Recovery and utilization of phosphorus in wastewater by magnetic Fe3O4/Zn-Al-Fe-La layered double hydroxides(LDHs)

Layered double hydroxides (LDHs) are the promising minerals which can adsorb phosphates, moreover, phosphorus-containing LDHs can be used for fertilizers. In this study, we tested the ability of magnetic Fe3O4/Zn-Al-Fe-La-LDH to remove and recover phosphate from simulated sewage. Results demonstrated that a magnetic Fe3O4/Zn-Al-Fe-La-LDH dose of 1.2 g/l could achieve a maximum adsorption capacity of 169.5 mg/g under an initial phosphate concentration of 200 mg/L, an equilibrium contact time of 24 h and a pH of 4. The adsorption kinetics of phosphate onto the magnetic Fe3O4/Zn-Al-Fe-La-LDH were well governed by the pseudo-second-order kinetic model, and the adsorption data fit well to the Langmuir isotherms. After four adsorption-desorption cycles, the LDH still had an adsorption capacity of 31 mg/g, demonstrating its reusability. Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) analysis indicated that ion exchange occurred between phosphate and the interlayer anions, and that phosphate was inserted into the double structure of the adsorbent. ICP-OES analysis showed that the LDH was stable and did not release the metal ions into water. The phosphorus-adsorbed LDH (P-LDH) has the potential to be used as a slow-release fertilizer to increase the growth of the soybeans Glycine max. The absorption of the metal into the soybeans was also reduced, which proved that the application of this type of LDH is non-toxic to plants.