The bioaccessible and labile contents of As(Ⅲ) and As(V) in four typical soils in China assessed by different methods

Soil bioaccessible and labile As were extracted using in vitro, diffusive gradients in thin films (DGT) and chemical extraction methods. The results showed that As(Ⅲ) was readily converted to As(V) in the soils, which was promoted by alkaline conditions and higher content of easily reducible Mn. As(V) was not readily reduced to As(Ⅲ) in these soils. The effect of soil pH on bioaccessible As content by in vitro method and labile As content by DGT method were inconsistent among different soils due to the coincidence of As(Ⅲ) and As(V), with As(Ⅲ) being more readily mobilized in acidic environment while the opposite was true for As(V). The labile As extracted by phosphate was significantly correlated with that extracted by in vitro method, while the labile As extracted by the DGT was correlated with that extracted by the CaCl₂ method. The labile As extracted by the DGT was much lower than that by the in vitro and phosphate methods. The in vitro and phosphate methods extracted As in soils which is tightly bound to Fe oxides through dissolution, complexation and ion exchange. In contrast, the DGT method relied on the adsorption of soil labile As onto ZrO-based binding layer. The higher contents of soil Fe oxides and greater adsorption capacity for As led to the lower content of As measured by the DGT method. In conclusion, the in vitro and phosphate extraction are potential to be used to predict soil bioaccessible As. The DGT method is more suitable for ecological risk prediction.