79Se, one of the key radionuclides for nuclear waste disposal, threatens the quality of the environment, as well as human health. Therefore, it needs to be permanently isolated from the biosphere. The aim of the study was to investigate the effects of Fe(II)/Fe(III) on the removal of 79Se using bentonite in the pH range of 2.0–10.0 under oxic/anoxic conditions. Under oxic conditions, Se(IV) prefers to form inner-sphere complexes with Fe(III)-oxyhydroxide, derived from the oxidization of Fe(II) using oxygen. Interestingly, Se(IV) will interact with Fe(III) and form a poorly soluble ferric selenite at pH ∼4 under oxic conditions. Under anoxic conditions, however, the concentration of Fe(II) is closely related to the sorption process of Se(IV) on bentonite. When the concentration of Fe(II) was less than 1%, Fe(II) combined with the hydroxyl, forming Fe(OH)2, which generated a disproportionation at pH ∼8 and formed a new sorbent, Fe3O4. However, when the concentration of Fe(II) was increased to 5%, reduction precipitation was the primary way to remove Se(IV) in aqueous solution. XANES (X-ray Absorption Near Edge Structure) spectra showed that higher pH values are beneficial for the formation of the final thermodynamic reduction product, Fe selenide. These results suggested that Fe(II) significantly affect the Se(IV) sorption. Overall, this study confirmed the significant role of Fe(II) on the retardation of 79Se and on remediation for Se(IV) contamination in the hydrosphere. Display omitted •Fe(II) species could greatly enhance the sorption efficiency of Se(IV) via bentonite.•Formation of ferric selenite contributes to the sorption under aerobic condition.•Under anoxic conditions, Se(IV) can be reduced by a two-step sorption/reduction mechanism.•Fe selenide is the finally thermodynamic reduction product of Se(IV).•Fe(II) plays an important role on the remediation of Se(IV) contamination in groundwater. The significant role of Fe(II) on retardation of 79Se via bentonite and on remediation of Se(IV) contamination in groundwater was demonstrated.