**2. Heavy metals and their impacts**

There are more than 32 metals in the periodic table. Heavy metals are metals with high density and toxic effects even at very low concentrations, with a density of 5 g/cm3 or higher [7]. These metals enter water, nutrients, soil and air ecosystems from the earth through natural processes and through anthropogenic activities [8]. These are considered major pollutants because of their toxic effects on living organisms and their prevalence in the world. Al, Fe, Cr, Sb, As, Be, Cd, Cu, Pb, Hg, Ni, Se, Ag and Zn are classified as primary contaminant metals by the United States Environmental Protection Agency (US EPA) [9]. In addition, the Agency for Toxic Substances and Disease Registry (ATSDR) has included heavy metals on the priority chemicals list due to their potential threat to human health and their toxicity [10].

### **2.1 Arsenic and arsenic compounds**

Inorganic arsenic (As) is a gray-colored element with a molecular weight of 74.92 g/mol found naturally in the earth's crust. However, inorganic arsenic often coexists with other elements such as oxygen, chlorine, and sulfur. Inorganic arsenic compounds include arsenic pentoxide, arsenic trioxide, and arsenic acid. The combination of carbon and hydrogen with arsenic creates organic arsenic. Organic arsenic compounds include arsanilic acid, arsenobetaine and dimethylarsinic acid. Pure arsenic is rarely found in nature and is not very toxic, but it can be oxidized easily in humid air and become toxic by conversion to arsenic anhydride. Arsenic, which has four different oxidation steps, is mostly in the form of +3 and + 5 valences in nature [1, 11, 12].

There are many uses of arsenic compounds in industry. Arsenic acid (AsO4H3), arsenic trichloride (AsCl3), arsenic triiodide (AsI3), arsenic anhydride (As2O5), copper arsenide ([Cu3 (AsO3) 2) and very toxic compounds such as calcium arsenide [Ca3 (AsO4)5], lead arsenide [Pb3AsO4) and sodium arsenate (Na2HAsO4), which are highly water soluble, are used for the production of paint (green, yellow), glass, ceramics, and semiconductors. Arsenic compounds can also be used as insecticides. Sodium arsenate (Na2HAsO4, 7H2O) is used to kill some parasites in viticulture, fruit cultivation and potato growing. Arsenic compounds have also been used as chemical weapons. Lewisite containing arsenic is a volatile liquid (chlorovinyldichloroarsine) and Adamsite (diphenylaminechlorarsine) is a solid

#### *Environmental Impacts of Heavy Metals and Their Bioremediation DOI: http://dx.doi.org/10.5772/intechopen.95103*

substance used in the form of aerosol. These substances have vesicant, irritant and sternutatory properties. Arsenious anhydride or arsenic trioxide (As2O3) is one of the most toxic compounds among arsenic compounds. It is a white powder, dissolves quickly in alkalis, acids and hot water, and contains arsenous acid (H3AsO3). Since it is volatile, it easily enters through the lungs, but there is a risk of loss during toxicological searches. As2O3 was used as a rat poison since medieval times. Today, arsenic compounds are used in limited amounts (10%) as rodenticide. Arsine (AsH3), or arsenous hydrogen, is a colorless gas with an unpleasant garlic odor. It is released as a result of contact between inorganic arsenic compounds with hydrogen or the hydrolysis of metallic (zinc, aluminum, etc.) arsenides with water or an acid, especially during washing of some metal parts with acids (H2SO4). Ethyl arsine is formed by the interaction of various fungi with arsenic compounds. The toxicity of arsenic compounds generally depends on their solubility in water: Pure metal arsenic and inorganic compounds (sulfides) are insoluble in water and theoretically lack toxic effects, though in a humid environment, they can be oxidized to toxic compounds (As2O3). Water soluble inorganic compounds (arsenous anhydride, arsenide, alkaline arsenates) are very toxic; and arsenic hydrogen or arsine (AsH3) has a different toxicity than other forms of inorganic arsenic as a hemolytic poison. There is a reaction with thiol groups. Mold and dampness increase the toxicity of arsenic compounds. Organic arsenic compounds have lower toxicity, they are eliminated faster from organisms [1, 13–15].

Arsenic compounds block the thiol (-SH) groups of many enzymes that play a role in cell respiration and different metabolisms in organisms. By inhibiting the oxidative decarboxylation of pyruvic acid, they disrupt the metabolism of carbohydrates, lipids and amino acids and form a true "biochemical lesion". Since the skin and keratinized tissues are rich in sulfur, the accumulation of arsenic compounds here is directly related to their attraction to thiol groups [1]. The International Agency for Research on Cancer (IRAC) has classified inorganic arsenic as a Group 1 carcinogenic to humans [16]. The Environmental Protection Agency (EPA), on the other hand, classified it as a Group A human carcinogen. The EPA calculated the cancer slope factor for inorganic arsenic oral exposure as 1.5 mg/kg/day. No cancer inhalation studies are available in humans or animals for arsine. The EPA has not classified arsine for carcinogenicity. The EPA set limits on the amount of arsenic that can be released into the environment from industrial sources. For this, the amount of arsenic permitted in drinking water was determined as 0.01 ppm [17].
