Phosphate removal from aqueous solution using iron oxides: Adsorption, desorption and regeneration characteristics

Dynamics of phosphate (PO₄^3-) adsorption, desorption and regeneration characteristics of three lab-synthesized iron oxides, ferrihydrite (F), goethite (G), and magnetite (M) were evaluated in this study. Batch experiments were conducted to evaluate the impact of several adsorption parameters including adsorbent dosage, reaction time, temperature, pH, and ionic strength. The results showed that PO₄^3- adsorption increased with reaction time and temperature while it decreased with an increase in solution pH. Adsorption isotherm data exhibited good agreement with the Freundlich and Langmuir model with maximum monolayer adsorption capacities of 66.6 mg·g^-1 (F), 57.8 mg·g^-1 (M), and 50.5 mg·g^-1 (G). A thermodynamics evaluation produced ΔG < 0, ΔH > 0, and ΔS > 0, demonstrating that PO₄^3- adsorption onto tested minerals is endothermic, spontaneous, and disordered. The PO₄^3- removal mostly occurred via electrostatic attraction between the sorbate and sorbent surfaces. Moreover, the PO₄^3- sorption was reversible and could be desorbed at varying rates in both neutral and alkaline environments. The good desorption capacity has practical benefits for potential regeneration and re-use of the saturated particles in wastewater treatment systems.