The Alternation of Flood and Ebb Tide Induced Arsenic Release and Migration from Coastal Tidal Flat Sediments in Yellow Sea Wetlands: an Ex-Situ Study

Periodic floods and ebb tides driven by seawater affect arsenic (As) behavior in coastal wetlands, which is not fully understood. In this study, we combined DGT, high-throughput sequencing, and real-time quantitative polymerase chain reaction (qPCR) techniques for the first time to investigate the effects of flood and ebb tide on As transformation and mobility in a typical coastal tidal flat wetland by laboratory experiments. In the flood-tide stage, more As was released into the water solution, and As(III) concentration in water solution was notably higher (51 μg/L) than in the ebb-tide stage, especially in SA (sodium acetate addition) treatment. As-methylation also occurred in this stage. The release of As was attributed to the reductive dissolution of amorphous and crystalline Fe oxides bound As induced by microbes. Fusibacter, Sva1033, Bradyrhizobium, Ralstonia, and Desulfobacter were primarily involved in the reduction of As, Fe, and S. The copy numbers of arsC were 20–148 times higher than arrA, suggesting that the detoxification reduction pathway was the primary mechanism of direct As-reduction in the flood-tide stage. However, 2-D high-resolution DGT techniques revealed that the concentrations of As(III) and total As in pore water of sediments in the ebb-tide stage were higher than those in the flood-tide stage. The detected aioA gene indicated that As was oxidized in the sediment surface, resulting in As stabilization by crystalline iron minerals, whereas more As(III) were accumulated in pore water. The reduction pathway mediated by arrA may play a prominent role in the ebb-tide stage. This study provides new insights into As release processes in coastal wetlands in seawater environments, which further deepens our understanding of As biogeochemical cycling behavior in different natural environments.