Micro- and nanoplastics are now everywhere: in stormwater, sediments, biosolids, and the air we breathe. But to understand their real risks, scientists face a fundamental problem - how do you accurately measure particles that are often too small to see?

In this episode, the podcast host, Dr. John Ahn, talks with Dr. Balaji Rao, assistant professor of civil, environmental, and construction engineering at Texas Tech University, whose work focuses on contaminant fate and transport. From mercury in sensitive Tennessee watersheds to stormwater from urban industrial sites, Dr. Rao’s research traces where pollutants originate, how they move, and where they ultimately end up - and now, how microplastics and nanoplastics are showing up across these pathways.

The discussion also centers on why the detection and quantification of micro- and nanoplastics in real environmental matrices represent a significant scientific challenge, what current analytical methods can determine and what their limitations are. It is also discussed what happens when plastic particles enter stormwater systems, settle into sediments, and potentially move through biological food webs.

Topics discussed:

• How Dr. Rao’s journey from petrochemical engineering to environmental science led to microplastics research.

• Why complex environmental samples of stormwater, sediments, and biosolids complicates the detection and analysis of microplastics and nanoplastics.

• What “contaminant fate and transport” means for plastics released into the environment.

• Emerging analytical techniques to identify and characterize microplastics in mixed samples.

• Why measurement uncertainty matters for regulators, public health, and global risk assessments.

• How science, engineering, and society can better track, mitigate, and prevent plastic pollution.

This conversation is part of an ongoing series exploring the science, risks, and future implications of micro- and nanoplastics in both the environment and human health.

Learn more about Dr. Balaji Rao and his research: https://www.depts.ttu.edu/ceweb/faculty/Balaji_Rao/index.php

"Nanoplastics. A Systematic Risk Analysis for Human Health, Ecosystems, and the Environment"

https://allatra.org/global-research-center/publications/agrc.report.mnp.2026.04001

The podcast is hosted on the ALLATRA Global Research Center platform.  

This material is for informational and discussion purposes only. The statements, assessments, interpretations, conclusions, and hypotheses contained herein represent personal views of their authors as of the date of publication and do not constitute an official stance of the ALLATRA international organization or any other public association, organization, project, partner, or affiliated entity or person, unless expressly stated otherwise. 

Publication of this material provides an opportunity for open discussion and does not in itself imply institutional endorsement, confirmation, or scientific verification of the views expressed. The material does not constitute an official statement, expert opinion, or professional advice.

Full Podcast Transcript

John Ahn, Ph.D., MBA, Chief Scientist and Chair, ALLATRA GRC

Hello, welcome to today's podcast from the ALLATRA Global Research Center. My name is John Ahn, and I'm your host. Micro- and nanoplastics are being detected in more and more places: in the air we breathe, the water we drink, and the food we eat.

And yet, scientists still face one of the most fundamental challenges in this field. That is, how do we accurately detect and measure particles that are often too small to see?

Today, we're joined by Dr. Balaji Rao, assistant professor of civil, environmental, and construction engineering at Texas Tech University. Dr. Rao specializes in contaminant fate and transport – essentially understanding where pollutants come from, how they move through our environment, and where they ultimately end up. His research focuses on identifying and characterizing micro- and nanoplastics in complex environmental samples, including stormwater, sediments, and biosolids. And he's helping us address one of the most critical questions that researchers face: how can we ensure that the measurements we make are trustworthy?

Today, we'll explore the challenges of measuring micro- and nanoplastics, the pathways these particles take through our environment, and new technologies that may help us understand better their potential risks.

Dr. Rao, welcome, and thank you so much for being here with us today.

Balaji Rao, Ph.D., Assistant Professor of Environmental Engineering | Texas Tech University

Thank you, John. It's a pleasure to be here.

John Ahn, Ph.D., MBA, Chief Scientist and Chair, ALLATRA GRC

Before we dive into micro- and nanoplastics, I want to start with your own journey. I want to know: how did you find your way into environmental engineering, and what sparked your interest in understanding what happens to pollutants once they enter our environment?

Balaji Rao, Ph.D., Assistant Professor of Environmental Engineering | Texas Tech University

Yeah, it's a great question, and I would say that it was not an intentional pathway for me to explore environmental engineering and, in fact, do a PhD in environmental engineering. I grew up in the southern part of India in a big city called Chennai, and I did my bachelor's in chemical engineering and worked in an oil refinery—petrochemical refinery, in fact, which is now one of the largest, or probably one of the largest, petrochemical refineries in the world. And so, I never had an intention to do environmental engineering. My first exposure to plastics was in the petrochemical engineering company where there was a polypropylene unit that was making polypropylene pellets. But I'd never envisioned, you know, I would end up in environmental engineering.

I did work in the refinery for about three years and then I decided to do my higher education. I came to West Texas, to Texas Tech University. I did my master's in civil engineering, followed by a PhD in civil but focus on environmental engineering. At that time, my research area was focused on characterizing a thyroid hormone disruptor called perchlorate, which was used in rocket fuels as ammonium perchlorate. And the research was about whether the perchlorate that we find in the environment is primarily due to these manufactured uses or is there an alternative source. And it turns out that there was a natural source of perchlorate that is likely formed in our atmosphere and deposited in the terrestrial environment. It was a very interesting direction that the research took, which was not what we expected.

And following my PhD, I did a postdoc in Oak Ridge National Lab. There is a bit of history in Oak Ridge National Lab: it is the place where the uranium was extracted for the first atomic bombs. One of the processes that they were using was mercury to separate these isotopes, and mercury is a neurotoxin. And if it's not disposed of properly, it will end up in the environment. And that's what we saw in that location. And I switched gears from perchlorate into looking at a more classic contaminant, mercury, and how it moves through the environment.

And I don't know if you've been to that part of Tennessee, but it's a very ecologically sensitive place with a lot of streams and lakes.

And so, mercury tends to kind of stick to the environment, move across these biological compartments, move up in the biological trophic levels, bioaccumulates in the environment—kind of a segue to what happens when microplastics are released into the environment, likely. What I was working on was how mercury moves through the environment, how can we better detect some of the transformation processes this mercury undergoes that involve development of novel analytical techniques to characterize this.

Eventually, I got an opportunity to move back to Texas Tech, focused not only on addressing the problem of contaminants transport and fate in the environment, but also how do we mitigate it? And that's what got me really excited to come back to Texas Tech. This is not a typical path that a researcher takes – you don't generally come back to your alma mater – but that was an opportunity that I couldn't miss.

And I was focused on how do we mitigate contamination, particularly in water systems and in sediments. And one of the projects that we were working on – and I can talk about that in more detail – was, you know, stormwater from urban industrial environments, how they can carry pollutants in the particulate matter in the stormwater and how they could end up in the sediments. And that was my initial introduction to characterizing microplastics.

The original focus was on characterizing contaminants such as mercury, polycyclic aromatic hydrocarbons, which are typically found in these stormwater matrices.

But at that point, there was some interest in characterizing what we call as emerging contaminants, that included microplastics and PFAS, which are also called forever chemicals, in stormwater samples, and we started exploring and adapting analytical techniques to characterize these emerging contaminants. And that was my introduction to microplastics.

John Ahn, Ph.D., MBA, Chief Scientist and Chair, ALLATRA GRC

Microplastics they're everywhere, and they were not very popular for some time, maybe until about five years ago. I mean, it used to be, yeah, people wrote about it, there were articles, but they were very few. Now, like every day there's this new article about micro- and nanoplastics.

You touched on this briefly, but what really attracted your attention to these particles? I mean, what convinced you that developing – I mean, like, for example, developing a way to detect these particles, especially in like stormwater and whatnot in these complex matrices – it's very complicated. And so, what drew you into this topic of micro- and nanoplastics, and what convinced you that, you know, we need a reliable way to measure them, and this is one of the most important challenges that we need to tackle? What was that enlightening moment?

Balaji Rao, Ph.D., Assistant Professor of Environmental Engineering | Texas Tech University

Going back to that project on stormwater, one of the hypotheses that we wanted to test is – until we started working on this project, many of the techniques for characterizing stormwater focused on either bulk characterization of the water, without paying much attention to the particles that could be present in the water. And why does it matter?

And when I'm talking about particles, I'm not necessarily referring to microplastics, even though they may be present in the stormwater – and they are. I'm talking about these sediment-laden particles that are kind of eroded from the environment as the water moves through these channels and, you know, through these streams. And these particles have an affinity to basically grab contaminants as they move along with the water channel.

Our hypothesis was it's not enough to do a bulk characterization of the water, because what you're missing out on is this information in terms of how these particle-laden pollutants would behave. Particularly, how does the size distribution of these particles affect their fate and transport? And, perhaps from an engineering perspective, what does the size distribution of these contaminants in these particles mean in terms of how they could be effectively removed from current stormwater treatment systems?

What some of these treatment systems are could be as simple as, like, a retention pond that just settles… lets the particles settle down, or they could be more advanced where you could have, you know, cartridge-based filters to specifically remove contaminants that are present in the stormwater. The hypothesis was the particle size distribution has an influence not only on how contaminants behave in the stormwater, but how effectively they are removed as well.

Now, that has a strong correlation with microplastic research as well. Microplastics do not come in one specific size. They come in a distribution of sizes, potentially a continuous distribution. And that could affect how they move in the environment as well as how they are removed by current technologies that may not be specifically designed for microplastic removal, but they may still be removing some of these microplastic material.

Long story short, what we did find is the coarser particles that are typically greater than 20 microns, irrespective of whether it's microplastics or background sediment material, they tend to be effectively removed by these stormwater control measures, which is the good news. The not-so-good news is those smaller microplastics and potentially nanoplastics and these nanoparticles are likely not removed by these treatment systems. We need to start thinking about developing better engineered systems to remove these particle sizes.

John Ahn, Ph.D., MBA, Chief Scientist and Chair, ALLATRA GRC

Great. We're talking about these particle sizes, and maybe it's sometimes confusing to a lot of people. People talk about microplastics, nanoplastics…
Before we go deeper into micro- and nanoplastics, I mean, let's start with the basics. There’re these terms, micro- and nanoplastics, they're discussed together. And again, they're not exactly the same thing, right?

For our listeners who may not understand this difference, you know, why… First, what is the difference between the two, between microplastics? And second, why is detecting the smaller nanoplastics more difficult than its counterpart, the microplastics?

Balaji Rao, Ph.D., Assistant Professor of Environmental Engineering | Texas Tech University

The general definition of microplastics has basically an upper limit and a lower limit. The upper limit is well defined. It's defined as any plastic particle that is at or less than 5 millimeters in size. If any of its dimensions is 5 millimeters or less, then it just qualifies as microplastics.

Now, where nanoplastics begin, technically, is any particle that is less than 1 micron, or 1,000 nanometers, would fall under nanoplastics. That size is important because when you compare this with the wavelength of light – you know, green light is around 450 to 500 nanometers – that determines what we can see.

And microplastics, the larger microplastics, this is something around millimeter size to these larger micron sizes – for instance, our human hair is about 100 microns – we can certainly see those particles. But once we start looking into these smaller microplastics – anything less than, say, 10 or 20 microns, all the way to nanoplastics – it becomes increasingly difficult to see using visual microscopic techniques.

And also, to confirm whether these are microplastics, you need not only to visually identify these as microplastics; you also need to have a way in which you can actually say, "Hey, this does look like a microplastic and not the background sediment material," just like the example that I used in stormwater, where much of the particles that you would find in stormwater or in other environments are likely not going to be micro- or nanoplastics. It's literally like looking for a needle in a haystack.

These particle sizes are very difficult not only to see, but also to confirm that these are microplastics. And that requires a technique that typically combines some sort of a microscope with a spectroscopy-based technique, which can characterize down to the smaller microplastic sizes.

But then all bets are off when you want to probe down to the nanoplastic sizes. Some people… some agencies tend to indicate that the lower size for micro- and nanoplastics is all the way down to 1 nanometer, which currently we do not have techniques that can characterize environmental nanoplastics down to those really small nanometer sizes.

John Ahn, Ph.D., MBA, Chief Scientist and Chair, ALLATRA GRC

The general consensus is that the smaller the particle gets, the more harmful effects it has, just because it has, for example, more surface area compared to volume. And it's not just identifying these particles, but the measurement is also complicated by the fact that you're identifying these in very complex matrices, like, you know, biosolids or in human bodies.

Could you also walk through, like, what the procedures, like… what the limitations are? And can you give us more details on how these are measured and what the challenges are?

Balaji Rao, Ph.D., Assistant Professor of Environmental Engineering | Texas Tech University

That's a great question. I would say that the challenges depend on what matrix you're looking at. Drinking water is a relatively clean matrix. Typically, you don't have to do a lot of processing of the sample before you characterize it. I would compartmentalize detection techniques into three different stages.

One is the sample processing stage. That includes sample collection and how we process the sample. How do we load these samples that potentially contain these microplastics onto a platform? Typically, we would use a filter to load these microplastics or to separate them from the water and other constituents that are present in the environmental sample.

And then you have the analytical technique that actually detects and identifies these micro- and nanoplastics.

And then you have a post-processing techniques where you get these signals and these spectra, as you will, from these analytical techniques. And then you match it with an existing library of some sort. And then you say that, "Oh, this looks like polypropylene" or "this looks like polyethylene," and so on. All these different stages come with their own challenges.

When it comes to a simple matrix – I call it a simple matrix from an analytical perspective, like drinking water – in the initial sample collection and processing stage, the only thing you have to be mindful of is that the sample does not come into contact with any plastic material. You use non-plastic material to collect the samples, and you use apparatus such as vacuum filtration apparatus to load the samples onto a filter that does not have plastic materials. More often than not, that's the main concern with that.

But when we talk about more complicated samples on the other part of the spectrum – in environmental samples like biosolids or sediments – you also have to deal with a really large amount of other constituents that are present in the sample, like organic matter, sediment particles, inorganic particles that, just visually, may even look like potential microplastics, or microplastics could be embedded in them so you can't really see them. It's not only enough to properly collect these samples, but these samples may have to undergo some sort of what we call as digestion techniques to remove these background materials that are not plastic, and then potentially apply some sort of physical separation technique, like density separation, to concentrate these microplastics that are present in these samples.

And then you go to stage two, where you start applying the analytical technique, which is common for all these different matrices. And then you also have to deal with the post-processing, where you have to compare it with libraries and so on. During this entire process, what you come across is, as the sample complexity increases, you need to really adapt your techniques to minimize interferences, minimize contamination, while also making sure that the analytical techniques and the post-processing techniques can be validated across different labs.

That is part of the current issue with the analytical characterization of microplastics: if I take the sample that I collect in Lubbock, if I send it to Washington to another lab and ask them to characterize the microplastics, and I characterize the same sample in my lab, it's likely that we come up with completely different results depending on the techniques that we use, depending on the processing methods we use, the digestion techniques we use, the analytical techniques we use, and the post-processing applications that we use for these samples.

John Ahn, Ph.D., MBA, Chief Scientist and Chair, ALLATRA GRC

Is that why different groups get different results? I mean, you have sometimes like, “Oh, it's the same sample,” but are they getting different results because of, oh, they have a different digestion procedure or they're using different techniques? Is that why some of the groups are getting different results even though they're sort of looking at the same samples?

Balaji Rao, Ph.D., Assistant Professor of Environmental Engineering | Texas Tech University

Absolutely. And that is part of the problem which the analytical community is trying to address. And they are starting with something that's relatively easy to deal with, the low-hanging fruit and more important exposure route, which is the drinking water.

The drinking water is relatively less complex. And there's been a recent inter-lab study – it's published out there – where they show that size matters when it comes to confidence across these multiple labs in identifying… in coming up with the same results, in other words.

Particle sizes – microplastics particle sizes that are less than, say, 20 or 50 microns – all bets are off when it comes to characterizing these plastics and validating the results across labs. But those larger microplastics, there seems to be a lot more confidence across the labs that, as long as we follow these specific protocols, we should end up with the same microplastic distribution. And those results – like, you know, X amount of polypropylene and Y amount of polyethylene – they seem to match across the labs.

But when the particle sizes are lower, in the smaller microplastics – I mean, certainly nanoplastics – then all bets are off. You may get completely different results among the labs. There is too much variability among the labs to say that, "Okay, this is what we would see in the sample."

John Ahn, Ph.D., MBA, Chief Scientist and Chair, ALLATRA GRC

For those small particles – I mean, I mentioned earlier that, you know, a general consensus seems to be that as the particle gets smaller and smaller, it gets more toxic, but it's even harder to detect.

What is needed? Do we just need a standardized method that really works, that's proven? Or do we just need better instruments? Do better instruments need to be developed? What is needed to detect those very small nanoplastics, which are very toxic to humans and other living organisms?

Balaji Rao, Ph.D., Assistant Professor of Environmental Engineering | Texas Tech University

We do need a lot more of these, kind of, interlab studies, because there were some interesting conclusions from these interlab studies and recommendations on how we can improve in characterizing these really smaller microplastics and nanoplastics. And what we need is more research efforts, starting with more funding, on how we can work together and develop more standardized and validatable methods, particularly for these smaller microplastics and nanoplastics.

And that starts with the scientific community kind of working together across different research groups – not working in silos that you would often see papers that, even in drinking water, everyone seems to use different methods. It is very hard to compare results from these different research papers. What we need to do is sit together and come up with methodologies that we try and standardize, and that way we can validate, you know, what these methods, you know, what they can do and what they cannot do.

Now, I do need to say that when it comes to nanoplastics, some of these methods, no matter how much we sit together and work on it, it’s not going to work. Like traditional micro-IR spectroscopy techniques have limitations in terms of what spatial resolution they can characterize. I would say anything less than 10 microns, no matter how much effort you put into it, you may not see validatable results across the labs if you use this technique.

There are other techniques, like Raman spectroscopy, that have been proven to characterize down to 1 micron. There is a little bit of an improvement when we apply these Raman spectroscopy techniques, but then nanoplastics is something that is really, really new at this point, and there is no agreement on what are the analytical techniques to use for nanoplastic characterization. Standardization is still a long way off when it comes to nanoplastics.

I would say the low-hanging fruit for standardization of methods, likely in the next five years or so, is likely going to be at the microplastic scale – potentially down to 1 micron – if we can all agree upon: "This is the method, this is the approach, and this is how we need to process the sample and analyze the sample, and characterize the data that is obtained from the analytical instrument." That would be a big achievement. But if we want to talk about nanoplastics, I think we are still a bit off from that.

John Ahn, Ph.D., MBA, Chief Scientist and Chair, ALLATRA GRC

Great. Yeah, thanks for sharing your insights on the measurements. Now, I want to move on from the measurements to how micro- and nanoplastics – where they come from, where they go, and where they end up in the human body.

Now, I want to talk about the pathways of micro- and nanoplastics. In a way that when most people talk about plastics, they think about, you know, bottles and bags just floating in the ocean, right? But many of the largest sources of micro- and nanoplastics, they come from everyday activities of people that you rarely talk about.

What are, like, what are some of the most important sources of micro- and nanoplastics that people might not realize are really contributing to this problem?

Balaji Rao, Ph.D., Assistant Professor of Environmental Engineering | Texas Tech University

You're right. The major sources are, you know, obviously there are plastic materials that we come across on a daily [basis]. Whenever we go to... you know, drive across the road, you see these plastic bags kind of flying across. And so, one wonders what happens to these plastic materials, because these are designed to be durable. They are a big contribution of microplastics into the environment. And these are some of the common sources – like our daily plastic materials, plastic containers, plastic bags, textile materials. These are what we call as the major potential sources of microplastics into the environment, or derived microplastics, into the environment.

Some of the less obvious sources – I would say one of them that people might not think about is paper cups. You would think, what do paper cups have to do with plastic sources? It actually turns out that for paper cups to hold liquid, they need to have some sort of plastic lining inside those paper cups. Otherwise, they will not hold liquid. And so, those plastic linings could be made of a polyethylene type [of material], and they could leach microplastics, particularly if you put hot liquids such as coffee into these paper cups, and they could be leaching plastic materials into those liquids, and you could be drinking directly those microplastics as you have this coffee in these paper cups. These are less obvious sources.

Other less obvious sources are paints. We often don't think about microplastics when we talk about paints, but paints have a significant amount of microplastics in them. In fact, there's been a recent article that talks about how paints have been understudied as a microplastic source – even though they have percentage levels of microplastics, they is, kind of, manufactured to have microplastics, particularly water-soluble paints are manufactured to do that. And so, it's not inconceivable that during the application and during the normal aging process of this paint, and wear and tear, there would be a release of microplastics into the environment, which has not been properly accounted for.

And I would say, when we talk about sources of microplastics, we start with something obvious – plastics that we can see with our naked eyes, or what obviously becomes what we tend to lean towards as major sources – but there are these hidden sources of microplastics, such as these paper cups and paints, which we need to evaluate more on how these sources contribute.

Eventually, when we talk about the impacts of microplastics, we are talking about how it affects our health. I think what I was talking about is the less obvious sources of microplastics could be a pretty significant source – exposure route, rather – when we talk about consumption. A plastic bag in the environment may [be] potentially contributing a lot of microplastics in the vicinity of where the plastic bag is, but we may not be in direct contact with those microplastics. But whereas a paper cup, which may not have a lot of microplastics in it, but you are directly coming into contact with that. In other words, it's not just about what is the total source of these microplastics in the environment. How we come into contact with these different microplastic sources is equally, if not more important, when we talk about the impacts of these microplastics.

John Ahn, Ph.D., MBA, Chief Scientist and Chair, ALLATRA GRC

You're alluding to the fact that there's a source of micro- and nanoplastics, and of course they don't just stay in one place, they move around, they go everywhere.

For our listeners, could you walk us through how these micro- and nanoplastics move around the air, soils, water, and wastewater systems, including, like, in our stormwater and biosolids along the way? And how do they eventually end up in humans? And for humans, what are the very well-established methods? I know like, you know, we're breathing, we're eating all these microplastics. And which ones are not really well-established, or which ones are not well known?

Just walk us through that journey. Like, plastic is released – how do they travel through the environment, and how does it end up with us? Just paint that picture for us.

Balaji Rao, Ph.D., Assistant Professor of Environmental Engineering | Texas Tech University

Yeah, great questions. And let me try to break it down into a multiple-part process on how this would evolve. The fate and transport of contaminants in the environment – when we talk about this, not just from plastics, but contaminants in general – both physical and chemical characteristics of these, in this case, microplastic particles, would matter.

What do we mean by the physical attributes? The key physical attributes that we care about – we talked a lot about this – is the size. It is logical to infer that the smaller the size, the more widely they will distribute into the environment. Now, that could also mean that they would get diluted into the environment, but it just gives you the extent in which they would impact the environment around you. The larger the sizes, they may not move through the environment as much as the smaller micro- and nanoplastics.

Then you talk about the density of those microplastic particles. The more dense they are – when we talk about sediment or aquatic environments – the chances are they would end up in the sediment. And then you may not be directly exposed to it, but those benthic organisms and the fish that feed in those ecological environments, they may start accumulating the micro- and nanoplastics. And if we eat those, you know, fish, then we would get indirectly exposed to the micro- and nanoplastics.

These less dense microplastics may end up directly in your drinking water sources, because they are basically floating in the water and they are easily moving across the water compartments, and eventually end up in drinking water sources. And some of these have been known to pass through the drinking water treatment systems, and they end up in your – you know, as you open the drinking water tap, they may end up in your drinking water as you consume the drinking water. That matters as well.

Then the chemical attributes matter too. In terms of chemical attributes, we talk about hydrophobicity of these plastics. And what we mean by that is – hydrophobicity is another term of indicating whether… how much it likes to stay in water as opposed to staying away from water, in very layman's terms. The hydrophobicity determines whether these plastics are sticky. They tend to stick with particles as they move through the environment, or they are less sticky and they tend to stay in the water. And as I mentioned earlier, they cause different sorts of transport through the environment and lead up to different kinds of exposure in the environment.

Recently, there's a lot of focus on the aging of these microplastics. One of the things that these microplastics – as they age in the environment, they undergo some physicochemical changes. Particularly, they start incorporating more, potentially, oxygen groups into their surface material, and they could change the charge of these microplastics, and that could change the behavior in the environment as they transport through these different environmental compartments. All these attributes of micro- and nanoplastics matter.

And interestingly, the techniques that we discussed earlier are not capable of simultaneously evaluating all these different attributes. Like spectroscopy-based techniques can give you particle size distribution – it does have limitations when we talk about nanoplastics – but it can give you particle size distribution along with the polymer chemistry that is associated with the particle size distribution. But it does not give you information on the charge of those particles. To some extent, it would not give you information on how weathered these particles are.
And some of the concerns of micro- and nanoplastics, which I don't think we got a chance to discuss much about, is the additives and pigments and dyes that are incorporated into these plastic materials, which may cause a larger adverse impact on ecological and human health than the plastic material itself.

John Ahn, Ph.D., MBA, Chief Scientist and Chair, ALLATRA GRC

Yeah, it seems like it's a very complex problem that you have to measure. I mean, you're challenged by size, you have all kinds of chemicals attached to it, and you have all kinds of different charges, different – I'm guessing – heterogeneity within the particles. It's a very complex process, and it seems like we don't have all the tools that are necessary, or at least the standardization to measure all this.

Looking ahead, what new analytical tools or what new technologies are you most excited about, or is there anything in the pipeline that you know about? And how could they improve the ability to measure these particles accurately and consistently at scale? Is there something that you know, or something you're excited about, in terms of measuring all these micro- and nanoplastics in complex matrix systems?

Balaji Rao, Ph.D., Assistant Professor of Environmental Engineering | Texas Tech University

In my opinion, a single technique cannot measure all the attributes that we just talked about. And the attributes that are, again, key to microplastic characterization in the environment is definitely the size, the morphology or the shape, and, you know, the potential charge that the microplastic carries, and additives… the polymer chemistry of course, and additives that are associated with this microplastic. A single technique – to my knowledge is, even the most cutting-edge techniques that we've come across – is not capable of doing all of this together. It's likely going to be a combination of techniques to do what we would really like to characterize when we talk about microplastic impacts in the environment.

Having said that, much of the research – microplastic research groups have focused on characterizing the size distribution along with the polymer chemistry and the challenges that we addressed when we talked about characterizing the small microplastics and nanoplastics – is something that there are some novel emerging techniques that seem to address it that has got me very excited.

Two of them that I would like to mention is based on what we call as the pump-and-probe technology. Basically, it overcomes the size limitation by using, say, for instance, IR spectroscopy where you use IR light to hit the particles and to differentiate whether it's microplastic or whether it’s a background material. But you still have the spatial limitation. You overcome the spatial limitation by using a probe which could be a visible light, which has a much smaller wavelength. In general, the smaller the wavelength, the smaller the particle size you can see – that's why we use X-rays to look at really atomic scales, because X-rays have really small wavelengths, so you can look into these really nano and atomic scales.

The same principle is applied here where you adapt these IR- and Raman-based techniques, combine it with highly spatially resolved techniques, such as optical photothermal IR spectroscopy. And something that can really do nanoparticle size is the technique that couples atomic force microscopy with IR spectroscopy to characterize down to the nanosize scales.

Now, we can indirectly characterize these nanoparticles using thermal-based techniques, which… the principle behind that is you take a sample and you increase the temperature in the absence of oxygen – a technique called pyrolysis – and you break down those plastic materials, and then you look at the breakdown products to infer what type of plastic it's made of. This works at even the nanoscale because size does not matter for this technique; it's more about how much mass of plastics you have in your sample. That is the limitation, not the size.

But the not-so-good news is you do not get information on the size distribution. All you can say is you have X amount of polypropylene or polyvinyl chloride and so on; you do not get information on the size distribution. And so, that is a catch-22 situation that we are dealing with when it comes to characterizing these different attributes of both micro- and nanoplastics. And at this point, I don't see a single technique doing everything that we want from these characterization activities. It's likely going to be a combination of techniques.

John Ahn, Ph.D., MBA, Chief Scientist and Chair, ALLATRA GRC

And I think what we need is a standardized method. And it looks like we do have, or at least we have some idea how to measure even, like, smaller particles. And in order to have a standardized process with the right techniques – how do we get there?

I mean, how do we move towards having a standardized solution so that whenever we read from paper to paper from different research groups, we have the data that is reliable and reproducible? How do we get there?

Balaji Rao, Ph.D., Assistant Professor of Environmental Engineering | Texas Tech University

That is an excellent question. I have thought about this for quite some time now, and I don't think there is a perfect answer to this. Coming back to drinking water, which is where many of these validation studies have focused on, they found out ways in which they can make standard samples embedded in pellets. That you could basically… It's a mixture of plastic materials of known size distribution in this pellet that is dissolvable in water, and you can put that pellet into the water, and then you create this sample that contains this mixture of microplastics, and then you send it across labs to validate whether they are getting an accurate distribution of this plastic.

Now, that works nice for a drinking water sort of matrix. But when we talk about more complicated matrices, like biosolids and sediments, and when we talk about human biological samples or fish tissue samples, I think that we are not there in terms of what represents a standard material. I don't think there are easy answers to that.

One approach that we use in our lab in terms of validating our technique is to intentionally spike known amounts of plastic material into these biosolid samples and look at the recovery. Now, if I spike 100 particles of polyethylene, do I recover those 100 particles of polyethylene as I process these samples into these three different stages as I mentioned from the digestion to the analysis and the post-processing? If I am not able to recover all the 100 particles that I spiked, then along these characterization steps, something is causing me to lose those particles and I need to revisit where I’m losing those particles. That’s an indirect way in which we try to validate our methods.

Now, I know NIST is working into… which would be the National Institute of Standards and [Technology] – they are working into developing materials that contain a known amount of microplastics that mimics these environmental matrices. And using those standard materials, then the labs can validate whether their techniques are capable of characterizing these microplastics. That's still in the pipeline, to the best of my knowledge. But once these materials are available, I think this will really be helpful to the microplastic research community.

John Ahn, Ph.D., MBA, Chief Scientist and Chair, ALLATRA GRC

In terms of these samples for standard measurements, I mean, you're talking about pure, meaning these virgin plastics that are fragmented to a certain size distribution, right? A lot of plastic, I mean most of plastic we found, I mean it’s not virgin. I mean, it's covered with all kinds of lipids and proteins and, you know, whatever is in the environment. Would that complicate, you know, in terms of getting the right standards, or not?

Balaji Rao, Ph.D., Assistant Professor of Environmental Engineering | Texas Tech University

It absolutely does. I think the very first validation is to use the standard plastic materials. And if you don't get good recoveries with that, your method is already failing the very first test.

But moving forward, you're absolutely right. We need to find ways in which we can spike environmentally relevant plastic that has undergone these natural weathering processes into the sample matrices, and then check if our methods still can characterize these microplastics accurately.

And this goes back to my discussion about developing standard materials that already have micro- and nanoplastics in them with all the lipids and proteins that could interfere with it. I think we are still way off on that.

At this point, spiking these plastic materials – whether they're aged or virgin – into these biological matrices is probably the best approach. I can't think of ways in which… like, for sediments, we have standard materials that have been validated across labs, and so you have this confidence that this has X amount of mercury in it, and if you were to measure in your lab, you should get pretty much the same results if everything goes well. We are not there yet.

John Ahn, Ph.D., MBA, Chief Scientist and Chair, ALLATRA GRC

And as a final question – I mean, micro- and nanoplastics, they are complex to measure and they are everywhere. It has very complex matrices, and maybe it feels overwhelming for many people.

As a final message, what would you like our listeners to take away from today's conversation? What would that be if you had one message for our audience?

Balaji Rao, Ph.D., Assistant Professor of Environmental Engineering | Texas Tech University

I would say let's not panic. Plastics are everywhere; they are ubiquitous. Certainly, they seem to have some impact based on the studies that have been done. But to the extent to which these impacts are causing potentially these, you know, human health issues, there is still a lot of uncertainty in that.

And much of the uncertainty stems from the bulk of our discussion here in this podcast, which is analytical methodology to accurately characterize microplastics, not only in the environment but also within our bodies, it is still an evolving research.

I would take it with a grain of salt whenever you come across newspaper reports or research articles that talk about, you know, "there is a spoonful of plastics in an average human brain." The chances are there are some plastics in your body, but to what extent you are affected by those plastics, and what do they mean in terms of your health issues – there is still a big question mark on that. We don't have to panic, but we do need to be mindful of plastic pollution.

And I would do the things that you have control over – like, for instance, avoid plastic materials, don't use plastic containers, certainly don't heat plastic containers, avoid paper cups that are lined with plastic materials. These are the kind of things that you can do right now to minimize your exposure. And as the science and technology evolves in microplastic characterization and mitigation, hopefully we reach a point where we are able to minimize, if not eliminate, the adverse impact of plastic exposure in the environment and in human health.

John Ahn, Ph.D., MBA, Chief Scientist and Chair, ALLATRA GRC

Yeah, thanks so much, Dr. Rao, for your message and thanks for being here with us today. And you know, you really helped us understand one of the biggest challenges in micro- and nanoplastics, and that is how to accurately detect and characterize these tiny plastics as they move through our environment and ultimately reach our cells. Thank you very much, Dr. Rao.

Balaji Rao, Ph.D., Assistant Professor of Environmental Engineering | Texas Tech University

Thank you very much, and I appreciate the time taken to have this podcast.

John Ahn, Ph.D., MBA, Chief Scientist and Chair, ALLATRA GRC

Yeah. And thanks a lot again. And for our listeners, I thank you for joining us. And if you'd like to explore today's topic more in depth, you'll find the links to Dr. Rao's work, along with the ALLATRA Global Research Center's review, the report called "Micro- and Nanoplastics: A Systemic Risk Analysis for Human Health, Ecosystems, and the Environment." There will be [links] in the description below. Thanks for listening, and until next time.