Biodegradation is part of the cycle of life. There are certain elements in the world around us that are constantly being recycled. These days of course we all attempt to recycle rubbish where possible, but nature has already been doing it since time immemorial.
There are four cycles that you learn about when you study environmental chemistry – the carbon cycle, the nitrogen cycle, the oxygen cycle, and the sulfur cycle.
By far the most important of these is the carbon cycle. In nature, of course, carbon is the element of life. This is why in chemical terms, carbon chemistry is also known as organic chemistry – it is the chemistry of living tissue. So whether it is carbohydrates (lignin in trees), lipids (triglycerides present as either vegetable oils or animal fats, or proteins or enzymes, they are all carbon-based and will all degrade.
Perhaps the noticeable exception here is wood. Although some carbohydrates are very biodegradable (sugars and starches), there are some that aren’t, most noticeably wood. If it gets wet, of course, it degrades (this is what rotting is) but dry wood will last for a very long time.
The difference between the properties of the various carbohydrates is simply the types of linkages in the molecules’ carbon backbone.
But of course this is offset by the fact that wood burns, whereas other carbohydrates don’t. A simplistic representation of this may be:
CH2O + O2 = CO2 + H2O
That is, carbohydrate (wood) + oxygen = carbon dioxide + water.
Interestingly, if we reverse the process that happens when the wood burns, we come up with
CO2 + H2O = CH2O + O2
As it happens, this process is photosynthesis, the natural process that converts carbon dioxide and water into wood and oxygen. This is the simplest and most important process that is part of the carbon cycle.
Another example is something rotting, or “going off”. You know what happens – you’ve left the milk out and when you come to it, it is all puffed up and bloated. When you open the lid you are met with a blast of rancid smelling gas.
This gas is in fact due to Volatile Fatty Acids (VFAs) from the milk fats. In fact, any naturally occurring oil or fat will produce the same result when in contact with water and oxygen. And of course this process happens very quickly – milk that is not refrigerated will begin this process within 24h. In fact the degradation of oils and fats is probably the most rapid biodegradation process that there is. Tip a bottle of rancid milk out by your front door today, and within 3 or 4 days the rancid smell will have disappeared.
The process may be represented as (using acetic acid as a simple carboxylic acid):
CH3COOH + 2O2 = 2CO2 + 2H2O
Note that soil is probably the most efficient catalyst for this process that there is, as it is full of the bacteria that catalyse this process. It was very common during the American Cilvil War, for example, for weather and erosion to uncover naked skeletons of soldiers that had been buried in shallow graves only a few months hence.