**2. General characteristics of heavy metals**

The term "heavy metals" refers to all metals except Al, Na, Ca, Mg, and K, i.e., to all metals that have a density higher than 5 g/cm3 . It includes a number of physiologically important elements such as Fe, Cu, Zn and Mn, then highly toxic Pb, As, Hg, Cd, Sb, Cr(VI) and less toxic Au, Ag, Mo, Cr(III) and Co [3]. The physiological and toxicological effects of these elements represent a collection of very different mechanisms.

Even at very low concentrations, they pose a threat to the environment and human health, because they are not biodegradable, so heavy metals are the cause of one of the most serious pollution problems. The most important nonessential heavy metals which affect the surface water systems are cadmium, chromium, mercury, lead, arsenic, and antimony [4].

Heavy metals present in pesticides and therapeutic agents are additional pollution sources. Burning of fossil fuels containing heavy metals and increasing industrial applications of metals such as metal galvanizing, paint and varnish industry, and mining and chemical industries are the main source of pollution of water systems by heavy metals.

Heavy metals are transported with waste water at the place of discharge and contaminate water sources downstream from an industrial site. In water, heavy metals have the ability to bind to the surface of microorganisms, from where they are transported inside the cell where they can be involved in chemical reactions and change chemically.

The majority of known techniques can determine the total amount of heavy metal ions. In addition, laboratory techniques that are routinely used for the analysis of metal ions, such as atomic absorption spectrometry, inductively coupled plasma mass spectrometry, anodic stripping voltammetry, and X-ray fluorescence spectrometry, require sophisticated equipment, pretreatment of samples, or qualified operators.

However, today it is known that only certain oxidation states of biologically available metal ions pose the greatest risk to human health and the environment. For example, "Cr(III) is an essential nutrient required in insulin action and sugar and fat metabolism, while Cr(VI) is believed to be highly toxic and carcinogenic" [5].

#### **2.1 Mechanism of heavy metal toxicity**

Metals and metalloid ions can be divided into three groups according to their toxicity. The first group includes metals (metalloids) that are toxic at extremely low concentration, such as lead, cadmium, and mercury. "Metals of the second group (arsenic, bismuth, indium, antimony and thallium) are less toxic, i.e., they are toxic only in higher concentrations. The third group includes metals (metalloids) of essential importance, such as copper, zinc, cobalt, selenium and iron, which are necessary for different chemical and biochemical processes in the body, and are toxic only above a certain concentration." Concentration window "of these heavy metals is somewhere between toxic and maximum permissible limits" [6].

**Table 1** gives critical concentrations of some heavy metals in natural waters according to EPA [7].

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and DNA splitting [9].

**Table 1.**

rivers, lakes, and oceans.

food and water with metals.

*2.1.1 Essential metals*

*2.1.1.1 Iron*

The toxic effects of heavy metals can be the result of changes in numerous physiological processes at the cellular or molecular level caused by the inactivation of the enzyme. It can also occur as a result of the blocking of functional groups of metabolically important molecules or by replacing the essential elements and disturbing the integrity of the membrane. A rather frequent consequence of heavy metal poisoning is the production of reactive oxygen species (ROS) due to interference with the transport activities of electrons, especially the chloroplast membrane [8]. This increase in ROS exposes cells to oxidative stress that leads to peroxidation of lipids, biological damage of macromolecules, membrane decay,

**Metal Max. allowable concentration (μg/ml)**

Mercury 0.002 Arsenic 0.5 Lead 0.5 Copper 0.6 Cadmium 0.04 Zinc 5

*Critical concentrations of some heavy metals in natural waters according to EPA*

They can penetrate into the organism in elemental form, in salt form, or as organometallic compounds, wherein the process of absorption, distribution, deposition, and elimination depends on the form in which the metal is present. Metals are very toxic because they are either in ionic form or within the compound, soluble

The mobility of heavy metals in water is particularly affected by the pH of water, the presence of hydrated forms of Mn and Fe, the concentration of carbonates and phosphates, as well as the content of organic matter. In addition, if the medium is very acidic and increased redox potential, the mobilization of Cu and Pb occurs, and under the reduction conditions, the hydroxides Mn and Fe are mobilized.

Heavy metals which are mostly the subject of research and monitoring in water and also generally in the environment due to their pronounced toxicity are arsenic, chromium, lead, mercury, and cadmium, while zinc, cobalt, copper, iron, and manganese are also interesting because they belong to the group of essential elements. The level of toxicity for some of these heavy metals is at or slightly above the concentration in which they are naturally found in nature [10]. Heavy metals occur in the environment naturally or as a result of human activities. Natural sources include volcanic eruptions, weathering (acid rock drainage), and discharge into

Anthropogenic sources of heavy metals have emerged with the development of society. For example, the release of metal from the dishes causes contamination of

Iron belongs to a group of essential metals and is crucial for a number of synthetic and enzyme processes in the human body. Most of the iron in our body exists as part of the hemoglobin molecule or myoglobin molecule. In addition to the

in water, and easily absorbed by living organisms [3].

*Biosensors for Determination of Heavy Metals in Waters DOI: http://dx.doi.org/10.5772/intechopen.84139*


**Table 1.**

*Biosensors for Environmental Monitoring*

chemical composition information.

lead, arsenic, and antimony [4].

systems by heavy metals.

change chemically.

fied operators.

carcinogenic" [5].

according to EPA [7].

**2.1 Mechanism of heavy metal toxicity**

mechanisms.

**2. General characteristics of heavy metals**

to all metals that have a density higher than 5 g/cm3

available concentrations of heavy metals in water but also assessing their biological effects, such as toxicity or cytotoxicity, which are sometimes more important than

The term "heavy metals" refers to all metals except Al, Na, Ca, Mg, and K, i.e.,

ologically important elements such as Fe, Cu, Zn and Mn, then highly toxic Pb, As, Hg, Cd, Sb, Cr(VI) and less toxic Au, Ag, Mo, Cr(III) and Co [3]. The physiological and toxicological effects of these elements represent a collection of very different

Even at very low concentrations, they pose a threat to the environment and human health, because they are not biodegradable, so heavy metals are the cause of one of the most serious pollution problems. The most important nonessential heavy metals which affect the surface water systems are cadmium, chromium, mercury,

Heavy metals present in pesticides and therapeutic agents are additional pollution sources. Burning of fossil fuels containing heavy metals and increasing industrial applications of metals such as metal galvanizing, paint and varnish industry, and mining and chemical industries are the main source of pollution of water

Heavy metals are transported with waste water at the place of discharge and contaminate water sources downstream from an industrial site. In water, heavy metals have the ability to bind to the surface of microorganisms, from where they are transported inside the cell where they can be involved in chemical reactions and

The majority of known techniques can determine the total amount of heavy metal ions. In addition, laboratory techniques that are routinely used for the

analysis of metal ions, such as atomic absorption spectrometry, inductively coupled plasma mass spectrometry, anodic stripping voltammetry, and X-ray fluorescence spectrometry, require sophisticated equipment, pretreatment of samples, or quali-

However, today it is known that only certain oxidation states of biologically available metal ions pose the greatest risk to human health and the environment. For example, "Cr(III) is an essential nutrient required in insulin action and sugar and fat metabolism, while Cr(VI) is believed to be highly toxic and

Metals and metalloid ions can be divided into three groups according to their toxicity. The first group includes metals (metalloids) that are toxic at extremely low concentration, such as lead, cadmium, and mercury. "Metals of the second group (arsenic, bismuth, indium, antimony and thallium) are less toxic, i.e., they are toxic only in higher concentrations. The third group includes metals (metalloids) of essential importance, such as copper, zinc, cobalt, selenium and iron, which are necessary for different chemical and biochemical processes in the body, and are toxic only above a certain concentration." Concentration window "of these heavy metals is somewhere between toxic and maximum permissible limits" [6].

**Table 1** gives critical concentrations of some heavy metals in natural waters

. It includes a number of physi-

**140**

*Critical concentrations of some heavy metals in natural waters according to EPA*

The toxic effects of heavy metals can be the result of changes in numerous physiological processes at the cellular or molecular level caused by the inactivation of the enzyme. It can also occur as a result of the blocking of functional groups of metabolically important molecules or by replacing the essential elements and disturbing the integrity of the membrane. A rather frequent consequence of heavy metal poisoning is the production of reactive oxygen species (ROS) due to interference with the transport activities of electrons, especially the chloroplast membrane [8]. This increase in ROS exposes cells to oxidative stress that leads to peroxidation of lipids, biological damage of macromolecules, membrane decay, and DNA splitting [9].

They can penetrate into the organism in elemental form, in salt form, or as organometallic compounds, wherein the process of absorption, distribution, deposition, and elimination depends on the form in which the metal is present. Metals are very toxic because they are either in ionic form or within the compound, soluble in water, and easily absorbed by living organisms [3].

The mobility of heavy metals in water is particularly affected by the pH of water, the presence of hydrated forms of Mn and Fe, the concentration of carbonates and phosphates, as well as the content of organic matter. In addition, if the medium is very acidic and increased redox potential, the mobilization of Cu and Pb occurs, and under the reduction conditions, the hydroxides Mn and Fe are mobilized.

Heavy metals which are mostly the subject of research and monitoring in water and also generally in the environment due to their pronounced toxicity are arsenic, chromium, lead, mercury, and cadmium, while zinc, cobalt, copper, iron, and manganese are also interesting because they belong to the group of essential elements. The level of toxicity for some of these heavy metals is at or slightly above the concentration in which they are naturally found in nature [10]. Heavy metals occur in the environment naturally or as a result of human activities. Natural sources include volcanic eruptions, weathering (acid rock drainage), and discharge into rivers, lakes, and oceans.

Anthropogenic sources of heavy metals have emerged with the development of society. For example, the release of metal from the dishes causes contamination of food and water with metals.

#### *2.1.1 Essential metals*

#### *2.1.1.1 Iron*

Iron belongs to a group of essential metals and is crucial for a number of synthetic and enzyme processes in the human body. Most of the iron in our body exists as part of the hemoglobin molecule or myoglobin molecule. In addition to the vital importance it has for most living organisms, iron is potentially toxic at high concentrations. The effect of iron on aquatic organisms and their habitats is mostly indirect. Combined direct and indirect effects of contamination of the aquatic environment cause a decrease in biodiversity and number of fish. In aqueous solutions, the Fe3+ ion is in the form of the aqua complex, Fe(H2O)6 3+, which is quite hydrolyzed (hydrolysis starts at pH 1). Hydrolysis of Fe(III) ions depends on the type of ionic environment, temperature, and the presence of other substances. The results of the researches show that the most important chemical types are found in hydrolyzed solution.

#### *2.1.1.2 Copper*

Copper is a microelement of outstanding biological importance and is part of essential metabolic pathways. Copper ions play a key role in active centers of oxidoreductases, such as superoxide dismutase (Cu, Zn-SOD), [5], an enzyme important for maintaining a low level of free radicals in the cell, thus protecting biomolecules such as proteins and lipids from the pathological conditions.

Copper deficiency can cause anemia, because insufficient amount of copper causes poor absorption of iron, reducing the number of red blood cells. The lack of copper also reduces the amount of white blood cells and therefore the resistance of the organism to diseases. In general, copper is not considered to be a major ecotoxicological problem, but its widespread distribution and exposure to exhaust gases are certainly the reasons why copper is involved in the structuring of ecosystems. Copper is found in three oxidation states, Cu+ , Cu2+, and Cu3+, with the Cu2+ form being the most common. The most mobile forms of copper are Cu2+ and CuOH+ . In the aqueous environment, copper is found in three basic forms, as suspended, colloidal, and dissolved. The accumulation of copper in the aquatic environment results in the primary exposure of aquatic organisms. Aquatic organisms can accumulate dissolved copper by direct absorption through the body surface, while colloidal forms of this metal are introduced into the body by ingesting contaminated food.

#### *2.1.1.3 Zinc*

Zinc participates in the structure of many enzymes and is an essential element. It is attached to insulin and plays a significant role in the metabolism of nucleic acids and amino acids, DNA replication, and gene expression. However, like all other essential metals, zinc in higher concentrations is toxic to living organisms. Zinc can bioaccumulate in fish, and the degree of bioaccumulation usually depends on the exposure mode, as well as the conditions prevailing in the observed aquatic environment. Conditions that may affect the toxicity of zinc (but also other heavy metals) in the aquatic environment are the content of Ca and Mg, the pH of water, the content of the hydroxide (alkalinity), and the content of dissolved natural organic matter, i.e., humic substances.

#### *2.1.1.4 Cobalt*

The required amount of cobalt in the body is about 5 mg for vitamin B12 to avoid anemia. In general, cobalt has low toxicity. Gastrointestinal (digestive tract) absorption of soluble cobalt compounds is estimated to be 25%. However, cobalt is toxic to humans. When cobalt has been used as an additive in beer (for foam stabilization), severe biventricular heart failure and a high mortality rate were observed in heavy beer drinkers [11].

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*Biosensors for Determination of Heavy Metals in Waters DOI: http://dx.doi.org/10.5772/intechopen.84139*

ment of anemia that cannot be treated with iron.

*2.1.2 Toxic metals*

and large cities.

*2.1.2.2 Mercury*

*2.1.2.3 Chromium*

dichromate (Cr2O7)

be carcinogenic [13].

*2.1.2.1 Lead*

Long-term inhalation of cobalt dust irritates the respiratory tract and can cause chronic bronchitis, and cobalt salts can cause benign dermatosis. Cobalt occurs in oxidation states 0, +1, +2, +3, and +4, and most of its compounds have an oxidation number +2 and +3, of which the cobalt(II) compounds are more stable. Most cobalt(II) compounds have an ionic character (halides and numerous Co(III) complexes). Cobalt is relatively a nonreactive metal. It does not oxidize under dry and humid conditions at normal temperatures. It binds to halogen elements by heating. Cobalt is used in the production of artificial fertilizers and so can be found in higher concentrations in soil and water. It is also used in medicine, in the treat-

Lead in the environment mainly comes from anthropogenic sources such as combustion of fossil fuels, landfills and fires at landfills, waste industrial sludges,

It is found in the form of sulphates, sulphides, and carbonates. It is considered the leading environmental pollutant and is increasingly endangering the living world, especially the surrounding areas of large industrial plants, frequent roads,

The intensity of the adoption of lead depends on its concentrations in soil, soil pH, organic matter content, ratio of cations and anions, and other environmental factors. Human is exposed to toxic effects of lead by consuming food and water that are contaminated with this heavy metal but also by inhaling particulate matter with lead content. Absorption over the skin is only possible for tetraethyl and tetramethyl lead. Lead is rapidly absorbed into the bloodstream and binds to red blood cells in the form of Pb2+, and via blood about 90% is deposited in the bones in the form of Pb3(PO4)2. In the case of acidosis (increased acidity), the mobility of lead from the bones in the form of Pb2+ which has a toxic effect on the central nervous,

Mercury vapors and organic compounds of mercury are very strong poisons. Harmful substances are released by combustion of fossil fuels, and the risk of pollution threatens also due to increased use of mercury in industry and agriculture [12].

In its compounds, chromium exists in several oxidation states: from bivalent to hexavalent. In solutions, chromium can occur in trivalent and hexavalent forms. Hexavalent chromium is usually present in the compounds as chromate (CrO4)

easily enters the biological membranes. Therefore, this form of chromium is considered carcinogenic. Because chromium(VI) is toxic, carcinogenic, and mutagenic to living organisms, damages the liver, and causes lung congestion, skin irritation, and the formation of ulcer, it needs to be removed from the wastewater before their release into natural recipients. On the other hand, trivalent chromium, Cr(III), is 300 times less toxic than chromium(VI). Chromium is a vital nutrient for many animal and plant species, but it can also cause allergic reactions on the skin and can

<sup>2</sup><sup>−</sup> ion. Cr(VI) is toxic due to its high degree of oxidation and

<sup>2</sup><sup>−</sup> or

phosphate-based fertilizers, pesticides, and exhaust gases from vehicles.

circulatory, and immunological systems and kidneys can occur. [10]

#### *Biosensors for Determination of Heavy Metals in Waters DOI: http://dx.doi.org/10.5772/intechopen.84139*

Long-term inhalation of cobalt dust irritates the respiratory tract and can cause chronic bronchitis, and cobalt salts can cause benign dermatosis. Cobalt occurs in oxidation states 0, +1, +2, +3, and +4, and most of its compounds have an oxidation number +2 and +3, of which the cobalt(II) compounds are more stable. Most cobalt(II) compounds have an ionic character (halides and numerous Co(III) complexes). Cobalt is relatively a nonreactive metal. It does not oxidize under dry and humid conditions at normal temperatures. It binds to halogen elements by heating. Cobalt is used in the production of artificial fertilizers and so can be found in higher concentrations in soil and water. It is also used in medicine, in the treatment of anemia that cannot be treated with iron.
