Residence time effects on arsenate sorption on an allophanic Andosol

The mobility and bioavailability of As in agricultural soils are controlled by sorption onto minerals. The effect of the residence time on the reaction between arsenate and allophanic Andosol, a typical agricultural soil in Japan, remains unclear. Therefore, we investigated the reaction between arsenate and allophanic Andosol and its components allophane and ferrihydrite over several months, based on the desorption experiment and X-ray diffraction (XRD) measurements in the long-term sorption experiment. The reaction between arsenate and three sorbents continued for approximately 150 d. The XRD patterns of allophane and ferrihydrite that reacted with arsenate did not change during the long-term sorption experiments under near-neutral pH conditions, suggesting negligible formation of surface precipitation. Furthermore, the irreversibility of arsenate sorption onto allophanic Andosol and allophane increased with extended residence time. These results suggested that the slow sorption of arsenate onto the materials could be ascribed to internal diffusion.