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 (RFe:Mn) and investigated their performance and selective adsorption mechanisms for the synchronous removal of I– and F–. By optimizing the RFe:Mn ratios, an FMBO with RFe: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 SO42–, 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.