Environmental contamination of heavy metals

The term heavy metal refers to any metallic chemical element that has a relatively high density and is toxic or poisonous at low concentrations. Heavy metals are conventionally defined as elements with metallic properties and an atomic number >20. The most common heavy metal contaminants are Cd, Cr, Cu, Hg, Pb, and Zn.
Heavy metal refers to any metallic chemical element that has a high specific gravity and high relative atomic mass.

Physical properties of metals

Luster (shininess), high melting point, high density (heavy for their size), good conductors of heat and electricity, non-degradable, malleable(hammered into thin sheets) and ductile (drawn out into thin wires).

Kinds of metals

Alkali metals -lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs).

Alkaline earth metals - beryllium (Be) magnesium (mg), calcium (Ca), strontium (Sr), barium (Ba) and radium (Ra).

Metalloids - boron, silicon, germanium, arsenic, antimony, tellurium, and polonium

Heavy metals- platinum, silver,  gold, copper (Cu), nickel (Ni), chromium (Cr), iron (Fe)  cadmium (Cd), lead (Pb), silver (Ag) and mercury (Hg)

Categories of heavy metals

•   Macro-nutrient elements - cobalt (Co), copper (Cu),  zinc (Zn) and iron (Fe).

•    Micronutrient  elements -copper (Cu), nickel (Ni), chromium (Cr) and iron (Fe)

•     Highly  toxic elements - cadmium (Cd), lead (Pb), silver (Ag) and mercury (Hg)

•     Precious elements -platinum, silver and gold

•     Essential metals - Iron, zinc, copper, chromium, cobalt, Molybdenum, selenium

•     Beneficial metals - Silicon, manganese, Nickel, boron, vanadium

•     Detrimental metals - Mercury, cadmium,  lead, arsenic, chromium
Heavy metal pollutants– different names-Globally distributed pollutants, Conservative Pollutants(effectively permanent), Chemical time bombs, Immutable pollutants.

Biological activity of metals
They Inhibit enzymes, Inhibit synthesis of proteins, Cross membranes by passive diffusion/pinocytosis, Target /critical organ for most metals is kidney.

Target organ toxicity of metals –

•         Zinc --àblood-àhemotoxicity

•         Arsenic -àliver-àhepatotoxicity

•         Mercury, Lead  -àbrain -àneurotoxicity

•         Cadmium-àKidney,lungs-àNephrotoxicity, pulmonary toxicity

Sources of heavy metal pollutants
The sources of heavy metals include mining, Smelting, Metallurgical Industries, Corrosion, Waste disposal, Fossil fuel combustion, Agriculture & forestry.

Toxicological properties of heavy metals- Heavy metals are persistent with long residual and half life and the soil residence time is more than 1000 years. They bioaccumulate and biomagnifies thro’ food chain. They show Chronic and sub-lethal effects at low concentration. They are synergistic in their effects and also have teratogenic and carcinogenic  properties.

Toxic diseases of heavy metals

•Aluminium has been associated with Alzheimer's and Parkinson's disease, senility, and presenile dementia.

•  Arsenic exposure can cause cancer, abdominal pain, and black foot disease.

•  Cadmium exposure produces kidney damage and hypertension

• Lead and mercury may cause joint diseases and ailments  of the kidneys, circulatory system, and nervous system

•  Nickel can cause damage to lung, liver and   kidney.

•  Chromium can cause lung damage and cancer.

Mercury contamination        

All forms of mercury are poisonous. Mercury is persistent and cycles globally.  Mercury continues to be widely used and can be toxic to CNS, lungs and kidneys. Mercury is an oldest poison with most potent neurotoxic and fetotoxic effects.        
 Natural sources - Volcanic eruptions,Rock weathering and Natural fires.

Anthropogenic intentional sources - Folk medicines, cosmetics,dental amalgams and vaccines

Anthropogenic inadvertent sources - Mercury mining, smelting and use,burning fossil fuels, coal-fired power plants, municipal / medical waste incineration,cement production and chlorine-alkali production.

History of mercury poisonings

        Minamata Bay, Japan

         In the 1950’s, large amounts of organic mercury were dumped into Minamata Bay in Japan. Mercury-contaminated fish were consumed by pregnant women.Many children that were born from these women had severe nerve damage. Later referred as Fetal Minamata Disease.        
          Iraq 
    Children born to mothers who consumed grain contaminated with organic mercury walking at a later age than non-exposed children.

       Faroe Islands
     Mercury exposure was caused by contaminated whale meat. Children born to mothers with high body levels of mercury scored lower on brain function tests than mothers with low body levels.

         Methyl mercury is the most dangerous form of mercury. Inorganic mercury bio-transformed in sediments into methyl mercury  by aquatic microbes and bio-accumulated through aquatic food chain in larger predatory fishes such as tuna, mackerel, shark, grouper. Methyl mercury is bio-persistent(lipophilic) and  crosses placenta and  the blood-brain barrier and  concentrates in CNS. It enters human  body through fish consumption.

Human exposure to mercury

• Breathing air containing mercury vapors.
• Drinking water contaminated with mercury.
• Eating fish or shell fish contaminated with mercury.
• Touching liquid mercury in the event of a spill.

Health effects of mercury

• Disruption of the central nervous system 
• Damage to brain, lungs and kidneys.
• Damage to chromosomes and DNA.
• Allergic reactions, resulting in skin rashes, tiredness headaches and vision problems.
• Negative reproductive effects, such as sperm damage, birth defects and miscarriages

Cadmium contamination

Cadmium is highly toxic and non-essential heavy metal. It accumulates in liver and kidneys. Cadmium is a potent enzyme inhibitor.

Sources of cadmium contamination

•         Mining and metallurgical operations

•         Electroplating industry

•         Manufacturing PVC plastics

•         Ni-Cd batteries, paints , pigments and dyes

•         Fertilizers and pesticides

•         Anti-corrosive agent for steel, iron, copper, brass and other alloys.

•         Photo voltaic devices and TV screens.

Human health  effects

•         Cause diarrhoea, stomach pains and severe vomiting

•         Itai- itai disease results in bone fracture

•         Kidney dysfunction leads to chronic renal failure

•         Reproductive failure and even infertility

•         Damage to the central nervous system

•         Damage to the immune system

•         Psychological disorders

•         DNA damage or cancer development

Environmental effects of cadmium

•         Cadmium -rich sludge can pollute surface waters as well as soils.

•         Cadmium strongly adsorbs to organic matter in soils.

•         Soils that are acidified enhance the cadmium uptake by plants.

•         Cadmium can accumulate in the plant eating animals , especially when they eat multiple plants.

•         In aquatic ecosystems, cadmium can bioaccumulate in mussels, oysters, shrimps, lobsters and fish.

         

Environmental contamination of Lead

Approx. 94% of the Pb in the atmosphere is derived from heavy motor traffic in urban areas. The aerial dispersion may lead to higher levels of Pb in rivers, seas and soils. Lead gets into soil through paint, dry and wet depositions from the atmosphere and industrial effluents and solid discharges. Lead combine with colloidal particles found in natural water. Urban runoff cause pollution of surface and ground water. Consumption of such untreated water may lead to Pb poisoning.Lead is a non-essential element and a cumulative neurotoxin. Lead is also a general metabolic poison.

Human health effects

•         Acute exposure  results in kidney damage

•         Chronic exposure  causes interstitial nephritis of kidney, neurological and reproductive dysfunctions. Lead inhibits haemoglobin synthesis and cause anemia

•         Lead poisoning results in plumbism, colica pictonium, saturnism and  painter's colic. Lead breaks the blood-brain barrier and interferes with the normal development of brain in infants

Environmental contamination of Chromium Cr

Chromium is an abundant element of earth’s crust. The trivalent (III) and hexavalent (VI)compounds of chromium are great industrial importance. Chromium(III) is an essential nutrient.

Human health effects of chromium

—  Acute chromium toxicity upset stomachs, and cause  kidney and liver damage

—  Chronic chromium toxicity cause cancers of respiratory tract.

Environmental effects of chromium

—  Chromium  in air will eventually settle and end up in waters or soils

—  Chromium in soils  adsorbs to soil particles and percolates to ground water

—  Chromium  may adsorb on sediments and become immobile in water .

Arsenic contamination

A toxic, non-essential and  semi-metallic element. Cumulative   poison and carcinogen. Trivalent arsenites are  more toxic than pentavalent arsenates

Sources of Arsenic contamination

•         Pesticides, herbicides

•         Combustion of coal

•         Mining, smelting of gold, lead, copper and nickel

•         Production of iron and steel

•         Leachate from abandoned gold mines

•         Tobacco smoke

Human health effects

•         Acute exposure causes severe vomiting, diarrhea and gastrointestinal damage.

•         Carcinogen: Lung cancer, Skin and liver cancer, cancers of the bladder and kidneys

Cobalt contamination

Cobalt has a melting point of 1495oC, boiling point of 2870oC, specific gravity of 8.2 g/cm3 at 20oC, atomic number of 27, atomic weight of 58.933 g/mol.Cobalt is hard, ductile and lustrous bluish-grey in color. Cobalt is an essential element. It is an integral component of vitamin B12. It is therefore required for good health.

Sources of cobalt

•         Mining and refining of cobalt

•         Production of alloys

•         Production of tungsten carbide

•         Jewellery producers

•         Ceramics, porcelain producers

•         Metallurgical industries

Uses of cobalt

•      Manufacture of many alloys  used in gas turbine, aircraft engines, corrosion resistant alloys, high speed steels and cemented carbides.

•         As catalysts for the petroleum and chemical industries

•         Used  as pigment in porcelain, pottery, glass, tiles, and enamel jewellery.

•         Radioactive isotopes, cobalt-60, is used in medical treatment and for food preservation

Human health effects of cobalt

Cobalt stimulates the production of red blood cells, thus, used to treat anemia. At high concentration it may damage human health. In soil, it may adsorb on soil particles and is only mobilized under acidic condition. Cobalt is also adsorbed on sediments.Cobalt may accumulate in plants and in the bodies of animals that eats these plants. Cobalt is not known to bio-magnify up to the food chain.

Zinc contamination

Zinc is a lustrous bluish-white metal whose atomic number and atomic mass are 30 and 65.37 g/mol respectively. The density is 7-11 g/cm3 at 20oC with melting and boiling points at 420oC and 907oC respectively. Zinc is brittle and crystalline at room temperature but becomes ductile and malleable between 110oC and 150oC. It is used for galvanizing steel and in the preparation of alloys.

Occurrence of zinc

Zinc is the most abundant element in the earth crust. It occurs naturally in air, water and soil. Zinc is an essential trace element required for the function of over 200 metallo-enzymes. Zinc is also important in the regulation of DNA and RNA synthesis via interaction with DNA binding protein. It also plays a critical role in hormone-receptor interaction.

Human health effects of zinc

Acute exposure causes loss of appetite and fatigue,decreased sense of taste and smell, stomach cramps,nausea, vomiting and anemia. High exposure damage the pancreas.

Uses of zinc

Zinc is used as the negative plates in batteries. Zinc is utilized for roofing in building construction. Zinc oxides is used as a white pigment in paints. Zinc is used as pigment in plastic ,cosmetics, photocopier paper, wallpaper, printing inks. Zinc is utilized as a catalyst during rubber production. Zinc is used as drugs, as anti-oxidants and protect against premature aging of the skin and muscle of the body.

Environmental effect of zinc

Zinc in water (from waste water of industrial plants) increases the acidity of waters. Some fish can bio-accumulate Zn in their bodies. Zinc in fish can bio-magnify and enter human food chain. Insoluble zinc in soils can contaminate groundwater.