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Microplastics recovery from soil by density separation: application at pristine and UV-aged particles differing in surface properties
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Researchers assessed how UV aging affects the recovery of polyethylene terephthalate and polystyrene microplastics from sandy loam and silt loam soils using density separation, achieving average recovery rates of 81% regardless of polymer type or aging state, while documenting that hydrogen peroxide treatment used in the protocol can alter the surface oxidation signature of aged particles.
Numerous methods are available for sampling, extracting, and detecting microplastics (MP; 0.001–5 mm) in soil. However, previous studies mostly focused on pristine MP and changing surface properties due to ageing (e.g. surface photooxidation) that will impact transport in soils are not considered. This study primarily aimed at MP recovery from soil by density separation, focusing on the impact of UV-aging on recovery rates while monitoring MP surface properties to avoid analytical artifacts. Thus, we mixed sandy loam and silt loam topsoil with both, pristine and UV-aged polyethylene terephthalate (PET) and polystyrene (PS) MP, produced from PET bottles and PS plates in three size classes. MP particles were recovered by density separation, using oversaturated NaCl and NaI, solutions, followed by H2O2-treatment to oxidise soil organic matter. Recovery rates were determined gravimetrically after filtration of the supernatant. MP surface properties were characterised by FTIR, Nile red staining, contact angle (CA), and XPS analysis before and after recovery. Recovery rates averaged 81.0%, ranging from 56.5% to 89.7%, with no defined differences between MP type, size, and variant (pristine, UV-aged). Before recovery, FTIR analysis revealed an increase in carbonyl groups, Nile red staining showed a darker colour, CA was significantly decreased, and XPS indicated an increase in surface O/C ratio for UV-aged MP. After recovery, FTIR and Nile red analysis showed no changes, CA of UV-aged MP, however, was found to be increased, probably due to H2O2-treatment that may have removed the oxidised surface layer. In summary, the separation procedure applied was found to successfully recover pristine and UV-aged MP from both, sand and silt soil, regardless of MP type tested (PET, PS).
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