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.