Efficient preparation of red mud-based geopolymer microspheres (RM@GMs) and adsorption of fluoride ions in wastewater.

In this paper, red mud-based geopolymer microspheres (RM@GMs: 75-150 µm) was prepared by dispersion-suspension-solidification method to remove fluoride ions (F). It was found that RM@GMs still had good mechanical properties and better F removal effect at RM content reached 80 % of the total solid mass. The batch adsorption experiment results showed that the F concentration (< 1.5 mg/L) reached the drinking water standard in 45 min at pH = 2 and RM@GMs dosage was 1 g/L. RM@GMs showed maximum adsorption capacity of 76.57 mg/g for F, and the adsorption kinetics and isotherm fitted the pseudo-second-order kinetic and Langmuir isotherm model, respectively. RM@GMs exhibited excellent dynamic separation effect at the flow rate of 4 mL/min and column height of 1 cm. In addition, RM@GMs had good selectivity for F in the competitive adsorption experiments and followed an order of: PO > > SO ≈ NO ≈ Cl. In real seawater, natural surface water and tap water, RM@GMs still had excellent F removal effect. The adsorption mechanism revealed that RM@GMs removed F mainly through the synergistic effect of adsorption and ion exchange. Therefore, this paper provides the potential value for the large-scale utilization of RM in the application of F-containing wastewater.