A smarter way to melt down plastics?

Benjamin Thompson

Welcome back to the Nature Podcast. This week, a new method for breaking down plastics...

Shamini Bundell

...and updates from the Nature Briefing. I'm Shamini Bundell...

Benjamin Thompson

...and I'm Benjamin Thompson.

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Benjamin Thompson

Hundreds of millions of tonnes of plastics are produced each year, but only a fraction of these will be recycled to create another product. A huge amount will end up buried in landfill or simply incinerated. But this week in Nature, a team have demonstrated a new method of breaking down the long polymers found in certain types of plastics into their monomer building blocks as a potential way to reduce plastic waste. Now, I did reach out to the paper authors, but sadly, they weren't available to speak to me in time for this week's show. So instead joining us to talk about the research is Claire Hansell, a senior editor here at Nature, who handles a lot of the material and chemistry papers published in the journal, particularly those relating to plastics. Claire, how are you doing today?

Claire Hansell

I'm alright, how are you today?

Benjamin Thompson

Yeah, I'm doing very well. Thank you very much. And so we're here today to talk about plastics. And before we get into the new paper, let's take a broader view. And maybe you can give us a flavor then of some of the approaches that are being taken to try and give plastics and the polymers they're made off a second life.

Claire Hansell

I mean, reduce, reuse, recycle is the first thing that people should be doing. But that is more of a behavior thing than the actual a science thing. And approaches vary, depending on what the type of polymer is, there are some which are more easily recycled than others, you can simply heat the things and that is a way of getting some useful products back out. But it's maybe not the most effective. There are other people working on enzymatic technologies. So you get an enzyme, which is really good at breaking up some classes of polymer and chopping it up into its constituent monomers. And then there is upcycling where you treat the polymer chemically in some way to generate products, which are not necessarily the constituent monomer. But they might be a small molecule or a slightly larger molecule, which ideally has a higher value than the original plastic, hence, the upcycling moniker.

Benjamin Thompson

I mean, you mentioned heating there and one method that's getting increasing attention is heating, specifically pyrolysis. And I guess its name gives a bit of a sense as to what it's about. But for those not familiar with it, what is pyrolysis?

Claire Hansell

So pyrolysis in a more controlled way than just lighting a match and watching it burn, is where you exclude all the oxygen from your huge, great reactor vessel, put in a load of energy in the form of heat, enough to overcome the bonds between carbon and carbon. And that just breaks your carbon-carbon bonds in your long polymer chain, chops it right up into smaller, medium-sized molecules that might be gases in the form of potentially even monomers to make that polymer, again, that might be slightly-longer chains, such as fuels. But the important thing is that there's a whole mixture, you can't necessarily just get monomer, or get an appreciable amount of monomer. And the yields of any individual component can be quite low. So you end up with a mixture of potentially quite useful stuff, but still a mixture.

Benjamin Thompson

And this method of pyrolysis then of potentially breaking down the polymer chains into their monomer building blocks is something being looked at in a new paper this week, but it's maybe coming at it from a slightly different angle. What's this research doing that maybe hasn't been done before?

Claire Hansell

Indeed, so in its essence, yes, this paper is still reporting pyrolysis of various types of plastics, but it takes a fairly, what I would call an engineering view. And the way in which they do this is they take a carbon felt like a mat of fibers made out of carbon, so it's porous, so things can travel through it, but importantly, it's made of carbon, so it's conductive. So you can heat the top of it using electricity, but not the bottom, which is in contact with solid plastic. The heating is pulsed, so it's very short amounts of heating, which makes a thermal gradient so your plastic is on the bottom, and it melts because that carbon felt is warm, wicks its way up through the carbon felt, and with the pulses of heating, it is then depolymerized. Once it gets to the top, the gases are whisked away and you get all of the benefits of pyrolysis in terms of the very high temperatures. And you can depolymerize your plastics, but fewer of the side reactions and because the heating is pulsed, it is both more energy efficient, but also it keeps the temperature and the temperature gradient across the felt in the 'Goldilocks zone'. So just right, to make sure that you're melting your plastic, wicking it all the way up to the top. And you're not overheating it to bias formation to unwanted side products or anything like that.

Benjamin Thompson

I'm looking at this paper and it almost seems like essentially they're making a candle, right? So the plastic of the bottom is getting wicked up through the carbon felt towards the heat at the very top and getting sort of broken down into the monomers, which are then taken away. I mean, this approach does seem fairly straightforward.

Claire Hansell

Yeah, absolutely. And this group has an awful lot of experience with making this carbon felt and using that with dual heating. So you get very high temperatures very quickly, but in a controlled way, the method also doesn't use any catalysts, which is attractive in terms of not needing to make one, or use it and then get rid of it later. And yeah, it works quite well, and is pleasingly simple in terms of not using loads of components.

Benjamin Thompson

And what sort of yields then is this method getting it in terms of percentages of plastic breakdown? And how does it compare to conventional pyrolysis, I guess?

Claire Hansell

Well, it really depends on which plastic type they look at. So they look at a variety of plastic types in the paper. So for example, PET, so polyethylene terephthalate, polypropylene, polyethylene, these are all fairly high volume plastics, the PET works best. But that said that is the plastic type, which is most readily recycled today. I think the yields of polypropylene is about 40% monomer. And then they produce a few other gaseous products as well, which is better than conventional pyrolysis. But it really depends on what exactly you do. But that's quite a high yield, when you're trying to capture a gas from possibly a complicated mixture.

Benjamin Thompson

But it has to be said that this is rather early days, because pyrolysis is, you know, being investigated on an industrial scale. But this new method is maybe more of the sort of tentative-steps stage.

Claire Hansell

I mean, I think the fact that you are dependent on the surface area of a carbon felt, and in the paper, it's a couple of centimeters square, it is a very lab-scale demonstration, the fact you're dependent on the surface area, is something that will need to be overcome with future work. And yes, pyrolysis on an industrial scale, is huge volumes. And how do you go about dealing with huge volumes of plastic, when it all needs to go through a thin layer of carbon felt is definitely something that needs to be worked out in future.

Benjamin Thompson

And you mentioned that they've tried this on a few types of plastic and maybe plastic that's a bit more amenable to being broken down into its constituent building blocks. But of course, there are a range of plastics with different attributes, some of which might not be so amenable towards this method.

Claire Hansell

Absolutely. So what they've done is they've looked at linear polymers only, which does cover a fairly wide range of waste plastics that are used in commercial situations or packaging. They haven't yet looked at any crosslinked ones, where you've got chains of polymer, which are linked by bridges, thermosetting plastics, for example. And they haven't looked at biomass or natural polymers yet. Whether it would work for those or not, who knows.

Benjamin Thompson

And of course, it's, you know, hard to predict the future. But for someone who sees a lot of papers involved in this area, breaking down or recycling of plastics, do you think that this method could potentially be a growth-area of research, perhaps?

Claire Hansell

I think it's always great to have new ideas, perhaps even from people who don't have a traditional polymer background. So this group has an awful lot of expertise in using flash heating for all sorts of things. And they've applied it in this instance to polymer degradation and polymer waste and therefore brought a different slant or a different idea to the table, which may well have legs and other people pick this up and run with it. I think it's really interesting to have a new idea like this or to have a different approach, which is complementary to the many other ways that people are looking at degrading or recycling plastics.

Benjamin Thompson

Nature's Claire Hansell there. To read the paper by Dong et al. look out for a link in the show notes.

Shamini Bundell

Coming up the bizarre genomes of yellow crazy ants. Right now though, it's time for the Research Highlights with Dan Fox.

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Dan Fox

Mastering the swing set at the local playground is a rite of passage for many children. Now physicists have modelled the movements needed to swing without outside help in greater detail than ever before. The team modelled swinging by analysing a simplified human body that moves smoothly and adjusts its movements as it swings higher. The model predicted that to get the most efficient push, the swinger needs to lean backwards while moving forwards and the lowest part of the swing. But as the swing traces of bigger arc, it becomes more efficient to lean back while moving backwards and approaching the top of the arc. The researchers observed ten college students who volunteered to use a playground swing set up in a laboratory. Without any training, the participants instinctively adjusted their movements in good agreement with the models theoretical predictions. Decide for yourself if that paper has reached new heights by reading it in Physical Review E.

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Dan Fox

How do you attract a mate? Dress up nicely? Get a haircut? Maybe a little spritz of perfume? Well, things may not be so different for male orchid bees, as researchers have found they use perfume to attract potential partners. Scientists have long known that male orchid bees gather mix of fragrant chemicals from flowers and other natural sources and store their harvest in specialized hind-leg pockets. But the reason for this complex behavior has been unclear. To unpick this, researchers collected perfume from wild-caught green orchid bees. They then doused trapped males with the fragrant chemicals before observing their interactions with females. Although the perfume had little effect on the bees courtship rituals scented individuals mated with more females than did unscented ones. Males that wore perfume also produced more offspring than their scentless counterparts. The authors say the findings provide the first evidence that male orchid bees concoct and release perfume blends to lure females for mating. If you like the smell of that research, you can find the paper in Current Biology.

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Shamini Bundell

Finally, on today's show, it's time for the Briefing Chat, where we discuss a few articles that have been highlighted in the Nature Briefing. So Ben, what have you got for us this week?

Benjamin Thompson

Well, last week on the show, I talked about supermassive black holes. And in that case, it was a story about two supermassive black holes coming closer to each other. I'm gonna have another supermassive black hole story, because I just think they're awesome, right. And this is a story that I read about in Scientific American, but it couldn't be more different. So instead of two of them, this is one supermassive black hole, and it appears to have gone rogue, it appears to be on the run across the cosmos.

Shamini Bundell

There's a black hole flying through space. Firstly, I did not know that that was possible. And secondly, that sounds slightly worrying, really.

Benjamin Thompson

I mean, I don't know in the cosmic scheme of things, how worrying it is, I don't think it's coming straight for us. But I will say that, that sightings of these are very rare. And researchers have put forward evidence that this is one in Astrophysical Journal Letters. Now the other week, I talked about how seemingly at the center of all galaxies, there are supermassive black holes, researchers believe, but to find one on its own is unusual, if that's what it is. And in this case, it was kind of found, you know, by chance, via an image taken by the Hubble Telescope.

Shamini Bundell

So usually, you know, black holes, hard to spot because they don't emit or reflect light, really. So you have to sort of see them by their impact on other things. And I can imagine in a, in the center of a galaxy, you kind of know where you're looking. But if there's a black hole just sort of running through space, how do you spot that?

Benjamin Thompson

Do you know what, that is actually a great question. And it turns out that in this case, what was seen on this image was a streak. Now, these are usually just artifacts, maybe cosmic rays hitting the Hubble Space Telescope. But in this case, the researchers had a bit of a closer look using some telescopes here on Earth. And it turns out that the streak is actually a bunch of young blue stars stretching 200,000 light years across the cosmos.

Shamini Bundell

So they've seen these stars is the idea that, you know, this black hole with its sort of massive gravitational pull is just sort of pulling stars along behind it and that's what we can see?

Benjamin Thompson

Well, potentially not quite. So what the researchers kind of think, is that actually, as this black hole is barreling along, it is compressing, shocking gas ahead of it, and forming stars, which are left kind in a wake behind it. And what is neat is that this wake of stars is pointing kind of like an arrow, if you will, to a galaxy about 7.5 billion light years away from Earth, that is showing no signs of having a giant black hole at its center. Now, it's obviously tempting to put two and two together. But researchers are interested to know like, is this what this might be?

Shamini Bundell

But how does a galaxy go about losing a black hole? You'd have thought that'd be quite hard to misplace really.

Benjamin Thompson

Agreed. And nobody really knows just yet. There's a lot of ideas. So when I talked about two black holes merging, coming together, no one's actually seen that final merging process, right. So what happens there isn't exactly clear, when they start to spin around each other. When they start to do this orbiting dance. Maybe one just gets flung away, who knows, right? Maybe a third supermassive black hole gets involved. And it's almost like billiard balls, like snooker balls knocking into each other, and to come together and smack that third one away. And so looking at the age of the stars, it looks like this supermassive black hole escaped about 40 million years ago, so not that long in the cosmic scheme of things. And it is absolutely motoring along at 1,600 kilometers per second. And its mass is many, many, many, many, many times that of our Sun.

Shamini Bundell

This sort of scales of this giant cosmic snooker game are just wild but actually in the vastness of space, this must have been a relatively sort of tricky spot.

Benjamin Thompson

Yeah, and I think researchers are quite excited about this. Obviously, they need to prove that this is what it is. And so things like the James Webb Space Telescope, the JWST, will have a look at it in more detail to confirm what's going on. But it could give some really interesting insights into galaxies. Because, as we say, if these are normally found at the center of galaxies, do they put the brakes on star formation? Do they actually increase star formation? What does them being there do in terms of how galaxies form and evolve, and this sort of thing. So there's loads more stuff that we could learn from this. And also potentially, there's a lot more of these to find, right? If this was found by chance as a little streak on a Hubble image, maybe there are more streaks out there that are actually, you know, rogue, supermassive black holes on their journey to wherever it is they're going. Well, listen, that's my story this week, let's move on to you Shamini, what have you got to discuss?

Shamini Bundell

Okay, so how's this for a segue? We're going from a huge, massive, supermassive black hole down to the humble and small ant, or actually a yellow crazy ant is what the species is called, to be precise. And this is a story that I had been reading about in Nature, there was a paper in Science, and it's about some really unusual ant genetics.

Benjamin Thompson

Right. Okay, so I must confess, I don't know very much about ant genetics. So what's been found then? And what's so unusual about it?

Shamini Bundell

Okay, so ant genetics, just so you're aware, are already weird. That's the sort of baseline. Ants, wasps are these sort of eusocial insects, they have this sort of particular, unusual genetic structure. So this is weirdness on top of what to us might already seem to be weirdness. So some researchers were looking at some genetic markers in some ants for I think, for another study, and they saw that males seemed to carry two copies of these genetic markers. Now, that wouldn't be weird for us, right? Because we have two copies of almost every gene in ourselves. Yeah, we have one copy from one parent, one copy from the other parent. Male ants typically don't have two copies of a gene, they typically don't have two sets of chromosomes, because male ants usually are formed from unfertilized eggs.

Benjamin Thompson

Okay, so that then begs the question, where is this other set coming from then? And how does it work in these what yellow crazy ants, you say?

Shamini Bundell

Yellow crazy ants, exactly. So basically, the researchers sort of spotted this and like, ooh, that's weird what's going on there, they looked at a cell from the male ant, it's only got one copy of the genome, look at another cell also only got one copy of the genome. what they realized is that these male ants in this species are chimeras. They are a combination of two different lineages.

Benjamin Thompson

And this presumably has raised eyebrows then for these researchers. Tell me more about these lineages, then where do they come from?

Shamini Bundell

Yeah, so as I said, usually an unfertilized egg would become a male, and a fertilized egg would become a queen or a worker in this sort of genetic system of ants. But in this case, you have got a sort of egg meeting with a sperm. But what happens is the nuclei of the cells don't fuse together. So you get you know, the one copy of the genome from the egg, the one copy from the sperm, and you've got different cells with the different genomes in so this is what it is to be a chimera, which usually chimeras are accidental. So I think tortoiseshell cats are chimeras that's an example where you can kind of see the different cells because it results in different patterning. But this is the first time where a species is sort of obligately, it has to be an always is from what they can see the males are chimeras, and that's how their system works.

Benjamin Thompson

So a very, sort of weird, standout bit of genetics then in this particular group of ants, this must be quite exciting for researchers.

Shamini Bundell

So one researcher quoted in this article, Daniel Kronauer, who actually we've interviewed before, described it as "...a piece of biology that’s unparalleled as far as we know". And there was a sort of puzzle here. There was a previous paper that definitely raised some questions about what was going on with the genetics of these ants. And one of the co-authors of that study described this as "...finally having cracked the enigma that kept me awake at night for much of my PhD."

Benjamin Thompson

Goodness.

Shamini Bundell

So people are definitely pretty pleased to have solved this puzzle. And potentially there could be some applications here because yellow crazy ants are actually hugely invasive. And there was an example on the David Attenborough documentary Planet Earth II where it showed these yellow crazy ants attacking a crab on Christmas Island, and yellow crazy ants have spread to loads of islands across Asia, causing problems. So there could be a way that understanding their quirky biology in this way, could help us maybe control their numbers, maybe even this chimerism is what's helping them be so effective at spreading and causing such a problem, but it's all as usual up in the air at the moment. And the paper author says, you know, there's so much we don't know.

Benjamin Thompson

Well, I do love our animal odyssey chats, Shamini, and that is a neat one, and listeners if you'd like to learn more about the stories, then look out for links in the show notes, where you'll also find a link on where you can sign up for the Nature Briefing itself and have more stories like these delivered directly to your inbox.

Shamini Bundell

Well, that is all for this week. But just before we go very quick reminder, there is still time to vote for us in this year's Webby Awards. Oooh.

Benjamin Thompson

Oooh indeed, yeah, that's right. We're up for two awards. And also we're up for the associated People's Voice Award, which is voted for by listeners, listeners, like you may be right now...

Shamini Bundell

Just like you, so if you have a couple of minutes to spare and you want to cast your vote for us, look out for a link on where you can do that in the show notes.

Benjamin Thompson

And as always, don't forget you can keep in touch with us on Twitter, we're @naturepodcast, or on email podcast@nature.com. I'm Benjamin Thompson.

Shamini Bundell

And I'm Shamini Bundell. Thanks for listening.