Magnetic and structural characteristics associated with the transformation of As(v)-coprecipitated ferrihydrite to hematite: implications for magnetic enhancement in soils and sediments

The transformation of metastable ferrihydrite to stable hematite has been linked to magnetic enhancement in soils and sediments. However, the magnetic and structural characteristics associated with the transformation of arsenate (As(V))-coprecipitated ferrihydrite to hematite remain unclear. Herein, we have investigated the structural and magnetic characteristics of fresh and aged As(V)-ferrihydrite coprecipitates. Magnetic characterization indicates that the high As(V) loading weakens the ferrimagnetic susceptibility of fresh Fe-As(V) coprecipitates by forming Fe-AsO4 complexes. In the low As(V)-loading system (Fe/As(V) >= 40), the increase in As(V) concentrations accelerates the growth of hematite particles with an expanded unit cell parameter c, and enhances magnetic susceptibility by changing both the surficial and internal lattice arrangements of hematite crystallites. In contrast, in the high As(V)-loading system (Fe/As(V) <= 20), the transformation of ferrihydrite is greatly retarded and the formation of poorly crystalline ferrimagnetic maghemite significantly enhances the magnetic susceptibility in the transformation products. Our findings provide new insights into the enhancement of magnetic properties of iron (oxyhydr)oxides in high arsenic soils and sediments, which are also essential for developing effective immobilization and long-term storage strategies for arsenic using iron (oxyhydr)oxides.