Seasonal variations of arsenic redox speciation and release in the intertidal and subtidal zones of the Yellow Sea, China: in situ evidence.

Owing to seasonal changes and tidal processes, the environmental behavior of Arsenic (As) in coastal tidal flat wetlands is unclear. This study investigated the speciation and mobility of As at the sediment-water interface in the intertidal zone (IZ) and subtidal zone (SZ) during summer and winter. The DGT-As(III) in pore water increased with sediment depth, reaching maximum values of 10.3 and 2.0 μg/L at IZ and SZ sites in winter, respectively. The DGT-total As exhibited the maximum peak of 27.5 μg/L at the shallow layer in the IZ site, and of 11.6 μg/L at the deep layer in the SZ site in summer. The DGT-As(III) and DGT-total As at the IZ site were higher than at the SZ site. Seasonal changes affected the transformation of As speciation. DGT-As(III) was higher in winter than in summer in the two sites, which was attributed to the reduction of As by As-reducing bacteria (mainly respiratory reduction) and sulfate-reducing bacteria. However, DGT-total As was higher in the surface layer during summer and in the deep layer during winter at the IZ site, which was attributed to reduction and dissolution of crystalline and amorphous Fe minerals mediated by Fe-reducing bacteria. There were significant differences in the microbial composition between the two sites, and the microbial activity and As redox process were stronger at the IZ site than at the SZ site. This study reveals the different behaviors of As release in the intertidal and subtidal zones, shedding a light for controlling As pollution in coastal wetlands.