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

Abstract

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 SO 4 2− , NO 3 − , and HCO 3 − 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.