Influence of sintering temperature on orthophosphate and pyrophosphate removal behaviors of red mud granular adsorbents (RMGA)

In this research, red mud granular adsorbents (RMGA) were manufactured under different sintering temperatures (ST), using red mud as the main raw material. These RMGA were applied to remove orthophosphate and pyrophosphate from aqueous solution, and the different phosphate removal behaviors were investigated comparatively. The increase of ST could lead to surface area increasing and effective components decreasing on RMGA simultaneously, and the combined effect on RMGA for phosphates removal varied with the initial pH (pHi) in solution, because pH influenced the stability of RMGA structure to some extent. The competitive interaction of adsorption and precipitation implied the mechanism for phosphates removal in this research: the strong relationship between orthophosphate removal capacity and Ca2+ concentration in solution indicated that precipitation affected orthophosphate removal greatly, while adsorption was the dominant reaction for pyrophosphate removal. When orthophosphate and pyrophosphate coexisted in solutions, precipitation of orthophosphate was weakened due to complexation of metallic ions, and pyrophosphate was comparatively easily adsorbed because of the advantage on electrostatic attraction. The mechanism of phosphates adsorption on RMGA can be described as electrostatic attraction followed by ligand exchange, and some effective sites on RMGA could capture orthophosphate or pyrophosphate selectively.