The Chemistry of Pollution #4

We have seen that there are several mechanisms by which chemicals degrade in the environment.  There is chemical oxidation, UV oxidation, and biodegradation.

There are two types of biodegradation – aerobic and anaerobic.  By far the more common of these two processes is aerobic because there is lots of oxygen around.  But, as we are about to see sometimes anaerobic processes take over.

Despite the efficiency of aerobic biodegradation, it is essentially confined to natural compounds.  And sometimes we as humans put chemicals in the environment which are not natural – but are synthetic, and these compounds can sometimes be very difficult to break down.

These chemicals go by several terms – “recalcitrant”, “persistent”, or “refractory”

As it happens, there is a common thread amongst many of these chemicals.  So let’s look at a few and see if you can work it out.  Consider each of the following chemicals: dichlorodiphenyltrichloroethane (DDT), carbon tetrachloride, chloroform, hexachlorobenzene, chlorofluorocarbons, trichloroethylene.

That’s right – they are all chlorinated compounds.  Each of these is a chemical that was at one stage considered useful, but is now banned (although DDT because of its incredible effectiveness in killing malaria carrying mosquitoes, is still used in some parts of the world).

But the rest of them are no longer used industrially, except in laboratory applications.  You may have heard about the scare recently when some road workers found the residue of a truck spill on the Pacific Highway, north of Sydney that occurred in 1980.  The initial concern was that it had been carrying radioactive material, but mentioned in the fine print also, was that it had been carrying DDT and hexachlorobenzene.  And the nature of these chemicals is that they will not have degraded much in the 32 years since a truck accident.

Why is that?  Why are chlorinated compounds so difficult to degrade biologically?  The reason is simple chemistry.  Aerobic degradation of the chemical by bacteria is nothing more than biological oxidation.  The bacteria oxidise chemicals to turn it into CO2.  But the problem with chlorinated chemicals is that the chlorination makes them resistant to oxidation, for reasons that I will not bore you with (but if you really want to know, it’s to do with the electron withdrawing effect of the chlorine atom).

But there is a second problem with chlorinated compounds.  They are heavier than water.  That is if they get into waterways, they sink to the bottom.

Now we’ve all seen the mess created by the Exxon Valdeez, and the recent oil spill in the gold of Mexico – but at least the oil floated, so it was accessible to clean up.  If the chemical in question sinks to the bottom, it is very inaccessible, and will sit there for many years, poisoning aquatic life and concentrating up through the food chain.

So this is why chlorinated chemicals are essentially a thing of the past – even trichloroethylene which was an exceptionally good drycleaning fluid is no longer used.

Even the humble prewash – Preen – had to be reformulated, because it contained trichloroethylene.

So what does happen to these compounds?  Well they mostly broken down by a completely different mechanism – anaerobic biodegradation.

Anaerobic biodegradation as the name suggests, occurs in the absence of oxygen.  These processes are very slow and take a long time – which is why these chemicals persist so much in the environment.

We have all experienced anaerobic degradation – because when you smell something that is rotting and smells really bad thing you know it has become anaerobic.  That is, the aerobic processes ran out of oxygen, and the system had to change over to an anaerobic mechanism.  And the reason they stink is simply because the chemicals they produce (mostly sulphides) have very strong odour – for example, rotten egg gas.

so when you drive past the carcass of a rotting kangaroo and you can smell it for the next 5 km, it is because in the internals of the animal the bacteria have run out of oxygen, and the system has become anaerobic

We have seen the shift away from chlorinated compounds in the insecticide industry.  The use of organochlorine (there’s that word again) insecticides are a thing of the past – these days, at least in Australia, they have mostly been replaced by the synthetic pyrethroids ( which are all chemicals ending in “thrin”).  And the advantage of the pyrethroids is that they are ultimately extracts of a natural compound from the pyrethrum daisy, and they are therefore biodegradable.

1841cookie-checkThe Chemistry of Pollution #4