Removal of copper and lead ions by red mud-metakaolin based geopolymer microspheres: Insights into the efficient preparation and adsorption mechanisms

This study used red mud and metakaolin as the main raw material, with sodium silicate as the alkali activator. Red mud-metakaolin based geopolymer microspheres (RM-MGMs) with good sphericity and high mechanical strength are prepared via dispersion-suspension-solidification method. By investigating the adsorption performance of RM-MGMs calcined at different temperatures for Cu²⁺ and Pb²⁺, the uncalcined RM-MGMs exhibit excellent heavy metal cation adsorption due to their abundant surface hydroxyl groups and high ion exchange capacity. The optimal adsorption conditions for Cu²⁺/Pb²⁺ were adsorbent dosages of 1.5/0.3 g/L, pH of 5.3-5.6/5.0, respectively. Cu²⁺ adsorption reached equilibrium at 960 min. Pb²⁺ showed faster adsorption rate due to its smaller hydrated ionic radius and reached equilibrium at 60 min. Kinetic results indicated that the pseudo-second-order model better describes the adsorption process of RM-MGMs for Cu²⁺/Pb²⁺. Adsorption isotherm models showed that increasing temperature favors the adsorption of Cu²⁺/Pb²⁺ by RM-MGMs. At 318 K, the adsorption capacities of RM-MGMs for Cu²⁺/Pb²⁺ reach 0.86/1.43 mmol/g, respectively. In competitive experiments, the removal rates of Cu²⁺/Pb²⁺ by RM-MGMs both exceed 80%. RM-MGMs achieve adsorption of Cu²⁺/Pb²⁺ through synergistic mechanisms including ion exchange, electrostatic attraction and complexation.