Health risk of exposure to organo-metallic compounds

Shishir Reza | Published: October 25, 2019 20:06:22

Waste is any product of metabolism that is not required for further metabolic process and is therefore excreted from the body. Waste is directly linked to human development -- both technologically and socially. The composition of different types of waste varies depending on time and location, with industrial development and innovation being directly linked to waste materials. Examples of this include plastic and nuclear technology. Some components of waste have economic value and can be recycled. Chemical waste is a waste that is made from harmful chemicals.

A variety of toxic organo-metallic compounds are found in the environment. The toxicological properties of some organo-metallic compounds - pharmaceutical, organo-arsenical fungicidies, and tetraethyl lead antiknock gasoline additives have been used for many years. Careful consideration should be given to the potential toxicities of many relatively new organo-metallic compounds that are now being used in semiconductors, as catalysts, and for chemical synthesis. Organo-metallic compounds often behave in the body in ways totally unlike the inorganic forms of the metals that they contain. This is due in large part to the fact that compared to inorganic forms, organo-metallic compounds have an organic nature and higher lipid solubility.

Perhaps the most notable toxic organo-metallic compound is tetraethyl lead, Pb(C2H5)4, a colourless, oily liquid that was widely used as a gasoline additive to boost octane rating. Tetraethyl lead has a strong affinity with lipid and can enter the body by all three common routes of inhalation, ingestion and absorption through the skin. Acting differently from inorganic compounds in the body, it affects the central nervous system with symptoms such as fatigue, weakness, restlessness, ataxia, psychosis, and convulsions. Recovery from severe lead poisoning tends to be slow. In cases of fatal tetraethyl lead poisoning, death occurs as soon as 1 or 2 days after exposure. Metal carbonyls, regarded as extremely hazardous because of their toxicities, include nickel tetra carbonyl (Ni(CO4)), cobalt carbonyl, and iron pent carbonyl. Some of the hazardous carbonyls are volatile and readily taken into the body through the respiratory tract or through the skin. The carbonyls affect tissues directly and break down to toxic carbon monoxide and products of the metal, which have additional toxic effects.

The metal-carbon bond in organo-metallic compounds is generally of a character intermediate between ionic and covalent. Primarily, ionic metal-carbon bonds are encountered either when the metal is very electropositive (as in the case of the alkali metals) or when the carbon-containing ligand exists as a stable carbanion. Carbanions can be stabilised by resonance (as in the case of the aromatic cyclopentadienyl anion) or by the presence of electron-withdrawing substituent (as in the case of triphenylmethyl anion). Hence, the bonding in compounds like sodium acetylide and triphenylmethyl potassium is primarily ionic. On the other hand, the ionic character of metal-carbon bonds in the organo-metallic compounds of transition metals, poor metals, and metalloids tends to be intermediate, owing to the middle-of-the-road electro negativity of such metals. In organo-metallic compounds , most p-electrons of transition metals conform to an empirical rule called the 18-electron rule. This rule assumes that the metal atom receives from its ligands the number of electrons needed in order for it to attain the electronic configuration of the next noble gas.

Radioactive decay is the process by which an unstable atomic nucleus loses energy by emitting ionising particles or radiation. The emission is spontaneous in that the nucleus decays without collision with another particle. This decay or loss of energy results in an atom of one type, called the parent radionuclide, transforming into an atom of a different type, named the daughter nuclide. For example: a carbon-14 atom (the "parent") emits radiation and transforms into a nitrogen-14 atom (the "daughter"). This is a stochastic process at the atomic level, in that according to quantum mechanics it is impossible to predict when a given atom will decay. However, given a large number of similar atoms the decay rate, on average, is predictable.

The SI unit of activity is the becquerel (Bq). One Bq is defined as one transformation (or decay) per second. Since any reasonably-sized sample of radioactive material contains many atoms, a Bq is a tiny measure of activity; amounts on the order of GBq (gigabecquerel, 1 x 109 decays per second) or TBq (terabecquerel, 1 x 1012 decays per second) are commonly used. Another unit of radioactivity is the curie, Ci, which was originally defined as the amount of radium emanation (radon-222) in equilibrium with one gram of pure radium, isotope Ra-226. At present it is equal, by definition, to the activity of any radionuclide decaying with a disintegration rate of 3.7 × 1010 Bq. The use of Ci is presently discouraged by the SI (Internet4).

Nuclides produced as daughters are also called radiogenic nuclides, whether they themselves are stable or not. A number of naturally occurring radionuclides are shortlived radiogenic nuclides that are daughters of radioactive primordial nuclides. Other naturally-occurring radioactive nuclides are cosmogenic nuclides, formed by cosmic ray bombardment of material in the Earth's atmosphere or crust.

In practice there is no such thing as zero radioactivity. Not only is the entire world constantly bombarded by cosmic rays, but also every living creature on earth contains significant quantities of carbon-14 and most (including humans) contain significant quantities of potassium-40. These tiny levels of radiation are not any more harmful than sun light. The hazards to people and the environment from radioactive contamination depend on the nature of the radioactive contaminant, the level of contamination, and the extent of the spread of contamination. Low levels of radioactive contamination pose little risk, but can still be detected by radiation instrumentation. In the case of low-level contamination by isotopes with a short half-life, the best course of action may be to simply allow the material to naturally decay. Longer-lived isotopes should be cleaned up and properly disposed of, because even a very low level of radiation can be life-threatening.

High levels of contamination may pose major risks to people and the environment. People can be exposed to potentially lethal radiation levels, both externally and internally, from the spread of contamination following an accident (or a deliberate initiation) involving large quantities of radioactive material. The biological effects of external exposure to radioactive contamination are generally the same as those from an external radiation source not involving radioactive materials, such as x-ray machines, and are dependent on the absorbed dose.

The biological effects of internally deposited radionuclides depend greatly on the activity and the biodistribution and removal rates of the radionuclide, which in turn depends on its chemical form. The biological effects may also depend on the chemical toxicity of the deposited material, independent of its radioactivity. Some radionuclides may be generally distributed throughout the body and rapidly removed, as is the case with tritiated water. Some radionuclides may target specific organs and have much lower removal rates. For instance, the thyroid gland takes up a large percentage of any iodine that enters the body. If large quantities of radioactive iodine are inhaled or ingested, the thyroid may be impaired or destroyed, while other tissues are affected to a lesser extent. Radioactive iodine is a common fission product; it was a major component of the radiation released from the Chernobyl disaster, leading to nine fatal cases of pediatric thyroid cancer and hypothyroidism. On the other hand, radioactive iodine is used in the diagnosis and treatment of many diseases of the thyroid precisely because of the thyroid's selective uptake of iodine.

Waste Minimisation is an appropriate strategy to address the problems of industrial pollution. The objective of the scheme is to assist the small and medium scale industries in the adoption of cleaner production practices. Under the grant-in-aid scheme Industrial Pollution Abatement through Preventive Strategies, a component of "Waste Minimisation in Small Scale Industries" is being implemented through National Productivity Council and other agencies in Bangladesh. So far 118 Waste Minimisation Circles have been established in 41 sectors in 17 geographic locations throughout the country. The activities undertaken under this include the following: establishing and running Waste Minimisation Circles in clusters of small scale industries; capacity building in the area of waste minimisation/cleaner production through training and awareness programme; waste minimisation demonstration studies in selected industrial sectors; preparation of sector-specific technical manuals on waste minimisation; preparation of compendium of success stories on cleaner production/waste minimisation.

Globally waste management is a critical issue in maintaining environmental quality. Every day huge amounts of hazardous and toxic wastes are discharged into our environment. Owing to increasing industrialisation and ever increasing population, use of different types of chemicals has been increasing over the last few decades. This has increased generation of waste in both developed and developing countries making effective waste management a dire necessity.

Shishir Reza is an environmental analyst & Associate Member of Bangladesh Economic Association. shishiresrm@gmail.com

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