Introduction Critical discussion on heavy metals. Target organs by heavy metal pollutantsIndustrial uses and pollution sources of Mercury. Ef mercury Biochemical effects, toxicology and toxicity of mercury Biomethylation of mercury Control of mercury pollutants Treatment on mercury poisoning.

1. Toxicity of Metallic Species:
Mercury
DR. VIKAS A.THAKUR
K.B.P.COLLEGE, VASHI
(EMPOWERED AUTONOMOUS)

2. Conception of heavy metals
❑Critical discussion on Heavy Metals.
➢ A heavy metal is any metal or metalloid of environmental concern.
➢ This term originated with reference to the harmful effects of Cadmium,
mercury and lead all of which are denser than iron.
➢ According to the International Union of Pure and Applied
Chemistry(IUPAC), the term heavy metal may be ‘’meaningless term’’.
➢ Because there is no standardized definition for a heavy metal.
➢ Some light metal or metalloid are toxic while some high density metals
are not.

3. Critical discussion on Heavy Metals.
➢ For example, Cadmium is generally is considered as a heavy metal with an atomic number
of 48 and Specific gravity of 8.65.
➢ While gold is typically non-toxic even though it has an atomic number of 79 and specific
gravity of 18.88.
➢ For a given metal, the toxicity varies widely depending on the allotrope or oxidation state
of the metal.
➢ Hexavalent Chromium(Cr6+) is deadly poisonous while Trivalent Chromium(Cr3+) is
nutritionally significant in many organisms including humans.
❖ Although there is no clear definition of what a heavy metal is, density in most cases taken
to be a defining factor.

4. ❖ Heavy metals are thus commonly defined as those having specific density more than 5
g/cm3.
❖ The main threats to human health from heavy metals are associated with exposure to lead,
Cadmium, mercury and Arsenic.
❖ Arsenic is a metalloid but it is usually classified as a heavy metal.
❖ Heavy metals have been used in many different areas for thousands of years.
❖ Lead has been used at least 5000 years, early applications including building materials,
pigments for glazing ceramics & pipes for transporting water.
❖ In ancient Rome, lead acetate was used to sweeten old wine and some Romans might have
consumed as much as a gram of Lead a day.

5.  Heavy metals are generally defined as metals with relatively high
densities, atomic weights or atomic numbers.
 The criteria used and whether metalloids are included vary depending on
the author and context.
 Example. In metallurgy, heavy metal may be defined on the basis of
density.
 While in Physics, distinguishing criteria might be atomic number.
 While in Chemistry, it is concerned with chemical behaviour.
 More specific definitions has been published but none of these have
been widely accepted.
 This definition covers 96 out of 118 elements. Only Mercury, lead and
bismuth meet of them.
 Density Criteria range from 3.5 g/cm3 to above 7 g/cm3.
 Atomic weight defination can range from greater than Sodium(At.wt.
22.98), greater than 40(excluding S and f –block metals, stay with Sc, or
more than 200(Mercury onwards).

6.  Although several adverse health effects of heavy metals have been known for a long time,
exposure to heavy metal continues.
 It is even increasing in some parts of the world, in particular in less developed countries.
 Though emissions have declined in most developed countries over the last 100 years.
 Cadmium compounds are currently mainly used in re-chargeable batteries of Nickel-
cadmium.
 Cadmium emissions have increased dramatically during 20th Century, one reason is that
Cadmium containing products are rarely recycled but often dumped together with house
hold waste.
 Cigarette smoke is a major source of Cadmium exposure.
 Recent data indicates that adverse health effects of Cadmium exposure may occur at lower
exposure levels than previously anticipated.
 Primarily in the form of kidney damage but possibly also bone effects and fractures.
 Many individuals in Europe already exceed this exposure level.
 Therefore measures have to be taken to reduce Cadmium exposure.

7. ❖ From the Environmental pollution point of view, metals are
broadly classified into following three categories.
➢ (1). Non-toxic but accessible.
➢ (2). Toxic but non-accessible.
➢ (3). Toxic and Accessible.
❖ The third category of potentially toxic and relatively
accessible metals have attracted more attention from the
point of view of environmental pollution and Public health.
❖ These toxic metals occur in very small quantities in the
earths crust(less than 1000 ppm).
❖ Hence they are called ‘’Trace metals.’’

8. ❖ They are further arbitrary subdivided on the basis of their densities.
❖ Those having densities below 5 g/cm3 are called ‘’light metals’’.
❖ Those with densities above 5 g/cm3 are designated as ‘’heavy
metals.’’
❖ Thus metals like Hg, Pb, cu, Cd, Zn, Ni etc. are generally known as
‘’toxic heavy metals.’’
❖ Surprisingly, even metalloids like As and Sb are also generally
considered under this category.
❖ Thus a term heavy metal is a misnomer term.
 Although the toxic heavy metals are present in the earths crust in
trace levels, anthropogenic activities such as Industrial processing
and use of metals, alloys and metallic compounds disperse them into
the environment.
 This disturbs natural level.

9. Heavy Metals present in the Effluent
Sr.No. Industry Heavy metal present in effluent
1 Chlor-alkali Cd,Cr,Cu,Pb,Zn,Hg and Se
2 Electroplatting Cu, Cr, Zn, Ni
3 Leather Cr
4 Batteries Pb,Cd,Hg
5 Paints & Dyes Cd, Cr, Cu, Pb, Hg, Se
6 Textiles Cr
7 Paper and Pulp Hg
8 Petroleum Refining Zn, Cd, Cu, Cr, Pb
9 Fertilizers As, Cd,Cr, Cu,Pb,Mn, Hg, Zn
10 Motor Vehicles Cd,Cr,Pb, Hg, Se, Zn

10. Heavy Metals present in the Effluent
Sr.No. Industry Heavy metals present in the
effluent
12. Mining & Metallurgy As, Cd,Cr,Cu,Hg, Se,Zn
13. Explosives As, Hg, Pb, Cu
14. Pesticides As, Hg, Pb, Se

11. Target Organs Affected by Heavy Metal pollutants
Sr.No
.
Heavy metal Pollutants Target organ
1. (As), Hg, Mo, Se Liver
2. (As), Cd, Hg, Pb Blood
3. (As), Hg, Pb Brain
4. (As), Cd, Hg Lungs
5. (As), Hg, Pb, Cd Kidney
6. Cd, Se Bones, Teeth

12. Toxicity of Metallic Species: Mercury(Hg)
 Mass : 3.3011x 1023kg
 Density : 5.427 g/m3

13. Toxicity of Metallic Species: Mercury(Hg)

14. Mercury : Introduction
 Mercury is a non-essential trace metal constituting
0.00003% by weight of the earth’s crust.
 It is extracted mainly from its sulphide ore,
Cinnabar(HgS)
 Mercury, both in metallic form(Quick silver) and as
the sulphide ore (HgS) played a prominent role .
 In therapeutics, Alchemy and folklore.
 Mercury chloride or calomel got popularity as a
Laxative.
 Mercuric oxide, Salicylate and chloride salts were
used as antiseptics.

15.  Red Mercuric sulphide, mercuric benzoate,mercurous acetate and mercurous
iodide were used from treating syphills.
 Mercurin is still used as diuretic to a limited extent.
 Mercury is now mostly used as a filling materials for dental cavities as a Silver
amalgam.
 In Past mercury was used as in felt-hat manufacture and furr processing for minor
extent.

16. Industrial Uses and Pollution Sources.
❑ Mercury finds extensive use in
 Chlor-alkali industry(In the manufacture of Cl2 and NaOH)
 Electrical and Electronic industries(in the manufacture of mercury vapour lamps, fluorescent
tubes, batteries, electric switch gears etc.),
 Plastic Industry(in the manufacture of Vinyl Chloride.
❑ Paper and pulp Industry.
❑ Pharmaceutical Industry.
➢ Hence the effluent from these industries pose environmental hazard.
➢ Organo mercurial used as a fungicides for seed dressing in agricultural are also widespread
source of pollution

18. Sources of Pollution
 Combustion of fossil fuel is the main source of air pollution by Mercury.
 About 3000 tonnes of mercury is annually released into atmosphere by this source.
 The mercury contents of Bituminous Coals is in the range of 1-25ppb;
 Anthracite coal is 1100-2700ppb; Crude oil is 1900-21000ppb and tars in the range of
5,20,000ppb.
 Natural weathering processes and submarine vulcanism releases about 5000 tonnes of
mercury annually into the oceans.
 Although, Natural agencies are predominant suppliers of mercury in the environment, the
proportion contributed by man made sources is increasing rapidly.

19. Sources of Pollution
 It is estimated that the mercury electrodes used in Chlor-alkali industry all over the world
disperse 1 million pounds of mercury into the environment.
 Mercury in the form of Cinnabar(HgS) or in the metallic state is embedded in igneous rocks, in
particular, basalt and granite.
 These rocks contain about 0-080ppm mercury content.
 Every year about 800 tonnes of mercury are released to the environment through weathering of
rocks.
 Volcanic action is yet another natural source of contamination of environment by mercury.
 Volcanic lava is always rich in the constituents of igneous rock.
 Because of repeated volcanic eruptions, the mercury content of Lake Ontake (Japan) has shown
a steady rise.

20. Environmental Levels
 Information regarding environmental levels of mercury came only after the
discovery of Flameless Atomic Absorption Analysis technique.
 Based on mean concentration of mercury in Cereals, pulses, milk and fish, the
average intake of mercury through the normal diet of an adult is estimated to be
7.3μg/day.
 The mercury concentration in ambient air in some industrial area in our country
is found to be 0.7 μg/m3.
 The mean mercury concentration in water varies from 2 μg to 33 μg/L.
 The USEPA limits of mercury in drinking water is 2 μg/L.

21. Absorption and Transport of elemental and Inorganic Mercury
❖ Monoatomic elemental mercury in the vapour state , Hg(g), is absorbed from inhaled air by the
Pulmonary route to the extent of about 80%.
❖ Inorganic mercury compounds are absorbed through the intestinal tract and in solution through
skin.
❖ Although elemental mercury is rapidly oxidized to mercury (II), in Erythrocytes(Red Blood
Cells), which have a strong affinity for mercury.
❖ A large fraction of elemental mercury absorbed through the pulmonary route reaches the brain
prior to oxidation.
❖ Then it enters that organ because of the lipid solubility of Mercury(0).
❖ This mercury is subsequentially oxidized in the Brain and remains there.
❖ Inorganic Mercury(II) tends to accumulate in the Kidney.

22. Biochemical Effects, Toxicity and Toxicology.
 The extreme toxicity of mercury, though suspected some
centuries back, was fully appreciated in the year 1953.
 When 52 persons living in fishing villages along Minamata
Bay, Japan, died of a mysterious disease.
 The Investigations revealed that the victims had eaten Shell-
fish contaminated with mercury containing effluent from a
nearby factory.
 Shin-Nihon Chiso Hiryo Co. operating since 1949.

23. Biochemical Effects, Toxicity and Toxicology
 This unit was producing Vinyl Chloride and acetaldehyde by the catalytic conversion
acetylene using HgCl2 as a Catalysts.
 The methyl mercury compound present in the effluent wastes discharged into the
Minimata Bay .
 These were gradually bioconcentrated by the fish and Shell-fish by the Bay which
provided the Sea-food for many families along its shores.
 The ailment become known as ‘’MINMATA DISEASE’’.
 It is characterized by peculiar neurological disorders.
 It was ultimately attributed to the organomercurials in year 1958.

24. Biochemical Effects, Toxicity and Toxicology
 Another major environmental crisis relating to mercury poisoning came to be known in
the year 1972.
 When 450 Iraqui villagers died after consuming grain.
 These grains were treated with mercury containing pesticide.
 Emergence of such episodes and the discovery of sensitive analytical techniques helped in
further research on mercury poisoning.
 Mercuric ions acts as potent enzyme inhibitors, protein precipitants and corrosive agents.
 Mercury has a great affinity for Sulphhydryl(-SH) groups, though it combined with
phosphoryl, Carboxyl amide and ammono group also.

30. Biochemical Effects, Toxicity and Toxicology
 The biochemical toxicology of mercury depends on the chemical form and entrance route
in the body.
 Elemental mercury is being lipid soluble can be absorbed through the intact skin.
 However, negligible amount of elemental mercury are absorbed through the gastric
mucosa.
 Hence elemental mercury swallowed during accidental breakage of thermometer in one’’s
mouth is of lesser concern than swallowing the pieces of glass .
 Because elemental mercury is fairly inert and is excreted without any serious damage.
 Mercury has a fairly high vapour pressure(0.001mm at 180C and rises markedly to
0.27mm at 1000C.
 The vapour is quite toxic if it is inhaled.

31. Biochemical Effects, Toxicity and Toxicology
 Vapour can diffuse through the lungs into the blood .
 From blood it is passed to the brain resulting in severe damage.
 For this reason, mercury should be handled only in well-ventilated areas.
 And any spilled Hg should be cleaned promptly.
 Mercury vapour, on inhalation enters the brain through the blood stream.
 It can cause severe damage to the central nervous system.
 Mercury ions react with chloride ions forming insoluble Hg2Cl2.
 As our stomach contains fairly high concentration of chloride, mercurous compounds do
not pose any hazard.
 Inorganic mercury compounds mainly attack liver and Kidney.

32.  Mercuric chloride is corrosive and hence when ingested , precipitates proteins of
the mucous membrane causing ashch(sticky) appearance of mouth, pharynx and
gastric mucosa.
 The affected areas cause intense pains, profuse vomiting, diarrhea and shock may
also experienced.

33. Biomethylation of Mercury
❖ Organic mercurals like Methyl mercury(CH3Hg+ ) are the most toxic substances.
❖ This is due to their solubility in fat, the lipid fraction of the membrane and brain
tissue.
❖ Although the Minamata’s chemical company discharged Hg into Minamata Bay, yet
the fish in the Bay were having CH3Hg+.
❖ It proved that mercury or its salts could be converted into Methyl Mercury by
anaerobic methane synthesizing bacteria called as ‘’methanogenic bacteria’’ in
water.
❖ The process is called as bio-methylation.
❖ This conversion gets facilitated by Co(III) containing vitamin B12 Coenzyme..

34. Bio -methylation of Mercury
 A CH3 group bonded to Co(III) on the coenzyme gets transformed enzymatically by methyl
Cobalmin to Hg2+ giving rise to CH3Hg+. Or (CH3)2Hg
 CH3 Co(III) + CH3Hg+.
 + Hg Methyl Colbamin + CH3)2Hg
(Co(III) ATP
❑ Higher levels of Methyl mercury chloride are found in deep, rather than shallow sediments.
❑ This indicates that bio-methylation largely occurs under anaerobic conditions.
❑ Biomethylation proceeds most effectively in the pH range 5.5-6.5

35. Bio-methylation of Mercury
 Methyl mercury tends to concentrate among the food chain.
 Being more soluble in the tissues of simple organisms than in surrounding water, those
substances penetrates inside the body of simple organisms.
 Subsequently, they became a concentrated component of the more complex species.
 This feed on the simpler one.
 This process is called as bioamplification.
 In aqueous solution, Methyl mercury chloride ionises as
 CH3HgCl --------- CH3Hg+. + Cl-
 However, the equilibrium constant for the reaction is only 10-5.45.

36. Bio -methylation of Mercury
 This indicates that methyl mercury chloride is weakly dissociated.
 The undissociated methyl mercury chloride is apparently readily transported across the
body membranes.
 And collects in the fatty tissues.
 From there the chances of its expulsion are quite more.
 Any factor which shifts the equilibrium to the left increases the chances of
bioamplification.
 Bioamplification of methyl mercury chloride in fish increases with temperature.

37. Biomethylation of Mercury
 Hg2+
 CH3Hg
 PLANKTON
 INSECTS SMALL FISHES
 BIRDS LARGE FISH MAN

38. Bio methylation of Mercury
 The symptoms of mercury poisoning may set in at blood levels of
as low as 0.5ppm of CH3Hg+
❑ And may even lead to chromosomal segregation and chromosomal
disruption .
❑ Inhibition of cell division.
❑ A concentration of 6ppm in brain cells can cause irreversible brain
damage.
.

39. Environmental fate
 Though Mercury may enter the hydrosphere initially in the form of Hg2+, it is really
scavenged by organic matter and converted by anaerobic bacteria such as ‘’Clostridium
Cochlearium’’.
 Which is facilitated by Co(III) containing vitamin B12 coenzyme to produce more toxic
organic mercurial such as methyl mercury(CH3Hg+) and dimethyl mercury(CH3-Hg-
CH3).
 Dimethyl mercury is stable at high pH values, but dissociated to CH3Hg+ in the low pH
range which normally exists in the bottom muds of streams and lakes.
 As methyl mercury is soluble, it is readily incorporated into organisms in the aquatic
environment.
 And ultimately finds its way into the higher members of the food chain such as Man.

40.  Dimethyl mercury is formed from monomethyl mercury under alkaline conditions.
 Dimethyl mercury being relatively volatile can contaminate the air.
 The biological half life of methyl mercury is about 74 days as compared to 5 days for that
of inorganic forms.
 A daily exposure to 1 mg of mercury vapour per m3 of air can be found fatal for humans.
 Mercury is teratogenic and can induce abortions and embryo toxicity.

41. Control of Mercury Pollutants
 Most of the environmental mercury is derived from natural sources such as weathering
processes and terrestrial and submarine volcanic activity.
 Hence mercury pollution is a fact of life and we have to live with it.
 However, we can restrict the mercury pollution from man made sources.
 Though efforts were made to substitute Mercury with other materials in some industries,
Mercury will continue to be used in fluorescent lamp industry, in Chlor-alkali plants, in
dentistry and in electronic industry.
 Stringent legislations are being enforced in many countries to control mercury pollution
from man-made sources.
 The results are encouraging.

42. Control of Mercury Pollutants
❑ The other measures are.
❖ 1.Banning of alkyl mercury pesticides.
❖ 2. Restricting use of other mercurial pesticides in only selected areas. And
❖ 3.Encouraging new techniques to replace the use of mercury in Chlor-alkali plants in
controlling mercury pollution.
➢ Decontamination of mercury rich bottom sediments of rivers and lakes by covering with inert
adsorbent materials is also being experimental.
➢ However, mankind will never be able to control mercury pollution from natural sources with its
serious problems in the fishing industry.
➢ In the world, suffering from hunger, it is impossible to neglect or prohibit tuna and Shark as
food sources merely because their present mercury levels are currently higher than the current
safety limits.

43. Treatment of Mercury Poisoning
❖ Acute Mercury poisoning usually results from the oral ingestion of highly dissociated
inorganic mercury compounds. Or
❖ Inhalation of vapours of elemental mercury. Or
❖ From organic mercurial or even from mercurial ointments applied typically.
❑ Treatment consists of introducing in the stomach, a source of sulph hydryl-rich protein
such as milk or raw eggs and then performing Copious lavage(plentiful overflowing
sufficient washing).
❑ Sodium formaldehyde Sulphoxide if available, provides excellent local antidote.
❑ As soon as mercury in toxication is suspected, intramuscular injection of dimercaprol or
Penicillamine is given to chelate the mercury and accelerate its excretion.

44. Treatment of Mercury Poisoning
 In the absence of the metal-chelating agents, isotonic NaCl solution is infused in amounts
as much as 10 litres per day to produce a copious diuresis to protect the kidney from high
concentration of mercury.
❖ Chronic (lasting for long time) mercury poisoning can be treated by removing the patient
from the exposure area, supportive therapy and the use of appropriate metal chelating
agents such as dimercaprol, Penicillamine or N-Acetyl, D,L-Penicillamine.
❖ H H H CH3
❖ H- C – C –C –H H3C – C – CH – COOH (Penicillammine)
❖ SH SH OH (Dimercaprol) SH NH2