Traditional plastic recycling isn’t working very well. Innovations in plastic recycling technology are on the horizon that can make big improvements. Plastic recycling has long chopped up plastic into small pieces and melted it to make something else from it. In other words, it doesn’t change the chemistry of the plastic.
Recycling rates have always been low. The market for waste plastic has largely dried up in the wake of the Chinese crackdown. So researchers are looking for ways to break down plastic into something that’s no longer plastic.
Recycling plastic with enzymes
I reported earlier about plastic-eating bacteria. One discovered in 2016 in a Japanese waste dump makes two different enzymes.
The first, PETase, breaks the PET (polyethylene terephthalate) into MHET (mono-2-hydroxyethyl terephthalate). It’s the monomer component of PET.
The other enzyme, MHETase, breaks MHET down into still smaller molecules: terephthalic acid and ethylene glycol. These two building blocks make it possible to synthesize PET without petroleum.
Degrading plastic waste is a great benefit. Using the resulting chemicals to remake it is totally revolutionary. It creates a closed-loop production and recovery cycle for making new plastic.
Scientists at the University of Portsmouth, led by John McGeehan, engineered a version of PETase in 2018. It started to break down plastic in a few days. Now, they have announced linking the two enzymes into one super-enzyme that works six times faster. It’s an example of how scientists are learning about a natural process and engineering it in the lab.
Carbios, a French startup, developed a different enzyme that digests PET bottles within 10 hours. It works only at a temperature exceeding 70ºC. The new super enzyme works at room temperature. No bacterium could create such a large molecule.
McGeehan points out that academic researchers in partnership with a company such as Carbios might be able to bring efficient biological recycling of PET to market within the next two years.
McGeehan’s team has jointly published its latest article along with four institutions in the US. Research continues in not only studying combinations of enzymes to degrade PET quickly but also other plastics.
When polyurethane degrades, it releases a toxic chemical that would kill most bacteria, but one bug actually uses it for food as it works on the not often recycled plastic. Maybe all plastics will find a microbe that enjoys it.
Catalytic plastic recycling technology
Instead of exploring enzymes to break down plastic, researchers at Ames Laboratory developed an artificial catalyst to mimic enzymes. The team prepared a very porous silicon oxide surface with a platinum catalyst at each pore.
It doesn’t separate polyethylene into its monomers. Instead, the catalyst separates molecules containing from eight to thirty carbons. By varying the depths of the pores, researchers could control the average length of the resulting chains. In other words, they could predetermine the hydrocarbon molecules they wanted. That way, they could make some hydrocarbons that worked as lubricants or others that worked as fuel.
This process needs a lot more work before it can become practical. For one thing, the costly platinum catalyst only breaks up polyethylene. It will be necessary to find other catalysts that will break up other plastics.
And only something less expensive than platinum to make the process economically feasible. Also, it only works at fairly high heat and requires a supply of hydrogen. Once the procedure becomes flexible enough to handle any kind of plastic at reasonable cost, this kind of plastic recycling technology will certainly find uses.
Recycling plastic with microwaves
An international collaboration has developed a different catalytic method of degrading plastic waste. Researchers mixed pulverized plastic particles with a catalyst of iron and aluminum oxides and microwaved it.
This one-step process produced a large volume of hydrogen. Most of the solid residue turned out to be carbon nanotubes. The method is not only fast, but inexpensive and does not emit carbon dioxide.
This new plastic recycling technology uses plastic as a feedstock for producing two valuable commodities. When the world sees used plastic as a manufacturing material instead of a waste disposal problem, it should go a long way to solving plastic pollution.
This innovation in applied science resulted from a pure science project. Fragmenting highly conducting metals into smaller and smaller pieces results in a stage where the particles stop acting like metals. They decrease in conductivity while at the same time greatly increasing microwave absorption. This research promises to produce a highly effective method of heating catalysts for other applications.
If recycling plastics isn’t making sense, remake the plastics / John Timmer, Ars Technica. October 18, 2020
‘Molecular scissors’ for plastic waste / Science Daily. April 12, 2019
New super-enzyme eats plastic bottles six times faster / Damian Carrington, The Guardian. September 28, 2020
Turning plastic waste into hydrogen and high-value carbons / Oxford University. October 13, 2020