Chemical Characterisation of Agricultural Plastic Mulch Film and Assessment of Anthropogenic Chemical Release to Agroecosystem Compartments

Plastic is a prominent material applied throughout agricultural supply chains through cultivational practices to product packaging.

Mulch film is one such 'plasticulture' application where opaque film is stretched over the soil surface to form a protective barrier and maintain a humid microclimate in the crop / fruit soil-root zone.

Mulch film represents an anthropogenic chemical input source to agroecosystems, whose ecotoxicological legacy is yet to be assessed.

Manufacturers include a diverse multitude of chemical additives in plastic formulations to aid with manufacturing, improve functionality, retard degradation and alter aesthetics.

In most cases, additives are not chemically but physically bound to the polymeric matrix meaning over time they are free to be released to the surrounding agroecosystem.

To trace the extent of release and subsequent interaction of chemical additives within ecosystems, the formulation of the film must first be determined. This project falls within the physical sciences theme of the EPSRC, specifically relating to environmental chemistry.

This projects first aim was to develop methods for the chemical characterisation of agricultural plastic mulch film, for both low density polyethylene (LDPE) and biodegradable articles polylactic acid (PLA)/polybutylene adipate terephthalate (PBAT) mulch film.

Developed methods included the use of solvent-based extraction techniques, such as microwave assisted extraction (MAE) and dissolution-precipitation extractions, to separate the chemical additive content from the polymer system.

The chemical extracts were then screened using gas chromatography (GC) and mass spectrometry (MS) techniques for the identification and quantification of additives and non-intentionally added substances (NIASs). To compliment these findings, methods to characterise the polymeric composition of the mulch film were also developed.

These included analyses with fourier transform infrared (FTIR) spectroscopy and nuclear magnetic resonance (NMR) spectroscopy with reference to polymer standards.

With attained knowledge of the plastics chemical composition, assessment of chemical release to agroecosystem compartments, such as soils, became feasible through a targeted approach of chemical screening.

The second aim was to assess the short-term leaching behaviour of the characterised chemical constituents from mulch film.

For this, plastic mulch was incubated in water for 28 days and the leachate was screened for plastic derived chemicals using GC-MS and GC-flame ionisation detector (FID).

Characterised compounds were then identified as the most mobile from the plastic in the short term, during in field use. The third aim focussed on the lifetime of any released chemicals within a simulated agroecosystem compartment.

This was especially important to evaluate given mulch film may be retained in the soil after a growing season, meaning the chemical content will over time be released to the wider environment.

To assess chemical persistence, target additives were incorporated into soil columns and periodically sampled over a 56-day period.

A developed sonication assisted solvent extraction procedure, followed by targeted chemical characterisation, allowed the chemical stability within the soil matrix to be temporally evaluated. The incubations were undertaken in both a mixed additive system and for individual analytes.

This allowed for the identification of preferential degradation of certain plastic additives over others and to evaluate if additive stability is altered by mixed chemical exposure.