Plastic is a big word. It encompasses a dizzying array of material throughout the world, as anyone can attest, from yogurt containers and car parts to hard lawn chairs and flimsy grocery sacks all around us.
That also means a giant amount of waste products, from a floating island of plastic trash in the north Pacific to stacks of plastic waiting to be recycled or just crammed into landfills. While many of us might see a nearly insurmountable problem, Hongfei Lin also sees a vast, untapped resource.
Lin, associate professor of chemical engineering at Washington State University, and his research team are working on a plastic recycling solution that bypasses tedious and inefficient physical sorting and sequentially breaks down different types of plastics using chemical means. They’ve already had success with producing jet fuel and high-quality industrial lubricants from plastic waste such as milk bottles.
Meanwhile, other WSU scientists are studying how tiny plastic particles move through wastewater treatment systems, what effects those plastics have on soil health (and how much is even in soil), impacts of the global plastic waste trade, and ways to reduce agricultural use of traditional plastics by replacing them with soil-biodegradable options.
Plastic waste is a problem with a lot of angles. Each researcher approaches that conundrum in a different way, but all with the goal of a more sustainable world with less plastic making its way into the environment. Lin, for example, wants to see recycling scale up quickly, since only 9 percent of plastic is recycled now. It could lead to a more circular economy where used plastic becomes an asset.
“Waste plastics are a huge reserve,” Lin says. “If you don’t consider it a waste, it becomes a useful resource for many years.”
David Attenborough, a 94-year-old documentarian and natural historian, sums up the idea in his 2020 memoir, A Life on Our Planet: “By changing our approach to the use of our resources, a growing number of people believe that humanity could eradicate waste and come to mimic nature’s cyclical approach.”
From bottles to jet fuel
Plastics didn’t start out as a problem. It was a wonder material that was cheap and easily shaped into any number of items. Its first iteration, as celluloid, actually came from a desire to replace rare animal products, such as tortoiseshell, horn, and ivory.
The first fully synthetic plastic, Bakelite, was invented in 1907 and marketed for its insulating capabilities in a rapidly electrifying society. Following that breakthrough, the word “plastic” became a catch-all term for synthetic polymers—long chains of carbon atoms in repeating units constructed primarily of fossil fuel-based chemicals.
Because the polymers were strong, lightweight, and flexible, manufacturers quickly started identifying polymers for new forms of plastic. The need for materials in World War II led to a rapid, 300 percent explosion of plastic production: Plexiglas for airplane windows, nylon for parachutes, and thousands of other uses. The postwar period saw those new materials enter the civilian world and offer cheap, often disposable, alternatives for consumer goods.
“When people designed plastics, they wanted really good properties. For example, packaging materials need to be durable. It’s not easy to break and you probably want it to resist chemicals and water,” Lin says. “From the very beginning, the scientists designed the formula to use plastics for a long time.”
Those same beneficial properties were detriments when it came time to dispose of plastic. It’s hard to break the molecular bonds in polymers to form another useful product. Plastics decompose very slowly in natural environments, which is why recycling is a preferred option.
It gets even more complicated because of the variety of plastics. Almost everyone has seen the numbers one through six in arrow triangles at the bottom of plastic products, which show the composition of the plastic for recycling purposes. Those recyclable, numbered products make up 80 percent of all plastics.
Lin notes that people throw almost everything in the recycling bin, even when there are sometimes complex plastic recycling rules for each city or region.
“The grand challenge of recycling plastics is that when we collect those waste plastics from residential or industrial places, it’s already in a mixed, commingled state,” he says.
The recycling industry relies on physical sorting facilities, which are very large and capital intensive. Even when plastics are separated mechanically and processed, the resulting material isn’t great.
Disposable water bottles, for example, are shredded, heated, and then extruded to fibers for other applications. These processes tend to change the properties of the plastics, Lin says. After a water bottle is recycled and goes through the mechanical process, it degrades the material and it won’t be suitable for that application again. This is called downcycling.
A typical mechanical recycling process also can’t bring together two types of commingled plastic and make a new plastic. The different polymer composition, for example, of a water bottle and a milk jug, prevents melting them together and turning the result into a useful material. They are not compatible.
“To address this, there’s an alternative approach in chemical processing,” Lin says. “We break the plastics down to monomers and then use the monomers as a building block. This is almost the same as producing plastics from petroleum.”
The plastic types have different chemical bonds. Lin’s research is identifying specific catalysts that will break the bonds of a plastic type, without affecting the other plastic types. This approach removes the need for physical sorting while recycling a wide range of plastics.
“Our idea is to convert a mixture of plastics sequentially,” Lin says. “This really depends on the catalysis, and if you can design a highly selective catalyst for every step of the process.”
Lin and his fellow researchers in the Gene and Linda Voiland School of Chemical Engineering and Bioengineering, including former postdoctoral researcher Shaoqu Xie and Chuhua Jia (’21 PhD Chem. Eng.), have already proven the concept with the multilayer plastic films that are composed of polyethylene terephthalate (PET), common number one plastics such as water bottles.
Using a specific catalyst, they converted 90 percent of the polyethylene into jet fuel and lubricants in an hour at a moderate temperature.
The researchers used ruthenium (a “noble” metal in the platinum group) on carbon as the catalyst and a commonly used solvent. The conversion took place at a temperature of 220 degrees Celsius, which is more efficient and much lower than 500–600 degrees Celsius that would be used in pyrolysis.
Changing parameters such as temperature, pressure, and solvents can produce different products, Lin says.
“Once we deconstruct number one plastics, we’ll send the residue to the next stage and convert nylon and then to convert polyethylene in the final stage. We can gather useful products from each conversion unit and eventually could utilize all these waste plastics,” he says.
As the team works on applying the chemical process, they’ll also study the fundamentals for the specialized catalysts. For example, Lin wants to ensure that ruthenium and other key catalysts remain stable for a long time, after many uses.
He also wants the technology to get out into the world. “My passion is to grow and develop the technology in the lab, so it will mature and then commercialize,” Lin says. “It’s a pressing need and, if it’s cost-competitive, we reduce the use of more fossil fuels and help mitigate CO2 emissions.”
Lin and his WSU team collaborated with researchers from the University of Washington and Pacific Northwest National Laboratory, with support from the Washington Research Foundation and the National Science Foundation, on the catalytic approach to plastic recycling.
Lin says another purpose of his work is training a workforce for industries and research. “It’s not just products, but people. Renewing resources in a circular economy is the future.”
What’s in the water and soil?
Recycling plastic scrap can lead to a more sustainable world, but it’s clear that plastics are already widespread in the environment. WSU researchers Indranil Chowdhury and Markus Flury investigate just how plastic particles move through water and soil, and how much plastic is actually in soil.
“There is a lot of research going on about plastics in the ocean,” says Flury, a soil sciences professor working out of the WSU Puyallup Research and Extension Center. “We have a fairly good idea how much plastic is in the oceans and that we need to really address that issue. However, we don’t really know how much plastic is in the soils.”
Flury explains that plastic is more difficult to analyze in soil. Since plastic is carbon-based, it is hard to separate plastic from natural, carbon-based organic matter that’s already in the soil. He suspects the same widespread problem of plastic micro- and nano-particles exists in soil as in water, so Flury and others are looking at methods to analyze and quantify the plastic in terrestrial settings.
One way is to try to remove natural organic matter. The plastics would remain and can be filtered to identify what type of plastic it is, such as a polyethylene, polystyrene, or polypropylene.
“You can try to identify the type of plastics and then also see where it ultimately came from,” Flury says. “Polyester fibers are likely from clothing, dislodged during washing in your washing machine. That goes into the wastewater treatment plant and then into the biosolids that spread onto the soil.”
Many biosolids from wastewater treatment end up on soil, especially for agricultural use. They generally have positive effects, Flury says, particularly if applied to drier areas in eastern Washington, but questions remain about plastics in those biosolids.
Flury’s investigations into plastics in soil connect to Chowdhury’s research on plastic in wastewater and drinking water. An assistant professor of civil and environmental engineering in the Voiland College of Engineering and Architecture, Chowdhury has found some of the mechanisms that allow tiny pieces of plastic bags and foam packaging at the nanoscale to move through a wastewater and drinking water environment.
Silica surfaces, such as sand, are often used as part of drinking water filtration. Chowdhury and his research team found that silica has little effect on slowing down the movement of plastics.
Natural organic matter in water resulting from decomposition of plant and animal remains, on the other hand, can either temporarily or permanently trap the nanoscale polystyrene particles. Polystyrene is often found in packaging materials and disposable food containers.
Unfortunately, polyethylene, the most common plastic material, doesn’t easily bind with organic matter and slips through sand filters. Chowdhury wants to better understand the fundamental ways tiny plastic particles move, with the intent of capturing as much plastic as possible in wastewater and drinking water treatment systems.
“We look at the traditional filtration systems and how we can actually improve the filters to better remove this plastic,” Chowdhury says. “People have seen these plastics escaping into our drinking water, and our current drinking water system is not adequate enough to remove these micro and nanoscale plastics.”
A 2019 study found that people consume about the amount of plastic in a credit card each week. The health effects of plastic ingestion are still mostly unknown.
Flury says the impact of plastics on soil and plants is similarly mysterious.
“Is the impact as bad as in the ocean or do soils have more resilience toward plastic pollution? Does the plastic hinder plant growth?” Flury asks.
“At the moment, we don’t see that because the biosolids probably overwhelm the negative effect of the plastic in terms of plant growth,” he says. “But microplastics could potentially be taken up by plants.” To have an effect, though, would require pretty high concentrations of plastic.
Flury notes that plastic itself is inert and not really toxic, unlike many pesticides with toxic effects. Plastics, however, can absorb chemicals on their surfaces. Moreover, plastics often have additives, such as dyes or plasticizers to make them more malleable.
Some of these additives have been revealed as toxic, including bisphenol A (BPA). Pat Hunt, Meyer Distinguished Professor in the WSU School of Molecular Biosciences, has published several high-profile findings that BPA disrupts hormonal processes and causes genetic abnormalities. Many companies changed plastic products and removed BPA from their composition.
There are still many questions to answer about plastics in soil and plants, though. “We are really working on whether plastic particles can potentially be taken up by plants,” Flury says. “We have also done some work with earthworms, to see whether they are affected by plastics if they eat them.”
To mulch plastic
Plastics have a key role in both conventional and organic agriculture, so Flury’s investigations often intersect with horticulturist Carol Miles’s work on biodegradable plastic mulch.
Miles, a professor at WSU’s Northwestern Washington Research and Extension Center at Mount Vernon (NWREC), has experimented with plastic mulch for more than 20 years, studying its effects and testing improved products with industry partners.
Whether it’s at the bottom of fruit trees near Yakima or under organic strawberries in the Skagit Valley, plastic mulch retains water, keeps fruits and vegetables cleaner, and controls weeds. It’s an essential part of horticultural production, which is why soil-biodegradable plastic can offer a better alternative.
Agricultural plastic is about 3 percent of the plastic used worldwide, Miles says. Even though that’s a relatively small portion of all plastics, its contact with soil and proximity to water give it more importance.
“I saw agriculture as a model to study the use of plastic that’s intentionally put into contact with the soil and to see if there was an opportunity to look at alternatives to conventional plastic,” Miles says. “Could it be biodegradable?”
The answer is yes. A number of companies now produce plastic mulch that can decompose in soil, but it’s not without challenges.
Miles explains that some plastics might be called compostable, that is, they rely on the high temperatures of municipal composting. Home composting or degradation directly in soil doesn’t have enough heat, and thus a different biodegradable plastic product is needed for those situations.
Regional variations in soil temperature, moisture, and composition can also affect the behavior of biodegradable mulches. In Washington, cooler soils around Mount Vernon are far different from dry, warm eastern Washington soils.
Another obstacle is convincing farmers who have their eyes on the bottom line to shift to soil-biodegradable plastic mulch. Miles says the economics make sense for farmers, once you consider the full production cycle. Rather than removing conventional plastic mulch after each season—which can’t usually be recycled because it’s dirty—the biodegradable material can be tilled into the ground.
“Your upfront costs are two times greater for the biodegradable than the conventional plastic. But there are no additional end of season costs for the biodegradable plastic. It costs a lot less over the course of the season to use a biodegradable mulch,” Miles says.
Moreover, when buying a product, farmers need to be sure of crop productivity. That’s where Flury’s study of plastics and soil health and Miles’s study of crop yield make a difference.
“The hypothesis was, you know, that the bioplastics would potentially impact soil health and microorganisms,” Flury says. “We have not found any evidence for that at the moment. Biodegradable plastics seem to perform fairly well, and do not seem to have any negative impacts on soil health as far as we can detect or measure.”
The plastic circle
Tilling useful plastic mulch back into the soil, where it will safely degrade after providing benefits to crops. Breaking down plastic waste into valuable products, which will lower the amount of fossil fuels extracted. The circular economy can take us past a throwaway model to a more sustainable way of viewing a material like plastic.
The efforts at WSU continue to show that the circular economy will benefit everyone.
For example, the growing organic fruit and vegetable industry has an even greater need for plastic mulch for weed control without herbicides, while maintaining a favorable soil temperature.
Lisa DeVetter, associate professor and small fruits scientist at NWREC, teamed with economist Suzette Galinato, assistant director of the WSU School of Economic Sciences Impact Center, as part of a national research effort to support sustainable practices such as biodegradable mulches.
In an expansion of Miles’s work, they are testing biodegradable mulches that are made with fully organic-approved ingredients. Because existing soil-biodegradable mulches are made from starch and a blend of polymers that come from plant-based and synthetic sources, they do not meet the federal National Organic Program’s requirements for organic farming.
“I am very excited to work with new materials and develop technologies that could help sustainably grow organic, specialty crops in an economical way, while reducing plastic waste,” DeVetter says.
By reducing plastic waste, whether it’s covering the ground in a field of strawberries with biodegradable mulch or converting a mound of bottles and jugs, both the economy and the environment can win. Even with technological solutions, though, the plastic problem can seem to be mountain-size, literally in some cases.
“If you go to India, you will see trash mountains filled with plastics,” Chowdhury says. “They’re growing higher than 200 feet.”
As recycling techniques, such as Lin’s catalytic method, create new ways to deal with those piles of plastics, countries and people will see economic gains. That’s already happening with the global plastic waste trade.
As a WSU doctoral student, Yikang Bai (’21 PhD Socio.) found that the import of plastic waste was associated with growth in gross domestic product per capita in lower-income countries. Bai and Jennifer Givens of Utah State University analyzed 11 years of data, from 2003–2013, on the global plastics trade against economic measures for 85 countries. They found a positive impact of taking in those waste plastics.
Even though the plastic waste adds to the environmental burden of those importing countries, they are seeing financial benefits. With increases in plastic recycling, the circular economy could raise those countries’ standard of living even more.
The prospect of changing the approach to plastics inspires scientists such as Miles and Lin.
“I’m very enthusiastic that we can develop technology and make contributions toward a sustainable society. It makes my work feel valuable,” Lin says.
Everyone should play a part though, Flury says. “The solution to the plastic problem is multifaceted. One of them, for instance, is recycling. Another one is to reduce the use of plastics in the first place.
“We also need to reuse. Instead of a single use plastic bag, you have a multiple use plastic bag,” he says.
The change in mentality could make the biggest difference. Rather than plastic waste, we could have plastic scrap, upcycled into something new and useful or just turned back into the soil where it won’t harm the planet.
Plant plastics: Bio-based plastic research at WSU