## 1 Introduction  
Heavy metals are naturally occurring elements that have a high atomic weight and a density at least five times greater than that of water. Their multiple industrial, domestic, agricultural, medical and technological applications have led to their wide distribution in the environment, raising concerns over their potential effects on human health, as well as the environment itself. Their toxicity depends on several factors, including the dose, route of exposure, and chemical species, as well as the age, gender, genetics, and nutritional status of exposed individuals. Heavy metals, metal compounds and various minerals, which constitute an important portion of pollutants, are widely spread in lakes, rivers, oceans and their sediments. Heavy metals spread in waters as a result of natural and anthropogenic influences [1]. In addition to their toxic, acute, chronic and direct effects, heavy metals have indirect physiological effects on aquatic organisms, particularly on the eggs and young individuals of these organisms, on whom these effects are more severe. Heavy-metal pollution is rapidly increasing, which presents many environmental problems. These heavy metals are mainly accumulated in soil and are transferred to the food chain through plant grown on these soils [2; 3; 4]. Chromium (Cr), manganese (Mn), cobalt (Co), copper (Cu), zinc (Zn), molybdenum (Mo), mercury (Hg), nickel (Ni), tin (Sn), lead (Pb), cadmium (Cd), antimony (Sb), etc., are the main elements of heavy metals. Three kinds of heavy metal are of concern: toxic metals (such as, Hg, Cr, Pb, Zn, Cu, Ni, Cd, As, Co, Sn, etc. ), precious metals (such as Pd, Pt, Ag, Au, Ru, etc.) and radionuclides, such as uranium (U), thorium (Th), radium (Ra), americium (Am). Heavy metals are natural components of the Earth's crust. They cannot be destroyed or degraded. However, most of these heavy metals become toxic at high concentrations due to their ability to accumulate in living tissues. The removal of heavy metals from industrial wastewater is of primary importance. Cadmium, zinc, copper, nickel, lead, mercury, vanadium and chromium are often detected in industrial wastewaters [1; 5; 6; 7]. Iron is a functional part of hemoglobin found in the structure of red blood cells in humans. It is an element of vital importance and it is also found in the structure of cytochrome peroxidase, catalase enzymes and myoglobin. Surplus iron accumulates in the bone marrow, liver and spleen [8]. While there is a control mechanism in the human body for iron absorption, there is no control mechanism for the removal of iron from the body. When taken into the body in excessive amounts, iron may cause damage to the digestive system and enter the circulatory system, through which it may reach and cause damage to organs such as the liver and heart. Organs are damaged due to prolonged exposure to iron, which may result in death. Iron, found in colloidal form in water environments, may cause death in fish by densely accumulating in their gills. The iron accumulated in bodies of the fish is transferred to the human body as a result of the consumption of fish as food and may negatively affect human health [9; 10]. Iron toxicity is classified as corrosive or cellular. Ingested iron can cause direct caustic injury to the gastrointestinal mucosa, resulting in nausea, vomiting, abdominal pain, and diarrhea. Significant fluid and blood loss can lead to hypovolemia. Hemorrhagic necrosis of the gastrointestinal mucosa can lead to hematemesis, perforation and peritonitis. At the cellular level, iron impairs cellular metabolism in the heart, liver and central nervous system. Free iron enters cells and concentrates in the mitochondria. This disrupts oxidative phosphorylation, catalyzes lipid peroxidation, forms free radicals and ultimately leads to cell death [11]. Insufficient zinc intake negatively affects more than 300 enzymes, whereas excessive amounts may cause various forms of damage to organisms. Zinc chloride (ZnCl\({}_{2}\)) toxicity has been reported to cause acute renal failure.