Heavy metals are generally referred to as minerals with a higher density, atomic mass, or atomic number. In different texts there are different categories of heavy metals. In materials engineering for example these metals are defined in terms of their density while in physics the definition is based on atomic numbers. Of course chemists like to classify based on chemical behaviour.
Definition of heavy metals
Various other definitions have been published in different articles but none of them are widely accepted and only metals like mercury, lead and bismuth are the same in all of these definitions. The definition used in most sources is that minerals with a density greater than 5g/cm3 are classified as heavy metals.
Metals known from the past such as iron, copper, tin, and precious metals such as silver, gold, and platinum are known as heavy metals. From 1809 until today light metals such as magnesium, aluminum and titanium have been discovered along with some other heavy metals such as gallium, thallium and hafnium.
Some of these metals such as iron, cobalt and zinc are essential nutrients while others such as silver, indium and ruthenium are harmless but large amounts are toxic.
The physical and chemical properties of heavy metals must be determined more carefully because the higher density of these metals reduces their reactivity and they have fewer sulfides and hydroxides than the light metals. Although heavy metals such as tungsten can be easily distinguished from lighter metals such as sodium, some metals such as zinc, mercury, and lead have properties similar to light metals, and lighter metals such as beryllium, scandium, and titanium have properties similar to heavy metals.
Heavy metals have few resources on Earth’s crust but are found in various aspects of life today. For example they are used in disinfectants, cars, plastics, solar panels and cell phones.
Origin and nomenclature of heavy metals
The malleability of metals such as gold, copper and iron has been known since ancient times and was a factor in making tools and weapons in the past due to its malleability. All minerals discovered till 1809 had a high density and this weight was evident among these minerals but from 1809 onwards light metals such as sodium, potassium and strontium were also discovered. Their low density impeded the general method of identifying minerals, and at that time these light metals were called “metalloids”. Of course the name was later given to the elements which are difficult to describe as metallic or non-metallic.
The term heavy metal was coined as early as 1817 when German chemist Leopold Gmelin divided the elements into non-metals, light metals, and heavy metals. The density of light metals was between 0.58-58 and the density of heavy metals was between 258.308-25 g/cm3. The term later became synonymous with elements with the same atomic mass and atomic number. This is why heavy elements are sometimes used in place of the term heavy metal. It is noteworthy that this term is used in scientific texts for the different meanings that bear heavy metals.
Heavy metals in the periodic table
As we mentioned there are several standards for how to detect these metals. To better understand this issue heavy metals are given in the periodic table based on the number of diagnostic criteria in the figure below. As you can see, the metals mercury, lead, and bismuth are the same in all of these definitions.
The biological role of heavy metals
Small amounts of heavy metals, especially in the fourth period of the periodic table, are essential for the performance of some biological processes in the body. These items are listed below:
- Copper: carries electrons and oxygen
- Cobalt: cellular metabolism
- Zinc: hydroxyl (hydroxyl)
- Chromium: glucose utilization
- Nickel: cell growth
- Selenium: Antioxidant and hormone production
The fourth and fifth periods of the periodic table contain smaller amounts of the essential heavy metals because they are heavier and less abundant in nature. In the fifth cycle, molybdenum is used as a catalyst for redox reactions. In the sixth cycle, cadmium participates in the metabolic processes of bacteria.
properties of heavy metals
One of the characteristics of heavy metals is their toxicity, which we will discuss below. Also, most heavy metals have a high density, atomic weight and atomic number. Some metalloids, intermediates, lanthanides, and actinides also fall into the heavy metal category. Many sources cite metals such as mercury, lead, bismuth, and cadmium as toxic elements in heavy metals. Examples of heavy metals include the following:
Toxic properties of heavy metals
Heavy metals are believed to be highly toxic and harmful to the environment. It should be noted that some of these metals are toxic and others are toxic if present in large quantities. For example, elements such as chromium, arsenic, mercury, cadmium, and lead are among the highly toxic. The elemental or combined form of these elements and their distribution in the environment would be toxic. For example, chromium with an oxidation number of +6 is as toxic as mercury vapor or mercury compounds. These five elements have a strong propensity for sulfur absorption and in the human body they bind to enzymes that control the rate of metabolic reactions through thiol (SH-) groups.
The resulting bond of sulfur and the metal prevents the optimal functioning of the enzymes and causes harm to human health and sometimes death. Carcinogenic chromium and cadmium also cause problems with the spine and joints. Lead also harms the central nervous system. In the following we will study the effects of these minerals with some of the other minerals in more detail.
Dangers of heavy metals
The danger of heavy metals is due to their “bioaccumulation” in the human body and other organisms. Bioaccumulation means that the concentration of a chemical in an organism increases over time compared to its concentration in the environment. This increase in concentration occurs when the compounds are absorbed and stored faster than the compounds are broken down (metabolically). Heavy metals can enter the domestic water supply system through industrial wastewater or enter soil, lakes, groundwater, and rivers due to acid rain. The three most polluting heavy metals are lead, cadmium, and mercury.
The impact of lead on the environment
Depending on the duration and extent of human exposure to lead, different biological effects occur that affect the fetus and infant more than adults. Large amounts of this substance cause problems with hemoglobin production, kidney and digestive system damage, joint damage, and the nervous system.
Lead in the environment comes from natural and human resources and people are exposed to it through food and climate. Lead was also used in ancient paints. Of course, food, water, and soil are among the main sources through which people can be exposed to lead.
The effect of cadmium on the environment
Cadmium is one of the heavy metals that if absorbed remains in the bodies of living organisms such as humans for years and even decades. If a person is exposed to cadmium for a long time, he will suffer from kidney problems. In large amounts, this substance can cause lung problems, cancer, and bone defects. In addition, high blood pressure in animals and heart problems can be considered among the effects of cadmium on the environment.
Cadmium is produced during zinc extraction and processing because the metal is present in the ore along with zinc and is one of the main uses in the production of batteries, nickel-cadmium alloys and electronic components.
Mercury’s effect on the environment
Mercury is a toxic substance with no known function in the human body and is rarely found in living organisms. Mercury poisoning can cause problems such as miscarriages, miscarriages, and birth defects. This substance is a pollutant that has complex chemical and physical properties. The main natural source of mercury is outgassing under the Earth’s crust and its release through volcanoes or evaporation of surface water. Intensive mineral exploration releases mercury into the Earth’s atmosphere.
Needless to say, the use of this substance is common in many industrial processes and different products such as batteries, thermometers, and lamps. In addition, this substance is used as an amalgam in dentistry.
Heavy metal application
Heavy metals are used in almost every aspect of life today. Of these, iron has the largest share of these minerals. About 20% of the consumer goods in human life are made of platinum. Some of the common applications of these metals are related to their general properties such as electrical conductivity and “reflectance”. The main properties of heavy metals such as density, toughness and high strength have also led to their increased use.
In addition to the above, the positive and negative biological properties of these minerals are also effective in their use. The electronic arrangement of the atoms of these metals is another factor in their use. For example, the “d” or “f” semiconductors in many intermediate metals allow the lanthanides and actinides to form color compounds. Several heavy metals including cerium, platinum, and bismuth have different oxidation numbers and are therefore used as catalysts. The weak overlap of 3d or 4ft orbitals in elements such as iron, cobalt, nickel, and some lanthanides and actinides leads to the magnetic properties of these elements. In general the use of heavy metals can be classified into the following six categories.
Application by weight or density
In sports, mechanical engineering, military industry and nuclear science, high densities of heavy metals are used. An example of such an application is golf, which uses metals such as tungsten and alloys of copper and copper to lower the center of mass of golf clubs. In the military industry, tungsten is also used to bond equipment.
An application based on durability and strength
The strength and durability of metals such as chromium, iron, nickel, copper, zinc, molybdenum, tin, tungsten, lead, and alloys of these metals have led to their use in the manufacture of sculptures, various devices, appliances, utensils, pipes, buildings, automobiles, and many industries.
For example, copper, zinc, tin, and lead are mechanically weak metals, but they are used to prevent metal corrosion.
The application depends on the biological and chemical role
The lethal properties of some heavy metals such as copper, platinum, osmium, ruthenium, and arsenic have led to their use in anti-cancer treatments. Other metals such as antimony, bismuth, gold and iron are also used in perch. In addition, copper, zinc, silver and gold are also used in the manufacture of antiseptics.
Small amounts of heavy metals are used to control algae growth in cooling towers. They also use the chemical role of heavy metals in automobile catalysts and in the refining process.
The application depends on the optical and coloristic properties
The color of glazes and glazes of ceramics as well as the colors of plastics and inks can be made with the help of heavy metals or their compounds. These metals include chromium, cobalt, copper, zinc, selenium, zirconium, molybdenum, iridium, gold, lead, tungsten, and uranium. The reflective properties of some metals are also used in the construction of mirrors and astronomical instruments.
The application depends on the electronic and magnetic properties
Heavy metals or their compounds can be found in a
For electronics, electrodes, coils and solar panels. For example, many electrical appliances in homes are wrapped in copper wire. Metals such as copper and silver have also been used in fuel cells in the past.
Application in nuclear properties
Some heavy, high-atomic metals are used in medical imaging, electron microscopy, and nuclear science. For example, metals such as tungsten or cobalt make up the anode portion of X-ray tubes, or metals such as lead, gold, palladium, platinum, and uranium are used to form conductive coatings in microscopes.