application in crops. Furthermore, elemental arsenic is used in alloys with copper and lead, while gallium arsenide and arsine have high electron mobility and are used in superconductors, magnets, light‐emitting and chips, and many other applications in the electronic industries.

On the other hand, lead is not a good electric conductor. It is used in construction, ammunition, metallic alloys, cable sheaths, lead‐acid batteries, pigments, fuses, and X‐ray and radiation protection. Lead‐contaminated potable water causes various undesirable effects, such as anemia, kidney and brain damage, infertility, miscarriage, and changes in behavior (hypersensibility, impulsivity, and aggressiveness). Despite oral exposure being the most common, anal, dermal, respiratory, and genital routes are also possible and result in the same kind of effects. This metal is well known for its chronic toxicity, easily damaging the central nervous system even at very low doses. In acute poisoning, the person shows signs of muscle weakness, pain, diarrhea, constipation, weight loss, bloody urine, etc. In industrial wastewaters, lead concentration ranges from 200 to 500 mg/L. According to WHO and EPA standards, its levels must be critically reduced to achieve 0.05–0.10 mg/L before discharge (Arbabi et al., 2015). In order to mitigate lead pollution, several methods for industrial wastewater treatment have been adopted, such as precipitation, adsorption, ion exchange, reverse osmosis, coagulation, and electro‐coagulation (Arbabi et al., 2015).

      Mercury, the only toxic metal present in liquid form at ambient temperature, is found in all living things and plays a crucial biological role. Nevertheless, it must remain at low levels, otherwise leads to toxic effects. The usual symptoms for acute exposure are fever, diarrhea, and vomiting, while chronic exposure causes tremors, nausea, stomatitis, pink disease, parageusia, and kidney malfunctions. Historically, gold amalgam (mercury alloy) was commonly used for gilding in architecture and fine arts. For example, in Saint Petersburg, Russia, at least 60 workers died from gilding the main dome of Saint Isaac's Cathedral (Emsley, 2011). Nowadays, industrial waters containing mercury

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