Assistant Professor

Chemistry

Gushkara Mahavidyalaya

 Gushkara, West Bengal, India 

Introduction

As we know all, today we are enveloped with many problems at the global scale, such as pollution, over-population, etc. But toxic effects caused by the heavy metals appear as a new challenge for the all concern. All living organisms whether plants or animals or even micro-organisms are ultimately composed of cells, which in turns are composed of macromolecules. These macromolecules, i.e. Carbohydrates, Lipids, Fats, etc. are composed of elements. If there occur even a slight variation (both excess and less) then occurs problem, which describe in technical terms as Toxicity.

 All chemicals are potentially toxic if organisms are exposed to a sufficiently large dose. Among the naturally occurring elements, the ones that have most frequently been associated with toxicity from environmental exposures include the heavy metals such as silver (Ag), Cadmium (Cd), Iron (Fe), Mercury (Hg), Molybdenum (Mo), Nickel (Ni), Lead (Pb), Tin (Sn) and Zinc (Zn).

All of the naturally occurring elements are ubiquitous in the environment in at least trace concentrations. This means that there is a universal contamination of soil, water and air with all of the natural elements. This contamination will always detectable, if the analytical – chemical methodology has detection limits that are sufficiently small. Some instances of elemental “pollution” are natural in origin. Such occurrences often involve surface exposures of minerals containing large concentrations of particular toxic elements, resulting in the pollution of soil, biota and water. In some cases, pollution by toxic natural elements that exceeds caused by anthropogenic emissions.

Some of these elements are required by plants and / or animals as essential micronutrients, e.g. Cu, Fe, Mo, Zn, etc. Under certain environmental conditions, however, these same elements may bioaccumulations to toxic concentrations and causes ecological damages.

 However, natural pollution is usually local in extent. It was not until the modern era of industrialisations that there was a widespread environmental contamination by toxic elements (and by other types of pollutants). The most polluted areas in this regard are north-eastern United States, Central Europe, Shanghai, and metropolitan cities of India. That’s why there is great to stop these heavy metal toxicity in these areas and to prevent them to spread to unaffected areas.  

Heavy Metals in Environment

All metals including heavy metals are universal in origin but they vary in proportions. Concentration of heavy metals in soils and rocks are generally larger than water and are generally larger in organism. However, it’s interesting that since the chemical form of elements dissolved in water is usually relatively available for uptake by biota, even a seemingly small aqueous concentration may exert powerful toxic effects. In contrast, the relatively large concentrations in soil and rocks are largely insoluble. Hence, in solid substances such as soils and rocks, the availability of toxic elements to plants and animals is much less than might be suggested by the “total” concentration, and there may not be a toxic effect.

Most elements are consistently present in small concentrations in the environment, e.g. Cd, Cr, Co, Cu, Pb, Hg, Mo, Ni, etc. However, all of these elements are potentially toxic, and they can affect many species of plants and animals at water-soluble concentrations smaller than about 1 ppm. In contrast, other elements can occur in large concentration in some compartments of the biosphere. Most notable in this respect is Fe (3-4 % of the Earth’s crust). But this is regarded as “unavailable” because it is present in insoluble form. 

The concentration (ppm) of different heavy metals in the environment is tabulated below:

Metal	Rocks	Water	Plants	Fishes	Animals	Soils
Ag	0.04 – 0.25	0.0003	0.01 – 0.8	0.04 – 0.1	0.009 – 0.4	0.05
Cd	0.028 – 0.22	0.0001	0.1 – 2.4	0.1 – 3	0.1 – 3.2	0.35
Cr	4 – 90	0.001	0.03 – 10	0.03 – 2	0.002 – 33	70
Co	0.1 – 35	0.0002	0.005 – 1	0.006 – 0.05	0.005 – 1	8
Fe	17 – 56	0.5	70 – 700	9 – 98	3 – 380	40
Hg	0.012 – 0.29	0.0001	0.005 – 0.02	0.4	0.02 – 0.45	0.06
Mo	0.2 – 2.6	0.0005	0.06 – 3	1	0.07 – 0.7	1.2
Ni	0.5 – 150	0.0005	1 – 5	0.1 – 4	0.7 – 1.2	50
Pb	3 – 24	0.003	1 – 13	0.1 – 15	0.2 – 30	35
Sn	0.5 – 3.5	0.000009	0.2 – 2	Absent	0.01 – 2	4
Zn	20 – 120	0.015	20 – 400	9 – 80	75 – 240	90
As	1 – 13	0.0005	0.2 – 7	0.2 – 10	0.007 – 1.6	6
V	45 – 250	0.0005	0.001 – 0.5	0.3	0.002 – 0.03	90

Mechanism of Toxicity by Heavy Metals

The mechanism of toxicity of metals is frequently damaged to an enzyme system. This occurs when metal ions bind to the enzyme and caused a change in its 3-D structure, with a resulting change or loss of its specific catalytic function. Another common mechanism of toxicity is damaged to DNA via metal binding, resulting in genetic damages by a disruption of transcription, by an inability to produce specific proteins (especially enzymes), or by some other toxic effect. Symptoms of acute toxicity to biota can include abnormal patterns or animals of growth and development, impaired reproduction, diseases and death. Chronic toxicity can also occur, but the symptoms are more difficult to detect and may only consists of small decreases in productivity.    

Toxicity is influenced by number of factors. These are listed below.

1. The Chemical Form of Toxic Elements:

The form of a substance may have a profound impact on its toxicity especially for metallic elements, also termed heavy metals. For example, the toxicity of mercury vapour differs greatly from methyl mercury. Another example is chromium. Cr³⁺ is relatively nontoxic whereas Cr⁶⁺ causes skin or nasal corrosion and lung cancer.

The chemical activity of substances also varies greatly. Some can quickly damage cells causing immediate cell death. Others slowly interfere only with a cell's function. For example:

a.Hydrogen cyanide binds to the enzyme cytochrome oxidase resulting in cellular hypoxia and rapid death.

b. Nicotine binds to cholinergic receptors in the central nervous system (CNS) altering nerve conduction and inducing gradual onset of paralysis.

2. The Chemical Environment:

This factor can reduce or exacerbate toxicity. For example, (a) the co-occurrence of calcium in a large concentration may decrease the toxicity of many metals; (b) soil organic matter and clay can bind and this partially immobilized ionic metals; (c) acidity frequency increases solubility and therefore enhances the exposure to metals; (d) Depending upon the element, oxygen concentrations and redox potentials may have large influences on total solubility and speciation of ions.

3.  Differences in Susceptibility among Individuals, Population and Species:

These differences may be related to: (a) different exposure pathways, which are a reflection of environment influences on toxicity; (b) Varying susceptibility among life-history stages or sexes; (c) Genetically based differences in physiological susceptibility, as in case of metal-tolerant plant ecotypes.

Effect of Heavy Metals

In this content, influences of heavy metals are described.

1. Lead:

Lead is one of the most toxic metals known. It is absorbed through precipitation or is supplied as dust, the content of which are released due to action of organic acids. Lead is a toxic metal used in a variety of products and materials. The main sources of lead exposure are lead based paints, gasoline, cosmetics, toys, household dust, contaminated soil, industrial emissions. Its use in various products, such as paints, gasoline, etc., has been considerably reduced nowadays. The main sources of lead exposure are lead based paints, gasoline, cosmetics, toys, household dust, contaminated soil, industrial emissions [1].When lead is absorbed into the body, it can cause serious damage to vital organs like the brain, kidneys, nerves, and blood cells. Lead poisoning is especially harmful to children. Lead poisoning can cause: 

1. Brain, liver, and kidney damage
2. Slowed development
3. Learning or behavior problems
4. Lowered intellect (or IQ)
5. Hearing loss
6. Restlessness
7. Headaches
8. Stomachaches
9. Nausea
10. Tiredness

2. Cadmium:

The kidney is the critical target organ following long-term exposure to cadmium. Cadmium accumulates primarily in the kidneys and its biological half-life in humans is 10–35 years. High intake of cadmium can lead to disturbances in calcium metabolism, the formation of kidney stones and effects on bone. Osteomalacia (softening of the bones) and osteoporosis may occur in those exposed through living or working in cadmium-contaminated areas; for example, in an area of Japan where soil was contaminated with cadmium from zinc/lead mines, itai-itai disease (characterized by osteomalacia, osteoporosis, painful bone fractures and kidney dysfunction) used to be widespread [2].

Plants can take up airborne cadmium as well as absorb it from the substrate. The indications of damage to the plants are noticeable in the pigmentation and growth rate. In some plants low concentration of cadmium have been reported to promote growth. 

3. Zinc: 

Zinc is a transition metal usually present in nature in its divalent state. It is considered an essential mineral as it is necessary for the production of hundreds of enzymes throughout the body. But  increased levels of zinc can become toxic. Acute toxicity (ingesting more than 200 mg/day of zinc) can cause:

a. Abdominal pain, nausea, vomiting and diarrhoea.

b. Other reported effects - these include gastric irritation, headache, irritability, lethargy, anaemia and dizziness [3]

Studies showed that increase in zinc concentration in plants was due to heavy industrialization.

4. Mercury:

Mercury is a heavy metal of known toxicity, noted for inducing public health disasters in Minamata Bay, Japan. Plants collect it through rainwater or dust particles. Major sources of mercury pollution include anthropogenic activities such as agriculture, municipal wastewater discharges, mining, incineration, and discharges of industrial wastewater. [4]. Mercury exists in three oxidation states: Hg° (metallic), Hg⁺ (mercurous) and Hg⁺⁺ (mercuric) mercury. The latter forms a variety of inorganic as well as organometallic compounds. Elemental and methylmercury are toxic to the central and peripheral nervous systems. The inhalation of mercury vapour can produce harmful effects on the nervous, digestive and immune systems, lungs and kidneys, and may be fatal. The inorganic salts of mercury are corrosive to the skin, eyes and gastrointestinal tract, and may induce kidney toxicity if ingested. Symptoms include tremors, insomnia, memory loss, neuromuscular effects, headaches and cognitive and motor dysfunction also noted [5]. 

5. Arsenic: 

Arsenic is a natural component of the earth's and a heavy metal.  It exists as organic or inorganic compounds. It is highly toxic in its inorganic form. Drinking water may get contaminated by use of arsenical pesticides, natural mineral deposits or inappropriate disposal of arsenical chemicals [6].

arsenic poisoning may occur by ingestion or inhalation of high levels of arsenic. Symptoms of arsenic poisoning may include:

1. Red or swollen skin
2. Skin changes, such as new warts or lesions
3. abdominal pain
4. Nausea and vomiting
5. Diarrhea
6. Abnormal heart rhythm
7. Muscle cramps
8. Tingling of fingers and toes

6.  Chromium:

With the increasing use of Cr, the adverse effects of chromium compounds in human health were being defined. Chromium occurs naturally by the burning of oil and coal, petroleum from ferro cromate refractory material, pigment oxidants, catalyst, chromium steel, fertilizers, oil well drilling and metal plating tanneries. Anthropogenically, chromium is released into the environment through sewage and fertilizers [7].Little toxic effect is attributed to trivalent chromium when present in very large quantities. Both acute and chronic toxicity of chromium are mainly caused by hexavalent compounds.  Cr exists in several oxidation states but the most stable and common forms are Cr(0), Cr(III) and Cr(VI) species. Cr(VI) as being highly mobile is toxic, while Cr(III) as less mobile is less toxic. The cause of Chromium Toxicity is generally due to repeated exposure either through oral ingestion (acute) or skin contact and inhalation (which is mostly chronic).

Common signs and symptoms of Chromium Toxicity include [8]:

1. Severe gastrointestinal irritation or ulcers
2. Nausea, vomiting, and diarrhea
3. Fever
4. Vertigo
5. Cramping of muscles
6. Intravascular hemolysis
7. Circulatory collapse
8. Liver damage

Conclusion 

Man with his intelligence has almost mastered the nature. But this so called “mastering the nature” has now caused the several problems like pollution, deforestation, terrorism, etc. of which the first one is our concern. Due to heavy metal pollution, plants may go on affecting severely resulting into the secondary effect on animals including human beings. If there is harmful effect on human beings such as health problems, environmental problems no one can live healthy.  Thus checking and controlling of heavy metals in environments is one of the main issues and requirements of today.

References

1. Gerhardsson L, Dahlin L, Knebel R, Schütz A. Blood lead concentration after a shotgun accident. Envirn. Health Perspect. 2002; 110(1),115–117 

2. Genchi G , Sinicropi M. S, Lauria G , Carocci A, Catalano A, Review The Effects of Cadmium Toxicity Int. J. Environ. Res. Public Health. 

3. Fosmire G J, Zinc toxicity, Am. J. Clin. Nutr., 1990; 51, (2), 225–227 

4. Chen CW, Chen CF, Dong CD. Distribution and Accumulation of Mercury in Sediments of Kaohsiung River Mouth, Taiwan. APCBEE Procedia. 2012; 1, 153–158

5. Environmental Health Department, Ministry of the Environment, Minimata Disease: The History and Measures, Ministry of the Environment, Government of Japan, Tokyo, Japan, 2002.

6. Jaishankar M, Tseten T, Anbalagan N, Mathew B. B, Beeregowda K. N. Toxicity, mechanism and health effects of some heavy metals. Interdiscip Toxicol. 2014 ;7(2), 60-72. 

7. Ghani A. Effect of chromium toxicity on growth, chlorophyll and some mineral nutrients of Brassica juncea L. Egyptian Acad J Biol Sci. 2011; 2(1), 9–15 

8. Baruthio F. Toxic effects of chromium and its compounds. Biol Trace Elem Res. 1992 ;32,145-53.