Kinetic, equilibrium and thermodynamic studies for phosphate adsorption to magnetic iron oxide nanoparticles

• Phosphate sorption to magnetic iron oxide nanoparticles had a maximum sorption capacity of 5.03 mg P g−1.
• Phosphate sorption was relatively constant at an acidic solution pH.
• Phosphate sorption to magnetic nanoparticles showed endothermic nature of sorption process.
• Magnetic nanoparticles could be used as adsorbents for phosphate removal with regeneration and repeated use.

Phosphate (P) removal by magnetic iron oxide nanoparticles was investigated using kinetic, equilibrium and thermodynamic experiments. The results demonstrate that phosphate sorption to the magnetic nanoparticles reached equilibrium at 24 h with the maximum sorption capacity of 5.03 mg P g−1 under given experimental conditions (initial P concentration range = 2–20 mg P L−1; adsorbent dose = 0.6 g L−1; reaction time = 24 h). The phosphate removal was relatively constant at an acidic solution pH (3.0–3.1 mg P g−1 at pH 2.0–6.0), whereas the phosphate removal decreased sharply as the solution pH approached a highly alkaline condition (0.33 mg P g−1 at pH 11.1). Thermodynamic tests indicate that phosphate sorption to the magnetic nanoparticles increased with increasing temperature from 15 to 45 °C, indicating the spontaneous and endothermic nature of sorption process (ΔH0 = 39.17 kJ mol−1; ΔS0 = 156.35 J K−1 mol−1; ΔG0 = −5.88 ∼ −10.57 kJ mol−1). The results indicate that the pseudo second-order model was most suitable for describing the kinetic data. Regarding the equilibrium data, the Freundlich and Redlich–Peterson isotherms were fitted well. This study demonstrates that magnetic iron oxide nanoparticles could be used for phosphate removal from aqueous solutions with regeneration and repeated use.