By Sally Brown, University of Washington
Abstracts of these resources are available in the searchable Information Portal offered to Northwest Biosolids members.
An overview of microplastic and nanoplastic pollution in agroecosystems
Effects of microplastics in soil ecosystems: Above and below ground
Polystyrene nano- and microplastic accumulation at Arabidopsis and wheat root cap cells, but no evidence for uptake into roots
Transfer and transport of microplastics from biosolids to agricultural soils and the wider environment
Transport of nano- and microplastic through unsaturated porous
media from sewage sludge application
Those of you of a certain age likely remember the scene in the Graduate where Ben (Dustin Hoffman) is given that word of advice to guide him through the future: ’Plastic’. He didn’t listen, but apparently the rest of the world did. We’ve all read about the horrors of marine plastic. I’ve seen it firsthand in plenty of faraway places. Snorkeling in Indonesia and watching the plastic bags float past. Plastics of all shapes and sizes end up interfering and generally harming aquatic systems. Rafts of plastics also serve as habitat- wouldn’t be my first choice in TripAdvisor but -. These plastics also end up in terrestrial systems. And, like everything else, they end up in wastewater. The grit screen at the plant will take out all of the bigger stuff. What we are typically left with are the tiny pieces, the micro and nano plastics. Your laundry is a primary source. Previous libraries on this topic (October 2018, July 2017) talked about what polyester fabrics shed the most fibers and gave general information on this topic. There have been some more papers since then and so it is time for an update. Spoiler alert- if you don’t want to hear about more scientists crying wolf- stop reading and pick up a good book instead.
The first article in the library is an overview. It is quantitative and helpful if you want to learn about the topic. I’m not saying it is all sweetness and light, but at least it is informative. The section that I found most informative was the one on degradation of plastics and a comparison between bioplastics and traditional plastics. Bioplastics have built in fracture points that allow for the materials to become smaller and more susceptible to microbial degradation. Take Table 1:
That is helpful.
The discussion on biosolids, though more quantitative, sounds more to me like the first article. They quote another study that suggests that each dry ton of biosolids contains between 9 and 63 kg of microplastics. They note that the US has much less restrictive rules covering biosolids than Australia, don’t give a lifetime maximum loading rate for biosolids in the US, but say that lifetime loadings of microplastics from biosolids may be as high as 9-63 tons per hectare (that puts a lifetime of 1000 tons per hectare). At an annual application of 10 tons per hectare that gives you a 100 year lifetime. I guess you’re healthier from all of those biosolids.
They note that these particles in soil can provide surfaces for pesticides to bond, in some cases enhancing degradation of both materials. They note that like at sea, the microplastics from worm castes are coated with microbes. Two studies of worms with microplastics showed different results; no effect to skinny worms that died prematurely. Those skinny worms were left to survive in a system that contained 28% PE microplastics. That is a lot of plastic. Studies with high rates of plastic mulch residues ruined the appetites of soil microbes (reduced CO2 evolution). They noted plant uptake is unlikely and talk about nanoparticle studies as a surrogate. They also say that it is entirely possible that microplastics won’t impact functions and fertility of soil. If you want a solid overview, this is a good article.
Article #3 presents results from a real study- not just an overview. The scientists added biodegradable plastics, plastic fibers from clothing and HDPE to soil. They looked at rye grass germination and growth and earthworm response. They mixed in the plastics at different rates. Here it is tough because I don’t really get the relative rates. They give densities for the two plastics but not the clothing fibers (what you would find with biosolids). The fibers were added at a rate of 10 ppm of fiber or 0.001%. That sounds awfully high to me. I did a rough calculation below, but am not sure of my numbers.
At any rate, with the fibers, they saw a slight decrease in germination and growth of the grass and no change for the worms. They also say that they saw changes in soil aggregation and not in a good way. If you read the abstract, you are sure that the sky is falling.
Article 4 is from the home team. At least Markus Flury is from WSU and currently Andy Bary is working a lot with him. Here they looked at the potential for plant uptake of nano- and microplastics in growth media. While the plastics accumulated at the root tips, they stayed outside of the plant tissue. So it is true that they can migrate to the root surface, at least in solution. Not getting into the root is what you would expect and we can thank Dr. Flury for demonstrating that this is the case.
Article 5 is the only one in the series that actually measures these materials in biosolids. It is based on observations from three application sites in Ontario, Canada. Biosolids from two different plants were applied to different fields. Concentrations of microplastics are presented in numbers of fibers rather than ppm. They ranged from 8700 to 14000 pieces of plastic per kg of biosolids. I have no clue if that is a little or a lot. One field had a history of biosolids applications prior to the current study. The field with the prior applications had higher microplastics to start with than the other fields. The kicker here is that after application, the microplastics essentially disappeared from the soils. The authors suggest that they eroded off of the soil into nearby creeks and so were exported to aquatic systems. This sounds strange to me as the first two papers pointed out that the microplastics partition heavily to the biosolids and not to the effluent. Why this would be different in a soil system is not clear. Also, regulations (and they do exist in Canada) have setbacks to prevent movement of N and P in the biosolids to aquatic systems. This is a very recent paper, also in a well-respected journal. Nevertheless, the possible explanations for the disappearance of the plastics make no sense.
Especially if you look at the last paper in the library. This is a lab study where micro and nano plastics were added to biosolids, the biosolids were then placed on top of porous media in columns and water (14 pore volumes) was poured through. These guys cheated and marked the plastic particles with inorganic tracers that made the finding and counting much easier. They found about 99% retention in the biosolids for microplastics. Nanoparticles were associated with soluble organics in the biosolids, allowing for about 50% of those particles to move. Maybe the authors from paper #4 should partner with these guys. They could do some geocaching (https://www.geocaching.com/play) and find those lost particles.
Take home from this library is to wear natural fibers or wash your synthetics less often. We have no clue what the actual impacts of plastics from biosolids on soils and terrestrial systems actually are. Take heart in the fact that we’ve been wearing synthetics for years and all of the studies on biosolids have shown benefits - nylon or not.