A method for extracting soil microplastics through circulation of sodium bromide solutions

Microplastics (MP) have been recently found in soil environments. These MP might have adverse effects at high concentrations and thus efficient extraction and analysis of MP from soil is needed. Here we propose a new method of NaBr solution circulation for extracting soil MP. A device for the circular extraction of soil MP was developed. This device included a separation, vacuum filtration, and solution recovery system. It was then utilized to test separation efficiency of soil MP with three economic and environmentally friendly extraction reagents: NaCl, CaCl2 and NaBr solutions. The separation was tested with ten different types of polymers, three different size classes and three different shapes of MP. Extraction with NaBr showed the highest recovery rates ranging from 85% to 100%. After extraction the samples were treated with H2O2 and analyzed by micro-Fourier transform infrared spectroscopy. The developed method was assessed for its potential influence on MP and no significant changes in the integrity of multiple MP were found. Finally, the established method was used to analyze MP in four types of soil: farmland, yellow-brown, paddy and floodplain soil from the suburb of Shanghai. Results showed that the mean abundance of MP was 136.6-256.7 item kg-1. Various MP including PP (40%), PE (35.5%), Acrylic (15.6%), PET (6.7%) and PA (2.2%) were found. With this paper, we provide an alternative method through NaBr solution circulation for the extraction of soil MP.