Microplastics

You may have heard that the ocean is full of plastics, and while full is an excessive word, there are huge amounts of plastics there, thanks to humans inability to look after some things when they have finished using them. Homo litterus is what we are. You may even have heard that these plastics degrade in light, and form microscopic particles that are having an adverse effect on the fish population. If that is it, as they say, “You aint heard nothin’ yet.”

According to an article in the Proceedings of the National Academy of Science, there is roughly 1100 tons of microplastics in the air over the Western US, and presumably there are corresponding amounts elsewhere. When you go for a walk in the wilderness to take in the fresh air, well, you also breathe in microplastics. 84% of that in the western US comes from roads outside the major cities, and 11% appear to be blowing in from the oceans. They stay airborne for about a week, and eventually settle somewhere. As to source, plastic bags and bottles photodegrade and break down into ever-smaller fragments. When you put clothes made from synthetic fibers into your washing machine, tiny microfibers get sloughed off and end up wherever the wastewater ends up. The microplastics end up in the sludge, and if that is sold off as fertilizer, it ends up in the soil. Otherwise, it ends up in the sea. The fragments of plastics get smaller, but they stay more or less as polymers, although nylons and polyesters will presumably hydrolyse eventually. However, at present there are so many plastics in the oceans that there may even be as much microplastics blowing out as plastics going in.

When waves crash and winds scour the seas, they launch seawater droplets into the air. If the water can evaporate before the drops fall, i.e. in the small drops, you are left with an aerosol that contains salts from the sea, organic matter, microalgae, and now microplastics.

Agricultural dust provided 5% of the microplastics, and these are effectively recycled, while cities only provided 0.4%. The rest mainly come from roads outside cities. When a car rolls down a road, tiny flecks come off the tyres, and tyre particles are included in the microplastics because at that size the difference between a plastic and an elastomer is trivial. Road traffic in cities does produce a huge amount of such microplastics, but these did not affect this study because in the city, buildings shield the wind and particles do not get lifted to the higher atmosphere. They will simply pollute the citizens’ air locally so city dwellers merely get theirs “fresher”.  Also, the argument goes, cars moving at 100 k/h impart a lot of energy but in cities cars drive much more slowly. I am not sure how they counted freeways/motorways/etc that go through cities. They are hardly rural, although around here at rush hour they can sometimes look like they think they ought to be parking lots.

Another reason for assigning tyre particles as microplastics is that apparently all sources are so thoroughly mixed up it is impossible to differentiate them. The situation may be worse in Europe because there they get rid of waste plastics by incorporating them in road-surface material, and hence as the surface wears, recycled waste particles get into the air.

Which raises the question, what to do? Option 1 is to do nothing and hope we can live with these microplastics. You can form your own ideas on this. The second is to ban them from certain uses. In New Zealand we have banned supermarket plastic bags and when I go shopping I have reusable bags that are made out of, er, plastics, but of course they don’t get thrown away or dumped in the rubbish. The third option is to destroy the used plastics.I happen to favour the third option, because it is the only way to get rid of the polymers. The first step in such a system would be to size reduce the objects and separate those that float on water from those that do not. Those that do can be pyrolysed to form hydrocarbon fuels that with a little hydrotreating can make good diesel or petrol, while those that sink can be broken down with hydrothermal pyrolysis to get much the same result. Hydrothermal treatment of wastewater sludge also makes fuel, and the residues, essentially carbonaceous solids, can be buried to return carbon to the ground. Such polymers will no longer exist as polymers. However, whatever we do, all that will happen is we limit the load. The question then is, how harmless are they? Given we have yet to notice effects, they cannot be too hazardous, but what is acceptable?