Comparison of antimony and arsenic behavior in an Ichinokawa River water–sediment system

This study has demonstrated the similarities in the geochemical behavior of Sb and As under relatively oxic condition within a river system of Kamo-Ichinokawa in Japan. Speciation data in both water and solid phases were obtained by HPLC–ICP-MS and X-ray absorption fine structure analyses, respectively. The solid–water distributions of Sb and As are similar at all sampling points. Scavenging of the two elements from the aqueous phase was attributed to their adsorption onto iron (hydr)oxides. The two elements were finally incorporated into the river sediments where the host phases of Sb and As were determined by XAFS and sequential extraction experiments. The contribution of Sb and As adsorption onto particulate matters in the river water was relatively small, suggesting that the two elements are conservative under river water oxic conditions (Eh > 200 mV). The distribution coefficients of Sb and As for river water and river sediments were within a factor of 4.7 under oxic conditions. In our model calculations, however, their distribution coefficients largely differed by more than a factor of 25 to 50 under reducing conditions (Eh < 90 mV). This could be extended to infer the origin of As in natural systems under oxic conditions based on the As/Sb ratio where Sb and As behave similarly.