Prediction of microplastics particles size-frequency distribution via the stochastic modelling of their formation and filtration on the net

To assess microplastics abundance in the aquatic environment different filtering tools (nets, meshes and filters) are generally applied.  As a result, their naturally occurring size-frequency distributions are altered because of removing of items smaller than the mesh size. This hampers comparison of the results between studies utilized filters and nets of different mesh size. To assess the process of MPs generation and filtration, a stochastic model of macroplastic destruction coupled with the model of MPs filtration on the net was proposed. The stochastic model of macroplastic destruction incorporates empirical parameterizations of fracture position and fracture probability as a function of particle shape. The model sensitivity was tested in respect to the input parameters: the initial number of particles, the initial size of the macroparticles, the number of fracture steps (number of generations, i.e. the final ‘age’ of the particles), and the number of independent ‘sources’ in the final particle set.

The simulation results were compared with the available publications as well as with the data collected at Lake Onego. This allowed us to achieve qualitative agreement between the modelled and the observed distributions based on the similarity of the shape of size-frequency distribution curve in log-log scale.

Large particles, which have all three dimensions larger than the mesh size, are retained by the net efficiently. For others the probability of particle retention by the net depends on the particle shape, smallest and largest dimensions, and particle orientation in space.

To simulate the actual filtration process on the net, a mathematical model of filtration process was developed. The model passes a given set of three-dimensional particles (in quasi-elliptic approximation) through a two-dimensional net with a given cell size. Randomly given parameters determine the position of a particle in space, the size of its projection on the two-dimensional plane (net), and the position of the particle centre with respect to the corners of the cell (net). 

The results of plastic breaking simulation were coupled with a stochastic MP particle filtration model. This allowed us to show qualitatively how the shape of size-frequency distribution of MP particles is altered after the filtering through the net with a mesh size close to the lower boundary. This information can be used to compare the results obtained in studies utilized different neuston nets and filters, which is one of the most relevant tasks in the assessment of environmental contamination by microplastics on a global scale.

The study was supported by the Russian Science Foundation grant number 19-17-00035.