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

Abstract

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.