Heavy metals
Heavy metals are dense metallic elements that can pose serious health risks to living organisms when released into the environment through mining, industrial activities, and waste disposal. Key heavy metals include lead, mercury, cadmium, and thallium, which are mostly located at the bottom of the periodic table. While some of these metals are rare, lead is abundant and has historical significance; it was used in ancient times for food containers, contributing to lead poisoning. These metals are capable of interfering with metabolic functions, resulting in health issues such as impaired nervous system functioning, birth defects, and even death.
Heavy metals can mimic essential lighter metals in the body, leading to toxic substitutions in biochemical processes and the formation of defective metalloproteins. Chronic exposure to these elements can result in various health problems, including neurological damage, kidney issues, and anemia. Specific heavy metals have different sources and effects: for instance, cadmium is linked to lung and kidney damage, mercury can cause severe neurological disturbances, and thallium has long-lasting toxic effects due to its binding properties. Detoxification strategies do exist, such as the use of chelating agents, which help remove heavy metals from the body. Understanding the impact of heavy metals is crucial, given their persistent presence in the environment and potential for human exposure.
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Heavy metals
DEFINITION: Dense metallic chemical elements
Heavy metals released into the environment by mining, industry, and disposal of wastes can poison living organisms by interfering with metabolic functions. Such poisoning can cause impaired nervous system functioning, birth defects, and death.
The chemical elements lead, mercury, cadmium, and thallium are located together at the central bottom portion of the periodic table. Although mercury, thallium, and cadmium are fairly rare, lead is an abundant element on earth. All are dense, soft metals (mercury is a liquid at room temperature) that have a high affinity for chemically bonding with the element sulfur. They occur on or near the earth’s surface as sulfur-containing minerals (sulfides) that are insoluble in water. This lack of water-solubility kept the heavy metals isolated from life-forms as they were evolving prior to the onset of the technological era, at which point humans began to mine and purify these useful elements. Once the heavy metals and their ions became more abundant in the environment, human beings—particularly mine workers and workers in certain industries—were at risk for toxic exposure to elements their bodies were not well equipped to process.
The ions of heavy metals resemble beneficial lighter metals such as zinc, calcium, magnesium, and iron in terms of their diameters and charges. This enables heavy metals to substitute for the beneficial elements and thus reside in the body over time. These lighter metals are much more abundant in the environment, and so life evolved with them, even employing them in critical roles.
The strong ability of heavy metals to bond to sulfur is significant because body biochemicals known as metalloproteins are composed of lighter metals such as zinc bonded to sulfur. (Sulfur is a normal constituent element of proteins.) In cases of heavy metal intoxication, defective metalloproteins are biosynthesized, employing an incorrect toxic metal that is strongly bound to sulfur. Although the bonding ability and even the size and charge of the heavy metal are appropriate for fitting into the structure of the biochemical, the biochemical fails to function as required, leaving the organism deficient in some vital way.
The degree of poisoning depends on the level and duration of exposure to the toxic element. Organisms have limited detoxification defenses, including proteins called metallothioneins, which scavenge metal atoms by virtue of having a large number of sulfur atoms in their structures. Medical intervention is possible in certain cases; for example, chelating agents are drugs designed to scavenge metal atoms and make them easier to eliminate from the body in the urine.
Lead
Lead has been mined since antiquity and occurs in one form as the common mineral galena. Lead is a soft, easily worked metal, and it was often used to make food containers in ancient Roman times. Historians believe that acidic wine and other food leached lead from these vessels to cause lead poisoning, resulting in neurological damage that contributed to the decline of the Roman Empire. In modern times, the use of lead in paints and as an antiknock agent in gasoline helped to spread lead throughout the environment, although its use in these capacities has been largely phased out in the United States. Lead was also once used extensively in plumbing, and some older structures still have plumbing systems that deliver drinking water through lead pipes. Lead is currently used on a vast scale in batteries.
Lead in soil is absorbed by crops and works its way up the food chain. Lead dust from crumbling paint has been implicated in learning impairments and violent behavior in children. The nature of this neurotoxicity is not well understood. Other effects of chronic lead exposure include kidney damage and anemia, which results from lead’s interference with the production of hemoglobin, the iron-containing component of blood. Lead is able to displace calcium in bone, where it can remain in the body for years.
Cadmium, Mercury, and Thallium
Cadmium, a fairly rare metal, occurs in zinc-containing ores and is found in high concentrations near zinc smelters. It is used industrially in batteries and metal coatings. Airborne cadmium gets into soil, from which, like lead, it is absorbed into crops, including tobacco. Smokers consume about twice the cadmium each day that nonsmokers receive from the environment. Results of cadmium poisoning include lung and kidney damage and painful damage to the joints known in Japan (where cadmium pollution has been severe) as itai-itai (ouch-ouch) disease. A diet high in calcium can help to limit cadmium poisoning.
Mercury is used in industry in many ways. In the past, the improper disposal of mercury caused poisonings, such as what took place at Minamata Bay in Japan in the 1950s. Mercury use is being phased out where other compounds can safely be used, such as in pesticides. Many mercury compounds are volatile and easily absorbed by the body. Several can cross the blood-brain barrier; therefore, key symptoms of mercury poisoning involve vision and hearing disturbances, loss of coordination, and tremors. Mercury poisoning can also cause miscarriage and birth defects.
The effects of thallium poisoning are similar to those of poisoning involving the other heavy metals. Thallium, a by-product of zinc and lead production, can behave chemically similar in some ways to the vital element potassium. Unlike potassium, however, once it has been absorbed from the environment, it binds to sulfur, from which it is slowly released, effectively exposing the individual over long periods. Thallium was once used as a rat poison, but owing to its toxic nature such use is now forbidden in the United States.
Bibliography
Cox, P. A. The Elements on Earth: Inorganic Chemistry in the Environment. New York: Oxford UP, 1995. Print.
Elnabi, Manar K. Abd, et al. "Toxicity of Heavy Metals and Recent Advances in their Removal: A Review." Toxics, vol. 11, no. 7, 2023, p. 580, doi.org/10.3390/toxics11070580. Accessed 17 July 2024.
Kaim, Wolfgang, and Brigitte Schwederski. Bioinorganic Chemistry. 2md ed. Hoboken: Wiley, 2006. Print.
Mirsal, Ibrahim A. “Major Types of Soil Pollutants.” Soil Pollution: Origin, Monitoring, and Remediation. 2nd ed. New York: Springer, 2004. Print.
Silva, J. J. R. Fraústo da, and R. J. P. Williams. The Biological Chemistry of the Elements: The Inorganic Chemistry of Life. 2nd ed. New York: Oxford UP, 2001. Print.
Singh, Veer, et al. "Toxic Heavy Metal Ions Contamination in Water and Their Sustainable Reduction by Eco-Friendly Methods: Isotherms, Thermodynamics, and Kinetics Study." Scientific Reports, vol. 14, no. 7595, 31 Mar. 2024, doi.org/10.1038/s41598-024-58061-3. Accessed 17 July 2024.
Timbrell, John. “Environmental Pollutants.” Introduction to Toxicology. 3rd ed. New York: Taylor, 2002. Print.
Wang, Lawrence K., et al., eds. Heavy Metals in the Environment. Boca Raton: CRC, 2009. Print.