Synchronous Removal of Fluoride and Iodide on Fe–Mn Binary Oxides Interface: Competitive Adsorption Behaviors and Mechanism

The coexistence of high-concentration iodide (I^–) and fluoride (F^–) in high-iodine groundwater increases the occurrence of endemic diseases such as iodine-induced disorders and fluorosis, and their simultaneous removal has rarely been investigated. In this study, we developed cost-effective Fe/Mn binary oxides (FMBOs) with different Fe/Mn molar ratios (R_Fe:Mn) and investigated their performance and selective adsorption mechanisms for the synchronous removal of I^– and F^–. By optimizing the R_Fe:Mn ratios, an FMBO with R_Fe:Mn = 0.5:1 was developed to achieve synchronous removal of I^– and F^– with efficiencies of 67.7 and 80.7%, respectively, when the initial concentrations of I^– and F^– were 200 μg/L and 1.5 mg/L, and the FMBO dosage was 1.0 g/L. As the pH increases, the removal efficiency of I^– and F^– by FMBO decreases. According to the results of X-ray photoelectron spectroscopy (XPS), high-performance liquid chromatography–inductively coupled plasma mass spectrometry (HPLC-ICP/MS), and X-ray absorption spectroscopy results (XAS), FMBO is mainly composed of Fe(III), Mn(IV), and Mn(III). Mn oxide is mainly responsible for the heterogeneous oxidation of I^–, whereas Fe oxide dominates in the adsorption of I^– and F^–. Based on density functional theory (DFT) calculations, the adsorption of SO₄^2–, NO₃^–, and HCO₃^– was achieved via the formation of Fe–O bonds; moreover, the adsorption of I^– and F^– was attributed to the formation of Fe–I/F bonds. This study provides insight into the site-specific mechanism involved in I^– and F^– adsorption onto low-cost FMBO in realistic high-iodide groundwaters with complex coexisting anions.