Biodegradation #3

In broad terms there are two types of biodegradation – aerobic and anaerobic, with and without oxygen respectively.

By far the more common is aerobic, mainly because there is plenty of oxygen around and also aerobic biodegradation is fast. For this reason, this approach is used quite a lot industrially. Typically, an industrial waste treatment plant will involve a chemical dosing step, followed by a biodegradation plant. This normally simply involves a large vat with aerators. The aerators provide the oxygen for the bugs to do their stuff, and they successfully remove both the carbon (as CO2) and nitrogen (as N2). Essentially, aerobic digestors are pretty foolproof.

And these are used in sewage treatment plants – massive treatment vats with banks of aerators blast air through the waste day and night. These plants have a substantial amount of sludge, of course, and this sludge is tapped off at various points and sent to an anaerobic digestor. The anaerobic digestor consumes this solid sludge with about 96% efficiency, and converts it all to methane, ammonia, and hydrogen sulphide.

Interestingly, I recently heard of one of these facilities that is being set up commercially, using food waste, to generate methane that can be used as fuel to generate power.

So the world around us is an incredibly efficient biological reactor, using several mechanisms, to recycle pretty much everything.

Biodegradation #2

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.

More tomorrow

 

Biodegradation #1

One of the most commonly asked questions with regard to any chemical that is used in the marketplace is “is it biodegradable”?

This applies to insecticides, detergents, dry-cleaning fluids, herbicides and so on. And the reason is that today we have an understanding of ecosystems that we didn’t have 50 years ago, and the need for any chemical that we place into the environment to biodegrade, and therefore have no lasting impact on the environment.

50 years ago, the very opposite was the case. You see. another word for “biodegradable” is “unstable”. And back then, that was a bad thing. Back then we wanted chemicals to last for, well, forever if possible. When we sprayed a surface insecticide we wanted it to last forever. And this was the great advantage of DDT. As a chlorinated insecticide it was extremely stable (ie not biodegradable), and so it could be blasted here there and everywhere, knowing that we wouldn’t have to come back for a while.

But as time marched on, we started to realize that these chemicals were starting to accumulate in the environment, and in some cases they have caused disruption of ecosystems that was never originally intended.

So now you pretty much cannot use a chemical in the marketplace if it is not biodegradable. And this even applies to dry cleaning fluids. It used to be the case that when you had your clothes dry cleaned, the chemical of choice was 1,1,1-trichloroethane, a triply chlorinated solvent that was also contained in the original formula of Preen aerosol. That now is out the door, to be replaced by something that is biodegradable. I’m not sure how well these newer chemicals work, however, as I tend not to get things dry cleaned. One thing I do know, however, is that today’s Preen is not a patch on the old stuff. Tomorrow we’ll start lookin at exactly how biodegradation works in world around us.

What was Gas Smell?

Reports came in overnight of a gas smell in Perth’s SW suburbs.
What was it? Well, the gas companies say it wasn’t them – and I believe them.

Here’s what I think happened. Both natural gas and LPG are odourless. But this represents a safety hazard, as if there is a gas leak you want people to be able to smell it. So for that reason, a chemical is added to the gases to give it a smell. The chemicals they use for this are mercaptans.

Mercaptans possess the unique quality of being the smelliest chemicals in existence. The human nose can detect them at tiny, tiny concentrations- far too low to present any health concerns.

So people associate the smell of mercaptans with gas (LPG or Natural).

But mercaptans can come from other sources as well, most notably rotting organic matter in anaerobic environments. And the most common place for this is rubbish tips. As putrescible matter (ie food scraps) is bulldozed into the ground it starts decaying anaerobically. This produces mercaptans which make their way up through the soil and into the atmosphere.

Under normal circumstances they get blown away and you never smell them, but if there are the right circumstances – warm weather, still air, and temperature inversions, they can accumulate to the level where you can smell them.

A few days ago Perth was blanketed in a smoke haze, with smoke from as far away as Albany apparently. This was due to a temperature inversion, and I’m betting the same thing has happened in this case.

Is it toxic? In a word, no – these gases have an intense smell, and can be detected by the human nose at very, very low concentrations – far too low to present a toxic hazard

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.