Pore-forming mechanism of granular red mud by microwave activation and its application in organic dyes adsorption from aqueous solution

Red mud was mixed with starch and granulated, then roasted by microwave to form microwave-activated granular red mud (GRM) adsorbent. In this work, pore-forming mechanism of microwave-activated GRM and its effect for the removal of a cationic dye (methylene blue, MB) from aqueous solution were investigated. The results of dielectric properties of GRM and red mud in microwave field reveal that the addition of starch can effectively improve the microwave absorbing properties of the GRM sample. In addition, it is found that microwave-activated GRM adsorbent contains more pores and larger BET surface area than conventional-activated GRM adsorbent. The pore-forming mechanism of GRM by microwave activation is explained by the existence of temperature gradient and good absorbing properties of H2O vapor and starch. The adsorption experiment results show that the MB removal rate of the microwave-activated GRM can reach 82% within 15 min, while the conventional-activated GRM requires 180 min to reach the same MB removal effect when the initial MB concentration is 60 mg L-1. Langmuir model is found to represent the adsorption isotherm of MB on GRM better than Freundlich model. The adsorption capacity at 25 degrees C is 16.005 mg MB/1 g GRM. Thermodynamic parameters depicte spontaneous and endothermic nature of adsorption. The kinetics study shows that pseudo-second order model matches well the kinetic data.