SUBMITTED BY: NAYAB ZULFIQAR
Heavy metal is a collective term that is applied to a group of metalloids and metals that have atomic density larger than 6g/cm3 and is poisonous or toxic even at low concentrations. It is a term that is widely applied to those elements that is often associated with toxic and pollution problems such as Pb, Cd, Hg, Cu, Cr, Ni, Zn, Pd etc (Duffus, 2002).
? According to Appenroth (2010) heavy metals include three elemental sub-groups that are as follows:
1. Whole transition elements
2. Rare earth elements that are subdivided into actinides series and lanthanides series
3. Elements from heterogeneous groups including Pb, Ta, Po, Te, Bi, Sn, As, In, Sb, Ga, Al and Ge
? According to Dufus (2002) “heavy metal” is a poor and loosely defined term that is used for “bad metals” for environment.
? According to Barakat (2011) metals with greater than 5 g/cm3 density are called heavy metals.
? Walker et al (2012) have proposed that a metal is considered heavy if its density larger than 5 compared to water.
PROPERTIES OF HEAVY METALS:
• Heavy metals occur at the bottom in periodic table
• Heavy metals have a high atomic weight and atomic number
• Heavy metals have a high density
• Heavy metals are toxic in nature
• Heavy metals can’t be destroyed or degraded and that’s why persistent in nature
CLASSIFICATION OF HEAVY METALS:
There are 3 main classes of heavy metals (Ward, 1995):
• Essential heavy metals;
• Non-essential heavy metals;
• Toxic heavy metals
ESSENTIAL HEAVY METALS:
Numerous heavy metals exist as essential heavy metals for animals and plants for maintaining optimum health when they are present in growing medium at low concentrations and are called as micronutrients e.g. Co, Zn, Cu, Mn, Fe, Ni and Mo. (Marschner et al 1991; Marschner 1995; Welch 1995).
NON-ESSENTIAL HEAVY METALS:
Non-essential heavy metals mimic essential metals and cause disruption of enzymatic and cellular mechanisms. For example zinc that is essential nutrient replaced with cadmium, potassium replaced with thallium, phosphate replaced with arsenate, and manganese replaced with iron. (Baselt 2004)
TOXIC HEAVY METALS:
These essential heavy metals become toxic when concentration limit is overreached and then instead of micronutrients, term toxic heavy metal is used. Some important toxic heavy metals include iron, mercury, aluminum, cadmium, arsenic and lead. These metals enter into body through different routes like inhalation, ingestion, or dermal routes of exposure either by contaminated drinking water or by contaminated food (Rusyniak et al 2010). Exposure to these toxic heavy metals generate oxidative stress that may cause various kind of neurological disorders, cancers, kidney function damage, and endocrine abnormalities (Mudgal et al 2010).
SOURCES OF HEAVY METALS:
Both natural and anthropogenic activities are responsible for release of heavy metals into environment.
Heavy metals enter into the environment through naturally occurring phenomena such as weathering of rocks, volcanic eruption, mineral deposits and forest fires etc (Nriagu 1989).
Heavy metals release into environment through human activities such as through the use of pesticides, fertilizers; through metallurgical activities such as smelting, mining, metal finishing and waste disposal of these activities; through paints; through burning of fossil fuels; through industrial operations etc (Bradl 2002).
HEAVY METAL POISONING:
Poisoning of heavy metal refers to the heavy metals accumulation in lethal amounts, in body’s soft tissues. Many heavy metals like iron, zinc, manganese, copper, chromium, arsenic, lead, cadmium are essential for body functions in small amounts but when the concentration of these metals exceeded from their permissible limit they cause poisoning and cause serious damage to body (Berglund et al 2001).
TYPES OF POISONING:
Heavy metal poisoning may be acute or chronic depending upon the heavy metal and the amount that is exposed, your age, health and nutritional status.
It results from exposure to any metal for a short time period at higher levels. Acute exposures can be toxic and results in serious health impacts such as numbness, feeling confused and sick and even death. For example, a toy that is coated with cadmium or lead when swallowed by children may cause serious symptoms such as neurotoxicity (Lidsky et al 2003).
It results from exposure to heavy metals at lower levels for a long period of time. Symptoms of chronic poisoning develop slowly with the passage of time but may be very severe such as headache, constipation, tiredness, weakness, muscle and joint pain etc. For example, regular ingestion of fish that have higher levels of arsenic or mercury in tissues of their body leads toward serious health impacts on nervous system (Bhan et al 2005).
ROUTES FOR HEAVY METAL ENTERENCE IN ENVIRONMENT:
There are three main routes for the entrance of heavy metals in the environment (Shrivastav 2001):
• Atmospheric particulates deposition
• Disposal of sewage effluents and sewage sludge that is metal enriched
• Mining processes by product
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SUBMITTED BY: NAYAB ZULFIQAR