RFBR programme for supporting young scientists, Project No. 18-35-00553 (2018-2020) [finished]
Annotation: Ocean pollution by microplastics has been closely studied by the scientific community in recent decades. At the moment, there is a problem of insufficient knowledge and experimental research on the dynamics of microplastic particles in the marine and freshwater environment which is necessary for understanding its transport and possible sinks. The project is aimed at the experimental investigation of the microplastics settling and the study of the influence of the particle shape on the nature of its dynamics. The analysis of the microplastics movement in conditions of convective mixing will be carried out for the first time. The project work plan includes experiments with microplastic particles of complex shapes encountered in the marine environment (films, fibers, fragments) under conditions of different buoyancy flux and comparison of obtained data with semi-empirical dependences of sedimentation rate on particle size available in the literature. Based on the results of the project, the most suitable approximations of the microplastics settling rates will be proposed which can be subsequently used both for estimating values characteristic of natural conditions and for numerical modeling of microplastics transport on a local and global scales. Experimental data on the deposition of microplastics is the starting point for parametrizing the processes of biofouling and flocculation, calculating the lifetime of particles in the water column.
Results and activities: The project made it possible to supplement existing knowledge about the processes of microplastics settling under stationary conditions and under conditions of convective mixing by means of laboratory experiment. In total, 83 measurements of the settling velocity of flat microplastic particles and 74 experiments with synthetic fibers under stationary conditions were carried out. Video recording allowed us to capture the complex nature of the secondary movements of flat particles and the features of the settling of thin fibers. Based on previously published and experimental data, an approximation of the settling velocity of flat microplastic particles was obtained. To parameterize the settling of fibers with a length of 0.5-5 mm, it was proposed to use the obtained average value of 1.4 mm/s. In the second set of experiment a qualitative description and video recording of the movement of fibers and a quantitative assessment of their velocities under conditions of a destabilizing buoyancy flow caused by heating from below were carried out. Fibers with negative buoyancy showed multidirectional movements (settling, rising, moving in the horizontal direction) with velocities ranging from 0.9 to 6.9 mm/s. On the other side, convective mixing did not significantly affect the motion of flat particles – particles continued to settle at velocities close to those observed under stationary conditions. Project results and quantitative estimates can be directly applied to the development and verification of numerical models of microplastics transport in the marine environment.
Video records of laboratory experiments are available at the figshare (https://figshare.com/) – international platform for storing and sharing scientific data, with the assignment of DOI:
- Settling of flat particles and fibers (2019), https://figshare.com/collections/Microplastics_settling/4886211; DOI https://doi.org/10.6084/m9.figshare.c.4886211
- Settling of fibers in the presence of thermal convection, 1 experiment (2020), https://figshare.com/collections/Microplastics_settling_convection_experiment_1/4886220; DOI https://doi.org/10.6084/m9.figshare.c.4886220
- Settling of fibers in the presence of thermal convection, 2 experiment (2020), https://figshare.com/collections/Microplastics_settling_convection_experiment_2/4906203; DOI https://doi.org/10.6084/m9.figshare.c.4906203