High-efficiency removal of fluoride ions in real fluorine-containing wastewater using red mud-metakaolin based geopolymer microspheres

Abstract

To address the problem of fluoride pollution and red mud accumulation, this study prepared red mud-metakaolin geopolymer microspheres (RM-MGMs) using dispersion-suspension-curing method, maximizing the utilizing of 60% red mud. The RM-MGMs prepared by this method exhibit excellent mechanical strength and the fluoride adsorption capacity, while simultaneously overcoming the difficulty in recovering traditional powder materials in actual wastewater treatment. The F⁻ removal performance of <75 μm and 75–150 μm RM-MGMs at the dosage of 1 g/L and the pH of 1 for the simulated fluoride-containing solution indicated that the residual F⁻ concentration met the World Health Organization (WHO) standard (F⁻ concentration < 1.5 mg/L) within 30 min and equilibrium was reached within 90 min. The results showed that RM-MGMs with smaller particle sizes had a faster adsorption rate and less intra-particle diffusion resistance, demonstrating a higher adsorption capacity. The adsorption kinetics and isotherm models of <75 μm and 75–150 μm RM-MGMs for F⁻ conformed to pseudo-second-order kinetic model and Langmuir model, with maximum equilibrium adsorption capacity (Q_max) of 175.15 mg/g and 172.25 mg/g respectively. The actual wastewater (with 190 mg/L F⁻ concentration) treated by RM-MGMs decreased to 9.58 mg/L within 1 min, meeting the national discharge standard (F⁻ concentration < 10 mg/L) and reaching the WHO standard within 45 min. In addition, RM-MGMs with the dosage of 1.50 g demonstrated outstanding dynamic separation performance at 1 mL/min flow rate. The excellent fixation effect of RM-MGMs for F⁻ demonstrates its eco-friendliness, providing a dual treatment scheme for addressing fluoride pollution and utilizing red mud.