Feature image: Structure of DDT. Chemical structures help determine how a chemical would move in the environment
Toxicologists love chemical structures because structures tell a lot about a chemical, just like genes tell a lot about a person. And chemicals, like people, are also shaped by the environment. Therefore, to understand how long pollutants persist, and how they move and distribute, it is important to know their chemical structure and environmental condition.
How are pollutants distributed?
If a pollutant/compound is water soluble, it is likely to be found in water. If it isn’t water-soluble, it will likely bind to soil or enter an organism (or, if it is added to water, it will just remain on the surface and not mix with water). In general, compounds containing many oxygen and hydrogen molecules (-OH) like water and compounds with many carbon and hydrogen molecules (-CH) don’t. And if the bonds between the molecules of a compound are weak, they are likely to enter and disperse in the atmosphere.
Thus, by checking the concentration of a pollutant in different matrices (water, organic compounds and air), scientists can determine how the pollutant would be distributed in the environment. Most pollutants are distributed in multiple matrices (example: soil and water if slightly soluble).
How long do pollutants persist?
Pollutants can be broken down by light, heat, water, oxygen, acids, bases, microorganisms, and enzymes in plants and animals. However, some pollutants are persistent in the environment because their chemical structure makes them more resistant to degradation. These pollutants often have many halogen atoms (chlorine, fluorine etc.). They bind tightly to soil and can accumulate in organisms. While toxicologists prefer pollutants that are degraded easily, the degradation process can sometimes ‘activate’ a pollutant and make it more toxic (more on this later!).
How do pollutants move?
Pollutants that are bound to soil or other organic materials (i.e., they are no longer bioavailable) will be unable to interact with and affect organisms. But a change in pH, temperature and/or humidity can weaken the bonds and make these pollutants mobile again. An increase in temperature and/or humidity can cause pollutants to vaporize and enter the atmosphere. Pollutants of smaller size move further and generally, move from areas of high concentration to low concentration. Wind speeds, water currents, snowfalls, and rainfalls decide how far pollutants get transported in the environment.
Principles of Ecotoxicology (4th Edition) by C.H. Walker, R.M. Sibly, S.P. Hopkin and D.B. Peakall