Podcast Transcript
[00:00:00] Nataliya Shcherbatyuk: Hello and welcome to the Mulch Matters podcast where we will explore the intriguing world of mulch and its impact on agriculture and the environment, as well as update you on the latest research about soil-biodegradable mulch and recycling options for plastic mulch. I am your host, Dr. Nataliya Shcherbatyuk, and I am a communications specialist for the project, “Improving end-of-life management of plastic mulch in strawberry system”. In each episode, we’ll dive into the latest research, trends, news, and insights on why mulch matters and how we can improve plastic mulch end-of-life options. We’ll also branch out and discuss other plastics as well as talk to researchers, experts, and practitioners in the field who will share their insights and experiences on how to use mulch effectively in different settings.
[00:01:05] Nataliya Shcherbatyuk: Welcome back to another episode of Mulch Matters podcast, our guest today is Cynthia Sigira, so let’s welcome her. Hello Cynthia. How are you today?
[00:01:17] Cynthia Bosibori Sigira: I’m doing good. How are you?
[00:01:19] Nataliya Shcherbatyuk: Oh, I’m great. I’m happy to see you. Well, I’m happy to hear you because though I see you, our listeners aren’t able to see you, so I’m happy to hear you.
[00:01:30] Cynthia Bosibori Sigira: That is awesome.
[00:01:32] Nataliya Shcherbatyuk: So, Cynthia let’s start with you telling us a little about, a little bit about yourself, a little bit about your background, and then how you ended up doing the work you’re doing right now, and the focusing on your research that you’re currently doing.
[00:01:51] Cynthia Bosibori Sigira: Yeah, awesome, so as you have all heard, my name is Cynthia. I’m a PhD student at North Carolina State University. My major is in Forest Biomaterials, and I’m minoring in Chemical Engineering. And how I ended up doing this research? So I’ll just go back to my undergrad days. So, I did my undergrad in Civil and Environmental Engineering back in my home country, in Kenya, and I was more leaning towards the environmental engineering side of things, where I was interested in sustainability and solid waste management.
[00:02:27] Cynthia Bosibori Sigira: And in my final year, I was fortunate enough to do a project on anaerobic digestion of agricultural waste not necessarily agricultural plastics. But that really sparked an interest in me to further into research, and then I applied into grad school, got into grad school, and then again, I was fortunate enough to be offered a position to work on this project by my advisor.
[00:02:53] Cynthia Bosibori Sigira: And I have been enjoying my research so far in this area, so that is how I ended up in this project.
[00:03:01] Nataliya Shcherbatyuk: This is fantastic. How many years have you been in grad school already?
[00:03:05] Cynthia Bosibori Sigira: So, this is my second year, technically. I started out in Washington State University for my Master’s in 2024. And then my advisor moved to North Carolina State, and then we moved together, so this is my second year doing, um, my PhD. I transferred into a PhD program instead of continuing with my Master’s.
[00:03:25] Nataliya Shcherbatyuk: Congratulations. That is exciting.
[00:03:28] Cynthia Bosibori Sigira: Thank you.
[00:03:30] Nataliya Shcherbatyuk: Nice. And I know that your work highlights the rapid expansion of plastic mulch, and you’re working a lot now on the plastic mulch, and I’m talking specifically about plastic mulches used in agriculture. So, from your perspective, what are the biggest benefits that are driving this growth?
[00:03:52] Cynthia Bosibori Sigira: Really good question. So, I think most of us know about the goals, and one of the goals is food security. And I think, or I believe, that the use of plastic mulch films achieves food security by offering several benefits and meeting global food demands. So, one of the main benefit driving this growth is, , the increase in crop yields, and some of the studies have shown that the use of plastic mulch films leads to up to 60% increased crop yields, which I think is a really big number. And how the plastic mulch films lead to higher crop yields, including the plastic mulch films lead to improved soil water retention. They have excellent weed management. They maintain soil temperature for crop growth, and the list goes on and on and on from literature. And so, because of these reasons, there’s an increase in the crop yields, and I believe that’s why people are increasingly using plastic mulch films from way back, I think in the 1950s or the ’60s.
[00:05:01] Nataliya Shcherbatyuk: I see. Yeah. And, you know, during one of your talks that I was listening to, you mentioned that many farmers are still relying on landfilling, burning, and burying used mulch. So, can you tell me if you know what some of the major environmental concerns that we need to think about are?
[00:05:25] Cynthia Bosibori Sigira: Great question, so I am just going to mention the environmental and human health risks according to the different types of disposal methods. Some of the effects overlap and all that, but I’ll just try my best to talk about each. So, let’s say for instance, landfilling, um, one of the major hurdles is it leads to the environmental persistence and accumulation of the plastics through leaching. And when we talk about burning, um, burning leads to the formation or, like, the release of compounds such as dioxins and furans, which are quite toxic, or let me say, highly toxic to human health, and sometimes can be carcinogenic. And then burning also leads to the production of gases such as carbon dioxide, nitrogen oxides, which are known to cause global warming, which I think, again, is not really good. And then when we talk about burying or in field burying of the films, again, similar to landfilling, it leads to the accumulation of plastic. But in this case, if you’re burying the films in the soil, um, in the agricultural soil, it leads to microplastic and nano plastic accumulation in the soil, and this affects, um, growth-promoting bacteria, it damages the soil structure, and then it leads to the, um, root growth retardation and development, which, um, I mean, we know we use plastic mulch films to increase crop yields, but if we are burying the films in the fields, then that leads to a decreased crop yield. And another method that is ma…is also used is stockpiling, um, especially if farmers do not know where to put their plastic mulch films after use. And again, this leads to accumulation, and the small plastic fragments can be carried by water and soil erosion, causing plastic pollution. So, all in all, all of them have different environmental and human health risks, um, depending on the disposal method.
[00:07:28] Nataliya Shcherbatyuk: Interesting. Well, and now I would like to move more into the details of your project that you’re working on. Okay. And I know that your research is focusing on pyrolysis and this is as a sustainable end of life of plastic mulch option. But for listeners who are not really familiar with it, how would you explain pyrolysis and why it shows promises for agriculture plastic waste?
[00:07:58] Cynthia Bosibori Sigira: Great question. So, this takes me back to last semester when I was doing it when I was taking this class on, um, research, and I remember one of the prompts in the class was explaining as if you’re explaining it to a five-year-old. So, the analogy I came up with was imagining cooking something using a slow cooker. So I relate the pyrolysis reactor to the slow cooker, and your materials are like your waste plastics as your food, uh, like the raw food, and then you put your raw food into your slow cooker, and then you close the slow cooker, and then you wait for maybe 30 minutes, an hour, depending on how long you’re going to cook your food. And then after the two, after maybe the two hours, you get your product, which is food, but in this case, in pyrolysis, we are obviously not eating the products. In pyrolysis, after the couple of hours or, like, the couple of minutes, you end up getting products such as fuel for—at least plastic pyrolysis—fuel, char, and some non-condensable gases, which, um, all of these products can be used for various purposes, and I think I’m just going to talk about that in the next, uh, in the other part of the question, but yeah, um, and I think that is just how I can explain what pyrolysis is.
[00:09:24] Nataliya Shcherbatyuk: Yeah. Well, we can have, like, a visual picture or imaginary idea of pyrolysis now much better, of course. And, you know, you talk about pyrolysis as being promising, um, you know, use for plastic waste or decrease in plastic waste. But even though it’s that promising, there seems still seems to be a gap, especially in research. There is very little research that is focusing on this work, specifically when we’re talking about agriculture, mulch film work. So, can you expand a little bit on why do you think this area has been overlooked and what makes these films so challenging to recycle?
[00:10:13] Cynthia Bosibori Sigira: That is a great question. So, as you mentioned in the question, um, the plastic pyrolysis shows good promise, and it shows good promise because it can handle high contaminant levels compared to other recycling methods. It can handle mixed streams of plastics, and I also mentioned about the products which are quite useful for different purposes. I wouldn’t really say the area has been overlooked, um, because there has, there have been a lot of publications recently on recycling plastic mulches. And, um, I think just like any other thing that is manufactured or, like, developed, there are benefits at the start. You can see the benefits and then maybe slowly over time you start noticing, oh, maybe this thing has an issue. Let’s say for instance there were chemicals back in the day which were developed and they had really good benefits, and then along, along the life cycle, people realized they have some detriments, and I think that’s what is, that’s what is happening here as well for the agricultural mulches. And the research that has been ongoing shows that, hey, plastic mulches are good. They are increasing our crop yields, but they also have some negative effects, specifically for the conventional polyethylene mulches or other plastic mulches. And right now, researchers are working towards, um, finding sustainable end-of-life solutions and/or alternatives to these mulches.
[00:11:41] Nataliya Shcherbatyuk: Yes. Interesting. So, there is a lot of work being done in this area, and we should be, should be seeing more evidence and results coming up.
[00:11:52] Cynthia Bosibori Sigira: Yes.
[00:11:53] Nataliya Shcherbatyuk: That’s great. And I think one of the most interesting aspects of your work is that you’re actually working in real-world conditions, and you’re looking at. And correct me if I’m wrong, but I’m, I’m, you know, from, based on what I’ve learned from your talks and, uh, you know, what I could find about your projects is that you work in including in the UV exposure, heat, soil contamination, and additives. And can you tell how these contribute and affect pyrolysis outcomes?
[00:12:30] Cynthia Bosibori Sigira: Great question, and yes, I do work with real world, quote-unquote, “real world” mulched films, which are impacted by all of these factors. And these factors do play a role in the recycling part in terms of, like, the challenges to recycling because, um, for instance, these mulched films, they are contaminated, and in one study it showed that the contamination can be bet…in between 30 to 80% by weight. And this contamination is from soil, plant debris, agrochemical contamination, and all that. And, um, for those who are familiar or unfamiliar with the plastic mulched films, they are manufactured with additives to improve their properties in terms of, um, field how they, how they perform in the field, field performance. And, um, all of these factors together with UV exposure, they do affect pyrolysis outcomes.
So, I recently finished working on a study on the impacts of these conditions, and my main conclusion from the study was that these factors do actually impact, um, the chemical properties of the plastic mulch films in the pyrolysis setting, and it also impacts the product composition. So, in my study, I was mainly focusing on oil production and also the presence of soil contamination and additives, um, and how they do impact the, the oil production and the product composition, as I mentioned before. And my results showed that the oil yield will be lower compared to, um—the oil yield while pyrolyzing the end-of-life mulch films will be lower compared to pyrolyzing, for instance, pure polyethylene. And this is due to the presence of, um, contamination, um, UV and all that.
So, for instance, if we have a lot of additives and contamination, this leads to higher ash content, and then higher ash content, that means you’re getting a lower oil yield. So, that was one of the things I noticed or like I got from my, from my results. And then the presence of additives and soil contamination, fertilizer residues, and UV degradation, led to the presence of compounds other than purely hydro…uh, hydrocarbons, because when we’re working with, I’m working with specifically polyethylene mulch films, you anticipate getting just pure hydrocarbons from the chemistry of polyethylene. Right.
But because of the additives which have different chemical, structures, and due to degradation, degradation can be from photooxidation, which leads to, um, these hydrocarbons in the plastics being converted into other chemical compounds. And so, for my pyrolysis compounds, I did see some of these compounds from photooxidation and soil and all that. So what this means for the product quality is that the resulting oil will definitely, the resulting oil, at least for the oil, because that was the main focus of my research, will have all these other hydrocarbon all of these other compounds, which if, for instance, you want to use the fuel for transportation fuel, you really don’t want impurities in that fuel. So, what this mean for someone who’s in the industry. They’ll have to do some downstream processing to upgrade the oil and then maybe use the oil for transportation or use the oil as feedstock, um, monomer for new plastic production. So that is in a nutshell. I think I talked a lot about that, but that’s how these factors influence the product quality and the yield.
[00:16:22] Nataliya Shcherbatyuk: That’s quite interesting. Yeah. You’ve, you’ve got a quite interesting project, I have to say that. Yeah, thank you. That’s a very big picture project as well. There are so many details about it that need to be considered. Yeah. And I also know that you study how catalysis interacts with degraded or contaminated plastics. Is that correct?
[00:16:45] Cynthia Bosibori Sigira: Uh, yes, I do.
[00:16:46] Nataliya Shcherbatyuk: Oh, fantastic. Then can you tell what potential catalysis, um, have in well, when we’re speaking about, you know how it can be improved in the process and what challenges remain that need to be scaled in?
[00:17:07] Cynthia Bosibori Sigira: Um, well, thank you for the question, the catalysis part is something I’m, I’m really interested in working on I just started doing a little bit of literature review to see how that works, and, um, I’m planning to dive extensively into catalytic pyrolysis here in a couple of months, and I’m excited to see the results. So, there are different catalysts used in plastic pyrolysis. They range from— at least for listeners who know the catalyst—they range from conventional catalyst such as zeolites, um, silica alumina, and then there has been an improved interest in trying to get catalysts from biomass, or at least agricultural biomass, because they are more sustainable and renewable and cheaper, and that is the area I want to get into to see the potential of agricultural-derived catalysts.
And you also asked about, how the catalysts improve the process, some of the main reasons why people use catalysts in pyrolysis is, one, they lower the temperature requirements of the process, pyrolysis is a very energy intensive process requiring temperatures up to 900° Celsius, which are really high and so if a person in the business wants to cut some costs, using catalysts will be really good because it lowers the temperature requirements.
And then another thing, uh, another good thing with catalysts is that, um, they improve the selectivity towards certain hydrocarbon ranges and types. So. for instance, some of the studies have looked into using catalysts to increase the production of naphtha range hydrocarbons. These hydrocarbons in TSO naphtha range are like C-5s to C-12s—um, that is just the carbon number, the C-5 to C-12s. They can be used in production of new plastics, so that’s why some people are interested in getting that range of hydrocarbons. And then other studies have focused on maybe producing more aromatic hydrocarbons for other purposes, and all that. So different studies have different, um, aims for the use of catalysts.
And then another thing or another benefit of using catalysts in plastic pyrolysis would be reducing wax formation. I recently just did a pilot study of doing, of a…actually doing the pyrolysis with a big reactor, and one of the challenges I faced was a lot of wax formation, and I was not using a catalyst. So, with the catalyst, it breaks down the high molecular weight hydrocarbons, which lead to the wax formation by breaking down these high molecular weight hydrocarbons, it leads to the formation of lower molecular weight hydrocarbons, which condense into oil instead of wax. So those are some of the benefits that, um, one can accrue with the use of catalysts.
And I know you also asked about the challenges that remain in scaling, in se…uh, in scaling this up. So, what I gathered is scaling up catalytic pyrolysis, right? So some of the challenges in scaling up the use of catalytic pyrolysis, especially for since we’re talking about the agricultural mulch films, the ones that contain contamination and everything, it would be catalyst deactivation as a result of the presence of contaminants. So these contaminants, um, the catalyst-contaminant interaction I’m still trying to get more into it and study and see how that happens. Like just the chemistry behind catalyst interacting with the contamination. The contaminants lead to the catalyst being deactivated, so, you know, catalyst stops, the catalyst stops working, and you end up facing the same problems, well, that are similar to when you were not using the catalyst.
And then another challenge to scaling up this process would be char deposition. So, when you burn your plastics, um, in pyrolysis, and I don’t know if I mentioned this before, but in pyrolysis, we’re burning the plastics in the absence of oxygen. So, we use an environment using nitrogen to prevent formation of harmful products, um, or harmful gases. And in this process, there’s char formation, and the char is deposited on the catalyst surface, and then it leads to the catalyst activity being reduced. So again, you go back to the challenges you were facing before using the catalyst.
And then another challenge would be feedstock variability. So there are so many different types of mulch films, at least plastic mulch films being used in the agricultural system. It’s just not polyethylene. Some people use PVC or other plastics. And so my study might be on, on low-density polyethylene, and I can find one catalyst that works really good for my feedstock and also the contamination, but someone else might be working on a different plastic or someone in the industry who’s doing plastic pyrolysis for agricultural plastics might be getting feedstock from different sources. Right. Yeah. So, you might, yeah, you might suggest, “Hey, you can use this catalyst,” but it might not work well for the feedstock. So that is one of the challenges.
And then economic uncertainty, this is more on the business side. It’s, is it really worth investing in catalytic pyrolysis? What will my return on investment? So again, becomes challenging in scaling up. And environmental concerns, policy and regulatory frameworks, which are some things I’m also really interested in learning about, I believe could be challenges in scaling up the catalytic pyrolysis.
[00:23:04] Nataliya Shcherbatyuk: That’s, that’s very, again, you’re working on a very interesting project.
[00:23:09] Cynthia Bosibori Sigira: Thank you.
[00:23:11] Nataliya Shcherbatyuk: And so your work aims to support the circular economy for agriculture plastics. Is that correct?
[00:23:19] Cynthia Bosibori Sigira: Yes, it is.
[00:23:20] Nataliya Shcherbatyuk: Okay. So how do you see or what a successful circular system would look like for mulch films? And what challenges, or not necessarily challenges, but more like what kind of changes, and I’m talking about, you know, technical or policy or even industry. So, what kind of changes do you think are needed to get there?
[00:23:46] Cynthia Bosibori Sigira: Oh, great question. For me, this is probably gonna be a long answer because Ii’s good when I think about my research, I think about the intersection of my, you know, my, my research is at the intersection of all of these three spheres, from the technical side, policy side, industry side. Or that’s how I try to think about my career and where I want to be. I want to be able to intersect all these three. And when we look, um, at the agricultural plastics, um, a successful ci…circular system, from my own opinion, um, means that we’ll be moving away from the fragmented end-of-life pathways.
So, for instance, we’re just landfilling, stockpiling, open burning, and we’re moving away from that, and we’re moving towards a system where the material value or the feedstock value is retained for as long as possible. So, for me, a truly circular system will need the collaboration of different stakeholders in the industry space, technical space, and policy space. So, I’m just gonna give examples on how, like maybe this collaboration might look like.
So, for instance, at the farm level, the farmers can work to ensure efficient collection after use and proper preprocessing or cleaning if necessary. And then once the farmers do that, it, the, the, the collected mulch films, they move from the farmers to multiple recycling pathways. So maybe the films move to different, um, recycling pathways depending on the contamination level. I know in this podcast, we’re mainly talking about, um, pyrolysis, but, um, depending on the contamination level, there can be different other recycling methods.
So cleaner films which have been highly preprocessed, and the contamination have, has been removed, can go into mechanical recycling, because mechanical recycling requires at least <5% of contamination. Mm-hmm. And then they could be, they could be converted into new plastic products. And then the heavily contaminated or degraded films, they can go into chemical recycling or the pyrolysis that we’re talking about, and it can be converted into fuels, chemical feedstocks, non-condensable gases, etc. And then, um, so that is how I see moving from the farmers to the recycling and then, you know, the policies pretty much, and close everything together.
If there’s a policy that is supporting recycling, then all of this is really possible. If, if, if, if, um, industry people are being given incentives to recycle, then it’s easy for the farmers to do, we’re going to, um, preprocess or, like, remove our mulch, take it to the recycling facility. The recycling facility is gonna do what it’s going to do, and then, um, we get the final products to the other people in the industry who are working on producing fuel or new plastics. And, um, I just would like to mention that as much as we would all love one universal solution to the agricultural plastic mulch, it’s really not possible at times because we have factors such as the location of where you are, and the resources available, and what can be done right now, or at least what I’m seeing is being done even for this project that we’re in. Um, people are matching the waste stream to the most appropriate recovery pathway. And by doing so, by doing so, it’s, you know, you’re looking at a holistic approach where all of this—
[00:27:20] Nataliya Shcherbatyuk: Right.
[00:27:21] Cynthia Bosibori Sigira: —spheres are intersecting and helping each other out, some of them also asked about some of the changes that are needed, and I, for me, I would answer it from, from that perspective of what I’m seeing that, um, happening right now because, two years ago, at this moment in time, I really did not know anything about pyrolysis or plastic pyrolysis. So, I’m still a baby in this, in this space, but what I’ve learned in the past two years during my graduate studies is that some of the changes needed, um, first of all, from the technical side, would be manufacturing the plastics, um, in a way that they are easier to recycle. So, for instance, not having heavily loaded additives in…into the plastic mulch films because that impacts the recycling. And then another thing that can be done or is being done is the improvement in preprocessing techniques to remove contamination. For instance, right now we’re working with mulch films that were, um, dry washed to remove the soil contamination.
And then another technical thing that can be done is developing highly selective catalysts, which I’m seeing it’s being done. I really don’t know much about that from the industrial perspective because I haven’t really worked in the industrial space yet. And then LCA or life cycle assessment studies on emissions because that is one thing that people are really concerned about pyrolysis or maybe mechanical recycling sounds good, but what are the emissions? Because it’s not zero percent emissions. There is a percentage if you compare it to any other recycling, um, method.
And then from the policy side, as I already mentioned, some of the things or like some of the policies that can be put in place include incentives on plastic recycling, because with incentives, um, you’re attracting investors in that area, and people will be more willing, or like business people will be more willing to recycle plastics or invest their money in that. And then one thing that I’m really, really interested about and interested in is using science-based evidence to make decisions on regulations. Because if you get some, um, results from scientists or from the research area and then use that to make decisions, I think that is really helpful. And then I think the policy space is also really doing good, good work in putting out clear standards and I think they need to continue doing that. I’m pre…There’s a lot of talks in the policy space regarding pyrolysis, so hopefully it will be clear in the next couple of months or years on how to move forward with pyrolysis.
And I think the technical and the industry side kind of go hand-in-hand. Like for instance, for the industry side, it can just go beyond sales of the agricultural mulch films. How can we make more rec…uh, the mulch films more recyclable? And then I think the industry is also doing good work, um, in working together with researchers to see how to solve the issue of, uh, agricultural mulch films challenges or, like, the end-of-life management of this agricultural plastic mulch films. I know that was a really long answer, but I hope it made sense.
[00:30:43] Nataliya Shcherbatyuk: That was very insightful, and I think, again, your project is very interesting. It has, like, endless opportunities, and I think this is fascinating and this is great. And I wanted to tell you a huge thank you for spending this time with us, and letting us learn more about pyrolysis and what you’re working on. And I am definitely looking forward to having you back on our podcast soon with more results, more details and data about your project.
[00:31:22] Cynthia Bosibori Sigira: Thank you so much. It was great being, um, being your interviewee in this podcast I’ve really enjoyed, my time here, and I hope some of the information I shared will be helpful to our listeners.
[00:31:37] Nataliya Shcherbatyuk: Thank you so much, Cynthia. Thank you.
[00:31:39] Cynthia Bosibori Sigira: You’re welcome.
[00:31:40] Nataliya Shcherbatyuk: That’s it for today and until the next episode. You can find more information by following us on Instagram and LinkedIn by @mulch_matters and going to our websites (www.smallfruits.wsu.edu) and choose ‘Mulch Technologies’. This work is supported by Specialty Crops Research Initiative Award 2022-51181-38325 from the USDA National Institute of Food and Agriculture. Any opinions, findings, conclusions, or recommendations expressed on this podcast are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.
Intro and outro music credit to Zakhar Valaha from Pixabay