Engineering Chemistry S Datta

576 ENGINEERING CHEMISTRY
Lead is mainly emitted from the automobiles as it is added to gasoline to improve
antiknock properties particularly in the form of tetraethyl lead. Presence of hydrocarbon group
helps it to pass through the membranes in our body; it mainly concentrates in blood, bones etc.
It reacts with the active —SH groups of enzymes producing haemoglobin and hence leads to
anaemia.
Similarly, mercury also reacts with —SH group of different essential proteins and inhibits
enzymatic activities. Toxicity of mercury depends on its state. Pure mercury, insoluble inorganic
mercury compounds have lower toxicities whereas soluble salts of mercury, mercury vapour
and organic mercury compounds like dimethyl mercury etc. cause mercury poisoning. Its
harmful effects include sore gums, loose teeth, brain damage, irritability and emotional
disturbances.
Mercury in natural water reservoirs are metabolised by certain micro-organisms forming
dimethyl mercury which may be eaten up by small fishes and thereby it may enter the food
chain called biomagnification and ultimately reach higher animals.
The best remedy for these heavy metals is treating with chelating agents like ethylene
diamine tetraacetate which removes heavy metals from the blood by forming complexes.
COO –
–
OOCH2C
CH2
–
OOCH2C
EDTA
CH COO –
2
N—CH 2—CH 2—N
Pollution from internal combustion engines
One of the major pollution of the atmosphere is due to the internal combustion engines
of the automobiles. Fuels used such as diesel, petrol etc., contain largely the hydrocarbons
with other S and N containing impurities. Burning or incomplete burning of these fuels causes
release of CO 2
, CO, SO 2
, NO 2
, hydrocarbons etc. All these pollute the atmosphere, form smog
and cause damage to lives.
This type of pollution can be minimised by using (i) better and pollution-free fuels,
(ii) using engines with better performances, (iii) using catalysts for the conversion of pollutants
to less harmful substances by joining a device in the exhaust system of an automobile. The
device is known as catalytic converter (Fig. 27.1), (iv) by mixing exhaust gases with more
air for better combustion.
From the
engine
CO/CO 2
NO X
CH X Y
Thin coating of catalyst
on the ceramic
Out to
the air
N 2
N 2 O
O 2
Ceramic block with many
fine tubes
Fig. 27.1 Catalytic converter.