Mechanisms of antimony ageing in soils: An XAS study

Antimony (Sb) retention in soils increases in the long term, resulting in lower soluble Sb concentrations and thus a decreased environmental risk of Sb. However, the mechanisms responsible for Sb ageing in soils are not clear, although this is necessary to understand, describe and predict Sb ageing processes in soils. To do so, the long-term retention of added Sb(V) to nine soil samples with contrasting soil properties and to a model iron (Fe; hydrous ferric oxide or HFO) and aluminium (Al; hydrous aluminium oxide or HAO) hydroxide under aerobic conditions was studied with X-ray absorption spectroscopy (XAS). The solid-liquid distribution coefficients (KD) of the soils increased by a factor of 2–4 between 2 weeks and 6 months. In addition, ageing of Sb was more pronounced in the Sb(V)-spiked HFO than in the soils, while no ageing of Sb(V) in the HAO was noted. The added Sb(V) did not change redox speciation throughout six months of aerobic incubation and no indications of antimonate precipitates with Fe or calcium (Ca) were found. Conversely, it was found that with time, Sb(V) was increasingly retained by the HFO and by the soils, likely through micropore diffusion and/or through substitution of Sb(V) into surface Fe octahedra in ferrihydrite. In addition, in the soils, the Sb(V) surface speciation on Fe hydroxides changed with time leading to higher proportion of edge-sharing bidentate Sb⋯Fe complexes. These results suggest that Sb(V) ageing in soils is controlled by different mechanisms simultaneously, i.e., by slow micropore diffusion and/or substitution processes that remove Sb(V) from the reactive surface of the Fe hydroxides and by slow surface processes, with slow sorption reactions more prevalent for edge-sharing bidentate Sb(V) than for corner-sharing bidentate Sb(V) surface complexes.