Arsenic speciation transformation in soils with high geological background: New insights from the governing role of Fe

In soils, the speciation transformation of As were inherently related to the behaviors of iron (oxyhydr) oxides. It is poorly understood that the effects of the transformation of iron (oxyhydr) oxides coupled with As speciation transformation during dissimilatory Fe(III) reduction (DIR) involving with humic substances (HS) as electron donor or shuttle in soils with high arsenic geological background. In this study, the relationships between the transformation of iron (oxyhydr)oxides and As speciation transformation were investigated according to the response between continuously As speciation monitoring and iron (oxyhydr) oxides identification during DIR in the soils. The results showed that F4 (arsenic incorporated with amorphous iron (oxyhydr)oxides including ferrihydrite and schwertmannite) and F5 (arsenic incorporated with crystalline iron (oxyhydr)oxides including hematite and magnetite) were the main source and sink for As(III)Dissolved during DIR. During the incubation period, Fe(II) was the dominant driving force for the reduction of As(V) in the water-soil system. The XRD analysis indicated the changes of iron oxides such as ferrihydrite, schwertmannite, hematite and magnetite were closely related to the release and reduction of As, and those iron oxides could play governing roles for As speciation transformation during DIR in soils. Different from the known mechanism in low As concentrations, a limiting effect of As concentration on iron oxides transformation was found in our incubation experiments using soils with high As geological background (∼1000 mg/kg). This work provides new insights for Fe as governing role in As speciation transformation in soils with high arsenic geological background by firstly identifying the corresponding iron (oxyhydr)oxides in operationally defined arsenic speciation incorporated with iron oxides.