Occurrence of Organotin in Bottom Water of the Yangtze River Estuary and Its Behavior in the Seasonally Hypoxic Zone

Abstract:

In the summer of 2019， field sampling and observation were conducted at 12 sites and one anchor station （at the seasonally hypoxic zone） along a longitudinal section from the Yangtze River （Changjiang） estuary to the East China Sea. The water quality data in the vertical profile and bottom water samples were collected. After sample filtration， solid phase extraction and ethylating derivatization， the concentrations of nine organotin compounds were detected by gas chromatography-mass spectrometry. The purpose of this study was to analyze its spatial distribution and tidal variation in the Yangtze River estuary， and to explore its behavior in the low oxygen area outside the estuary. The results show that the concentrations of dissolved organotin in the bottom water of the Yangtze River Estuary ranges from 33.9 to 203 ng?L^-1 （as Sn）， with an average of 125 ng?L^-1. Tributyltin （TBT） is detected in only 42% of the samples， and the highest concentration of TBT is 53.1 ng?L^-1. The detection rates of the other eight organotin compounds were 92% to 100%. The methyltin concentrations are （36.7 ± 16.2） ng?L^-1， accounting for 20% to 40% of the total concentrations of dissolved organotin species. The concentrations of butyltin are （33.1 ± 22.0） ng?L^-1， accounting for about 16% to 45%. The concentrations of phenyltin are （55.7 ± 12.4） ng?L^-1， accounting for about 26% to 59%. In the estuarine turbidity maximum zone and the coastal seasonally hypoxic zone， as well as in the tidal transition stage from flood tide to ebb tide， the concentrations of dissolved organotin species in bottom water peak， which is mainly attributed to the increase of dissolved TBT and mono-substituted organotin compounds. It is also found that the concentrations of triphenyltin （TPhT） and monobutyltin （MBT） increase with the decrease of dissolved oxygen saturation. The tide-driven resuspension and reductive dissolution in the bottom boundary layer lead to pulsed exposure of organotin such as TBT and TPhT in the bottom water， which may pose ecological risk to marine benthos and deserves further attention and research.