[Experiment on Recovery of Phosphorus from Aqueous Solution by Calcium Doped Fe₃O₄]

Adsorption is an economical and effective method for recovering phosphate from wastewater. In order to improve the adsorption capacity of Fe₃O₄ for phosphate and for easy separation from water under the action of an external magnetic field, CaO₂ was used in this study as an oxidant to partially oxidize Fe²⁺. A phosphorus recovery adsorbent, Ca doped Fe₃O₄ (CMIO), was prepared and was characterized by X-ray diffraction (XRD), X-ray fluorescence (XRF) and vibrating sample magnetometer (VSM) techniques. The results showed that CMIO had a Ca²⁺ doped Fe₃O₄ crystal structure with a saturation magnetization of 38.82 emu·g^-1, which was easily separated from water by using an external magnetic field. The phosphorus adsorption capacity of the CMIO decreased with an increase of pH value. When pH=2 and T=25℃, the maximum adsorption capacity was 24.10 mg·g^-1, which is almost five times the adsorption capacity of pure Fe₃O₄. The phosphorus adsorption of CMIO was in accord with the Langmuir isotherm adsorption model, and the adsorption process followed the pseudo-second order kinetic model. The complexation of phosphate occurred on the inner surface of the CMIO to form a ≡Fe-Ca-P ternary complex, which can adsorb phosphorus. Compared with other anions in the aqueous solution, CMIO had good adsorption selectivity to PO₄^3-, and the adsorbed PO₄^3- could be desorbed by NaOH solution.The quality loss of the CMIO was less than 4% once, and multiple recycling was possible.