Pollution Due to Trace Elements.
Pollution Due to Trace Elements There are approximately 90 elements that are found in the earth’s crust. From these only 9 elements as Al, Fe, Ca, Mg, O, Si, Na, K, and Ti are over 99% by weight. The remaining 81 elements together, account for hardly 0.14% by weight. These 81 elements constitute the so-called trace elements. These elements play a vital role in plant and animal nutrition. These elements occur in Living tissues in minute amounts or traces. Virtually all these elements can be estimated with great accuracy and precision with the help of modern analytical methods. The various techniques are as follows.
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- Neutron activation analysis.
- X-ray fluorescence spectrometry.
- Atomic absorption spectrometry.
The trace elements are mobilized by three types of mechanisms i. e., primary, secondary and tertiary dispersions.
Essential Trace Elements.
An essential trace element should satisfy the following criteria.
- It should be present in all healthy tissues in all living things.
- The concentration of an essential trace element is fairly constant among various species.
- Its deficiency or depletion induces reproducible structural and physiological abnormalities.
- The deficiency symptoms or abnormalities disappear or reverse on replenishment that element.
- These biochemical changes due to deficiency can be cured when the deficiency is cured.
Among 40 naturally occurring elements detected in living organisms, about 25 of them seem to be highly essential for the higher animals and humans. These are as follows:
- Metals: V, Cr, Mn, Fe, Co, Cu, Mo, Zn, Na, K, Ca, Mg, and Sn.
- Non-metals: F, Cl, Br, I, H. C, N, O, Si, P, S, and Se.
some trace metals such as Pb, Cd, and Hg and metalloids like As, Sb, and Se considered being toxic.
Toxic Trace Elements.
The sources, uses, environmental levels, ecological, biochemical and toxicological effects of some typical trace elements are discussed in the following section,
Mercury is a non-essential trace metal. It constitutes 3 x 106 % by of the earth’s crust. It is extracted mainly from its sulfide ore i. e., cinnabar (HgS).
Compounds of Mercury (Hg) and Their Uses.
Mercury, both in the metallic form (quicksilver) and as the sulfide (HgS) plays a prominent role in the following aspects.
- Mercuric oxide, salicylate, and chloride salts were used as antiseptics.
- Red mercuric sulfide, mercuric benzoate, mercurous acetate, and mercurous iodide were used for treating syphilis.
- Mercury chloride or calomel has been used as a laxative.
- Mercury is now mostly used as filling material for dental cavities as silver amalgam.
- Mercury is still used as a diuretic to a limited extent.
Industrial Uses of Mercury (Hg).
Mercury finds extensive uses in the following industries.
- Pharmaceutical industries.
- The plastics industry in the manufacture of vinyl chloride.
- Electrical and electronic industries in the manufacture of mercury vapor lamps and fluorescent tubes, batteries, electric switch gears, etc.
- Paper and pulp industry.
- The Chlor-alkali industry for the manufacture of Cl2, and NaOH.
Pollution Sources of Mercury (Hg).
The effluents from the above-mentioned industries create an environmental hazard. The main sources of Hg as a pollutant are as follows.
Combustion of fossil fuels is the main source of air pollution by mercury and about 3000 tonnes of mercury is annually released into the atmosphere by this source.
The mercury contents of bituminous coal, anthracite coal, crude oil, and residual tars are in the range, 1-25 ppb; 1100-2700 ppb, 1900-21,000 ppb, and 520,000 ppb respectively.
Organomercurials used as fungicides for seed dressing in agriculture is also a widespread source of pollution.
- It is estimated that the mercury electrodes used in the Chlor alkali industry all over the world themselves disperse 1 million pounds of mercury into the environment.
- Natural weathering processes and submarine volcanism release about 5,000 tonnes of mercury annually into the oceans.
Control of Mercury Pollution.
Mercury pollution is a fact of life. We have to live with it. However, we can restrict mercury pollution from man-made sources. Stringent legislations are being enforced in many countries to control mercury pollution from man-made sources and the results are encouraging. Banning of mercury pesticides and restricted use of other mercurial pesticides in only selected areas should be done. We should encourage new technology to replace the use of mercury in chloralkali plants. Decontamination of mercury rich bottom sediments of rivers and lakes can be done by covering with inert or adsorbent materials. However, mankind will never be able to control mercury pollution from natural sources with its serious problems in the fishing industry.