All plastics, including mulch films, contain additives, which can provide specific properties to the plastic. There is a huge variety of additives available, with over 10,000 different compounds known to be used in the production of plastics. For example, metal and metalloid salts can be used as fillers and pigments. Organic additives are used to aid processing and use (lubricants, plasticizers) while others extend the lifespan of the product (antioxidants, UV absorbers). Additives are essential for the function of plastic mulch films, but they are not chemically bound to the films and can be readily lost to the environment. While some additives have minimal potential negative hazards, others are known to have detrimental effects on environment and human health. For example, phthalates, which can be taken up by plants and enter the food chain, are endocrine disruptors.

Although we know that there are potential hazards associated with additives, there is little information on the additive composition of plastic mulch film, the leaching of additives from films, or the fate of additives in agricultural soil. As part of this project we are working to:

1. Define the leachable additives present in plastic mulch films

We have conducted a survey of low density polyethylene (LDPE) and biodegradable plastic mulch films to determine the organic additive composition and concentration of metal and metalloid additive present in new films and the potential for these additives to leach. We have combined our findings with farm surveys of plastic mulch in China (from WP4) to provide a farm-level assessment of the chemical burden posed by long-term use of plastic mulch films.

2. Quantify degradation of additives across soil types

In soils from across the project partners (India, Vietnam, Egypt) and wider collaborations (Australia, Brazil), we have assessed the rate of degradation of three common additives: di(ethyl hexyl)phthalate, a plasticizer, octabenzone, a UV absorber, and tris(2,4-di-tert-butylphenyl) phosphite, an antioxidant. We aim to evaluate how soil properties such as texture, available carbon and microbial biomass influence the breakdown of additives by microbes and by oxidation. This will provide insight into soils which may be at risk of accumulating additives due to low degradation rates.

Soils from different countries used in additive degradation study.

3. Determine the fate of additives from plastic mulch film during use

We have utilised the multi-country mulch film common experiment to quantify the input of additives from plastic mulch film into agricultural soils, and therefore potential uptake into crops. First, we have started with the film itself, to identify the additives lost, and thus target additives and degradation products present in soil. We are then analysing the soil and crops using high resolution mass spectrometry to determine if additives are accumulating over the sequential growing seasons. We aim to evaluate how different climates effect the input of additives into soils under field conditions, and whether they accumulate over growing seasons.

Plots from common agronomy experiment after maize harvesting showing film remaining at the end of the growing season. Top: LDPE mulch film; bottom biodegradable mulch film.

Techniques

The analysis of additives in plastic mulch films, soils and plants is undertaken in the Organic Geochemistry Unit, School of Chemistry, University of Bristol. We host part of the National Environmental Isotope Facility (NEIF), which gives unique access to a range of chromatographic and mass spectrometric techniques. For additive analyses, we are using microwave assisted solvent extraction followed by solid phase extraction. We focus on the low molecular weight organic additives amenable to gas chromatography as they are more likely to leach from the film. Gas chromatography-mass spectrometry (GC-MS) is used for the identification of additives in plastic mulch film and pyrolysis/gas chromatography-mass spectrometry (Py/GC-MS) to investigate degradation of the polymer itself. To detect additives in environmental matrices such as soil and plants, we use gas chromograph-quadrupole time-of-flight mass spectrometry (GC-QTOF), to give enhanced sensitivity. 

Microwave extraction (left) and gas chromograph-quadruple time-of-flight mass spectrometry instruments (right).

As part of the project, we have been welcoming PhD students and early career researchers from international partners to train in these techniques.