Heavy metal refers to any metallic chemical element that has a relatively high density and is toxic or poisonous at low concentrations. Also, they cannot be degraded or destroyed. Heavy metals are naturally occurring elements and are present in varying concentrations in all ecosystems. There is a huge number of heavy metals. They are found in elemental form and a variety of other chemical compounds. Those that are volatile and those that become attached to fine particles can be widely transported on very large scales. Each form or compound has different properties which also affect what happens to it in the food web, and how toxic it is. Human activities have drastically changed the biochemical cycles and balance of some heavy metals.
Recently, different heavy metals act as the central atom of artificially designed “bioinorganic” catalysts for special chemical transformations. Moreover, the researcher finds precious noble elements like gold, silver, iridium, rhodium, or platinum. On the other hand, many of them, e.g., mercury, cadmium, arsenic, chromium, thallium, lead, and others, classically represent the “dark side of chemistry”; they exert toxic effects already at low concentration.
Heavy Metals/metalloids include lead (Pb), cadmium (Cd), mercury (Hg), arsenic (As), chromium (Cr), copper (Cu), selenium (Se), nickel (Ni), silver (Ag), and zinc (Zn). Other less common metallic contaminants include aluminium (Al), caesium (Cs), cobalt (Co), manganese (Mn), molybdenum (Mo), strontium (Sr), and uranium (U).
Sources of heavy metals: It has two types of sources. Such as:
l Volcanic eruption
l Forest fires
l Weathering and abrasion of rocks
l Particles released by vegetation
l Aerosols formation overseas etc.
Anthropogenic sources, e.g.
l Sewage sludge
l Fossil fuel combustion
l Industrial processes
l Organic and inorganic fertilizers etc.
Effects of Heavy Metal:
l Effects on the Environment : Heavy metals are toxic to soil, plants, aquatic life if their concentration is high in the compost.
l Effects on Soil: Heavy metals indirectly affect soil enzymatic activities by shifting the microbial community which synthesizes enzymes. Heavy metals exhibit toxic effects on soil biota by affecting key microbial processes and decrease the number and activity of soil microorganisms. Conversely, long-term heavy metal effects can increase bacterial community tolerance as well as the tolerance of fungi such as arbuscularmycorrhizal (AM) fungi, which can play an important role in the restoration of contaminated ecosystems.
l Effects on water: Two major origins are the culprits of water contamination: urbanization and industrialization. The metals are transported by the runoffs from villages, towns, cities and industries which accumulate in the sediments of water bodies. Even if traces are transported to water bodies, they might still be very toxic to human beings and other ecosystems.
Toxicity of heavy metals depends on a lot of factors such as which metal is present, the nature of the metal, the biological role of the metal, the organism exposed and the period of the organism’s life when it is exposed. If one organism is affected, this will affect all the food chain. Since humans are usually the last of the food chain, this will affect us more as we would have accumulated more heavy metal as the concentration increases along the food chain. Both industrial and domestic wastes usually are expelled into the sewage system.
l Effects on air: Natural and anthropogenic activity has caused the release of particulate matters (PMs), especially, fine particles, and dust. Particulate matters that are present through natural activity are released through sand storms, volcanic activity, soil erosion and the weathering of rocks. While particulate matters present to human activity are released through industrial activity, burning of fossil fuels, vehicle exhaust, smelting and more. The particulate matters can precipitate severe health problems and also cause infrastructure deterioration, the formation of acid rain, corrosion, eutrophication due to particulate matters falling in the water when it rains, and it can cause haze.
l Effects on Human health: Heavy metals become toxic when they are not metabolized by the body and accumulate in the soft tissues. Chronic level ingestion of toxic metals has undesirable impacts on humans and the associated harmful impacts become perceptible only after several years of exposure.
l Cadmium (Cd) is a well-known heavy metal toxicant with a specific gravity 8.65 times greater than water. The target organs for Cd toxicity have been identified as liver, placenta, kidneys, lungs, brain and bones.
l Zinc is considered to be relatively non-toxic, especially if taken orally. However, the excess amount can cause system dysfunctions that result in impairment of growth and reproduction.
l Copper (Cu) is an essential element in mammalian nutrition as a component of metalloenzymes in which it acts as an electron donor or acceptor. Conversely, exposure to high levels of Cu can result in several adverse health effects. Exposure of humans to Cu occurs primarily from the consumption of food and drinking water. Acute Cu toxicity is generally associated with accidental ingestion; however, some members of the population may be more susceptible to the adverse effects of high Cu intake due to genetic predisposition or disease.
l Lead (Pb) is physiological and neurological toxic to humans. Acute Pb poisoning May results in a dysfunction in the kidney, reproduction system, liver and brain resulting in sickness and death Pb heads the threats even at extremely low concentrations.
l Chromium (Cr) is the 10thabundant element in the earth’s mantle and persists in the environment as either Cr (III) or Cr (VI). Cr (VI) is toxic to plants and animals, being a strong oxidizing agent, corrosive, soluble in alkaline and mildly acidic water, toxic and potential carcinogens The toxicity of Cr (VI) derives from its ability to diffuse through cell membranes and oxidize biological molecules
l Mercury is toxic and has no known function in human biochemistry and physiology. Inorganic forms of mercury cause spontaneous abortion, congenital malformation and gastrointestinal disorders (like corrosive esophagitis and hematochezia). Poisoning by its organic forms, which include monomethyl and dimethylmercury presents with erethism (an abnormal irritation or sensitivity of an organ or body part to stimulation), acrodynia (Pink disease, which is characterized by rash and desquamation of the hands and feet), and gingivitis, stomatitis, neurological disorders, the total damage to the brain and CNS and are also associated with congenital malformation.
l Effects on plants: Some of these heavy metals e.g. As, Cd, Hg, Pb or Se are not essential for plants growth, since they do not perform any known physiological function in plants. Others e.g. Co, Cu, Fe, Mn, Mo, Ni and Zn are essential elements required for normal growth and metabolism of plants, but which elements can easily lead to poisoning when their concentration greater than optimal values. Uptake of heavy metals by plants and subsequent accumulation along the food chain is a potential threat to animal and human health.
l Mechanisms of remediating heavy metals: Recent researches, therefore, focused on the recovery of chemical species from acid mine drainage (AMD) and secondary sludge. This is aimed at recovering valuable resources and also enabling easier and safer disposal of the treated sludge. Hence, reducing their environmental footprints:
Major remediation processes are-
Disposal of metal waste to landfills and waste retention ponds/heaps lead to secondary pollution of surface and subsurface water resources. Integrated processes are two different methods to achieve a synergistic and an effective effort to remove heavy metals.
The author of this article is an Associate Editor, “The Environment Review” Magazine