The Chemistry of Pollution #3

We have seen that there are several processors by which material degrades in the environment.

Whereas metals are prone to oxidation by the oxygen in air, and plastics, fibreglass, rubber, clothes and other materials are prone to oxidation by UV light, by far the most common mechanism of degradation is biodegradation, specifically of organic material.

Stated simply, this explains how things rot.  When food goes off in the fridge, it is biodegradation.  When you see the carcass of a rotting kangaroo by the side of the road, that is biodegradation.

There are two types of biodegradation – aerobic and anaerobic.

As the name suggests, aerobic degradation occurs when there is oxygen present, and anaerobic degradation occurs when there isn’t.

The way it works is this – microorganisms (commonly referred to as simply bugs) convert large organic (carbon-based) molecules into carbon dioxide.  This is an aerobic process because they require the oxygen to make the carbon dioxide.  This is why if you accidentally leave a carton of milk on the table for a few days, it is all bloated when you discover it.  The expansion is caused by the CO2 that is generated by the degradation process.

This is an extremely important process which is widely used in the industrial sector.  The most common place is in waste treatment plants, both domestic and industrial.

By domestic, we are simply talking about sewerage plants.  In these plants, using a process known as the activated sludge process, naturally occurring bacteria convert human waste, and other domestic waste into CO2, nitrogen, sulphates, and biomass.  The role of the plant operator is to manage the plant such that there is sufficient oxygen available for the bacteria to do their job.  If you were to visit one of these plants you would typically see large blowers blowing air into the water like a giant aquarium bubbler in your fish tank.  This is simply to provide enough oxygen for the aerobic bacteria to do the job.

This biodegradation is also extremely important industrial process, and has become more so in recent years.  Now, when an industry wishes to process the waste from an industrial process, the use of biological plants (often in conjunction with chemical plants) is very common.  These of course are used widely mostly in areas where the waste is organic in nature – food processing and so on.

it is also used to regenerate contaminated groundwater or soil.  Where an industrial pollutant has leaked into groundwater or soil, often the best way to degrade it is the naturally occurring bacteria in the soil – all they need is enough oxygen to do the job.  A common approach at these sites, therefore, is to drill holes in the ground and insert huge air blowers which just blast area into the ground.  The air allows the bacteria to do their job (and their population increases as a result and the process becomes more efficient) and often all but the most recalcitrant chemicals (which will look at tomorrow) can be removed.  It is, for example, a common process where petroleum solvents are the culprit.

So what sort of chemicals can be biodegrade, and which ones can’t?

In general, any naturally occurring chemical will be prone to biodegradation – this includes even crude oil as we have seen before.  The danger, of course, is that synthetic materials are not prone to biodegradation readily.  Plastic bags are a good example – the polymer chains are extremely stable and resist the action of bacteria.  But they are not so much of a problem, as they are chemically inert – the worst to say about them is that they are unsightly if they cause litter – but there are no toxicity issues.

Of much greater concern, are synthetic chemicals, which were made (and put into the environment) in an age where people just didn’t care about whether they were biodegradable or not.  They were made, and used, because they were cheap, they did the job, and they were stable – that is they didn’t degrade this time.  So the very thing that made them attractive in the first place, now is causing a problem.

This is a large topic in itself, and targets chemicals in several different industries – and we will look at it tomorrow.

The Chemistry of Pollution #2

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?”

The Chemistry of Pollution #1

There are basically two types of pollution:

1.The right chemical in the wrong place.

2.The wrong chemical in the wrong place.

Everything is a chemical.  There is not one thing you look at, touch or feel that is not a chemical of some sort. But every chemical has its place – petrol belongs in the tank in your car, not on your front lawn, for example. In your car, it does the job it was made to do, but on your front lawn, it kills the grass and smells bad.

Essentially, the right chemical in the wrong place, is a naturally occurring chemical that is a pollutant because it is somewhere where it shouldn’t be.

A good example of this is the recent oil spill in the Gulf of Mexico.  The crude oil spewing out of the pipe made an awful mess, and killed wildlife, and impacted local industries.

But here’s the point – crude oil is a naturally occurring chemical.  All that black oily stuff squirting out from under the ocean floor is a natural chemical – it was made over many years from decaying organic matter by bacteria – so it’s as natural as flowers in the springtime.

Unfortunately, it’s not quite as attractive as flowers – and when it finds herself in the wrong place, it can make an awful mess, as the residents of Mississippi or Prince William Sound have found out (the Exxon Valdez)

The other type of pollutant are synthetic chemicals, that find their way into the environment for one reason or another.  These are unnatural chemicals, and as we will see, they can pose a great hazard due to the inability of nature to break them down.  In general terms these chemicals are either insecticides, fungicides, biocides, or cleaning agents.

These chemicals, often labelled “persistent”, “recalcitrant”, or “refractory” were invented because they had some very useful property, and it was their very stability – the fact that they lasted a long time – which initially made them attractive.

Most of these chemicals were developed in the golden age of science – the 1950s – when science was seen as having the answer to all the world’s problems.  Unfortunately, it was also a time when the understanding of toxicity, not only to humans, but to other forms of life, was not what it is today.

Log on tomorrow, and I’ll begin a series on the various types and sources of pollution, and the various mechanisms by which they are broken down in the environment.  If you have any specific ones you would like to know about, just leave a comment and I’ll have a look at it.

Cockburn Cement and Lime Dust

Cockburn cement is in trouble with the EPA for its emissions of sulphur dioxide and lime dust.

Sulphur dioxide is one thing, but personally I’d be more concerned about the lime dust. Here’s why.

They make the lime (which they use for making cement) by roasting calcium carbonate (which of course is limestone).

The reaction looks like this:

CaCO3  = CaO + CO2

Or, without the symbols:

Limestone = Quicklime + Carbon Dioxide

That is, the limestone is converted into lime (calcium oxide) and carbon dioxide (by being heated to 800°C).

Now since the whole point of the exercise is to make lime, it is obviously in Cockburn’s interest to catch as much of it as they can. The problem is that it is an extremely fine dust, and it is very difficult to capture it all, so some of it is released into the atmosphere, causing problems for the residents of Cockburn.

Well, what happens to the calcium oxide (industrial name “quicklime”) when it is released into the atmosphere?  To quote the amazing Harry Hoo, this present “two possibility

The first is that the quicklime reacts with carbon dioxide and the reaction above simply goes in reverse, so that we will reform the limestone (calcium carbonate).  This is a problem of sorts, since we know that limestone is extremely hard and if tiny particles of it get in the wrong places they could have a very abrasive effect.

But the second possibility, and in my view of greater concern, is if it reacts with water vapour in the atmosphere to form hydrated lime:

CaO + H2O  = Ca(OH)2

The reason that this is of concern, is that hydrated lime is extremely caustic (alkaline).  And unfortunately, of the two reactions, the second reaction is more favoured energetically, so most of the quicklime will be converted to hydrated lime.

I used to use this stuff quite a lot in a previous job, and lost track of how many pairs of trousers and shirts I destroyed with it.  It is an extremely fine powder, and so when you get it on your clothes, you don’t notice.  But when you wash your clothes, the highly caustic nature of this stuff just burns holes through the fabric.  It’s more of a problem with natural fibres, like cotton, but polyester cotton is not immune either.

The other problem with the hydrated lime might occur if you got it on to paint work.  It’s not so much a problem on modern cars, with their extremely hard and durable two-pack coatings, but it could certainly be a problem if it deposited onto your house paint, or other painted goods with perhaps an older style of paint on them.

In days gone by caustic soda solutions were a common formulation for paint strippers – and hydrated lime is almost as caustic as caustic soda.

So if you are in the plume of sulphur dioxide from the plant, the chances are you’re also in the plume of the lime, which of course is so fine as to almost be invisible.  So if you’re experiencing degradation of painted surfaces (or clothes) at an unacceptable rate, lime may be the culprit.

Sulphur Dioxide and Cockburn Cement

Sulphur dioxide stinks.  I should know – I come from Wollongong – the home of the Port  Kembla steelworks. We always knew when the winds blowing from the south because you could smell the steelworks.  That is, you could smell the sulphur dioxide.

It’s an awful, musty, industrial type smell, and it’s no wonder the residents of Cockburn are unhappy with Cockburn cement.  They have every right to be.  The issue is not toxicity, its quality of lifestyle, and the legitimate desire to walk outside and not feel as though you are in the middle of a manufacturing plant.

The reason that sulphur dioxide creates this feeling, is that it is a very common industrial gas.  There are essentially two sources of it.  The first is from the combustion of coal, which contains a fair bit of sulphur, and the second one is from the smelting of sulphide ores, a common thing in Western Australia.

This was a huge problem at the Kalgoorlie nickel smelter, and it created such angst in the town that they had to do something about it.  They chose a wet sulphuric acid process, whereby the sulphur dioxide is converted into sulphuric acid, which they can then sell, so they killed two birds with one stone.

Sulphur dioxide is still a problem at the Gidgee roaster, about 80 km to the north-east of Kalgoorlie.  I have actually been inside their main control room, where they have a very sophisticated weather monitoring system.  The centrepiece of the display is the wind data, and there is a no-go zone in terms of wind strength and direction, at which point they must shut the roaster down, as the fumes will be blowing over Kalgoorlie.

But back to Cockburn cement. Their sulphur dioxide come from the massive amounts of coal that the have to burn to beat their kilns hot enough to convert the limestone to lime (about 800°C).

But of all people they have the least excuse in terms of dealing with the problem, as one of the simplest methods to overcome it is a wet scrubbing method involving lime, which of course they have plenty of.