Ferrihydrite sulfidation transformation and coupled As(V) and Cd(II) mobilization under anoxic conditions

Ferrihydrite sulfidation is an important process influencing the environmental behavior of co-existent arsenate (As(V)) and cadmium (Cd(II)) pollutants in mining-impacted environments. However, the mineral evolution of ferrihydrite and the coupled mobilization behavior of co-existent As(V) and Cd(II) remain unclear. In this study, we have investigated As(V)-Cd(II)-bearing ferrihydrite conversion behavior induced by environmentally relevant concentrations of S(-II) (1 and 5 mM). PXRD, HR-TEM, and XAS results demonstrate that the co-existent As(V) and Cd(II) inhibit the conversion of ferrihydrite to secondary lepidocrocite (γ-FeO(OH)) and subsequently to goethite (α-FeO(OH)) at different S(-II) concentrations. Elevated As(V) and Cd(II) levels promote the formation of amorphous mackinawite (FeS) and pyrite (FeS₂). Lepidocrocite and greenockite (CdS) are the predominant secondary phases at 1 mM S(-II) but lepidocrocite and pyrite are dominant at 5 mM S(-II) when the As(V) and Cd(II) levels are low. These sulfidation transformation pathways reduce the mobilization of the co-existent As(V) and Cd(II). Cs-TEM and chemical extraction results reveal that substantial portions of Cd(II) and As(V) are incorporated into secondary pyrite and lepidocrocite, in addition to surface adsorption and greenockite precipitation. These findings not only enhance our understanding of the geochemical cycling of Fe(III), As(V), and Cd(II) in natural anoxic sulfidic environments but also may provide guidelines for developing effective remediation methods for As-Cd co-contaminated settings.