There are basically two types of chemical pollutants
When these pollutants get into the environment, what happens to them then? If they are broken down, how does it happen? What mechanisms are there?
Almost exclusively, chemicals are degraded in the environment by natural oxidation. The reason for this is simple – the two things that most promote oxidation – oxygen and UV light – are available in plentiful supply.
Essentially, one of four processors may occur:
1. Chemical oxidation.
In the vernacular, this is simply rust. We have all seen remains of old car bodies sitting in fields out in the countryside. This is simply where atmospheric oxygen, aided by water, oxidises the steel. This, of course, mostly applies to metals, and steel in particular. Aluminium also corrodes, but the difference by comparison to steel is that the oxide of aluminium is stable. As it happens, atmospheric oxygen has a more important role to play in terms of the degradation of chemicals than direct oxidation, which we shall look at when we consider biodegradation.
2. UV oxidation.
This process explains a great deal of what we see in the world around us. It explains the fading of paint, the perishing of rubber, the embrittlement of fibreglass and some plastics. it explains cracked dashboards in cars, the cracking of tiles, and the fading of colours in clothes that are left on the clothesline for too long.
We all know that if we spend too long in the sun wed get sunburnt. The reason is the UV light that the sun emits. UV light possesses sufficient energy to attack many chemicals, particularly if they are exposed for long periods of time. Manufacturers of plastic tanks that are designed for external use are aware of this and include UV stabilising chemicals in their manufacture. For some types of chemicals, notably plastics, it is the fastest route to degradation that there is.
Plastic bags buried in the ground last a long time – plastic bags exposed to the sun don’t last nearly as long.
Tomorrow we shall look at the last two mechanisms of natural degradation, in which we shall answer the question “when is something biodegradable?”