Spatial and temporal distribution of polyhalogenated carbazoles in sediments from the Yangtze River estuary and adjacent East China Sea

Polyhalogenated carbazoles (PHCZs) have been increasingly detected in marine sediment, raising concerns in recent years. In this study, sediment samples (42 surface and one core) were collected from the Yangtze River estuary and the adjacent East China Sea, and eleven PHCZs and unsubstituted carbazole were measured. The total concentration of PHCZs in surface sediments ranged from 0.19 to 2.49 ng/g dry weight (d.w.) (median 1.03 ng/g d. w.). The congener compositions of PHCZs in the surface sediment were dominated by 3,6-dichlorocarbazole (36-CCZ, 53.2%), followed by 3-chlorocarbazole (14.9%) and 3,6-dibromocarbazole (36-BCZ, 11.8%). Carbazole in the surface sediment ranged from not detected to 9.89 ng/g (median 1.25 ng/g), with a detection frequency of 81.0%. The spatial distribution of 36-CCZ in surface sediments exhibited a clear decline from the coast to offshore, while 36-BCZ showed the opposite trend. The depth profile of 36-CCZ was maintained at a relatively low and stable concentration (about 0.36 ng/g) in core segments from 1903 to 1951, followed by a steady increase to 1.5 ng/g in 2006. This increase coincides with the industrial and agricultural development in China that began in the 1950s. In contrast, the other detected PHCZs and carbazole maintained stable, low concentrations over time. These spatial and temporal patterns suggest that 36-CCZ in this area is predominantly from anthropogenic sources, while 36-BCZ has a natural origin. Toxic equivalent estimations indicated that dioxin-like effects for the observed PHCZs were low. These results provide useful information for understanding the origin of PHCZs and carbazole in this area.