Heavy Metal Toxicity And Its Impacts: A Review

By Subhangi Basu, Sarbari Dasgupta, Sumit Sarkar, Debkanya Sengupta (UG III, Department of Life Sciences, Presidency University, Kolkata)

Introduction

Heavy metals, according to IUPAC are defined to be of densities from about 5 gm/cm3 and atomic numbers above 40, excluding the Lanthanides and Actinides.1 Such metals are mined, changed in forms and discharged in environment through industries and human activities. Some are poisonous, harming blood, destroying bones and nerves in animals. Plants are also destroyed due to heavy metal toxicity.

Heavy metals may accumulate in an organism over time and some like mercury increases deposition across food chains. If unchecked, they can potentially destroy life.

Here, we look into the toxic effects of five different heavy metals and the ways in which they affect the flora and fauna of the world.

Mercury

Mercury (Hg) is a shiny, silver-white volatile liquid heavy metal at room temperature. Mercury is a ubiquitous and persistent pollutant with bioaccumulation ability in living organisms. Mercury vapours, mercury salts and organomercury compounds are among the most poisonous substances in our environment.

Mercury is found in rock in the Earth’s crust naturally, including in coal deposits. Human activities are responsible for most of the mercury that is released into the environment. There are various ways in which mercury can become airborne. These include burning coal and other fossil fuels by people for energy. In the United States, power plants that burn coal to create electricity are the largest source of emissions; they account for about 44 % of all manmade mercury emissions. Other causes of Mercury air emissions include burning oil that contains mercury, burning wood that contains mercury, burning mercury containing wastes, burning products that contain mercury, etc. This airborne mercury can fall to the ground in raindrops, in dust or by air deposition. Mercury use is all seen in fertilizer and fungicidal sprays, household bleach, acid, caustic chemicals like battery acid, household lye, etc., instruments containing mercury like thermometers, barometers, etc, dental amalgams, , lubrication oils, etc.

Mercury poses a devastating toxic threat to the flora and fauna.

Terrestrial plants are mostly insensitive to the harmful effects of mercury compounds; however mercury may affect photosynthesis and oxidative metabolism by interfering with electron transport in chloroplasts and mitochondria. Mercury also inhibits the activity of aquaporins and reduces plant water uptake.

In animals, mercury and its compounds are generally neurotoxic in nature.

• In fishes, decreased spawning and increased embryo mortality is seen. There are also adverse effects on development, difficulty in schooling and chances of death due to inability to survive extremely high levels of mercury.
• In birds, there are also lower reproductive success due to fewer, smaller eggs, altered chick behaviour and lower survival rates. There are alsocertain behavioural abnormalities, neurological problems and physiological problems and may lead to death.
• Documented impacts of elevated mercury levels in mammals include certain physiological problems like weight loss, impaired sensory and motor skills and may lead to death.
• High-level exposure to methylmercury in humans causes Minamata disease. This is generally due to consumption of fishes and shellfishes with high content of methylmercury in their bodies. Severe cases are characterized by Hunter-Russell syndrome. Methylmercury exposure in children may result in acrodynia (pink disease). Mercuric and mercurous salts affect the alimentary lining and the kidneys.

Arsenic

• Arsenic is one of the natural components of the earth’s crust and is found throughout the environment in the air, water and land.
• It is highly toxic in its inorganic forms. If people are exposed to elevated levels of inorganic arsenic through drinking contaminated water, eating contaminated food and smoking tobacco. Continuous exposure for a long period of time to inorganic arsenic, mainly through drinking-water and food, can lead to chronic arsenic poisoning that can cause skin lesions and skin cancer.
• Aquatic and terrestrial biota shows a wide range of sensitivities to different arsenic species. The type of toxicity and mechanism of uptake of arsenate by organisms differ considerably that explains why there are interspecies differences in organism response to arsenate and arsenite.
• Arsenic compounds can cause acute and chronic effects in organisms, populations and communities at concentrations ranging from a few micrograms to milligrams per litre, depending on species, time of exposure and end-points measured. The environment contaminated with Arsenic are characterized by limited species abundance and diversity. If arsenate levels are high enough, only species which exhibit resistance may be present.
• Inorganic arsenic is a carcinogen and is the most significant chemical contaminant in drinking-water globally. Arsenic can also occur in an organic form. Inorganic arsenic compounds are found in water, highly toxic while organic arsenic compounds found in seafood are less harmful to health. The immediate symptoms of acute arsenic poisoning may include vomiting, abdominal pain and diarrhoea, this can be followed by numbness and tingling of the extremities, muscle cramping and death, in extreme cases. The main and immediate symptoms of long-term exposure to high levels of inorganic arsenic through drinking-water and food are usually observed in the skin, and include pigmentation changes, skin lesions and hyperkeratosis. After a minimum five years exposure these symptoms can arrive and may be a precursor to skin cancer. In addition to skin cancer, long-term exposure to arsenic may also cause cancers of the bladder and lungs.
• The International Agency for Research on Cancer (IARC) has confirmed arsenic and arsenic compounds as potent carcinogenic to humans, and has also stated that arsenic in drinking-water is carcinogenic to humans.
• Other health effects that may be associated with long-term exposure or ingestion of inorganic arsenic include developmental effects are diabetes, pulmonary disease, and cardiovascular disease. Arsenic-induced myocardial infarction, in particular, can be a significant cause of excess mortality.
• Arsenic exposure has been found to cause “Blackfoot disease” in China, which is a severe disease of blood vessels leading to gangrene. This disease has not been observed in other parts of the world possibly due to Arsenic is also associated with adverse pregnancy outcomes and infant mortality, and with other significant effects such as, impacts on child health and exposure in utero and in early childhood has been linked to increases in mortality in young adults due to multiple cancers, lung disease, heart attacks, and kidney failures.
• A lot of studies have demonstrated negative impacts of arsenic exposure on cognitive development, intelligence, and memory.
• Arsenate compounds are known to affect oxidative phosphorylation by competition with phosphate. The environments with high phosphate concentration, arsenate toxicity to biota is generally reduced. Arsenate being a phosphate analogue, organisms living in elevated arsenate environments must acquire the nutrient phosphorous that helps them to avoid arsenic toxicity.
• The Environmental Protection Agency (EPA) has set a limit of 0.01 parts per million (ppm) for arsenic in drinking water.

Lead

Lead (Pb) naturally present, mostly as galena. Lead poisoning, rather than being a single isolated event, is a persistent and in some regions serious problem spanning most of human history. One of the earliest metals to be discovered and used by Man, lead has always been a popular choice for its versatility and ease of use. The Ancient Greeks considered lead as poisonous and Benjamin Franklin noted lead toxicity symptoms. There had been no exposure before human activities. Serious action and legislation took place only in the 20th century.

Metal-works are a major source. The use of tetraethyl lead as an anti-knocking agent in gasoline was historically the largest source. Lead ingestion occurs through food and water, with water contaminated through soldered pipe joints. A major source of lead pollution in India comes from lead batteries, much of which are used in motor vehicles. The poorly regulated production and recycling have resulted in contamination.

Effects of lead poisoning include:

• Food grown in soils with high lead content have significant amounts of lead, which passes onto humans. Lead hampers normal plant biochemistry, disrupting photosynthesis and interfering with nutrient absorption. Plants show significant decrease in water content and disrupted cell division. Root growth is hampered, causing problems in anchorage.
• Unsurprisingly, livestock is affected, as they live in man-made environments. Lead accumulates in bone, liver and milk. This is serious as such animals are the source of milk and meat. Lead deposits in bones and is harmful when used in fertilisers. Levels can be 4 times for cow milk than of humans under similar exposure.
• In case of water near industries, levels can be much higher than the threshold of 0.01mg/L. Some species die out at very low levels. This can cause the collapse of food webs; as such animals are the food of larger creatures. Scavengers or the breakers of dead organic matter also disappear, disrupting nutrient cycling. Fishes can tolerate much higher levels, showing symptoms like humans.
• For wildlife, wildfowl is the most affected, with a million annual deaths in USA by lead shots. Lead toxicity reduces efficiency of food-gathering, thus eggs of thinner shells are laid, putting future generations at risk.
• Lead irreversibly affects almost all body systems. Lead has classically been categorised as a chronic toxin, with effects seen after long exposure. Short-time exposure causes gastric problems, with deposits in liver and kidney. Chronic, or classical symptoms, concern blood, nervous system and bones. Hypertension is common; as are headaches, and even paralysis. Lead can also replace calcium in the bones. Possible effects include decreased male fertility and even cancer. Children are especially vulnerable in their periods of development. Chronic, low level exposure changes brain development- with IQ 4-5 points lower than average. Latent immunological damage and kidney fibrosis occurs. It is suggested that lead rewrites genes.

In 2016 the Institute of Health Metrics and Evaluation attributed about 5, 40,000 deaths globally by lead, with about 1, 65,000 in India.

In India, there are few studies regarding the effects and extent of exposure. The existing studies show high mean blood lead levels with 6.55-7.26µg/dL in blood in children and 11.18µg/dL in non-occupationally exposed adults. In children, toxicity occurs at 5µg/dL. Subsequently, a drop of 4 IQ points occurs in children.

Cadmium

Cadmium (Cd) is a widely distributed heavy metal. It’s found as a by product from Zinc industries and in nature mainly in sedimentary rocks. For battery production, it was vastly used. Cadmium is a very toxic element. In developed countries like in the US, cadmium intake is about 0.4 microgram/kg/day which is less than half of the US EPA’s oral dose. Of late, in solar panels, Cadmium is used (Cadmium telluroide). Cadmium and its compounds are highly carcinogenic (WHO 1992). It badly affects renal system as well as bones and slightly, lungs.

• Kidney is the major target and storage organ of cadmium. Industrial workers associated with cadmium directly or indirectly expose mainly by inhalation or polluted foods. The total concentration of cadmium in renal cortex from which renal effects occur is approximately 150-200ppm (microgram/g wet weight of renal cortex), both in humans and animals. Naturally cadmium binds with small cystein peptide including metallothionein which detoxifies excess Cadmium, otherwise Cadmium produces more reactive oxygen species (ROS) like peroxides or superoxides including inhibiting the activities of oxidant enzymes. Exceeding of the critical concentration of cadmium in renal cortex causes proteinuria. PCT can’t absorb some microglobulin, retinol binding protein. This renal problem can be irreversible with a low Glomerular Filtration Rate (GFR) if protein loss through urine occurs. It is followed by demineralisation of bones, forming kirney stones. Often it leads to heart failure, cerebral infraction, nephritis, nephrosis and diabetic patients are more susceptible to cadmium toxification.
• In Toyama, Japan after World War II, people suffered from Itai-itai disease (severe osteomalacia). Research studies stated that cadmium decreases bone-mineral density especially in post-menopausal women. This toxification alters Vitamin-D metabolism.
• Moreover, in mining, cadmium intake occurs through inhalation, though in comparison to renal affects, pulmonary effects are less. But according to research, breathing high level of cadmium causes severe lung diseases.
• It causes renal, lung, prostate cancer. In animals, cancer is also occurred due to cadmium poisoning. It is fatal for plants also. Researchers from Oregon University, U.S. have found that willows biomagnify cadmium and the birds and other animals which eat that will have a tendency to possess greater amount of cadmium in their body.

Chromium

Chromium (Cr) is generally detectable on the Earth’s crust in small quantities associated with other metals, particularly Iron. Naturally occurring Chromium is usually present as trivalent Cr (III) whereas hexavalent Cr (VI) is almost totally derived from human activities. (WHO 1990).

Due to its wide industrial use, chromium is considered a serious environmental pollutant. Humans introduce chromium into the environmental waters through electroplating factories, leather tanneries, animal feed, wood preserving, chromium plating, metal processing, alloy preparation, petroleum refining and manufacturing of automobile parts and textile manufacturing facilities. Chromium also enters groundwater by leaching from soil.

Chromium plays a key role in the biological life but above critical level, it is toxic, mutagenic, carcinogenic and teratogenic.

• Due to chromium accumulation, reduction in plant production along with toxicity in the nutritional contents and reduction of growth is observed. Toxic effects include alternations in the seed germination process, inhibition of enzymatic activities, impairment of photosynthesis and oxidative imbalances.
• Certain hexavalent chromium compounds have been found to cause cancers in humans, but the evidence to date indicates that carcinogenicity is site specific and limited to the lung and sinonasal cavity. It is dependent on high exposures, such as might be encountered in an industrial setting.

Conclusion

Heavy metals are certainly very toxic for the environment and its biotic as well as abiotic components. There are many ways of clearing heavy metal contamination from the environment. Some of them include:

• Excavation or removal of the soil contaminated with toxic heavy metals.
• Stabilizing heavy metals in the soil by adding chemicals to the soil (the process is called in situ fixation).
• Phytoremediation or growing plants which help contain or reduce heavy metal pollution.
• Bioremediation by the use of microbial bioabsorbents.
• Chemical precipitation, ultra-filtration, micro filtration, nanofiltration, electrodialysis, photocatalysis and reverse osmosis for the removal of heavy metals from the contaminated wastewaters.
• Coagulation/flocculation, electrocoagulation, electro-floatation and electro-deposition for the removal of heavy metals from contaminated water resources.
• Biosorption/bioaccumulation processes for removal of heavy metals from contaminated wastewaters.

Although it seems distant and nearly impossible, but if we, humans, try and implement any of the aforesaid methods or any other method for the heavy metal detoxification, we might be able to have a cleaner, greener and safe environment that would definitely positively impact the liveability on Earth.

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