specific weights more than 5 g cm-3 (Holleman and Wiberg,. 1985). There are about ... coordination chemistry, categorizes heavy metals as class B metals that come .... inorganic and organic fertilizers are the most important sources of heavy ...
Journal of Environmental Biology ISSN : 0254-8704 http://www.geocities.com/Lenviron_biol/
J. Environ. BioI. 26 (2 suppl), 301-313 (2005) Lenviron_biol @yahoo.com
Biological effects of heavy metals: An overview Rajesh Kumar Sharma and Madhoolika Agrawal Ecology Research Laboratory, Department of Botany Banaras Hindu University, Varanasi - 221 005, India (Received: 20 December, 2003 ; Accepted: 01 April, 2004)
Abstract: Heavy metals constitute a very heterogeneous group of elements widely varied in their chemical properties and biological functions. Heavy metals are kept under environmental pollutant category due to their toxic effeqJ$" on plants, animals and human being. Heavy metal contamination of soil results from anthropogenic as well as natural activitiei~'Anthropogenic activities such as mining, smelting operation and agriculture have locally increased the levels of heavy metals such as Cd, Co, Cr, Pb, As and Ni in soil up to dangerous levels. Heavy metals are persistent in nature, therefore get accumulated in soils and plants. Heavy metals interfere with physiological activities of plants such as photosynthesis, gaseous exchange and nutrient absorption, and cause reductions in plant growth, dry matter accumulation and yield. Heavy metals also interfere with the levels of antioxidants in plants, and reduce the nutritive value of the produce. Dietary intake of many heavy metals through consumption of plants has long term detrimental effects on human health Key words: Heavy metals, Sources, Effects, Plants, Human health
Introduction The term 'heavy metal' although often not rigidly defined is commonly held for those metals, which have specific weights more than 5 g cm-3 (Holleman and Wiberg, 1985). There are about 40 elements that fall into this category. Heavy metals are largely found in disperse form in rock formations. Industrialization and urbanization have increased the anthropogenic contribution of heavy metals in biosphere. Heavy metals have largest availability in soil and aquatic ecosystems and to a relatively smaller proportion in atmosphere as particulate or vapours. Heavy metal toxicity 'to plants vary with plant species, specific metal, concentra~ion, chemical form, and soil composition and pH, as many heavy metals are considered to be essential for plant growth. Similarly many heavy metals are essential trace nutrients of animals and human body (Wintz et al., 2002). Some of these heavy metals like Cu and Zn serve either as cofactor and activators of enzyme reactions e.g. informing enzymes/substrate metal complex (Mildvan, 1970) or exert a catalytic property such as prosthetic group in metalloproteins. These essential trace metal nutrients take part in redox reactions, electron transfer and structural functions in nucleic acid metabolism. Some of heavy metal such as Cd, Hg and As, etc are strongly poisonous to metal sensitive enzymes, resulting in growth inhibition and death of organisms. An alternative classification of metals based on their coordination chemistry, categorizes heavy metals as class B metals that come under non-essential trace
elements, which are highly toxic elements such as Hg, Ag, Pb, Ni (Nieboer and Richardson, 1980). These heavy metals are persistent and bioaccumulative, and do not readily break down in the environment or not easily metabolized. Such metals accumulate in ecological food chain through uptake at primary producer level and then through consumption at consumer levels. Plants are stationary and roots of a plant are the primary contact site for heavy metal ions. In aquatic systems, whole plant body is exposed to these ions. Heavy metals are also adsorbed directly to the leaves due to particles deposited on the foliar surfaces. Heavy metals enter the human body either through inhalation or ingestion, which is the main route of entry for general population. However, urbanization and traffic, industrial and agricultural activities, waste incineration and mining have significantly contributed to \"the entry of heavy metals through inhalation in human body. Heavy metals such as Cd, Ni, As and Cr pose a number of hazards to humans. Heavy metals are also potent carcinogens. Cadmium intake leads to itai- itai disease and mercury intake leads to minimata disease. Other heavy metals such as As causes poisoning due to drinking water contamination. This paper presents an overview of the research information on sources and effects of heavy metals on plants. Impact of heavy metals on human being is also s,ummarized. Essentiality of heavy metals: Some of heavy metals (Fe, Cu and Zn) are essential for plants and animals (Wintz et al., 2002). The availability of heavy metals in
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medium varies. The range of concentrations of different heavy metals in soil and crops is given in Table 1. Some of these heavy metals such as Cu, In, Fe, Mn, Mo, Ni and Co are called micronutrients (Reeves and Baker, 2000), and are only toxic when taken in excess of requirement (Monni et al., I2000; I' Blaylock and Huang, 2000). The toxic limit and recommended cor safe intake of heavy metals for human health are given in Table 2. Heavy metals are also called as trace elements due to their presence in trace (10 mg kg-1 or 1'1) or in ultra trace (1 ~ g kg-1 or 1-1) quantities in the environmental matrices. The essential heavy metals (Cu, In, Fe, Mn and Mo) play biochemical and physiological functions in plants and animals. Two major functions of essential heavy metals are: (a) Participation in redox reaction, and (b) Direct participation, being an integral part of several enzymes. Copper is an essential heavy metal for higher plants and algae, particularly for photosynthesis (Ouzounidou et al., 1992). Cu is a constituent of primary electron donor in photosystem I, the copper protein plastocyanin. Because Cu can readily gain and lose an electron, it is a cofactor of oxidases, mono- and di oxygenase (e.g. amine oxidase, ammonia monoxidase, ceruloplasmin, Iysyl oxidase) and of enzymes involved in he elimination of superoxide radicals (e.g. Superoxide dismutase and ascorbate oxidase). Several enzymes contain In, such as carbonic anhydrase, alcohol dehydrogenase, superoxide dismutase and RNA polymerase. line is required to maintain the integrity of ribosome. It takes part in the formation of carbohydrates, and catalyzes the oxidation processes in plants. line also provides a structural role in many transcription factors, and is a cofactor of RNA polymerase. Nickel is also recognized as an essential micronutrient for living organisms. Nickel is a component of the enzyme urease, and essential for its functioning and good health in animals. Other heavy metals such as Molybdenum are present in nitrate reductase. Mangnese plays an important role in reactions of enzymes like malic dehydrogenase and oxalosuccinic decarboxylase. It is also needed for water splitting at photosystem II, and for superoxide dismutase. In plants, Co complex is found in the form of vitamin B12. Iron is an essential element in many metabolic processes and is indispensable for all organisms. It is a component of haem-containing protein such as haemoglobin, myoglobin and cytochrome, and innumerable non haem-iron containing proteins with vital functions in many metabolic processes. Iron and Cu are
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found as components of protein, and catalyses redox reactions. Sources of heavy metals: Heavy metal pollution can originate from both natural and anthropogenic sources. Activities such as mining and smelting operations and agriculture have contaminated extensive areas of world such as Japan, Indonesia and China mostly by heavy metals such as Cd, Cu and In (Herawati et al., 2000), Cu, Cd and Pb in North Greece (lantopoulos et al., 1999), in Albania (Shallari et al., 1998) and Cr, Pb, Cu, Ni, In and Cd in Australia (Smith et al., 1996). Heavy metals originate within the Earth's crust, hence their natural occurrence in soil is simply a product of weathering process. (a) Natural sources of heavy metals: The most important natural source of heavy metals is geologic parent material or rock outcroppings. The composition and concentration of heavy metals depend on the rock type and environmental conditions, activating the weathering process. The geologic plant materials generally have high concentrations of Cr, Mn, Co, Ni, Cu, In, Cd, Sn, Hg and Pb. However, class wise the heavy metal concentrations vary within the rocks (Table 3). Soil formation takes place mostly from sedimentary rock, but is only a small source of heavy metals, since it is not generally or easily weathered. However, many igneous rocks such as olivine, augite and hornblende contribute considerable amounts of Mn, Co, Ni, Cu and In to the soils. Within the class of sedimentary rocks, shale has highest concentrations of Cr, Mn, Co, Ni, Cu, In, Cd, Sn, Hg and Pb followed by limestone and than sandstone. Volcanoes have been reported to emit high levels of AI, In, Mn, Pb, Ni, Cu and Hg along with toxic and harmful gases (Seaward and Richardson, 1990). Airborne emissions of heavy metals originate from forest and prairie fires (Ross, 1994). Volatile heavy metals such Hg and Se are part of carbonaceous matter produced during the fire. Wind dust, which arises from desert region such as Sahara, has high levels of Fe and lesser amounts of Mn, In, Cr, Ni and Pb (Ross, 1994). Natural vegetation emits heavy metals into the soil and atmosphere through leaching from leaves and stems, decomposition and volatilization. Many heavy metals have been detected in inland coastal areas due to seasprays and aerosols produced in oceanic activities. (b) Agricultural sources of heavy metals: The inorganic and organic fertilizers are the most important sources of heavy metals to agricultural soil include liming,
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Biological effects of heavy metals: an overview Table -1 : Heavy metal composition of typical uncontaminated soils and agricultural crops (Modified from Allaway, 1968). Heavy metals Cd Co Cr Cu Fe Mn Mo Ni Pb Zn
Range in agricultural crops (ppm d. wt) 0.2-0.8 0.05-0.5 0.2-1.0 4-15
Range in soil (ppm d. wt) 0.01-0.7 1-40 5-3000 2-100 7000-550000 100-4000 0.2-5 10-1000 2-200 10-300
15-100 1-100 1.0 0.1-10 15-200
Table - 2: Toxic limits and recommendedl safe intake of selected heavy metals for human health (Modified from Oliver, 1997). Recommended/ safe intake 15-25 ~g/day (adults) Max. tolerable intake: 70 ~ g/day 2-25 ~ g/day (children) 15-50 ~g/day (adults) 50-200 ~ g/day 2 mg/day (adults) 80 ~ g/day (infants) 40 ~ g/day (children) 20-282 ~ g/day (adults) 9-278 ~ g/day (children) Safe intake: 15 ~ g/day Recommended upper limit: 45 ~ g/day
Toxic limits 3 mg/day for 2-3 weeks 200 ~ g/kg of fresh weight
Heavy metal As Cd
Cr Cu
12 mg/day (adults) 150 ~g/day (children)
Pb
? 500 ~ g/I toxic concentration in blood. 250-550 ~ g/I (children) 150 ~ g/day
Zn
Table - 3 : Range of heavy metal concentrations (ppm) in igneous and sedimentary rocks (Cannon et al., 1978). Metals As Cd Cr Co Cu Pb Mo Ni Zn
Basaltic igneous 0.2-10 0.006-0.6 40-600 24-90 30-160 2-18 0.9-7 45-410 48-240
Shales and clays --0.0-11 30-590 5-25 18-120 16-50 --20-250 18-180
Granitie igneous 0.2-13.8 0.003-0.18 2-90 1-15 4-30 6-30 1-6 2-20 5-140
Black shales _n
