Photochemical
weathering leads to degradation of microplastics
and releases chemical additives, polymeric fragments, and/or byproducts.
This study evaluated the release kinetics of organotin compounds (OTCs)
from three different sized (10–300 μm) polyvinyl chloride
(PVC) microplastics under UV- and visible light irradiation. Four
OTCs, dimethyltin (DMT), monomethyltin (MMT), dibutyltin (DBT), and
monobutyltin (MBT), were found to release from PVC particles after
24 h leaching in darkness ranging from 2 to 20 μg·g-PVC–1. Under UV/visible light irradiation, only DMT and
DBT were detectable, whereas MMT and MBT were not detected due to
rapid photodegradation. The total tin concentrations (including organic
and inorganic tins) in the aqueous phase monotonically increased under
light exposure. By contrast, they reached plateaus after 24 h in darkness,
confirming the photodegradation of OTCs. A release kinetics model
was established and correctly interpreted the microplastics size effect
on the OTC release process. Finally, the impacts of salinity and dissolved
organic matter (DOM) were investigated. The release and photodegradation
of OTCs were both inhibited at high salinity conditions, probably
due to the enhanced readsorption of OTCs on PVC microplastics and
the formation of halogen radicals that were less reactive toward neutral
OTCs. The presence of DOM, however, increased OTCs release probably
because the excited state triplet DOM (3DOM*) formed and
reacted with OTCs from PVC microplastics.
Background: The Yangtze Estuary is a vital habitat and breeding ground for many rare species (e.g., Chinese sturgeon, Chinese paddlefish). From the view of marine ecosystem health, organotin compounds (OTCs) should be of great concern in this area based on their broad applications and potential threats. At present study, five OTC species were determined in 28 and 26 surface sediments from the Yangtze Estuary (including the Nearshore area, Estuarine Turbidity Maxima (ETM) zone and Plume zone) in 2015 and 2016, respectively. Meanwhile, four sludge samples were collected from the adjacent Shidongkou Wastewater Treatment Plant to perform a source apportionment. Results: The sum of OTCs presented a decreasing trend towards the open sea, averaging 4.8 ± 6.0 ng Sn g −1 dw in the Plume zone. While OTC levels in the Nearshore area (2.9-34.6 ng Sn g −1 dw) were similar with those in the ETM zone (5.6-36.0 ng Sn g −1 dw), and the sites belonging to the Deepwater Navigation Channel showed heavier contamination in the ETM zone than the Nearshore area. There are abundant suspended particles and organic matters in the ETM zone that can effectively capture the hydrophobic compounds. Besides, OTC contamination in shoreside zone was more serious than those in channel center. High OTC loads (average 633 ± 124 ng Sn g −1 dw) were also found in sludge samples from adjacent wastewater treatment plant. Conclusion: Land-based sources (e.g. sewage discharge, runoff) are probably one of the predominant pathways of OTCs entering the ocean. Combining with previous observation in 2014, OTC contamination in the same ten sites varied slightly with years. Risk assessment indicated that the concentrations of tributyltin (TBT) are sufficient to pose ecosystem threats especially in the ETM zone. Hence, OTC contamination issues in the Yangtze Estuary still can not be neglected.
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