Comparison of bismuth ferrites for chloride removal: Removal efficiency, stability, and structure

The chloride (Cl-) ions in wastewater are difficult to remove, since there is a lack of effective and recyclable chloride removal agents. In this study, the bismuth ferrites (BFO) are first explored as chloride removal agents, and their oxidation or precipitation effects on the Cl- removal are studied. In the BFO samples, the crystal phases of BiFeO3 and Bi2FeO4 have very limited contributions to the Cl- removal, while the enhanced removal ability is exhibited with the increase of Bi25FeO40. When the mass percentage of Bi25FeO40 reaches 70%, high Cl- removal efficiencies of 80-90% are achieved, which even exceeds 95% for pure Bi25FeO40. The morphology and structure characterizations reveal that the excellent Cl- removal performance of Bi25FeO40 is determined by its unstable chemical structure due to the sharing of Bi5+ and Fe3+ ions in the tetrahedral positions. The density of states and electron localization function of Bi25FeO40 demonstrate that its Fe and O bonding is weaker than those of BiFeO3 and Bi2FeO4, resulting in the easier breaking of the Fe3+/Bi5+-O bonds to release Bi5+ ions, which have oxidation ability for the formation of center dot OH and Cl center dot radicals to enhance the Cl- removal efficiency.