Unearthing the hidden plastic in garden compost

Compost and potting mixes have emerged as significant sources of plastic and biodegradable polymer residues in soil. While bio-based (such as polylactic acid (PLA), polyhydroxyalkanoates (PHAs) and polybutylene succinate (PBS)) and fossil-based (such as polycaprolactone (PCL), and polybutylene adipate terephthalate (PBAT)) biodegradable polymers are designed to rapidly degrade and are marketed as biodegradable, they require specific conditions to do so. Consequently, both conventional and biodegradable plastic polymers raise concerns that they may be harming the environment. Compost is widely used to improve soil properties, but the presence of conventional and biodegradable plastic polymers in compost is not well understood. This study used pyrolysis coupled to gas chromatography mass spectrometry (Pyr-GC-MS) to investigate the prevalence of selected plastic polymers in commercial potting mix/composts and homemade composts in Australia. Polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), polycarbonate (PC) and polyvinyl chloride (PVC) and the biodegradable polymers polylactic acid (PLA), polybutylene succinate (PBS), and polyhydroxyalkonaotes (PHA) were investigated. While PE and PVC polymer markers were assessed, they were not included in the results due to potential interferences. PP, PMMA and PET were present in only 1 sample at 0.18 mg/g d.w., 0.18 mg/g d.w. and 0.12 mg/g d.w. respectively, while PC was present in 2 samples, with a mean concentration of 0.04 mg/g d.w. and PS was below detection limits. PLA was detected in only one sample at 0.09 μg/g d.w., while PBS was present in all samples, with concentrations ranging from 0.02 to 0.39 μg/g d.w. PHA was found in two samples, at 0.03 μg/g d.w. and 0.09 μg/g d.w. and PCL and PBAT were not detected in any sample. Finally, the potential application of plastics to domestic gardens in Australia, through the application of compost/potting mix materials, was calculated to be between 0.02 and 0.6 g/m², based on typical application rates and plastic content.