Photo: TECS associate professor Sonja Salmon displays a piece of denim treated with enzymes
Written by Cameron Walker
Some problems are so big, so systemic, they can seem insurmountable. Take textile waste, which Americans currently generate at the rate of more than 11 million tons per year; textile wastewater, which is one of the world’s major environmental pollution problems; or carbon dioxide pollution from fossil fuel combustion, which is the driving cause of global climate change. Inaction is not an option in the face of these critical challenges, but we need a catalyst, an agent of significant change to turn the encroaching tide. Enter researchers like associate professor Sonja Salmon from the Department of Textile Engineering, Chemistry and Science (TECS) at the Wilson College of Textiles at NC State.
Salmon conducts her research at the intersection of enzyme technology and fiber technology.
“Enzymes are nature’s catalyst,” she said. “They’re biological materials, special kinds of proteins that can cause chemical reactions to occur in very gentle conditions…Enzymes are produced by all living things. In fact, a living thing cannot be a living thing without enzymes, because [they] are the catalysts that produce the molecules existing in and needed by living things . Enzymes are all around us and in us.”
In the human body, enzymes are responsible for breathing, digestion, metabolism, DNA replication and muscle, liver and nerve function, among many other things. You may be familiar with some enzymes found in our bodies, including lipase, which helps digest fats in the gut; amylase, which helps turn starches into sugars; and lactase, which breaks down the milk sugar lactose into the more digestible glucose and galactose.
“Enzymes can break down molecules,” Salmon said. “They can take big molecules and chop them into smaller ones. It’s what happens when you eat food; in digestion, you’re chopping down a big molecule into little ones so that your body can get energy.”
Enzymes are produced at an industrial scale by fermentation, much like the process of brewing beer — in fact, sometimes they even smell like beer. Certain enzymes are commercially available, but there are many more types of enzymes in nature that may prove to be useful for industry.
“You can use several different classes of enzymes, different categories of enzymes that work in different ways,” she said. “For any particular application, you have to find the match between what the enzyme does and what your goal is, and once you find that match, you work on optimizing it to make it work in the best way possible.”
Salmon’s research seeks to use enzymes together with fibers to solve some of the world’s most pressing problems.
“There are so many things that you can do with fibers,” she said. “Many fibers are produced naturally — you find them in the natural world. Your hair is made of fiber, cotton that you wear comes from a fiber [produced by] a plant. Nature still has a lot to teach us about what we can create and use.”
There are three main issues that Salmon’s sustainability research seeks to resolve: difficulties in textile recycling, the textile industry’s high water usage and inefficient and expensive filtration of wastewater and mixed-gas streams.
“We produce and consume and discard a lot of textiles, and they’re piling up as much as we like to donate them away,” said Salmon. “They end up in the landfill, where they just sit and don’t degrade. We lose a resource that way.”
There seem to be myriad ways to recycle unwanted textiles, including neighborhood drop-off bins, municipal pick-up efforts and in-store recycling boxes at retailers such as H&M. But the reality of textile recycling is more complicated. If a garment or other textile is not donated to a resale shop or consigned, the odds of it being recycled are pretty low. In fact, the latest figures from the Environmental Protection Agency (EPA) state that in 2017, only 15.2% of all textiles were recycled in the United States.
But why do we recycle such a small amount? It’s complicated — literally.
“One single shirt could be a polyester/cotton blend, it could be multicolored, it could have different kinds of finishes on it,” said Salmon. “That’s actually why these things get thrown away in the end. They’re so complicated, they’re hard to recycle.”
Current recycling technology is not made to handle such complexity. It is difficult and expensive to separate fibers from a mix of materials or those treated with different chemicals. But Salmon has found a way to streamline the process and ensure that more textiles are recycled on the back end — by using enzymes to selectively degrade some of the textile’s components.
“For example, in a cotton/polyester blend, I can use cellulase to degrade the cotton part,” she said. “Essentially, I’m cleaning the polyester and making that polyester portion easier to recycle. So I take the biodegradable part and chop it down, then I recover the synthetic part and make it something that can be reprocessed.” Salmon explains that the degraded cotton part forms a liquid containing simple sugars that can then be fermented into products like biofuels.
She calls the process biocycling, and it could be the key to saving millions of tons of textiles from being landfilled or combusted. After this process is perfected and adapted, her larger goal is to educate designers so they can create textiles on the front end that will be easier to recycle — essentially, designed to degrade.
“In textile processing, you’re using harsh chemicals like sodium hydroxide, which is commonly used in processing cotton, but cotton holds on to sodium hydroxide,” said Salmon. “The reason you end up having to use all this water is that you’re trying to wash out a chemical. If you can replace the sodium hydroxide and get the same benefit with something else, like an enzyme, the enzyme will wash out much easier and you don’t need that high pH to start with, [resulting in] water savings.”
Sodium hydroxide (NaOH), also called caustic soda, is not only difficult to wash out of cotton — it is also harmful to wildlife, so the high volume of effluent is prohibited from being discharged into groundwater. Replacing processing chemicals like NaOH with enzymes could save millions of gallons of water per year.
Last semester, Salmon introduced her class to the sustainable power of enzymes through a common material: denim. Over two days in the wet lab, her class processed raw denim with cellulase, which chewed into the cotton’s cellulose, turning it into a soft fabric and releasing some of the indigo dye, resulting in a salt and pepper appearance.
“As an educator, I want my students to understand before they walk out of the door here that enzymes are an option, and I want them to understand why enzymes are a good idea,” said Salmon. “Enzymes are made by renewable resources; they’re produced by fermentation in microorganisms…By using these biocatalysts, by using these enzymes, you have a chance to move away from harsh chemistry — reducing chemical load, reducing water usage, reducing the temperature at which you need to process things. Saving energy, saving chemicals, saving water is the triad of benefits that can come with using enzymes.”
Much of Salmon’s research centers on a new class of materials she calls biocatalytic textiles. One application for this innovative technology is filtration, which could help power plants and factories become more sustainable and greatly reduce their carbon footprints.
“I’m taking enzymes, which are special, highly effective catalysts, and I’m placing them on textile fibers,” she said. “Textile fibers have a high surface area, are lightweight and you can fabricate them in just about any type of shape you would ever wish to. In this way, you have the potential to create a new type of reactor, a biocatalytic textile reactor, that you can use for placing into a process stream.” A filter containing this catalyst could passively treat large volumes of wastewater without chemicals.
Salmon has an even bigger target, one that has the potential to make a powerful, positive impact on our planet.
“I’m immobilizing an enzyme called carbonic anhydrase on textile fibers,” she said. “The idea is to make a filter that can selectively remove carbon dioxide out of gas.”
Carbon dioxide (CO2) is recognized as one of the primary greenhouse gases; the accumulation of these gases in our atmosphere is driving climate change. The majority of human-caused CO2 emissions (87%) is the burning of fossil fuels, which powers most of our factories, our electricity and our transportation. The source of the energy that powers modern society is the very thing that threatens our planet and our way of life.
One way to reduce these emissions is through gas scrubbing, a technology that grabs the carbon in between combustion and release into the air. In a typical gas scrubbing system, explains Salmon, carbon dioxide (just one of a mix of gases in the exhaust) selectively reacts with a liquid, which is then heated to release the now-pure CO2, which can then be compressed and transported in pipelines to be sequestered underground. However, this technology is extremely expensive to implement, which means it is rarely used.
But Salmon has found what is potentially a much better way to capture the CO2 before it is released into the atmosphere. Her enzyme-treated filter would react CO2 with water, turning it into bicarbonate of soda — better known as baking soda. Her goal is that it would be a cheaper, easier process, that could help industry implement the CO2 management processes we so urgently need.
Climate change, industrial pollution, the waste generated by fast fashion…these problems can seem insurmountable because it’s hard to fathom billions of people voluntarily changing their lifestyles.
“That is actually why I care,” said Salmon. “That’s why I think I can do something here, because I also like to take a hot shower. I also like to turn on my own lights. I realize that at some point, you’re so accustomed to your lifestyle, that you just can’t give it up. We really have to have a different way to think about how we are going to manage this. We can’t roll back the clock and we can’t ignore it. So we have to innovate our way out.”
Salmon’s research is interdisciplinary, and she credits collaboration as a main driver in her work.
“Collaboration is key,” she said. “It is absolutely essential. The world is very complicated now; it’s not possible to know everything, and if you really want to make a difference, you have to collaborate with people. You are by far stronger as a team than you are alone. Even the process of innovation itself is deeply enhanced by collaboration; you may think you have a good idea but then, by talking to a few other smart people, your idea will get even better. It may change direction completely. You may land on something that you never thought of before.”
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