A predictive model to assess the accumulation of microplastics in the natural environment

The use of plastics inevitably leads to (micro-)plastics entering and accumulating in the natural environment, affecting biodiversity, food security and human health. Currently, a comprehensive and universally applicable methodology to quantify microplastic accumulation in the natural environment is lacking. This study proposes an integrated biodegradation model that provides the possibility to examine and compare the microplastic formation and accumulation of different polymer types in diverse natural environments. The proposed model derives carbon mass flow streams from experimental mineralisation curves (CO₂ evolution) of polymers and predicts the concentrations and residence times of the different plastic states during their biodegradation processes. The model allows for the description of the accumulation potential of polymers, as the time-integrated concentration of microplastics present in the natural environment during a timeframe of 100 years after a polymer enters the natural environment. The model is applied to estimate the accumulation potential of three polymers with different biodegradation profiles in soil: polybutylene succinate (PBS), polylactic acid (PLA) and polyethylene (PE). It is demonstrated that the dimensionless accumulation potential of PBS in soil is near zero (between 3.0·10^-4 and 0.002) which corresponds to a potentially very low level of accumulation. On the other hand PE shows a near maximum value of 1 which corresponds to the almost completely non-biodegradable character of this polymer in soil. PLA exhibits a wide range of values in between that of PBS and PE which reflects its reported relatively slow biodegradation in soil. The proposed model can be used to guide material selection in product design by quantifying the microplastic accumulation of these different polymer types. To demonstrate its use, plastic candy wrappers and agricultural mulch films were selected as case studies. Both case studies show that high biodegradation rates can limit or prevent microplastic accumulation in soil.