From intentionally used plastic films to soil microplastic contamination

Abstract

In modern agriculture, plastic material is intentionally used for different purposes. In 2020, it was estimated that about 7.1 105 t of plastic was used in European agriculture, whereas most plastic was applied in form of plastic mulch films to improve growing conditions, e.g., via temperature regulation or reduction of evaporation. Most of this plastic films are made from Light Density Polyethene (LDPE) with different physical (e.g., thickness between 15 and 200 µm) and chemical properties (e.g., different types and amounts of additives for UV stabilization). Plasticulture improves productivity but a growing number of studies indicate soil contamination with macro-, meso- and microplastic particles originating from plastic mulching films. The aim of this study is to compare the changes in plastic film properties and stability as basis for the fragmentation into meso- to microplastic following different environmental stressors applied to plastic films used for different purposes. A series of lab experiments was set-up to mimic natural UV radiation as well as mechanical stress. Overall, six different agricultural films were tested (two black LDPE mulch films, thickness 20 µm; two transparent LDPE films of small tunnels, thickness 180 µm; and two black-white LDPE asparagus mulch films, thickness 100 and 150 µm). In a first step, different UV light exposures time were used (Q-SUN Xe-1-SE xenon test chamber, TUV (300-400 nm)) to simulate LDPE aging as exposed to sunlight at the soil surface, in second step mechanical stress was applied during an abrasion test (20 g of standard soil were mixed with the degraded samples at 4 rpm for 61 days). Both treatments and their combinations were then analyzed regarding changes in plastic properties. Therefore, we used a 3d laser scanner confocal microscope (LSM) to analyze changes in plastic surfaces, a Fourier-transform infrared-attenuated total reflectance spectrometer (FTIR-ATR) to determine changes in the chemical compounds, an optical contact angle (OCA) to determine changes in hydrophobicity and finally tested changes in mechanical stability with a universal nanomechanical tester (UNAT). First results indicate that UV alone can affect the stability of the films, which is increased by mechanical stress. The FTIR-ATR spectra, especially from the thin black film, presented variation at the carbonyl band (1800-1600 cm-1) after the degradation test; LSM results for the degradation test didn’t show any significant change, while preliminary results from the mechanical stress present changes on its surface; for OCA preliminary results the thin black film showed a variation from 85.7o to 77.2o after UV degradation. Overall, the tests indicate the importance of a combination of UV radiation and mechanical stress for LDPE film degradation, which especially in case of the thin, black mulch film leads to a change in plastic properties paving the way for plastic fragmentation within months of environmental exposure, while the thicker foils are less affected within such timeframe. Hence, thin plastic mulching foils might improve agricultural productivity but on the cost of increasing soil plastic contamination.