Synchronous Moderate Oxidation and Adsorption on the Surface of γ-MnO₂ for Efficient Iodide Removal from Water

Long-term exposure to excessive iodine via drinking water presents health risks. Moderate oxidation of iodide (I^-) to iodine (I₂) has a better iodine removal effect than excessive oxidation to iodate (IO₃^-). This study combines computational and experimental methods to construct a heterogeneous interface with synchronous I^- moderate oxidation and I₂ adsorption to increase the total iodine removal. Compared to other forms of crystal manganese dioxide (MnO₂), theoretical calculations predict that MnO₂ with a γ-crystal structure has the lowest adsorption energy, that is, -1.20 eV, and a slight overlap between the conduction and valence bands, which favors electron transfer between I^- and Mn(IV) and I₂ adsorption. Thus, γ-type MnO₂ was designed by adjusting the precursor Mn sources and hydrothermal reaction conditions. The liquid chromatography-inductively coupled plasma-mass spectrometry and high-performance liquid chromatography confirmed that the total iodine concentration in water decreased from 173.7 to 36.3 μg/L after 2 h, with 200 mg/L γ-MnO₂ dosage lower than the national standard of 0.1 mg/L. A minute proportion of I^- in water was converted to IO₃^- (approximately 1.1 μg/L). The current I^- adsorbent performed better than previously reported ones. During iodine removal, most of the I^- migrated from water to the surface of γ-MnO₂, and the ratio of I^- to I₂ was determined to be 1:0.6 by X-ray photoelectron spectroscopy. This study evaluates iodine species transformation and an optimum strategy for heterogeneous interface design; it is promising for treating high-iodine groundwater.