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The xanthene fluorescent dyes usage for the microplastics in soil detection and for phytotests

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The paper examines the xanthene fluorescent dyes questions available to a wide laboratories range in order to detect microplastics with an average particle length of 157 ± 59 μm in soil samples and conduct phytotests using fluorescently labeled microplastics. For the research, soils with a humus content of 1.59 ± 0.15 % (P1) and 6.74 ± 0.11 % (P2) as well as suspension polyvinyl chloride (RusVinyl LLC, 157 ± 59 μm, white) were used. In order to study the possibility of selective staining of microplastics in the presence of soil particles, polyvinyl chloride microparticles, soil P1 and P2, as well as a mixture of soil P1 and polyvinyl chloride (5 % by weight) were stained with rhodamine B, rhodamine G, fluorescein and eosin Y in isopropyl alcohol (dye concentration — 200 mg/L, temperature — 100 °C, staining time — 2 hours with constant stirring on a magnetic stirrer) and washed with distilled water on a paper filter. To study the chemical polyvinyl chloride microparticles structure before and after staining attenuated total reflectance-Fourier transform infrared spectroscopy was used (spectrometer Tensor 37 (Bruker, Germany), attenuated total internal reflection MIRacle Pike attachment with a diamond-coated ZnSe crystal). Spectrophotometry and microphotograph analysis using ImageJ software were used to determine whether dye could be leached from polyvinyl chloride microparticles after repeated washing with water. To conduct the laboratory experiment to detect microplastics in soil a mixture of P1 soil and polyvinyl chloride microparticles (0.1 % by weight) was prepared and stained with rhodamine G. In order to reduce the amount of mineral particles and concentrate polyvinyl chloride microparticles, a separation technique was used due to the difference in density using binary solution of NaCl and Ca(NO3)2. Microphotographs were obtained using an optical microscope with an additional ultraviolet source (λ = 365 nm). Image areas measurements of detected microparticles projections, that are necessary for calculating the approximate mass of the pollutant, were carried out using ImageJ software. The possibility of using fluorescently labeled polyvinyl chloride microparticles for phytotests was established using the seed germination test in contaminated soil and studying seedlings using an optical microscope with an additional ultraviolet source. It was shown that after staining with rhodamine B and rhodamine G, fluorescence is observed in polyvinyl chloride microparticles both separately and in a mixture, since soil particles P1 and P2 do not acquire similar properties. When fluorescein and eosin were used, polyvinyl chloride microparticles and soil practically did not fluoresce. It has been established that coloring does not affect the polyvinyl chloride chemical structure. Dyes are not washed out of polyvinyl chloride microparticles after repeated washing with water. The laboratory experiment showed that it is possible to detect and quantitation polyvinyl chloride microparticles in soil at the 0.1 % concentration by weight with a relative error of about 30 %. It is possible to use fluorescently labeled polyvinyl chloride microparticles with rhodamine B and rhodamine G when conducting phytotests. This research demonstrates for the first time the possibility of selective staining of polyvinyl chloride microparticles with rhodamine B and rhodamine G in a mixture with soil due to the observed fluorescence and their detection in a concentration of 0.1 % by weight. The results obtained expand knowledge in the field of monitoring microplastics in soil and, since today there are no standardized methods for detecting this pollutant, it can be used in their development. Fluorescently labeled polyvinyl chloride microparticles with rhodamine B and rhodamine G are planned to be used in phytotests as part of experiments on the hygienic justification of the maximum permissible concentration of the pollutant in the soil.

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