As soon as a pollutant enters environment, agencies of transfer, transport and transformation start acting on it almost simultaneously.
While the pollutant is dispersed in the medium and diluted, it also undergoes abiotic transformation in which solar radiations; water and air play an important part. The pollutant may thus be broken down or altered in the abiotic environment itself. Abiotic transformation of pollutants is the only process available in the atmosphere as it lacks the biotic agencies which occur on land as well as in water. Abiotic transformation may alter or degrade the pollutant molecules in the following manner:
1. Photolysis:
Consequent upon the absorption of solar radiations pollutant molecules in the environment may react photo-chemically and acquire an excited state, themselves or by reaction with another chemical in excited state. This is very important process for pollutants in the atmosphere or substances near the surface where plenty of light is available.
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Direct absorption of energy in or near ultra-violet range of electro-magnetic spectrum may result in oxidation, hydrolysis, cleavage of various types of bonds and structural rearrangements. Photolytic decomposition of pollutants is dependent upon the energy of incident radiations, absorption spectrum of the molecule and the presence of photo- sensitizers in the environment.
2. Hydrolysis:
Hydrolytic dissociation of pollutants is a process which is of greater importance on land and in water. Broadly speaking, the hydrolysis of a pollutant is displacement reaction involving replacement of one of the functional groups of the molecule with that of hydroxyl-ion in presence of water.
RX + HOH – RCH + HX
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RCOX + HOH -RCOOH + HX
3. Other reactions:
A number of simple reactions occur when water or air carrying different pollutants mix and bring reactants together. Atmospheric air contains a number of gaseous pollutants which dissolve with condensing moisture in droplets of water or form a fine film of active material over the aerosol surface and yield chemically reactive matter. Water flowing over the soil and rocks collects products of decomposition of organic matter and other soluble substances. Abiotic transformation of a number of pollutants is brought about by these reactants.