Flame Retardant Could Be Made From Discarded Cocoa Husks

7:49 minutes

Cocoa husks with seeds removed.
Leftover cocoa husks with the seeds removed. Credit: Shutterstock

On cocoa farms around the world, cocoa beans are pulled from their pods, and the hard husks are discarded, leaving some 20 million tons of plant waste to biodegrade and potentially harm future crops. These husks are a source of lignin, a substance that gives plants their rigidity. It’s extremely abundant—but often wasted.

A new study published in the journal ACS Sustainable Chemistry and Engineering found that the lignin processed from leftover cocoa pod husks could have a new use as an ingredient in flame retardant.

In the study, the researchers applied a pretreatment process that allows lignin be chemically manipulated more easily. “[Our] lignin is pretty special, as it is very soluble in organic solvents,” said study co-author Dr. Nicholas Westwood, a professor of chemistry and chemical biology at St. Andrews University in Scotland, in an email.

Because of the lignin’s malleability, Westwood and his co-authors were able to add a flame-retardant molecule to the processed substance, and found that the modification increased its already naturally high ability to smother flames.

That’s just one possible application. While lignin hasn’t found widespread industrial use yet, scientists hold hope for it to become a greener alternative for fuel and a biodegradable plastic instead of just being leftovers. Processing biomass for food or fuel also produces a massive amount of lignin as a byproduct, which has been converted to materials like activated charcoal or carbon foam. “There are endless possibilities,” Westwood said.

​​Joining Ira to talk about lignin and its potential uses is Dr. Rigoberto Advincula, a materials scientist with the Oak Ridge National Laboratory and the University of Tennessee in Knoxville.

Correction 12/18/23: A previous version of this article quoted Dr. Westwood as saying lignin is soluble in organic solvents. He was referring specifically to butanosolv lignin, which is derived from a pretreatment process. The article has been updated accordingly.

Further Reading

Segment Guests

Rigoberto Advincula

Rigoberto Advincula is a material scientist at Oak Ridge National Laboratory and the University of Tennessee Knoxville in Knoxville, Tennessee.

Segment Transcript

IRA FLATOW: This is Science Friday. I’m Ira Flatow. On cocoa farms around the world, husks are stripped off cocoa beans, leaving 20 million tons of plant waste to biodegrade. These husks are a source of lignin, the substance that gives plants their rigidity.

A new study published in ACS Sustainable Chemistry & Engineering says that lignin from the leftover cocoa bean husks could be an alternative for lots of different fossil fuel-based products, like plastics and flame retardants. Joining me to talk about this is my guest, Dr. Rigoberto Advincula, material scientist with the Oak Ridge National Laboratory and the University of Tennessee in Knoxville. Welcome to Science Friday.

RIGOBERTO ADVINCULA: Well, thank you so much, Ira. I’m happy to be with your show.

IRA FLATOW: Thank you. Let’s talk about lignin. What are we currently use it for?

RIGOBERTO ADVINCULA: Well, lignin is actually a by-product of the pulp industry. It’s well known in the biosciences that you need lignin to hold the cellulose and basically give the rigidity in plants. But right now, a lot of it is thrown away.

IRA FLATOW: And so we want to save it from the cocoa husks?

RIGOBERTO ADVINCULA: Yes. So like any other agro-industry, they have a lot of biomass. That includes lignin and cellulose. And they’ve been finding that there are many uses for lignin for a variety of things, including plastics, flame retardants, adhesives, and so on.

IRA FLATOW: Give us an idea on how you extract it from the cocoa husks and other plants.

RIGOBERTO ADVINCULA: So basically, lignin binds the cellulose, hemicellulose derived from a big unit called fibril. And you have to do some chemical treatment dissolution with a solvent. And when you do that, you fractionate them basically into the cellulose and the lignin, which, in our language, basically, is a mess.

Lignin is a very heterogeneous molecule, or biomolecule. But when you process it or pyrolyze it properly, you get all sorts of very useful stuff that, for example, can be used for making further complex and very useful chemicals.

IRA FLATOW: You said it was a mess. What are the challenges of biorefining these materials? How do you get high-quality lignin?

RIGOBERTO ADVINCULA: Yes. So lignin is a class of what we call heterogeneous phenolics or alcohols. They are highly branched. And depending on the plant source, they can be very different in terms of the aromatic content and the amount of phenolic content.

So one has to carefully understand the heterogeneity or the variance of chemical intermediates you can extract from these various plant sources. So for example, with cocoa and coconut and wood, they have different fractions which has to be separated in order to use them for a variety of chemical intermediates.

IRA FLATOW: And you already mentioned the possibility of using them for plastic, flame retardants. Are there any other kinds of sustainable materials that might show up?

RIGOBERTO ADVINCULA: Yes. So cellulose obviously is a type of polysaccharide, the same as the class of starch that we eat. But cellulose is the stuff that is not easily digested. They’ve been finding many uses for cellulose beyond the paper industry, including nanomaterials, including different types of bioderived plastic.

On the other hand, the lignin is actually a rich source of chemical intermediates when derivatized, for example, with phosphonate groups or lots of nitrogen. That’s what makes it very useful, for example, in flame-retardant applications. So depending on the type of biomass, basically, you have to classify the type of lignin chemistry you can derive. And therefore, the biorefining, solvolysis, pyrolysis, these are all steps that are done in order to make them more useful.

IRA FLATOW: Now, we’ve heard about bioplastics. But bioplastics are still plastic, right? Does extracting lignin from cocoa husks solve this problem?

RIGOBERTO ADVINCULA: So the cellulose actually has been around for a long, long time. It’s basically your natural polymer– natural polymer, natural plastic, and therefore biodegradable, in many cases. Polylactic acid, which is derived from other plant or even bacterial sources, is actually a biodegradable plastic that is useful for packaging these days.

Lignin, on the other hand, is a different kind of plastic. Because of the phenolic and alcohol groups present, one has to do some type of condensation polymerization to convert them into what we call thermosets. So when we say thermoset, these are plastics that are not easily molded and reform with temperature.

A good example of a thermoset is epoxy. And epoxy has many uses, for example, in the fiber-composite industry. So in this case, lignin really is a rich, rich playground to do more chemistry to derive different types of plastics and materials.

IRA FLATOW: Is there a business then that needs to be set up to extract and reuse all these husks that are lying around?

RIGOBERTO ADVINCULA: Yes, good question. So a lot of the value depends really on the use of catalysts and the lower energy, meaning less cost to convert them into something useful. So for example, many companies, normally, they will just consider this as waste. But then the new chemistries that are being developed, even with some startup companies, they’ve been finding ways to efficiently extract the useful intermediate and therefore get more value out of it.

So a good example is the lignin can be derived into what we call monolignins, essentially very small molecules derived from lignin. These are closer to other types of aromatic intermediates that we use for solvents, for use in shampoo or plasticizers, for different types of chemical intermediates to make more complex molecules. So the more they can be purified into monolignins, the more they become useful as replacements for fossil-derived chemical intermediates.

IRA FLATOW: Fascinating, fascinating. And good to hear about this. And I hope we get to see some products made from the husk soon.

RIGOBERTO ADVINCULA: Yes. Yeah, exciting because that means you can take any waste– biomass waste and derive value out of it.

IRA FLATOW: Yeah, yeah. Well, that’s what we’d like to do. Thank you, Dr. Rigoberto Advincula, material scientist with the Oak Ridge National Laboratory and the University of Tennessee in Knoxville.

RIGOBERTO ADVINCULA: Thank you. My pleasure.

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