**2. Research review**

### **2.1 Toxic effect of some heavy metals**

Out of the 92 naturally occurring elements, 30 are recognized to be potentially harmful to humans. These are created by anthropogenic or natural processes, but the industrial discharge that is of concern in this case is of particular note [7]. Additionally, it is well recognized that the heavy metal pollution chain moves in a circular pattern from industry to atmosphere to soil to water to food to people. Heavy metals can be harmful even at relatively low levels, despite the fact that toxicity is a function of concentrations. The importance of human exposure, consumption, and absorption was emphasized, particularly in industrialized nations.

### **2.2 Chromium (Cr)**

One of the heavy metals whose concentration continually rises as a result of industrial expansion, particularly the growth of the chemical and tanning industries, is chromium. Electroplating, leather tanning, wood preservation, pulp processing, steel manufacture, and many other operations release chromium into the environment, and the concentrations of chromium and nickel in the environment vary greatly. The greater use of these two metals in emerging nations and their non-degradability raise serious concerns [1]. The human body is carcinogenic and highly soluble in hexavalent chromium. It is also well known that the metallurgies, refractory, chemical, and tannery sectors employ this same hexavalent chromium extensively.

### **2.3 Iron (Fe)**

A heavy metal in the first row of transition metals, iron is one of them. Although Fe2+ is also detected, Fe3+ is the main form that is seen. Iron serves as an oxidizing and reducing agent in the porphyrin enzyme of respiration (Vines and Rees).

*Environmental Impact of Heavy Metals DOI: http://dx.doi.org/10.5772/intechopen.108973*

Seawater contains roughly 3.5 ppm of iron, the fourth most prevalent element in the earth's crust [1]. It reacts fairly quickly. The transition metal (heavy metal) iron is by far the most common and significant one that has a purpose in living systems. Proteins that contain iron take part in the transport of oxygen and the transfer of electrons, respectively. There are other molecules, [4] whose job it is to transmit and store iron. Ferritin and albumin serve as the storage proteins in humans and many other higher animals.

### **2.4 Nickel (Ni)**

Nickel is the most useful element in soil and plant research. Nickel appears to be required for the growth of marine micro algae. The effect of food containing very low concentrations of nickel (e.g., 40.00 ug/g) includes impaired liver metabolism, decreased iron absorption, and decreased activity of many enzymes. The average concentration of nickel in the world's soil is 40.00 mg/g [7]. In the absence of the emission effect, dietary nickel intake was estimated to be 16,511 ug/day on average [8].

The toxicity of nickel is determined by the route of exposure and the solubility of the nickel compound. Epidemiology studies have shown that occupational inhalation exposure to nickel (Ni) dust can result in an increase in pulmonary and nasal cancer [9].

### **2.5 Zinc (Zn)**

Zinc is a heavy metal in the periodic table's first row of transition metals. Zinc is found everywhere and has been shown to be a growth factor in plants and some rodents. Its absence causes mottled leaf disease in fruit trees. Zinc is found in mammalian enzymes such as carbonic anhydrase. It is required for protein metabolism and appears to be involved in the production of chlorophyll in some way. Zinc is essential for plant growth due to its role in auxin formation and as a component of certain enzymes [9]. Zinc is required for the synthesis of the molecule tryptophan, from which auxin is produced.

Several crop disorders were reported in the early 1900s that have now been identified as zinc deficiencies [9]. A thorough investigation of zinc deficiency in all plants reveals, among other symptoms, some form of leaf chlorosis, mostly on veins and ranging in color from white to light green.

Zinc deficiency is common in soils with abnormally high levels of soluble or total phosphates. An early study on Tung's tree zinc deficiency in fluoride concluded that high phosphate in soils was an important factor reducing available zinc [10]. A relatively low concentration of the element in the body can cause heavy metal toxicity, most commonly intestinal distress.

### **3. Environmental pollution**

Through their bonds with sulfhydryl groups and the production of ROS, heavy metals cause toxicity in biological systems. In addition to oxidative stress and glutathione depletion, this results in the inactivation of important macromolecules. There are a number of events that take place once hazardous metals enter the body and are exposed to them, including interactions with or inhibitions of certain metabolic pathways [10, 11]. Multiple negative consequences on both people and animals are consequently seen. Congenital disorders, immune system problems, hormone changes,

particular organ dysfunctions, metabolic abnormalities, cancer, and congenital disorders are a few of these [11]. The presence of metals in the environment, food supply, and drinking water is therefore regulated by a number of international organizations. Studies on risk assessment examine if heavy metals are present in food and water. Nearly 21% of them had amounts that could be detected, it was discovered.

### **4. Solubility of metals and metal compounds**

Chemical speciation affects the environment's metal and metal compound solubility, bioavailability, and persistence; for some metals, speciation may affect the pattern of toxicity (e.g., inorganic arsenic versus organic compounds, inorganic and organic mercury compounds). The papers on exposure concerns and bioavailability and bioaccumulation explore the function of speciation in bioavailability and bioaccumulation within the environment as well as bio-accessibility to human receptors. It is typically believed that the potential toxicity of inorganic species is connected to the cat-existence ions in bodily tissues (in most cases, bound to a tissue ligand). The potential or availability of the metal for interacting at a particular biological target, such as may depend on the intracellular environment and kind of ligand or protein binding [2].

Solubility is one of the most important factors influencing metal and metal compound bioavailability and absorption. A metal compound's solubility is determined by its chemical species, the pH of its medium (H+ ions), and the presence of other chemical species in the medium (see the environmental chemistry paper) [12]. Except for silver, mercury, and lead, nitrates, acetates, and all chlorides of most metals are soluble. Except for barium and lead, most metal sulphates are also soluble. Most hydroxides, carbonates, oxalates, phosphates, and sulphones, on the other hand, are poorly soluble. Particle size is another factor that influences the absorption of poorly soluble compounds: fine particles are usually more soluble. Metallic lead in body tissues (as may occur after gunshot wounds) is most likely absorbed [2].

### **5. Measures of exposure to metals**

In terms of health assessment, the extent of a metal's exposure is best determined by measuring its internal concentration, and even better, the biologically effective dose at the target organ (as opposed to environmental concentration). However, for a variety of reasons, determining the internal or biologically effective dose of the metal at the target tissue is not always feasible. For example, the activity of the heme-synthesizing enzyme aminolaevulinic acid dehydrate (ALAD) in red blood cells is directly related to blood lead concentration and thus may be used as a surrogate for blood lead measurement. The use of biological indicators or markers of exposure, also known as "biomarkers of exposure, " is a method of linking a person's external exposure [13].

### **5.1 Material**

Polyethylene (plastic) bottles, 14 volumetric flasks (100.00 ml), glass funnel, filter paper, 14 beakers (500.00 ml), hot plate pipette, measuring cylinder, hydrochloric acid (HCl), nitric acid (HNO3), Atomic Absorption Spectroscopy (AAS), Conductivity Meter, Turbidity Meter, and pH Meter were used for the analysis.

### **5.2 Methods**

To avoid the risk, the sample was prepared and digested using standard analytical methods with nitric acid (HNO3) and hydrochloric acid (HCl) at relatively low temperatures, as reported by [1].

The powerful solvent used was aqua regia, a mixture of hydrochloric acid and nitric acid (10:1 V/V). 100.00 ml of each sample was measured and transferred to a 500.00 ml beaker, followed by 10.00 ml of hydrochloric acid (HCl) and 1.00 ml of nitric acid (HNO3). The mixture was then heated on a hot plate for about 3 hours at a relatively low temperature (200C-500C) (NB: Do not allow it to boil) until it was reduced to about 20.00 ml.

The mixtures were then cooled and filtered in a 100.00 ml volumetric flask using a glass funnel and filter paper, and then diluted to volume (i.e., distilled water was added to the mark of the volumetric flask).At the quality control laboratory, the mixtures were tested using Atomic Absorption Spectroscopy (AAS). Based on the above procedure, we reanalyzed the five different samples for the following heavy metals: chromium (Cr), iron (Fe), nickel (Ni), and zinc (Zn).
