Manganese Modified Activated Alumina through Impregnation for Enhanced Adsorption Capacity of Fluoride Ions

Activated alumina, with low cost to implement and consistent performance, has been widely used as a defluoridation adsorbent in contaminated groundwater. However, its application was hampered by an undesirable adsorption capacity. In the recent research, an innovative adsorbent (manganese modified activated alumina, MAA) was synthesized by impregnation method and showed a more significant adsorption capacity than that of freshly activated alumina. The scanning electron microscope, Brunauer, Emmett and Teller, X-ray diffraction spectroscopy, X-ray photoelectron spectroscopy, and Fourier transform infrared verified introduction of manganese oxides (MnOOH and MnO2) successfully, improvement of surface microstructure and nature of single-layer adsorption, which enhanced the adsorption ability. In a short period of adsorption, the MAA maximum capacity increased from 38% to 67%. Then, batch-scale kinetic and thermodynamic adsorption investigations were performed. The high correlation coefficients R2 (close to 1) of the quasi-second-order model, Langmuir kinetic model, and Langmuir isotherm model confirmed a better fit to the adsorption experimental data, further indicating that the diffusion of adsorbate was primarily governed by adsorption onto the active sites and the adsorption of fluoride on MAA was single-layer physical adsorption. This paper provides an approach to modifying activated alumina as a groundwater fluoride adsorbent.