The oceans are full of garbage. One of the most depressing facts about humanity’s degradation of the natural world is the presence of the Great Pacific Garbage Patch, a gigantic vortex of plastic and other marine debris that could be up to twice the size of the continental United States. Generally speaking, plastic takes around 450 years to completely degrade, and we’re dumping more of it into the oceans all the time – so this patch isn’t going away anytime soon.
Although our priority should be to clean up this mess and stop any more of it entering the geological cycle, there are other things science has conjured up that may also help. Writing in the journal Science Advances, a joint US-China endeavor has described a way in which this type of plastic can be converted into a source of fuel.
This is all based on the fact that plastics – combinations of hydrogen, carbon and oxygen atoms arranged in long chains – are made from fossil fuels, so it stands to reason that they can be converted into a type of fossil fuel. The authors decided to focus their efforts on polyethylene (PE), a simple chain molecule that is also one of the most commonly used types of plastic in the world.
“PE is the largest-volume plastic in the world, with annual production exceeding 100 million metric tonnes [110 million US tons],” the scientists, led by Xiangqing Jia, a researcher at the Chinese Academy of Sciences, write in their study.
PE takes a remarkably long time to degrade and requires vigorous chemical processes in order for it to react to anything – or be converted back into a fuel. Simply heating it doesn’t work, as the molecule chains (or “polymers”) will break down chaotically into many smaller variants, all of which have their own properties. Being aware of this, the team turned to catalysts, chemicals that accelerate reaction processes.
A visualization of the plastic garbage patches in our oceans. NASA's Scientific Visualization Studio
The team first uses an iridium-rich compound, which removes the hydrogen atoms from PE. The carbon atoms left behind begin forming double bonds with each other, which are more reactive to certain chemicals than the original single bonds.
The second catalyst, which contains a mixture of aluminum, oxygen, and rhenium, attacks this new vulnerability, and smashes the polymer apart. After this, the liberated hydrogen atoms are once again re-imprisoned back onto the fragmented polymer segments (“monomers”).
The end result of this concoction of chemical wizardry is to turn the original PE object – which in this case included a plastic bag, a bottle, and food packaging – into a fuel. Changing the amount of hydrogens and carbons on a hydrocarbon polymer literally transforms it into a completely different chemical, and this is precisely what this team has managed to achieve without using much heat, and therefore energy, in doing so.
A mixture of hydrocarbon monomers, here represented depressingly by oil pollution in the sea. huyangshu/Shutterstock
Changing the speed of the reaction, or the degree of initial catalytic fragmentation, allows the team to choose whether they want a liquid fuel to emerge at the end, or, alternatively, a type of wax. This process seems to produce relatively clean, low-pollution fuels, with respect to other fossil fuels. Not investing energy into excavating fossil fuels is also a good thing, environmentally speaking.
Hopefully, though, this doesn’t become the go-to future method for producing energy. Recycling plastic to be used as a fuel is a good idea, but really, we should be focusing on energy sources that have a low-to-zero carbon footprint, like renewables, and nuclear. Above all, we should stop wasting so much plastic in the first place.