Removal of selenium compound, especially hexavalent selenate ion, is one of the most difficult water treatment techniques and an effective method has not yet been established. In this research we propose a novel selenium removal process called Recycled Green rust/Ferrite Process, composed of an addition of Fe source, an alkaline conditioning of a concentrated slurry, an anaerobic chemical reaction stage, and a return of the settled slurry after solid-liquid separation. A mixed Fe2+-Fe3+ hydroxide compound, green rust (GR), reduces selenate to elemental selenium, while GR is consequently oxidized to magnetite Fe3O4. Therefore the black-colored magnetic slurry containing GR and magnetite contributes to the removal of selenate. Repeated batch-mode selenium removal tests showed that the residual selenium concentration gradually decreased with the number of batch test increased. Addition of ferrous sulfate or ferrous chloride (resulting Fe2+ concentration 600 mg/L) was eligible for Fe source for GR formation. Stable selenium removal was achieved when the number of batch test exceed 15 times. Optimum selenium removal ratio reached 99.95% where initial Se concentration 2.1mg/L decreased to 0.001mg/L. In contrast, little Se removal occurred when the combination of ferrous sulfate and elemental iron powder (each Fe2+ and Fe0 concentration was 300 mg/L) was used. During the reaction, oxidation-reduction potential leveled off -700mV vs Ag/AgCl and no dissolved oxygen was detected, indicating a strong reducing condition suitable for the reduction of selenate. Ninety percent of selenium disappeared as soon as the GR formed, and then selenate removal rate followed a first-order kinetics depending on the slurry concentration. These experimental results indicated that the proposed process served an advanced treatment technique for selenate-contaminated water.
