Removal of phosphate from aqueous solution by activated siderite ore: Preparation, performance and mechanism

Activated siderite ore (ASO) was prepared by annealing siderite ore at different temperatures for different durations. The performance of ASO on the removal of phosphate from aqueous solution and the effects of particle size, contact time, pH, initial phosphate concentration, temperature, and coexisting anions on phosphate removal were investigated. X-ray diffractometer, thermogravimetric analyzer (TGA), transmission electron microscopy (TEM), surface area analyzer, Fourier transformed infrared spectra and other techniques were utilized to characterize ASO before and after adsorption. The results showed that a large amount of CO2 was released meanwhile the siderite began to transform to hematite when the annealing temperature increased to 450 degrees C. The specific surface area of annealed product increased from 4.17 m(2)/g to 57.5 m(2)/g as the temperature came to 470 degrees C. Phosphate can be removed efficiently by ASO in a relatively wide pH range between 3.0 and 11.0. The effects of coexisting anions were not significant within the experimental concentration ranges except HCO3-. The adsorption process of phosphate on ASO fitted pseudo-second order kinetics model. ASO showed a high-efficient adsorption capacity of 9.24 mgig estimated from Langmuir isotherm at 30 degrees C. Results of TEM and FTIR analysis suggested that under the condition of pH=6.5 and phosphate concentration (100 mg/L), two different monoprotonated phosphate complexes exist at ASO, with one surface complex coordinated in a monodentate binuclear (bridging) fashion, and the other as a monodentate mononuclear complex. A small amount of co-precipitation is most likely to be another mechanism for the removal of phosphate. These fundamental data indicate that siderite is a promising precursor for preparing porous material at least for adsorption of phosphate. (C) 2017 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.