The effect of the ferrihydrite dissolution/transformation process on mobility of arsenic in soils:investigated by coupling a two-step sequential extraction with the diffusive gradient in the thin films (DGT) technique

Zhang, T. and Zeng, X. and Zhang, H. and Lin, Q. and Su, S. and Wang, Y. and Bai, L. and Wu, C. (2019) The effect of the ferrihydrite dissolution/transformation process on mobility of arsenic in soils:investigated by coupling a two-step sequential extraction with the diffusive gradient in the thin films (DGT) technique. Geoderma, 352. pp. 22-32. ISSN 0016-7061

Abstract

Ferrihydrite has been prevalently used in soil remediation as an effective amendment for in situ immobilization of arsenic (As). However, its poorly crystalline structure is unstable and can pose the risk of re-releasing As into soil. In this study, sequential extraction was coupled with a newly-established method, chelex-metsorb diffusive gradient in thin films (DGT), to study the ferrihydrite transformation/dissolution process and its effect on the mobility of arsenic in soil. Experiments in this work found that high soil moisture (70% SWHC) with a low soil redox potential (Eh) can significantly increase the rate of the ferrihydrite transformation/dissolution process compared to 30% SWHC. Soils with low pH and high available iron (Fe) content may also accelerate ferrihydrite transformation/dissolution, while soils with high clay fraction and high soil total organic matter (STOM) may inhibit the process. The amount of arsenic adsorption can also affect ferrihydrite transformation, even exceeding the effect of soil pH and dissolved Fe. Arsenic release was clearly observed in all three soils across all treatments, and it was also affected by changes in soil redox potential. More arsenic was released at high soil moisture (70% SWHC) roughly 7–15 d after the release of Fe. In addition, a partial arsenic fraction was transformed, along with ferrihydrite, from a combined As (F1–As) amorphous phase to combined As (F2–As) well-crystallized phase. These results suggested that ferrihydrite transformation/dissolution can affect the mobility of arsenic and that this phenomenon is more extreme at higher soil moisture levels.