Environmental Behavior of Coexisting Arsenite and Cadmium during Fe(II)-Induced Ferrihydrite Transformation under Anoxic and Circumneutral Conditions

Despite extensive studies on the environmental cycling of arsenite (As(III)) and cadmium (Cd(II)) in both engineered and natural settings, such as contaminated soils and sediments, the behavior of coexisting As(III) and Cd(II) during Fe(II)-induced ferrihydrite transformation remains unclear. Herein, we have investigated the mechanism controlling As(III) and Cd(II) mobilization during Fe(II)-induced ferrihydrite transformation at environmentally contaminated metal(loid) concentrations and pH values under anoxic conditions in both single and binary metal(loid) systems. Powder X-ray diffraction (XRD), Raman, and transmission electron microscopy (TEM) results demonstrate that goethite is the dominant secondary phase for the metal(loid)-free and single Cd(II) treatments, whereas goethite and lepidocrocite are the primary secondary phases for the single As(III) and binary metal(loid) treatments. The transformation of ferrihydrite decreases as the concentrations of As(III) and Cd(II) increase. The coexisting Cd(II) inhibits the oxidation of As(III) to As(V) during Fe(II)-induced ferrihydrite transformation. Chemical analysis reveals that surface adsorption and structural incorporation are the predominant mechanisms for As (III/V) and Cd(II) retention. Additionally, the presence of As(III) significantly enhances Cd(II) mobilization by restraining ferrihydrite transformation, whereas Cd(II) has negligible effects on the mobilization of As(III) due to the formation of As(III/V)–Cd(II)–Fe ternary complexes. This study can bridge the knowledge gap regarding the interaction between As(III) and Cd(II) in contaminated environments, thereby contributing to a deeper understanding of the fate of As(III) and Cd(II) under anoxic conditions.