Engineered Bacteria Might Help The Dream Of Mixed Plastic Recycling
We’ve all been there—standing by the recycling bin, holding some sort of plastic object, and trying to figure out if it can go in the bin.
There are many different types of plastic out there, from the film that wraps the meat at the grocery store, to the plastic in your milk jug. But they all differ in their ability to be recycled, and in the specific procedures and recipes that it takes to process them. Writing in the journal Science, a team of researchers describes a demonstration process that can break down a mixed bag of plastics, even dirty ones, and produce a single chemical output that could be used in industry.
The process starts with a catalytic oxidation process involving metal salts, an acetic acid solvent, heat, and oxygen. That process is essentially a “controlled combustion,” says Dr. Gregg Beckham of the National Renewable Energy Laboratory. The oxidation process breaks the plastics in the reaction into a blend of liquid chemicals. Then, that blend of products is fed to a strain of engineered bacteria that have been designed to be able to eat each of those chemical breakdown products, and use them to make a specified product.
Beckham says that in the initial experiment, they created two different products—one a biodegradable plastic, and one a precursor to a type of recyclable nylon—but the method could conceivably be adapted to any product that bacteria can be enabled to grow via synthetic biology. Beckham joins SciFri’s John Dankosky to talk about the demonstration, and the challenges of moving this technology out of the laboratory and into an operating recycling process.
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Gregg Beckham is a senior research fellow at the National Renewable Energy Laboratory in Golden, Colorado.
JOHN DANKOSKY: OK, so breaking down toxic forever chemicals– that’s a pretty big deal for the environment. But what about that jumbled mess of plastic waste that every household makes each week. There’s tons of different plastics out there, from the wrap on the grocery store vegetables to the plastic in your milk jug. And then you’re standing over the recycling bin wondering, does this go in?
Well, when it comes to recycling, not all plastics are created equal. Some can be recycled fairly easily, but others can’t. And each type has a different recipe or process. But now, writing in the journal Science, researchers describe a demonstration of a process to recycle lots of kinds of plastics, even all mixed together. It takes two steps. One is chemical, and the next is biological, involving engineered bacteria.
Dr. Gregg Beckham is lead author on that paper. He’s a researcher at the National Renewable Energy Laboratory in Golden, Colorado. Dr. Beckham, welcome back to Science Friday.
GREGG BECKHAM: Great to be with you, John. Thanks for having me today.
JOHN DANKOSKY: So first of all, why is it that some plastics can be easily recycled and others just can’t?
GREGG BECKHAM: The two primary plastics that can be recycled today are primarily single-use beverage bottles, which is a polyester, polyethylene terephthalate, often known as PET, and the other is high-density polyethylene, HDPE, which is typically found in, as you alluded to already, plastic milk jugs and things like this. Those are the two primary plastics that are going to be sorted out at a materials recovery facility. And those are then sent off to mechanical recyclers to then put those back into circulation as different materials.
Oftentimes, polyester PET bottles will go to carpet or textiles or clothing. And HDPE can be sometimes recycled several times into, again, to milk jugs or to consumer goods that contain things like laundry detergent. But beyond that, there are many other plastics, and most of them are not recycled at all.
JOHN DANKOSKY: And so you really do have to separate out these other plastics that are easily recycled from the ones that aren’t.
GREGG BECKHAM: Absolutely. And that is really the point of, when we put things into those blue recycling bins, they’re sent to those materials recovery facilities. And they’re really trying to fish out primarily those two types of plastics only from myriad other plastics that oftentimes are put into recycling bins.
JOHN DANKOSKY: Talk more about this new approach that you’re coming up with here. There’s two steps. What’s the first part?
GREGG BECKHAM: So the first part, we use a catalytic oxidation reaction. And that basically is a way to say we use a chemical catalyst, which is a metal salt, and oxygen from the air. And we put that together with a solvent– in this case, acetic acid, which is found in vinegar. And this is like a controlled combustion reaction.
We take that oxygen from the air, and we’re essentially using this catalyst to add it to the plastic. And in doing so, it breaks up all kinds of mixed plastics into molecules that are then oxygenated. And that means they’re more water soluble and more bioavailable for the next step, actually.
JOHN DANKOSKY: So then what happens to these liquids?
GREGG BECKHAM: So then we have this mixture of water-soluble, oxygenated compounds that are essentially breakdown products of these mixed plastics. And then we engineered a bacterium, which you can find in the soil, basically to be able to consume all of those breakdown products, or at least all of the ones that we can detect and measure. Imagine like a buffet. You’ve got all of the entrees and side dishes on this buffet. We’ve engineered this single bacterium to eat all of the options on the buffet. And what it does is it can turn all of those mixed products into a single target product that we sort of engineer in on the organism side as well.
JOHN DANKOSKY: So it’s like an omnivorous bacteria. It likes everything on the buffet.
GREGG BECKHAM: Absolutely. Yeah, that’s a great analogy.
JOHN DANKOSKY: So when you talk about a bacteria that is engineered, what exactly do you mean? I mean, how do you engineer it? What does it take?
GREGG BECKHAM: So we use techniques from molecular biology and synthetic biology to go in and modify the genome. Mostly, what we’re doing is we’re taking genes from other soil bacteria that are able to consume, say, building blocks of these plastics already in the soil. And we’re putting them into this one really robust, quite omnivorous, and making it even more omnivorous bacterium. So now, it’ll have all of the genetic machinery to make the enzymes needed to assimilate and consume all of the products from these mixed plastics oxidation reactions.
JOHN DANKOSKY: That’s so cool. I mean, we’ve talked on the program before. And when you were on the show last year, and we were talking about the future of plastics, we talked about enzymes breaking down plastics. Is this a different process than the one you described to us last time?
GREGG BECKHAM: So the enzymes that we have worked on in the past– and many other people around the world are working on those, it’s a quite exciting topic– those are only able to break down, like, your polyester plastics. What we’re looking at here now are plastics that are much less bioavailable. So there’s not a single enzyme that can break down all of these mixed plastics. We’ve now made all of those building blocks much more bioavailable, just like the building blocks from polyesters already are to enzymes.
And the cool part about using an organism now instead of an enzyme is that the organism can make hundreds of its own enzymes that are able to now go and basically pull in and consume all of those building blocks. And so it’s like the supercharged version of what we talked about last time on the program.
JOHN DANKOSKY: This is Science Friday from WNYC Studios. We’re talking with Dr. Gregg Beckham. He’s from the National Renewable Energy Laboratory in Golden, Colorado. And we’re talking about breaking down plastics into something that we can use.
So what happens now? The products that this bacteria make– can it be anything? Can we make some new product out of it?
GREGG BECKHAM: Yeah, that’s a really exciting, I think, next step. So in the current study, we demonstrated two products. The first one was this bacterium in particular, this soil bacterium. When it has a lot of excess food source around but not enough nitrogen or other nutrients, it will store the equivalent of bacterial fat.
That happens to be a polyester. It’s called polyhydroxyalkanoate. And it is already a product on the market, sold by some companies around the world for biodegradable packaging and packaging that can behave like some of the plastics we already use today from fossil fuels. But in this case, it’s biodegradable. And so that was one of the products we made, simply because it was easy. The bacteria, if we give it a lot of carbon, and we don’t supply it with a lot of extra nutrients, it will simply make that. And so we showed that would work.
We separately made another strain of the bacterium that was able to produce a building block of a nylon-like material, a nylon that is better performing than the nylons we make today and use as well as much more easily recycled. But that said, to your question directly, John, using the tools of synthetic biology, in sort of an ideal world, we would be able to make any target product we would want that you can have a bacterium make.
JOHN DANKOSKY: So this is– it’s remarkable. It sounds so exciting. Obviously, Gregg, there’s got to be a catch. What’s the catch?
GREGG BECKHAM: Yes, absolutely. As I mentioned earlier, there’s a long way to go. We have demonstrated this at the 100-milliliter scale, so a tenth of a liter, and on plastics that are about the size of a penny or so.
Can we put bottles? Can we put entire pieces of carpet? How do we scale this up? Is it economically viable? How do we best integrate all of the pieces of the process? Those are open questions.
And certainly, what do we make? That’s another open question. And all of those things need to be answered and addressed to basically take this to, hopefully, a commercially-viable process.
JOHN DANKOSKY: And commercially viable meaning you’ve got to find industry partners who can take whatever you make with these bacteria and want to make it into something else, or else there’s no economy there.
GREGG BECKHAM: Absolutely. Yeah. And not only does it have to be commercially viable from an economics perspective, but it absolutely had better be more sustainable than the linear economy of extract petrochemicals– make plastic, use them, put them in the landfill. So it absolutely needs to be better than that current linear approach that we have today.
JOHN DANKOSKY: Well, and I guess for a final question, though, that is really the big piece of this. We’re trying to adjust our economy, our energy economy, away from fossil fuels. The inputs in the future might not be the same byproducts of fossil fuel production. So how exactly does a process like this fit in as we start to shift away from the types of things we make and consume overall?
GREGG BECKHAM: It’s a great question, John. I hope in the next 50 years or so or next several decades that certainly bio-based building blocks that go into tomorrow’s plastics certainly should be a major emphasis of the research community, the industrial community, and generally of humankind. I view technologies like this as kind of a bridging approach where we’re going to continue making the plastics that we use today, probably for many, many years.
And hopefully, this can be a way to onboard a much bigger, essentially, funnel of types of plastics that we’re able to currently recycle. And hopefully, by doing this and then turning them into building blocks for recyclable plastics, we can then sort of usher in this transition. And hopefully, the technologies like this, many of which are being developed around the world, can help in that respect.
JOHN DANKOSKY: You sound positively, I don’t know, giddy about these prospects.
GREGG BECKHAM: Absolutely. We are extremely excited about this process concept. We think it’s very cool. The ability to recycle mixed plastics is just such a challenge. And right now, there’s only a few options, and they have a lot of issues with them at scale. And so I think any and all approaches should be looked at that are able to take in mixed plastics over just a single type of plastic, for sure.
JOHN DANKOSKY: Gregg Beckham is a researcher at the National Renewable Energy Laboratory in Golden, Colorado. Greg, thanks so much for taking the time to talk to us today.
GREGG BECKHAM: Thank you so much, John. Really appreciate it.
John Dankosky works with the radio team to create our weekly show, and is helping to build our State of Science Reporting Network. He’s also been a long-time guest host on Science Friday. He and his wife have three cats, thousands of bees, and a yoga studio in the sleepy Northwest hills of Connecticut.