Effects of microplastic aging on its detectability and physico-chemical properties in loess and sandy soil

Comparing pristine microplastic (MP) to aged MP collected from the soil environment, aged MP particles generally have a rougher surface and thus a greater specific surface area, higher crystallinity, more oxygen-containing functional groups, and lower tensile strength, as well as greater mobility and consequently a higher risk of leaching into the groundwater [1, 2]. The aged MP is produced due to the solar UV irradiation and soil abiotic and biotic processes [3]. Thus, pristine and aged MP may behave differently in the environment due to different surface properties and MP extraction from soil consequently must ensure to recover both, pristine and aged MP. Therefore, we aimed to develop a fast and efficient laboratory method to detect pristine and aged MP and to compare the recovery rate of aged and pristine MP. For this purpose, an experiment was conducted by adding pristine and UV-aged (UV irradiation period: 35 d) MP particles of two different types, polyethylene terephthalate (PET) and polystyrene (PS), at three sizes (S: 500 µm, M: 500-630 µm, L: 630 µm -1mm) to sand and loess soil at a concentration of 0.5% w/w. For extraction, a density separation was performed, using saturated NaCl (1.2 g cm^-3) and NaI (1.8 g cm^- 3) solutions, followed by H₂O₂ (33 %) treatment to remove soil organic matter. Recovered MP was quantified gravimetrically. The physico-chemical properties of pristine and aged MP before and after extraction were analyzed by Mid-FTIR for changes in surface functional groups and by contact angle (CA) determination for changes in wetting properties due to aging. We found that the recovery rates of pristine and UV-aged particles were the same for both PS and PET, with no significant differences between recovery from sand and loess soils. The average recovery rate of all samples was about 81 %. Pristine MP was hydrophobic (CA≥90) while aged MP was subcritically water repellent (CA>0 and <90°). In line, FTIR spectra indicated the formation of hydroxyl groups (i.e., polar sites) in aged MP. FTIR spectra before and after recovery showed no significant differences, however, CA of aged MP (size L) was increased to >90° after recovery, probably due to amphiphilic organic matter (OM) compounds sorbed during extraction. In conclusion, the tested recovery procedure did work for pristine and aged MP at all size fractions and loess and sandy soils, while the extraction procedure seems to result in an interaction between polar sites and released OM in case of aged MP.

Keywords: Microplastic, Pristine, UV-aged, Recovery, Identification, Quantification.

[1] doi: 10.1016/j.scitotenv.2018.02.079. [2] doi: 10.1016/j.watres.2021.117407.117407.[3] doi: 10.1002/jpln.202200136.