In this article we will discuss about:- 1. Introduction to Environmental Pollution 2. Air Pollution 3. Noise Pollution 4. Water Pollution 5. Soil Pollution 6. Nuclear Pollution.
Introduction to Environmental Pollution:
Environmental pollution is one of the most dreadful ecological crises to which we are subjected today. Ancient India sages quoted that in the current age of Kaliyug, the sins of mankind are being so bad that the earth and mankind would be destroyed in the near future.
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Today India, which occupies 7th place among the industrialized developing countries of the world, is provided with a good industrial infrastructure in several industries like chemicals, power, nuclear energy, petroleum, plastics, food, pesticides etc. A rapid progress in atomic and nuclear energy has added a huge amount of radioactive substances in the atmosphere.
Various industrial effluents and emissions — especially deleterious gases — are released into the air daily. Thus the environment is deteriorated to such an extent that it has crossed the critical limit and has become lethal to most of the organisms including human. The consequences of the industrial developments are that we find warnings everywhere as “Air unfit for breathing”, “Water unfit for drinking”, “Vegetable unfit for eating” and so on!
In fact, environmental pollution is the necessary evil of all man-made progress. It is not only the industrialized countries which are threatened with environmental pollution, but the menace is growing in India too. The developed countries dump lots of effluents into the environment, polluting the entire globe. Needless to mention here that pollution has been exported to developing countries by the developed countries of the world. Undoubtedly, US is the most polluted country of the world today.
Air Pollution:
Air pollution is the resultant of the direct or indirect change in the physical, chemical or biological characteristics of atmosphere which mainly results from gaseous emissions from industry, thermal power stations, automobiles, domestic combustions etc.
The main air pollutants are grouped into two major heads viz.:
(A) Natural source, and
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(B) Man-made source.
(A) Natural Source:
Such pollutants are created by the activities of the nature. The natural activities affect only limited area.
These natural sources are:
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i) Volcano which emits gases, soot, smoke, particulate matters etc.
ii) Forests fires.
iii) Cyclones, typhoon, thunderstorms.
iv) Fog.
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v) Decomposition of dead matters, vegetation, animals etc.
(B) Man-Made Sources:
Include industry, thermal power stations, automobiles, farming practices, domestic use equipments, nuclear weapons and tests etc.
Noise Pollution:
The word noise is derived by the Latin word nausea meaning unpleasant sound which causes discomfort. Noise may be defined as the wrong sound in the wrong place at the wrong time. Thus the noise is itself pollution which means the unpleasant sound produced in the atmosphere leading to discomfort or the health hazards.
Decibel & Hertz:
There are two basic properties of sound viz.:
A. Loudness, and
B. Frequency.
A. Loudness:
Loudness is the strength of sensation of sound perceived by the individual. It is measured in terms of decibel (i.e., dB). Loudness is also expressed in sones. One sone is equal to the loudness of 40 dB sound pressure at 1000 Hz. Just audible sound is about 10dB. Mumbai, New Delhi, Kolkata & Chennai usually have more than 90dB.
B. Frequency:
Frequency of sound is defined as the number of vibrations per second. It is denoted as Hertz (Hz). One hertz is equal to one vibration per second. We hear the sound between 16Hz (i.e., infra- audible) to 20,000 Hz (ultrasonic sound).
Sources of noise pollution are:
The main sources of noise are factories & industries, transportation (air, road, rail and ship) and community and religious and cultural activities. The main man-made sources in the urban area are automobile, factories, industries, train, aeroplanes. As far as our country is concerned, we are noisy people and every sentiment either religious, social, cultural or family activity is celebrated in a noisy way.
Loudspeakers and amplifiers are used at every occasion. God and Allah is listened only through loudspeakers. Nowadays Jagrans’ become a regular feature in the name of devis and has been spreading like a plague. The procession is become must for dancing and drinking on the occasion of marriage and also on election.
Effects of noise pollution are:
Noise is harmful to the body and mind. It causes irritation or annoyance, headache, constricts the arteries, increases the flow of adrenaline and forces the heart to work faster Continuous noise increases the cholesterol level resulting in the constriction of blood vessels which leads to heart attack and strokes.
The noise causes a number of physiological disorders due to imbalance in functioning of the body e.g., neurosis, anxiety, insomnia, hypertension, hepatic diseases, behavioural & emotional stress, increase in sweating, nausea, fatigue undesirable changes in respiration, peptic ulcer. It reduces depth and quality of sleep thus affecting overall mental and physical health.
Noise creates the disturbance in doing mental work and causes ill-temperament. A noise of 50-60 dB generally interferes with speech. Even a low level of noise from crowd, highway, TV, radio etc., causes the great annoyance for the balanced persons. The auditory fatigue occurs at the 90 dB with side effects as whisling and buzzing in ears whereas at 100dB, permanent loss of hearing occurs.
Mumbai & Kolkata are the noises cities in the world. Supersonic aeroplane creates a shock wave called sonic boon which produces a startle effect. That is more harmful than continuous noise. It may damage the window pans and building structures. It increases the heart beat rate of human foetus.
Water Pollution:
Water is generally available in two forms on our earth viz., marine (saline) water and fresh water. Saline water constitutes about 97.5% of the total water present on the earth and is found in oceans or seas. Fresh water is utilised by man and constitutes only 2.5% of the total water. Out of 2.5% only 0.5% is available for human use.
Normally water is never pure in a chemical sense but it contains impurities of various kinds i.e. suspended as well as dissolved. Water is itself a purifying agent but the alternation in its physical, chemical or biological characteristics which damages the water quality is called water pollution.
There are numerous sources of water pollution but the major sources are:
1. Sewage & other wastes,
2. Industrial effluents,
3. Agricultural discharges, and
4. Industrial wastes from chemical industries, fossil fuel (thermal power) plants and nuclear power plants.
Pollutants of Water:
The followings are the major water pollutants:
1. Organic pollutants,
2. Inorganic pollutants,
3. Thermal Pollutants,
4. Sediment, and
5. Radioactive materials.
1. Organic Pollutants:
Water carrying the organic pollutants has decreased level of dissolved oxygen (DO) because all these compounds can undergo bacterial degradation in the presence of oxygen. Such organic pollutants are oxygen demanding wastes, disease causing agents, plant nutrients, sewage, and synthetic organic compounds.
The main sources of such organic pollutants are excreta of an infected person, sewage system and agricultural runoff. The presence of synthetic organic compounds like fibres, detergents, paints, food additives in water add nasty taste, odour and colour to it.
2. Inorganic Pollutants:
These are mainly inorganic salts, mineral acids, metallic compounds, trace elements, organometallic compound and its main sources are industries. Many coal mines discharge substantial amounts of H2SO4 into groundwater through seepage. The organometallic compounds are highly toxic to aquatic life.
3. Thermal Pollutants:
The major sources of these pollutants are coal power plants, nuclear power plant, sewage, and industrial processes.
4. Sediment:
Sediment in water usually comes from natural process of soil erosion. Sediment is washed away during rainfall and runoffs. The rate of soil erosion is increased by the human activities like deforestation, agricultural runoff, construction processes and mining activities. Sediment interferes in the natural processes of aquatic organisms by reducing light penetration in water bodies.
5. Radioactive Materials:
The major sources are nuclear weapons, nuclear power plants, mining of radioactive substances, medical research & industrial applications. These radioactive materials can enter the food chain in aquatic ecosystems and completely disrupt the metabolic pathways.
Each year a large nuclear plant produces 5 tonnes of treated wastes. Radioactive wastes are put in stainless steel drums and stored deep underground. It is then surrounded by concrete and sealed. Low level radioactive wastes are put in drums and buried in shallow pits.
Effects of water pollution are:
i. The pathogens found in sewage are responsible for many diseases.
ii. It makes water unfit for drinking and domestic use.
iii. Decrease in dissolved oxygen (DO) leads to production of foul odour in water.
iv. The self-purifying ability of water is lost.
v. Industrial effluents have deleterious effects on living organisms and may cause death.
vi. Heated effluents discharged into water bodies may severely alter the aquatic system and life.
vii. Industrial effluents impart colour, foul odour and turbidity to receiving water.
viii. Thermal pollutants accelerate the activities of pathogenic organisms and thus give an impetus to their population.
ix. Excessive sediment decreases the depth of rivers, streams, lakes etc. and makes the water to overflow.
x. Sediment reduces the light penetration and lowers the photosynthetic activity of aquatic plants.
xi. Radioactive wastes present in water cause cancer, eye cataract, DNA breakage and carcinoma in man.
xii. Radioactive wastes destroy the biological immune system.
Marine Pollution:
Marine pollution is defined as the discharge of undesirable substances in oceans or sea water causing changes in physical, chemical and biological characteristics of water which might prove harmful to marine organisms and affect the usage of oceans or sea water. All that are carried by rivers are dropped into the sea. The discharge of oils and petroleum products and dumping of radionuclide wastes into the sea also cause marine pollution.
Sewage contributes excessive of nutrients in sea water which lead to fast growth of algae and some other sea weeds, preventing the sunlight to reach in depth. The major sources of marine pollution are sewage, industrial effluents, synthetic detergents, pesticides, fertilizers, heavy metals, oils and petroleum products, radionuclide wastes, plastics.
The most serious pollutants is oil. A single drop of oil spreads over a large area of water. Oil layer may isolate water from coming in contact with atmospheric oxygen. The shortage of oxygen inhibits growth of plankton which forms the basis of the food chain in aquatic ecosystems. About 285 million gallons of oil are spilled each year into ocean, mostly from transport oil tankers. About 50,000 to 2,50,000 birds are killed every year by oil.
The oil is soaked in feathers and the air of feathers is displaced and thus it interferes with buoyancy and maintenance of body temperature. Hydrocarbons and benzpyrene accumulate in food chain and consumption of fish may cause cancer. On 18.03.1967, a liberian tanker Terry canyon spilled over 60,000 tonnes of crude oil into the entrance of the English channel of the Great Britain. This oil splattered over to 160km of coastline killing fish and birds.
On 24.3.1989, the Supertanker Exxon Valdez spilled over 11 million gallons of oil into the clean water of Alska, USA where oil splattered over 1930 km of shoreline killing 100,000 seabirds. In 1973, the oil tanker, Cosmos Pioneer ran aground and 3000 tonnes of oil splattered to the Gujarat coast.
In 1974, an American oil tanker, Transhuron collided with one of the atolls of the Laccadives spilling 5000 tonnes of special furnace oil. In June 1989, M.T. Puppy (Maltese tanker) collided with a British vessel spilling over 5500 tonnes of furnace oil into the open sea off Mumbai. The massive oil slick in the Gulf in 1991 has been the largest so far spreading over 700 sq. km.
The pollution in Baltic Sea along the coast of Finland, has occurred largely from sewage and effluents from wood industries. The radionuclide wastes in sea include Sr-90, Cs-137, Pu-239, Pu-240. Many sea birds suffer from gastro-intestinal disorders due to the ingestion of plastic. A huge amount of plastic packing materials is being dumped into sea and many more are entered through rivers. The pollutants in sea are dispersed by turbulence and ocean currents and are concentered into the food chain.
Water pollution due to oil may lead to the following detrimental effects:
i. Oil layer reduces the rate of O2-uptake significantly by water.
ii. Oil coating does not allow the fish to respire and clog their gill slits.
iii. Oil layer decreases the light intensity penetrating deep in the sea.
iv. Natural insulating oils and waxes which shield the birds from water are destroyed by oil coating in sea water.
v. Several birds die because their fur and feathers become oil saturated and agglomerated.
vi. Oil spill also affects the terrestrial food chain.
Mercury Pollution:
Mercury is one of the water pollutants. Both organic and inorganic forms of Hg are highly poisonous. Mercury was found responsible for the Minamata epidemic disease that caused several deaths in Japan and Sweden. This disease was occurred due to consumption of heavily-contaminated fish. The source of Hg to the bay was a single chloride producing plant where HgCl2 was used as a catalyst.
In Sweden the widespread use of mercury compounds as fungicides and algicides in paper and pulp industries and in agriculture polluted the many rivers and lakes. The main source of Hg-containing effluents is chloral alkali plant. The paper & pulp industries also cause Hg-toxicity in Japan and Canada. The effluents from batteries, thermometers, fluorescent light tubes, high density street lamps and switches-making industries also contain Hg.
At the bottom of the water bodies, the mercury is metabolically converted into highly persistent methyl mercury compounds by anaerobic microbes. Methyl mercury is soluble in lipids and thus accumulated in fatty tissues of animals when eaten up. Methyl mercury ions may directly be accumulated in Fish. In Minamata bay all the mercury in sea-food was as organic methyl mercury compounds. Swedish fish eaters have also high Hg- content in their blood.
The main sources of lead to water are effluents of lead & lead processing industries. But Lead toys, paints, some plastic pipes, lead containing pesticides, food, beverages, ointments and medicinal concoctions for flavouring and sweetening also cause lead-pollution.
Cadmium- contaminated drinking water and its high level in rice causes the Itai-itai (ouch-ouch) disease.
In nature fluorine is found as fluoride. In Haryana & Punjab, consumption of fluoride-rich water from wells caused endemic fluorisis. High fluoride content water caused dental fluorisis in Andhra Pradesh and Rajasthan. Many people of Rajasthan have humped back only due to high fluoride content in water.
Prolonged intake of fluoride causes stiffness in bone joints particularly of spinal cord. Fluoride has an affinity with Calcium and thus gets accumulated in bones, resulting in the mottling of teeth, pain in the bones and joints and outward bending of legs from the knees. This is known as Knock-knee syndrome.
Soil Pollution:
Soil is actually the outer covering of the earth’s crust. It is the soil on which we live on and get our food supply. Unsustainable Agriculture, Rapid industrialisation, urbanisation and population explosion have resulted in soil pollution.
The undesirable changes in physical, chemical and biological characteristics of soil, which are harmful for all living beings, are collectively known as soil pollution. Soil pollution is quite different from air pollution and water pollution as in soil pollution pollutants remain in direct contact with soil for a relatively longer period.
Sources of soil pollution are:
There are various sources for soil pollution but the main sources are:
1. Man-Made Sources-
a) Agricultural Practices,
b) Disposal of solid wastes on land,
c) Mining activities,
d) Biological agents,
e) Radioactive pollutants, and
f) Heavy metal pollutants.
2. Natural sources like volcanoes, tsunami waves, storms near desert areas etc.
i. Excessive use of fertilizers and pesticide chemicals does not allow microbial flora and fauna in soil to flourish.
ii. Excessive use of nitrogen and phosphatic fertilizers makes the soil deficient in other micronutrients like Zn, Co etc., and causes nutrition imbalances.
iii. Soil fertility is reduced or adversely affected if pesticides remain in soil for longer period.
iv. Pesticides like DDT, dieldrin etc. are known to seep gradually through soil into ground water and thus contaminate public drinking water supplies.
v. Farm animals can even die by drinking stagnant water in fields contaminated with pesticide.
vi. People in contact with pesticides are extremely prone to get poisoned.
vii. Some of the industrial wastes are extremely toxic for organisms.
viii. The industrial as well as other wastes may be transferred to all organisms through food chains.
ix. Pathogens found in sewage are infectious to men and animals.
x. Solid urban wastes and industrial wastes produce foul and offensive odour.
xi. Construction materials and other garbage pile up at places and cause obstruction in day-to-day activities.
xii. Stray animals further spread the heap of wastes.
xiii. Heavy metals and other toxic substances (impurities from mining activities) can destroy beneficial microorganisms of the soil.
xiv. Rate of retention and absorption of heavy metals is higher in infants and children.
xv. When rain containing radionuclides falls on the soil, the radioactivity gets transferred to the soil due to its absorption by the soil particles.
xvi. Rain also adds radioactive contaminants into water bodies thus affecting the aquatic flora and fauna.
xvii. Radioactive pollutants can cause a number of undesirable diseases of digestive system if they enter our body through food chain.
Control of soil pollution are:
i. Adoption of sustainable agriculture having organic farming and use of bio-fertilizers, bio-integrated pest management and proper water management, composting etc.
ii. Adoption of suitable and proper industrial and urban wastes management.
iii. To educate the people about the wastes dropping at the proper place.
iv. Use of proper sanitation.
v. Adequate controlled use of heavy metals and toxic substances.
vi. Go for bioremediation of polluted soils, e.g. Thiobacillus ferroxidans is an iron and sulphur-oxidising bacterium. This bacterium brings about bioleaching of zinc, cobalt and nickel from sulphide rocks.
vii. Afforestation, social & agro-forestry,
viii. Use of plastic bags, containers and other plastic products should be discouraged.
ix. Non-biodegradable wastes can be recycled and used again.
x. Biomedical wastes should be carefully disposed off so that it does not create any health hazard.
xi. Use of incinerator which converts solid wastes into liquid and then into gaseous forms.
Nuclear Pollution/Radiation Pollution:
Radioactive substance releases invisible radiations which cause many deleterious effects on all living organisms directly or indirectly. These radio-active substances are radium, uranium, plutonium, polonium etc.
Low levels of ionising radiations have been emanating from natural resources since evolution but the level of exposure has increased enormously after the advent of nuclear weapons and the development of nuclear energy.
Nuclear pollution is a kind of physical pollution of the environment and it differs from air, water, and soil pollution in the respect that it can bring about physiological changes in the present and future generations of all living beings. There is no safe dose of radiation.
Sources of nuclear pollution are:
1. Natural Sources:
a) Solar and cosmic radiations.
b) Internal radiation which is released during the decay of some elements within our body e.g., decay of K in our muscles releases radiations within our body.
c) Radiations from radio-active elements.
2. Anthropogenic Sources:
Such sources are related to human activities:
a) Medical X-rays are extensively used for diagnostic purposes.
b) Radio-isotopes are administered to patients during radiation therapy to destroy the diseased cells. Radioisotopes are such isotopes of elements that emit ionising radiations.
c) The leakage from the core of nuclear reactors release nuclear radiations.
d) Radioactive wastes generated by nuclear power plants.
e) During the mining processes of radioactive ores of uranium and thorium, huge amounts of radioactive wastes are generated.
f) Radiations from researches on radioactivity e.g., a large no. of radioactive isotopes like C14, I125, P32 and their compound are largely used in scientific researches.
g) Nuclear accidents i.e., on 26.04.1986 the Ukrainian reactor at Chernobyl released a massive amount of radiations where innumerable people were died and several others suffered from cancer, leukamia due to radiation.
h) Nuclear disaster i.e., in 1945, during the course of IInd world war, the USA dropped atom bombs on the cities of Hiroshima and Nagasaki in Japan and till today the people suffer from the adverse effects of nuclear bombs.
i) A no. of nuclear explosions have already been made during the recent past is different parts of world. In an explosion, 50% of the energy goes to the blast, 33% as heat and the rest 17% or so to radioactivity. The most dangerous materials in radioactive fallout are Sr90 (Strontium-90) and Cs137 (Caesium-137) which contaminate the environment for many years.
i. The acute radiation exposures cause sudden death, death after some weeks, loss of hairs, bleeding from gums etc.
ii. High doses of radiation may damage to bone marrow and thus retard body’s ability to fight against infections.
iii. Brains of foetus is highly vulnerable to damage and may result in mental retardation.
iv. High doses of radiation cause blood haemorrhage and ultimately death of the organisms.
v. Radioisotopes released into the environment accumulate in the air, water and soil and enter human body through food chain and its accumulation in the body may cause leukemia, bone cancer, and hereditary diseases.
vi. Plants also show genetic changes due to exposure to radioactivity.
vii. Atomic radiation can char wood and even ignite it with 16 km from the site of explosion. The temperature increases so high and metals and minerals not only melt but will vapourise.
viii. The areas subjected to radiations have reduced biodiversity.
Bioremediation:
Bioremediation is the removal of pollutants from the biosphere by using the biological processes. The microorganisms in particular and some higher plants have the abilities to degrade, detoxify and even accumulate the harmful organic as well as inorganic compounds.
Thus Bioremediation is the name given to the use of bacteria and other small organisms and also higher plants to clean up or reduce unwanted concentrations of certain substances. The basic principle of bioremediation is natural attenuation (or intrinsic bioremediation).
Natural attenuation is natural occurring process to reduce the mass, toxicity, mobility, volume or concentration of contaminants in an environment without human intervention. In most of the cases the alternation or manipulation of physical and/or chemical properties of the contaminant or factors is used to enhance the natural processes. This is referred to as enhanced bioremediation.
There are two approaches for enhanced bioremediation viz.:
a) Biostimulation:
Primarily it depends on the modification of the environment.
b) Bioaugmentation:
It uses the addition of microbial cultures to increase biodegradation.
Some microbial degradation or transformation occurs only in the absence of oxygen. The dry anaerobic composting (Dranco) process converts the organic parts of biodegradable organic solid waste and refuse into energy in the form of biogas (Methane and carbon dioxide) and a humus like material by a group of anaerobic bacteria like methanogens (methane producing archeobacteria).
This Dranco process is used at Brecht, Belgium and Salzburg, Austria. The bioreactor which removes nitrate from water has been tested at Blankaart, Belgium. This bioreactor contains the methylotrophic bacteria like Methylophilus methylotrophus to carry out denitrification. Methanol is first added to bioreactor to support the growth of methylotrophs.
Bacteria like Pseudomonas cepacia are capable to biodegrade chlorinated hydrocarbons present in effluents of pesticide industries manufacturing DDT, heptachlor, chlordane etc. In Matera (Italy), waste water is treated anaerobically in a bioreactor that produces epoxy resins from epichlorohydrin and phenolics. A bacterium Acetobacter liquefaciens S-l is used to treat waste water in textile and dye industries in Hong Kong.
In bioscrubbers and biotrickling filters, multiple microbial communities are grown on solid surfaces to produce multilayered complexes called biofilms. When gas streams having organic pollutants are passed through biofilms/biofilters, the pollutants are degraded. Biofiltration has been used to treat gases given off by soybean toasters in Hengelo (Netherland) since 1989.
For the treatment of volatile organic compounds in air, some fungi like Candida tropicalis are exploited in biofilters. The mycelium of the fungi provides a large surface area and thus greater capacity to eliminate pollutants. A ceramics factory of Southern Germany uses biofilters to remove more than 99% of the ethanol and isopropyl alcohol released into air from drying ceramics. Airborne hydrocarbon vapours are readily treated with biofilters.
The crude oil spills at the sea are perhaps the most widely covered environmental incident which cause alarming threats to biodiversity and human health. The safest way of treating an oil slick is bioremediation where microbial surfactants are sprayed from the air which emulsify after mixing with the oil and oil spill is dispersed throughout the water body so thinly that it no longer remains hazardous.
Bacteria and yeasts can grow on several fractions of hydrocarbons as heptane, decane, hexadecane etc. Professor Ananda M. Chakrabarty, working at the University of Illinois Medical Centre, Chicago, USA has developed many strains of oil eating bacteria. He developed very efficient oil eating ‘Superbug’ using species of Pseudomonas through recombinant DNA technology.
The successful bioremediation of shorelines affected by the oil spill from the Exxon Valdez in Prince William Sound, Alaska (USA) in 1989 was the largest bioremediation project to date where more than 111 km of shoreline was treated. The slow-release formulations of inorganic nutrients, primarily ammonium nitrate and ammonium phosphate, were distributed throughout the oiled zone which stimulated the metabolism of indigenous hydrogen-degrading microorganisms and degraded both surface and subsurface oil 3 to 5 times faster than occurred at untreated test sites.
Petroleum hydrocarbons contaminants of land and groundwater were removed by air sparging and bio- venting. Air sparging is the injection of air to stimulate aerobic degradation and volatilization. Bioventing means the contaminated water is pumped to the surface and re-injected.
Microbial remediation of metals and radionuclides contaminated soil and sediments can be achieved by:
a) Immobilisation of metal in situ to reduce metal bioavailability and mobility, or
b) Removal of the metals and radionuclides from the contaminated soils and sediments.
Microbes like Thiobacillus ferroxidans remove metals from soils through metal solubilization or leaching. Bioleaching is used to recover Cu, Pb, Zn and uranium. Some surfactants like rhamnolipid produced by Pseudomonas aeruginosa showed specificity for Cd & Pb. Emulsion produced by Acinetobactor calcoaceticus helps in removal of metals.
Several toxic heavy metals may be removed from industrial effluents by using Pseudomonas putida, Arthrobacter viscous & Citrobacter spp. Uranium & thorium like radioactive metals are removed by Rhizopus arrhizus, Penicillium chrysogenum (radium), the yeast Saccharomyces cerevisiae (uranium). Bioleaching of Zn, Co, & Ni from sulphide rocks is brought about by the bacteria like Thiobacillus thiooxidans.
Phytoremediation:
Phytoremediation is the use of vegetation to remove, accumulate, degrade, contain or immobilize harmful pollutants from the soil or water. It relies on the plants ability to act as a solar-driven pumping and filtration system and enhances or stimulates the natural tendency of ecosystem to restore itself.
Some plant species have the unique ability to uptaken, tolerate and even hyperaccumulate heavy metals and other toxic substances from soil and water through their roots and concentrate them into roots, stems and leaves e.g., some aquatic weeds like Salvinia, Lemna, Azolla and Eichhornia, sedges like Typha latifolia and some herbaceous and woody flowering plants.
The alpine pennycress Thlaspi caerulescens when grown on zinc-contaminated soil, yields 30-40% zinc which is as high as highgrade ore. Thus this plant is bio-ore of Zn. A variety of tree Sebertia acuminata (Sapotaceae), native of New Caledonia, accumulates 20-25% nickel of its dry weight.
This plants bleeds a bluish green latex, i.e., sap. Phytoremediation processes are grouped under six main categories viz. Enhanced rhizosphere degradation, phytodegradation, phytoextraction (Phytoaccumulation), Rhizofiltration, Phytovolatilization and Phytostabilisation.