High-resolution characterization of arsenic mobility and its correlation to labile iron and manganese in sediments of a shallow eutrophic lake in China

Abstract

Understanding the status and dynamics of labile arsenic (As), iron (Fe), and manganese (Mn) across the sediment/water interface (SWI) is essential for As cycling in eutrophic lakes. This study conducted systematic monitoring of labile As, Fe, and Mn in water and sediments in regions of varying eutrophic levels in Lake Taihu to investigate Fe- or Mn-coupled cycling of As in sediments. We arranged high-resolution combined ZrO-Chelex diffusive gradients in thin films (DGT) probes in intact sediment cores to measure labile As, Fe, and Mn (i.e., DGT-labile As, Fe, and Mn) under static condition for 24 h. After retrieval, we rinsed each probe first with lake water and then with deionized water to remove any visible sediment particles for analysis. Results showed that DGT-labile As in overlying water and sediments was relatively higher in the middle or hyper-eutrophic regions compared to the mesotrophic region. Diffusive flux across the SWI further demonstrated that the internal dissolution of As in the middle or hyper-eutrophic region was stronger than the mesotrophic region, indicating that eutrophic environments may facilitate As release in sediments. We observed a positive correlation between DGT-labile As and Fe or Mn, which indicated that the reductive dissolution of As binding Fe/Mn oxyhydroxides in sediments was responsible for the internal release of As, which strengthens under eutrophic environments. Furthermore, the DGT-labile molar ratio of Fe/As or Mn/As had a significant positive relation with Fd, further confirming that labile Fe or Mn is much more relevant for sediment As cycling. Eutrophic environments facilitated reductive dissolution of As binding Fe/Mn oxyhydroxides in sediments, resulting in the release of more labile As in the middle or hyper-eutrophic region.