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Oil and water: How do you clean up?

Updated Wednesday, 21st July 2010

Sometimes, clearing up an oil spill actually makes things worse. Patricia Ash explains why.

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Dr Patricia Ash

We try and rescue as much of the oil as we can because it’s so valuable and we don’t want it out there in the marine environment.  We’re still using detergents.  They are being used more carefully but we are also still using physical methods.  Physical methods when they were first tried tended to be quite rough and there was too much clearance being done, and this was very much illustrated later on by the Amoco Cadiz oil spill off the coast of Brittany and there’s a huge area of marsh there and it was completely coated with oil.  I think it’s called the Grande Ile Marsh.  And immediately people set to work clearing the oil using spades, anything they could get their hands on and they completely denuded one area of the marsh which unfortunately subsequently led to it being subjected to a much faster rate of erosion by the sea.  And as we know now, these coastal saltmarshes are very, very important in helping to soak up huge tides and to take up that water so that it doesn’t actually inundate the farmland behind it.  So, the last thing we really want is to worsen the situation by being so drastic with clearing oil off a saltmarsh and therefore making the problem even worse, whereas if we left it alone in many cases, not all, the normal biological and physical processes would actually break down the oil.  But I have to emphasise not always because there have been other oil spills where despite natural processes occurring they’re very slow and oil pollution remains many years after the event.

But you ask why we can’t just hoover it up.  Well, there are many reasons.  First of all, the lighter components tend to make very thin layers anyway but wave action itself can cause emulsification of oil, you know that when you shake your oil and vinegar together, but it does all come right in the end.  But when you’ve got wave action going on for hours and hours, the emulsification stays and in fact in extreme circumstances it becomes an extremely stable emulsion and forms what’s known as a sort of chocolate or toffee mousse where you get horrible mousse-like aggregations floating on the surface of the sea.

They tried to use, well they did use the commercially available detergents which are substances that will emulsify oil.  The way that they do it is that each molecule of detergent has got two different ends.  So you imagine a molecule, you can imagine it as a blob if you want.  One end of the blob has got a hydrophobic end which loves oil and it wants to stick to it and be very close to it.  The other end of the molecule is hydrophilic and it loves water and it wants to stick to that.  So, essentially a detergent, it keeps water molecules apart, it makes them less likely to stick together and so the oil droplets stay as discrete droplets and they can’t, the oil can’t get away because it’s anchored into the water by the hydrophilic ends of the detergent and that’s how it works. 

The purpose of the emulsification is essentially to help the oil to essentially disperse and to make it easier to deal with.  Now, if you’ve got a wider surface area of the oil, in theory more of the lighter component should be able to evaporate more easily and the oil should be able to disperse and you’ve got a greater surface area for say naturally occurring microorganisms to set to work on it, because what we have to remember that actually oil is biodegradable but it does take quite a long time for that to happen because you need to have huge build-ups of the oil-eating bacteria in the sea which takes time.  There’s a huge research effort at the moment to investigate the use of bacteria and it has been found that there are specialist groups of bacteria.  For example, alcanivorax likes to eat the simple straight chain hydrocarbons and there’s another species which, I think it’s called cystoclasticus or something, which is a wonderful name, it prefers to live in sediments and to break down the oil there, but that’s a very long term thing.  What the public want is for the oil to be cleared away very quickly.

So, when it was found with Torrey Canyon that they could not collect the oil, that the oil was spilling out, they did an amazing thing, they actually got the RAF in to bomb the tanker and they pounded it with everything that they had and they even tried dropping napalm and chlorate onto it and it was quite a huge operation.  In the event they managed to burn off about 50,000 tonnes of the stuff.  So, although it was just under half it did help out. 

But unfortunately the worst thing that they did in hindsight was that they sprayed loads and loads of detergent all over the beaches and this was what’s known as a first generation detergent, it was quite toxic, so it did kill much of the normal invertebrate and fish populations in coastal areas and on the coast itself.  And there are reports in the literature of biologists finding a rock pool which had been contaminated by oil but it still contained the normal populations of fish, anemones, molluscs and so on, but after spraying all of the animals were dead, after being sprayed with the detergent because it was so toxic. 

The worst thing about oil disasters in my personal opinion is the huge effect it has on the animals that live in the sea, the microorganisms, the seaweeds, and in turn the livelihood of the people who depend on fishing and tourism for their incomes.  But of course we all feel very upset when we see these tragic images of the oiled birds, and recently we heard about a very badly oiled baby dolphin which was quite tragic, but what we don’t see of course is what’s happening deep in the sea and I don’t think we see anything like the real numbers of oiled birds, dead fish and so on, so that is a terrible tragedy.  And the oiling of environments such as saltmarsh that really are the breeding grounds of animals such as crustaceans say, crabs and fish, they lay their eggs close to shore, they develop into tiny free swimming larvae, all these little planktonic organisms are extremely vulnerable to toxic oil and indeed to the toxic detergent.

Of course we must remember that although oil is biodegradable and ultimately it was actually derived from living organisms, albeit millions of years ago, most of the components of oil are actually pretty toxic, especially to animal life.

9’04”

 

Oil and water

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