Formation of Nanoplastics from Biodegradable Plastic Mulch Films and Characterization of their Behavior in Soil
Hayes, D. G., A. Astner, H. M. O'Neill, B. R. Evans, S. V. Pingali, and V. S. Urban.  2017.  BioEnvironmental Polymer Society 24th Annual Meeting, Albany, CA, 20-22 September 2017.

Abstract:
Micro- and nanoplastics, (MPs and NPs), formed from fragments of discarded post-consumer plastic materials, present an increasing global burden for organisms in aquatic and terrestrial environment. Degradation and breakdown of plastics, triggered by UV radiation, hydrolysis, and oxidation, gradually results in MP and NP fragments and fibers that can enter the food chain through organisms such as fish, resulting in bio-accumulation. In the terrestrial environment, MPs and NPs can potentially leach into surface- and groundwater via runoff, resulting in uptake by plants and soil organisms, but their aggregation and partitioning between water and soil is poorly understood. A main source of terrestrial plastic fragments originate from thin plastic mulch films, made from low density polyethylene (LDPE), applied on top of the soil and used for cultivation of fruits and vegetables to reduce evaporative water and soil loss and weeds, and pest prevention. The global market size for plastic mulch is 4 million tons). Recycling of LDPE mulch films is very difficult due to contamination with herbicides, pesticides, and soil. Residual LDPE fragments persist in the soil for several years. As an alternative to LDPE films, biodegradable mulches formed from biodegradable polymers such as polybutylene co-adipate (or co-azaelate) co-terephthalate (PBAT), or PBAT-starch blends were developed, which can be plowed into the soil after crop harvest, providing a cost efficient and environmentally friendly disposal route. However, biodegradable plastics decompose slowly (over several months), and may form NPs that may persist in the soil for several months. We are investigating the interaction of NPs formed from biodegradable mulch films and soil particulates, to better understand the behavior of NPs in the soil and the role of NP-soil particulate interactions in the biodegradation mechanism. In this presentation, two goals are addressed: 1) the formation of NPs from PBAT/starch films; 2) detection of PBAT NPs and characterization of their behavior in soil environment by small-angle neutron scattering (SANS). We have shown that MPs and NPs are formed from PBAT/starch mulch films by a novel procedure including cryogenic milling, sieving and grinding. Particle sizes were characterized by analysis of SEM micrographs using Image J software and by dynamic light scattering. The average particle size of the NPs (smallest size fraction via sieving) was 366.583.23 nm. For characterization of NPs in soil, SANS is being employed using neutron contrast techniques. We have determined that the SANS signal for vermiculite, an artificial soil, disappears in the neutron beam when suspended in 66% D2O / 34% H2O. This study demonstrates the potential utilityof SANS to visualize NPs in a slurry combined with vermiculite particles. This finding may provide a pathway for prospective research in determination of NPs in terrestrial environment, particularly in soil.