Adsorption of orthophosphoric, pyrophosphoric, and tripolyphosphoric acids from aqueous solutions by calcined gibbsite.

In this research, gibbsite (GB) samples calcined at 200-1000°C (GB200-GB1000) were produced. These GBs were used to adsorb orthophosphoric, pyrophosphoric, and tripolyphosphoric acids from aqueous solutions. The properties (amounts of hydroxyl groups, specific surface areas, mean pore diameters, and solution pHs) of the GBs were investigated, and their adsorption capacities for phosphoric acids evaluated. The amount of hydroxyl groups (0.46 mmol/g) and specific surface area (295.3 m(2)/g) of GB400 were greater than those of the other GBs. The mechanism of phosphoric acid adsorption on the GBs was related to the amount of hydroxyl groups and specific surface area. The optimal pH for phosphoric acid adsorption by GBs was 2.0-3.0. Equilibrium adsorption was reached within 24 h. The adsorption processes followed a pseudo-second-order kinetic model (correlation coefficient, 0.998-0.999). The adsorption capacity increased with increasing temperature. The adsorption isotherm data fitted the Langmuir (correlation coefficient: 0.921-0.992) and Freundlich (correlation coefficient: 0.948-0.997) equations well. Our results will be useful when developing methods for the adsorption of phosphoric acids from aqueous solutions.