Mechanistic insights into effect of storage conditions of Fe-Mn co-oxide filter media on their catalytic properties in ammonium-nitrogen and manganese oxidative removal

Iron-manganese (Fe-Mn) co-oxide filter media are generally used to treat ammonium-nitrogen (NH4-N)- and manganese (Mn2+)-contaminated groundwater by catalytic oxidation. In this study, two storage method types, dry- and wet-storage, were used to preserve the filter media for five storage periods (10, 20, 40, 70, and 100 d). The morphology, structure, and elemental state of the filter media under different storage modes were analyzed. The scanning electron microscopy results indicated that the folds on the surface of the wet-storage filter media were smaller and tighter than those on the dry-storage filter media surface, whereas the X-ray diffraction patterns indicated that dry-storage could lead to a larger crystal size and/or improved Fe-Mn co-oxide film layer ordering than those of the media under wet-storage. The catalytic activity evaluation for NH4+-N and Mn2+ oxidation revealed that it was more difficult to recover catalytic activity after long-term storage of the filter media under the dry conditions than that under the wet conditions. The recovery time of the Mn2+ removal activity was much shorter than that of the NI-(+)(4)-N. The change in the catalytic activity of the filter media was closely related to the change in the structural water content in the oxide film. The longer the preservation time, the lower the interlayer water content in the oxide film. Based on the results of the preservation method influence on the catalytic activity, the role of structural water in the NI-(+)(4)-N and Mn2+ removal process was elucidated, which promotes the use of the Fe-Mn co-oxide filter media.