Fluoride adsorption on manganese carbonate: Ion-exchange based on the surface carbonate-like groups and hydroxyl groups

Abstract

Manganese carbonate (MnCO₃) nanowires and microcubes were controlled synthesized by a simple ethylene glycol (EG) mediated solution method. The volume ratios of EG and water has a decisive impact on the morphology of MnCO₃ products. With a decreased water volume, MnCO₃ products were transformed from microcubes to nanowires. The obtained MnCO₃ nanowires were characterized by X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, and nitrogen adsorption-desorption isotherm. The adsorption properties of the MnCO₃ products towards fluoride were investigated. The adsorption capacities of the nanowires were higher than that of the microtubes. According to the Langmuir model, the maximum adsorption capacity was 46.80mgg^-1 at pH 7.0. The adsorption capacity was 11.58mgg^-1 when the equilibrium fluoride concentration was just below the WHO guideline of 1.5mgL^-1. The kinetic data were well fitted to pseudo-second-order model. The fluoride removal was attributed to the ion-exchange based on the surface hydroxyl groups and carbonate-like groups, which was revealed by Fourier transform infrared absorption spectroscopy and X-ray photoelectron spectroscopy.

Keywords: Fluoride adsorption; Ion-exchange; Manganese carbonate; Surface carbonate-like groups; Surface hydroxyl groups.