Removal and recovery of phosphate from water by a magnetic Fe3O4@ASC adsorbent

The removal and recovery of phosphate from water by Fe3O4@alkali-treated calcium-silicate composite (Fe3O4@ASC) was investigated. The adsorbent was characterized by XRF, BET, XRD and zeta-potential analyses. The characterization results showed the successful synthesis of Fe3O4@ASC with a high specific surface area (129 m2/g). In batch experiments, the kinetic data and isotherm data fitted well to the pseudo-second-order model and Langmuir model, respectively, and the maximum adsorption capacity calculated by Langmuir model was 128 mg/g. The phosphate adsorption of Fe3O4@ASC performed well over a wide pH range from 2.5 to 13 and exhibited a good selective property even with 10 times higher molar concentration of other anions. Fe3O4@ASC could remove nearly 100% phosphate from real lake solution with phosphate concentration of 10 mg/L at the dosage of 0.25 g/L. In the column experiment, the breakthrough point of Fe3O4@ASC column was nearly 6000 mL for the initial phosphate concentration of 18.02 mg/L. Experimental data showed a good agreement with Yoon and Nelson model and Thomas model, the saturation adsorption capacity was 92 mg/g. Phosphate fractionation, XRD, and FTIR spectra analysis indicated that Ca-P precipitation generated during the phosphate adsorption process. Besides, the adsorbed phosphate could be successfully recovered by 2% citric acid solution, which implied that recovered phosphate could be reused as fertilizer.