A critical review of microplastic degradation and material flow analysis towards a circular economy.

The resilience and low cost of plastics has made their usage ubiquitous, but is also the cause of their prevalence and longevity as waste. Plastic pollution has become a great concern to the health and wellbeing of ecosystems around the world; microplastics are a particular threat, due to their high mobility, ease of ingestion by wildlife, and ability to adsorb and carry toxic contaminants. Material flow analysis has been widely applied to examine stocks and flows of materials in other industries, and has more recently been applied to plastics to examine areas where waste can reach the environment. However, while much research has gone into the environmental fate of microplastics, degradation strategies have been a lesser focus, and material flow analysis of microplastics has suffered from lack of data. Furthermore, the variety of plastics, their additives, and any contaminants pose a significant challenge in degrading (and not merely fragmenting) microplastic particles. This review discusses the current degradation strategies and solutions for dealing with existing and newly-generated microplastic waste along with examining the status of microplastics-based material flow analysis, which are critical for evaluating the possibility of incorporating microplastic waste into a circular economy. The degradation strategies are critically examined, identifying challenges and current trends, as well as important considerations that are frequently under-reported. An emphasis is placed on identifying missing data or information in both material flow analysis and degradation methods that could prove crucial in improving understanding of microplastic flows, as well as optimizing degradation strategies and minimizing any negative environmental impact.

The potential of aerial insectivores for monitoring microplastics in terrestrial environments.

Limited research has been conducted on microplastics in terrestrial ecosystems and biota, despite being some of the most ubiquitous environmental pollutants. We investigated the presence of microplastics (over 125 µm) in tree swallow (Tachicyneta bicolor) chicks (10 d. o.), an aerial insectivore whose diet involves terrestrial and/or freshwater sources. Swallows nested immediately downstream (300 m) of the discharge pipe of a large, urban wastewater treatment plant (WWTP) or at a rural conservation area (40 km apart). Anthropogenic microparticles (including microplastics) were identified in nearly all WWTP chicks (90%; N = 20) and reference chicks (83%; N = 20). All microparticles were fibers (100%) in the gastro-intestinal (GI) tracts of WWTP nestlings, whereas unexpectedly, they were more diverse in the GI tracts of reference chicks, with ~15% characterized as pre-production plastic pellets. The fecal sacs of most nestlings (90%) contained microparticles, and all were characterized as fibers suggesting their excretion by tree swallows. Compared to WWTP chicks, the reference chicks had more microparticles in their fecal sacs and larger particles (length, width) in their GI tracts, likely reflecting the more aquatic-based diet of the reference chicks fed insects caught adjacent to the nearby dam, compared to the more terrestrial-based diet of the WWTP chicks. The numbers of microparticles were not correlated between GI tracts and fecal sacs, nor with the chicks' condition or size (weight, organs, feathers). We recommend sampling macroinvertebrate prey to permit stronger conclusions regarding WWTPs as possible sources of microplastics for swallows, and to determine if such macroinvertebrates may be a non-lethal method to characterize microparticle diversity ingested by birds as presently identified in chicks' GI tracts. We conclude that sampling fecal sacs only, while not indicative of the diversity of microplastics ingested by terrestrial passerines (e.g., tree swallows), is useful for determining their exposure to microparticles.