Characterization of red mud granular adsorbent (RMGA) and its performance on phosphate removal from aqueous solution

Red mud granular adsorbent (RMGA), which was applied to remove phosphate from aqueous solution in this research, was manufactured with red mud (a solid waste from alumina industry) as the main raw material. Based on the combined adsorption–regeneration experiment, RMGA sintered at 1000 °C was selected for further characterization. Through the process of sintering, both the alkalinity and the surface of RMGA were ameliorated. X-ray diffraction analysis showed that the components with OH and SO4 were the functional groups for phosphate adsorption. The removal of phosphate by RMGA was weakly affected by the common coexisting ions in solution (such as Cl−, NO3- and SO42-), but strongly depended on pH and contact time. RMGA performed relatively better at pHi (the initial pH in solution) of 3.0–6.0, and equilibrium could be reached after 7 h. The kinetics studies presented that phosphate removal by RMGA mainly followed the pseudo second-order model. The initial phosphate removal rate was faster at lower pHi, since the electrostatic repulsion between RMGA and phosphate was enhanced as pH increased. The pH in solution rose during phosphate removal process, and the mechanism for phosphate removal was divided into two stages: the removal of phosphate within 1 h was mainly attributed to phosphate adsorption on RMGA; while that after 1 h was the combined effect of adsorption and precipitation, which resulted in the appearance of the maximum phosphate removal capacity (6.64 mg g−1) at pHi 5.00.

► Red mud is applied to prepare granular adsorbent (RMGA) for phosphate removal. ► Components with OH and SO4 are effective for phosphate adsorption on RMGA. ► Pseudo second-order model can describe the phosphate removal by RMGA well. ► The largest P removal capacity was obtained at pHi 5.00 after equilibrium. ► P removal includes two stages: adsorption and combined adsorption–precipitation.