Arsenic speciation and diffusion flux in Danshuei Estuary sediments, Northern Taiwan

• The sequential extraction method was used to study the As, Fe and Mn species in core sediments.
• The total sedimentary As concentrations in core sediments ranged from 5.0–19.0 μg g− 1.
• The As species were dominated by association with hydrous of Fe and Al phases, and residual phases.
• The As geochemical behavior in core sediments is influenced by the redox potential and Fe oxides.
• The sediment was a source of dissolved As and the diffusion flux ranged from 0.047–0.35 μg cm− 2 y− 1.

Sedimentary As, Fe and Mn species in core sediments from the Danshuei Estuary, an anoxic estuary, were studied using a five-sequential extraction method followed by chemical analyses of these elements. The sediment texture, redox potential and dissolved concentrations of these elements in pore water were also determined. Dissolved As diffusion flux was also calculated. Higher concentrations of sedimentary As were found at the middle and lower estuary stations. The sedimentary As species in the samples were dominated by three fractions: amorphous and poorly-crystalline; and well crystallized hydrous oxides of Fe and Al; and residual phases, accounting for 90% of the total As pool. The geochemical behavior of sedimentary As in the Danshuei Estuary is strongly influenced by the redox potential and Fe oxides. The dissolved As concentrations in pore water correlated inversely with the redox potential. The sediment served as a source of dissolved As and the diffusion flux ranged from 0.047–0.35 μg cm− 2 y− 1.