After reading this article you will learn about the toxic effects of heavy metals on human health and plants.
Toxic Effects of Heavy Metals on Human Health:
Metallic elements are an intrinsic component of the environment. Their presence is considered unique in the sense that it is difficult to remove them completely from the environment once they enter in it. With the increasing use of a wide variety of metals in industry and in our daily life, problem arise from toxic metal pollution of the environment have assumed serious dimensions.
Metal containing industrial effluents constitute a major source of metallic pollution of hydrosphere. Another means of dispersal is the movement of drainage water from catchment areas which have been contaminated by waste from mining and smelting units. The sources of chief toxic metals and their harmful effects on human health are given in Table 1.
1. Mercury (Hg):
Mercury a liquid-volatile metal, is present in air and water naturally as well as through industrial effluents. Mercury is toxic, potent and versatile poison commonly occur in water. Inorganic and organic mercury compounds enter the water bodies through industries where anaerobic microbes (bacteria) convert the mercury to methyl mercury compounds.
In general, mercury pollution causes headache, fatigue, lethargy and anxiety. But long duration stress of mercury with elevated concentrations becomes fatal. The burning example of metal toxicity is ‘Minamata disease’ caused by mercury poisoning in Japan. During 1953 to 1962 in Minamata Bay of Kyushu, Japan, the Minamata disease caused by methyl mercury compound, was in epidemic form. The cause of paralysis and death of about 100 people was the fish contaminated with mercury.
The mercury in the Bay had come from nearby chloride producing industry using mercury chloride (HgCl2). Methyl mercury compound is highly persistent and accumulates in the food chain especially in the fatty tissues of animals and man. Methyl mercury is absorbed through the gills by fish and this respiratory uptake is shown to be dependent on the metabolic rate of fishes. Mercury in nature exist as element whereas other heavy metals exists as cations in the form of salts or other chemical combination. Methyl mercury causes nervous disorder in marine animals at a very low level of dietary intake.
The symptoms of Minamata disease was included visual disturbance, mental deterioration, convulsion and finally death. Mercury also brings genetic defects (mutation) by causing chromosomes breaking and interference with cell- division. Mercury also occurs naturally in the sea as the result of weathering of mercury containing rocks. Discharges of industries making electrical goods, batteries, thermometers and pesticides contain mercury and effluents of chlor-alkali plants seem to be the major source of mercury pollutant.
2. Lead (Pb):
Automobiles are the chief source of lead toxicity. Lead compounds added to gasoline (as tetraethyl lead) to reduce knocking, are emitted into the air with the exhaust as volatile lead halides. Beside automobiles, some lead and lead processing industries are other main sources of lead pollution in water. From air lead easily reach to sediments in water and ultimately enters in food chain.
In some plastic pipes, lead is used as stabilizer and water may be contaminated in these pipes. Lead is known to accumulate in plants growing alongside highways in proportion to traffic density. Lead content on road-side atmosphere may be 2-20 times higher than that on non-road-side. Sedimentation from atmosphere contaminates soil and vegetation. Lead in road-side top soil may be 30 times that in non-road-side soil.
Lead pollution is highly toxic to plants than animals. Lead compounds reaches from water to plants, to animals and finally to human through food chains with gradual magnification. Lead toxicity is major industrial hazard causes liver and kidney damage, reduction in haemoglobin formation, mental retardation and abnormality in fertility and pregnancy.
The chronic lead-poisoning causes:
(i) Gastro-intestinal troubles which includes intestinal stress.
(ii) Neuromuscular troubles collectively called ‘lead plasy’.
(iii) Central Nervous System (CNS) troubles commonly called as ‘C.N.S. Syndrome’. It includes convulsion, coma and death.
3. Chromium (Cr):
Chromium is used in stainless steel tools alloy steel. It is also used in heat and corrosion resistant material in cast iron, metal plating, polythene bag and in leather tanning industries. The effluents discharge from these units contaminates the water of the area.
Recently in February 1999. The tube-well of a dying industry in Ghaziabad, U.P. was giving out yellow colour water containing high amount of chrome. Chrome is a poison and is known to travel across of kilometers in the ground water table. People living in the vicinity of that industrial area will definitely be suffered and affected by chromium toxicity. It causes respiratory and hypertension troubles in human.
4. Cadmium (Cd):
Industries engaged in extraction, refining, electroplating and welding of cadmium containing materials are the main sources of cadmium in air and water resources. Pesticides and phosphate producing industries also discharged cadmium. It is mainly emitted as vapour and in this state it quickly reacts to form oxide and chloride compounds. It is carcinogenic at very low levels and is known to accumulate in human liver and kidney. Cadmium toxicity develops hypertension and caused kidney damage, respiratory problems and heart ailment.
5. Zinc (Zn):
Zinc occurs in air around zinc smelters and scrap zinc refineries. Open hearth furnaces emit 20-30 gm. zinc/hr. in refining the galvanized iron scrap. Zinc in air mostly occurs as zinc-oxide fumes and it is very toxic to man, causes allergy and respiratory troubles.
6. Arsenic (As):
Arsenic is produced as a by-product of metal refining process. In industrial areas its concentrations may reach nearly 25-100 ng/m3. It is found to cause cancer in man. Arsenic toxicity breakdown the red blood-cells, damage kidney and cause jaundice in human. Arsenic accumulates also in plants when soluble form present in high amount.
Industrialization and poor in its waste management and reutilization procedures are the main factor of environmental pollution especially in developing countries. Waste materials treatment plants are either absent or malfunctioning, resulting the discharge of untreated or partially treated industrial effluents in water bodies.
Effects of Heavy Metals on Plants:
The heavy metals have adverse effects on plants. They reduces productivity, bring stunting in growth and loss in chlorophyll (chlorosis) and protein contents in higher aquatic plants These metals are incorporated permanently in plants and then are passed on to animals and human beings. The aquatic plants and animals are a good source of bio-purification processes going on naturally in rivers, ponds and lakes.
The plants are capable of absorbing these metals. It has been experimentally proved that heavy metals like copper, lead, mercury and zinc are easily absorbed by the plants. Plants absorb toxicants either directly from the atmosphere through the leaves, or from soil or water through the roots.
The usual pathway is through the leaves with gaseous pollutants. Studies on aquatic plants have shown that uptake of both toxic heavy metals and organohalogens is mostly initiated by a phase of rapid and passive absorption to the cell wall. Foliar deposition of metals is more important route of contamination than is root absorption.
In higher aquatic plants as Elodea and Eichornia, the uptake of water-borne toxicants by the stems and leaves is more significant than the absorption from sediments by the roots. It is also known to us that plants can generate atmospheric particulate matter in the form of metallic ions.
The absorbed heavy metals (Cu, Hg and Zn) can be discharged from leaf surfaces to the atmosphere. Experiments on Pisum sativum (Pea) Vicia fabae (Bakla) and seedling of Pinus sylvestris (chir) have clearly shown that heavy metals are easily absorbed by their roots and they travel all through the plants, and ultimately some of them passed out from the leaf surface.
The industrial wastes containing heavy metal pollute the water and this water is used for irrigation purposes. The land becomes mineralized and some of these harmful minerals are absorbed by the plants from the roots. These are released from plant surfaces in a particulate form and vitiate the atmosphere.
Various kinds of industrial effluents as paper and pulp and other waste may be reutilized or recycled for beneficial uses. Pollutants physically can be removed from waste by appropriate methods such as adsorption, electro dialysis, ion-exchange and reverse osmosis. CSIR, New Delhi has given techniques for removal of different metal pollutants from water. For example, mercury could be removed from chlor-alkali plant effluents by using mercury selective ion-exchange resin.
It has been shown that non-biodegradable toxic metals can be removed from water by one of the most promising and successful method, by the cultivation of metal tolerant algae and other aquatic plants. Algae is playing important role in minimising the heavy metal toxicity as a bio-transformer and bio-accumulator. Algae as sea weeds are the largest primary producer in aquatic ecosystem on the globe.
It makes the basis of biotic existence in water and grow abundantly even in extreme of conditions. Marine algae are subjected to a great deal of pollution from different sources. Metals as cadmium, mercury and lead accumulated from water by brown algae Fucus, Laminaria, Sargassum and Macrocystis.
The site of accumulation of metal is mainly cell wall. It is thought that tissues of some brown algae are able to tolerate relatively high levels of heavy metals and this increases their potential use as bio indicator for metal pollution. These tolerant algae protect themselves from metal toxicity by producing access amount of some secondary metabolites and fatty acids, amino-acids and hydrocarbons. Though phytoremediation has a long history but its industrial applications is recent. Lower to higher (vascular) plants, are being exploited and tested for their ability to clean up contaminated soil and water.
The process of recovery of toxic substances from soil or water contaminated with industrial wastes by plants is called phytoremediation. Some examples of lab experiments and field tests on removal and abatement of toxicants from soil and water by using plants are being given below.
Effects of heavy metal toxicity on many green algae (Chlorella, Dunaliella, Chlorococcum and Botryococcus) and blue-green algae (Nostoc, Anabaena,. Oscillatoria and Lyngbia) have been studied in batch and continuous cultures. Algal cells have remarkable ability to take-up and accumulate heavy metals from their external environment.
High concentrations of all metals, including those essential for growth and metabolism, exert toxic effect on metabolic machinery of algae. Algae are able to tolerate certain concentrations of metals through either of the following general mechanism; exclusion from cells and intracellular detoxification.
The intracellular metal detoxification mechanisms comprise binding with metal-binding proteins and peptides, binding and precipitations within the cytoplasm and/or vacuole. Some algae may convert mercuric or phenyl mercuric ions into metallic mercury which is then volatilized out of the cell and from the solution. Many plants accumulate high concentrations of amino-acid, proline when treated with elevated concentrations of copper, nickel, chromium and zinc. Schat et al., (199″ observed that metal- induced proline accumulation do not occur until the damage have been caused and consequently it do not apparently prevent metal toxicity.
Mehta and Gaur (1999) have shown that heavy metals chromium, copper and nickel induced proline accumulation in green algae Chlorella vulgaris and this proline is playing important role in ameliorating metal toxicity in the microalgae. Exposure of Chlorella vulgaris and Botryococcus spp. to elevated concentrations of Fe, Cu, Ni, Cr and Zn led to intracellular accumulation of high concentrations of these metals Concomitantly, accumulation of free proline, lipids and hydrocarbons occurred depending on the concentration of metals in the external culture medium or in the cell.
The greater the toxicity or accumulation of a metal, the greater the amount of proline and fatty acids in algal cells. Test metals also induced lipid peroxidation and show a protective effect of proline on metal toxicity through inhibition of lipid peroxidation.
Metal toxicity inhibits photosynthetic efficiency in terms of CO2– fixation, chlorophyll and protein formation in Botryococcus spp. Sub-lethal concentrations, (LC50) of Cu, Zn and Fe (1.0, 2.0 and 15.0 mg/ml, respectively) reduced the growth of algae by about 50%. Chlorella vulgaris accumulates high intracellular concentrations of metals when incubated in metal-enriched culture medium.
The build-up of high concentration of test metals in cell resulted in inhibition of growth rate. Accumulation of Cr is the highest followed in decreasing order by Cu, Ni and Zn. However, the order of metal toxicity is found to be Cu > Cr > Ni > Zn. The hierarchies of metal accumulation and toxicity are broadly similar, except for Cu and Cr; Cu is apparently more toxic than Cr in algae.
Two species of a hydrocarbons rich microalgae, Botryococcus (B. braunii and B. protuberans) confirm the toxicity of test metals used on cellular composition. In B. braunii, at LC50 concentration of each metal (Cu, Zn and Fe), dry weight (g/1) falls to 50%. Protein content reduced to 33%, 25% and 29% in Cu, Zn and Fe supplemented medium, respectively. Maximum reduction in chlorophyll and minimum in carotenoid content is well established in lower plants. However, 2-5% lipid stimulation is noticed. B. protuberans also shows similar trend (Table-2).
Among these test metals Zn and Fe is found conducive to lipid and hydrocarbon production in both the species and maximum inhibition of 14CO2-fixation is noticed in Zn (60%) followed by Cu and Fe supplemented cultures (Singh, 1992). As micronutrients, these metals are involved in several physiological activities of algae. Their deficiencies increases carotenoid and lipid accumulation in test organisms.
All the values are % of dry weight and means of three independent replicates. Cultures were harvested after 20 days of incubation. The metal uptake studies in green algae collectively show an initial rapid uptake and binding of cations onto the cell surface, followed by their metabolism dependent uptake. The requirement of higher concentrations of iron (Fe) to the plant may be due to involvement of Fe in various metabolic pathways and formation of iron-phosphate complex.
The toxic metals affect the productivity of plants causing alterations in physiological and biochemical processes through binding with their macromolecules as lipid, protein, polysaccharides. Growth inhibition of algae by metal toxicity and bioaccumulation of metals have been well documented by Vymazal (1987).
Various algae are excellent monitors of environmental pollution. Ulva and Enteromorpha are used in monitoring the water quality of estuaries. Heavy metal pollution of water (river, pond, lake etc.) is monitored by algae as Cladophora, Pithophora and Stigeoclonium. Cladophora is completely absent whereas Stigeoclonium (metal tolerant) shows abundant growth in waters polluted with heavy metals like Cd, Cu, Ni and Fe. Many species of algae Chlorella, Dunaliella, Scenedesmus, Cyclotella, Amphidium, Skeletonema, Chlorococcum and Pavlova are also used as indicators of toxic metals. Phytoremediation is emerging as most ideal alternative technology for removing metal pollutants from the soil and water.
Phytoremediation is nothing but the use of plants to degrade or prevent environmental pollutants. Besides organic compounds phytoremediation can be used to treat sites contaminated with heavy metals. Plants including bacteria, fungi and algae have shown capacities to uptake the metal ions. After uptake, these metals either accumulate or assimilated by them.
The bacteria like Thiobacillus sp. bring about bioleaching of Zn and Ni from sulphide rocks. Fungi like Rhizopus, Trichoderma and Aspergillus are shown to have bio sorption ability of heavy metals and these plants seem to play important role in detoxification of industrial effluents. Green (flowering) plants are also able to remove metals from contaminated soil and ground water.
Some strains of Brassica juncea accumulate heavy metals like Chromium (Cr) when growing in metal contaminated soils. Among higher plants some aquatic weeds as Salvinia, Lemna, Azolla, Eichornia and Elodea are also known to tolerate, uptake and accumulate heavy metals and other toxicants in their cells.
Green plants are not only the lungs of nature with unique ability to purifying impure air by photosynthesis evolving oxygen to sustain aerobic life in the biosphere, but it has also been demonstrated that they could be very useful in cleaning up the hazardous waste sites. Some of the limitations of phytoremediation can be overcome through genetic engineering. One can produce strains of plants that are resistant to heavy metals, have greater degradive capabilities and able to grow in many adverse kinds of environment.
Interestingly some of the vascular plants given in Table 3, love to grow where heavy metal ions especially Al, Pb, Cd, Cr, Hg, Cu, Fe, Zn etc. are present in higher concentrations in the soil water. The heavy metal loving plants called Metallophytes, can be planted where effluents are discharged near the industries. Metallophytes are also known as Metallocolus because they can tolerate the presence of excess of metals in the soils and plants that cannot tolerate the presence of even smallest does of metals are called Metallofuges.
Some common metallophytes are as species of Astragalus, Aster, Neptunia and Stanleya grow in selenium rich habitat. Viola calmineria (zinc voilet) and V. lutea indicates the presence of zinc in soil and has been used in many European countries for finding out zinc in soil and zinc deposits. Alsine verna and Arineria vulgaris are the other zinc tolerant species. Zinc oxide accumulates in their leaves up to the extent of 20-25%.
Plants like Viscaria alpina in Norway, Polycarpea spirostylis in Australia, Gymnocolea acutioba (Liver wort) in America, Bulbostylis barbata in Queensland (grow in soils containing up to 2000 ppm of copper) grow in the soil rich in copper. A fern like Asplenium adulterinum grow naturally on ultrabasic rocks and on industrial effluents containing nickel. Lychnis alpina grows over nickel deposits in Sweden. Damara ovata, Dacrydium calendonicum and Mutassa intermedia grow on iron rich soils in Scotland.
In Italy, Ilex aquifolium grows abundantly in aluminium rich soils. Besides above, Agrosistes tenuis, Eichornia crasspes, Festuca rubra, Solanum nigrum and many other grasses are also important metallophytes. These plants absorb and accumulate toxic metals from soil and water through the roots and later may be recovered from plant body. In such a simple and natural way these plants will help us to minimise and control the heavy metal pollution from soil and water.