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ISSE Project: Understanding Formation, Structural Changes, and Ultimate Fate of Terrestrial Microplastics

Team: Barbara Evans, Mechanical, Aerospace, and Biomedical Engineering/ORNL; Douglas G. Hayes, Biosystems Engineering/ORNL; Hugh M. O'Neill, Cellular and Molecular Biochemistry and Cellular and Molecular Biology;/ORNL

2016-2017 Funded Proposal

The increasing  global burden  of discarded  plastics and resultant microplastic  fragments is  a major environmental and health concern. Microplastics and nanoplastics, µm and nm-sized  particles that  originate from disposal and/or  fragmentation of discarded  and post consumer plastic materials, present emerging environmental and health problems. Public concern and research studies have focused on waters and beaches of oceans, lakes, and rivers ,where ingestion by aquatic invertebrates and shellfish can result in bio-accumulation progressing up the food chain. Few  studies have examined the effects of microplastics in terrestrial  environments.

A major source of terrestrial plastic fragments are debris from plastic mulches, thin films applied to the top of the soil layer in the cultivation of fruits and vegetables in agriculture, to reduce evaporative water loss, weed control, soil conservation, and pest prevention (global market size of 4 million tonnes). These films typically are cast from low density polyethylene (LDPE) and are not reusable, as weathering during a single growing season disrupts their structural integrity, resultng in loss of strength and fragmentation. They cannot be recycled  due to adherent soil and possible contamination with herbicides and pesticides, in addition to labor and transportation costs. They eventually disintegrate into fragments which are shed into the soil and surface water, where their effects on soil ecosystems are largely unknown. Earthworms ingesting 50 -150 µm LDPE were reported to grow more slowly than controls. LDPE is extremely recalcitrant  to biodegradation.

To address this problem, efforts have been made to render these materials either more readily degradable or to replace the LDPE with polymers that are intrinsically  biodegradable  under ambient soil conditions such as polybutyrate adipate teraphthalate (PBAT) or PBAT-starch blends. The biodegradable plastics can be plowed into the soil at the end of their use, providing a simple and inexpensive route for their disposal. Yet PBAT and other biodegradable plastics mineralize slowly (over several months), and are susceptible to abiotic hydrolysis, so they may form potentially harmful microplastics that persist in the soil. This situation is representative of the possible formation of microplastics from compostable plastics such as polylactic acid in composting environments.

The process of micro- and nano-plastic formation is not well understood for either conventional or biodegradable plastics, particularly in terrestrial environments. Although weathering of the plastics plays a role in the degradation process in soil, the nature of the relationship between above- and below-ground changes in the plastics is not fully understood. Through USDA funding (Specialty Crops Research Initiative [SCRI]), co-PI Hayes (SCRI Project Director) and a transdisciplinary team of collaborators are investigating the long-term impacts of repeated use of biodegradable mulches on soil-related ecosystems, production of vegetables, and their impact on farmers and consumers, as well as several additional stakeholder groups (http://biodegradablemulch.org). However, as the USDA-funded project does not study micro- and nano-plastics, the proposed project is different but complimentary to it.

We will investigate the mesoscale structures, molecular associations, and hydration of micro- and nano-plastics formed from plastic films commonly used in agriculture, LDPE and biodegradable  polymer blends, following  incorporation into the soil environment and weathering. We anticipate that the proposed  two-year project  will provide  fundamental knowledge of the changes in surface properties, water dynamics, and aggregation of plastics at a wide range of length scales in situ in soil that cannot be obtained with other techniques. This  unique  knowledge will  enable control  of plastic fragmentation and degradation in the environment, potentially guiding remediation and/or redesign of plastic materials to reduce environmental  impacts  and  improve performance.

Work Plan:

Task 1: Formation of micro- and nano-plastics in simulated soil
We will investigate the formation of microplastics from weathered LDPE and biodegradable PBAT/starch composite agricultural mulch films in an abiotic synthetic aluminosilicate-based soil containing water at controlled levels of hydration and pH  (and water and ambient air serving as controls). Macroscopic deterioration of plastic film specimen placed in petri dishes of simulated soil will be analyzed via optical light and laser confocal microscopies versus time, with analyses focusing upon the specimen and soil particles (to observe adsorption). Adsorbed microplastics will be isolated using solvent extraction of soil. Differential scanning calorimetry, FTIR spectroscopy, and gel permeation  chromatography will be used to probe changes in crystallinity and  molecular structure  (e.g., oxidation and depolymerization). We will test the  hypothesis that microplastics  form via a surface erosion mechanism.

Subsequent complimentary  experiments  will  be performed  in D20-artificial  soil  slurries. The effect of shear and water flow will be simulated using gentle mixing of the slurry. The  aqueous phases will be isolated. Aqueous solutions isolated as described and solvent extracts of  soil particulates will be analyzed for the presence of micro- and nano-plastics through dynamic light scattering and small-angle x-ray or neutron scattering (SAXS,or SANS respectively). Centrifugation will be used to isolate the micro- and nano-plastic subpopulations at different size ranges. The distribution of LDPE  between  residual plastic fragments, adsorbed soil particulates, and dispersed water will be evaluated  using NMR.

Task 2: Structural Analysis of micro- and nano-plastics by SANS
To understand the  complex  mechanisms  of fragmentation, environmental distribution, and degradation of conventional and biodegradable plastics, we will  use the highly  penetrating and non-destructive nature of neutrons to probe and compare the changes that occur to the two diverse plastics during  degradation. SANS will  provide information  on the structural changes that occur in the soil to embedded plastics over multiple length-scales ranging from mm to nm. When combined with contrast matching which takes advantage of the differences in the scattering length densities (SLD) of hydrogen and deuterium, it becomes possible to selectively probe individual components within a complex system. The SLD of the components are sufficiently well-separated that the scattering pattern of each component in the complex mixture can be separated without deuteration of any of them: LDPE and biodegradable mulch films (-10- 15% D20); and soil (aluminosilicates, -60% D20). The naturally different scattering by these constituents enables contrast matching to separate structural features of plastic mulch film samples obtained from Task 1 in the simulated soil. This integrated approach will pave the way
to compare the mesoscale and molecular structures of new plastic mulch films to those of field­ exposed samples and laboratory-treated samples to obtain new fundamental knowledge of the properties of these materials and how they change during fragmentation and degradation.