Adsorption of phosphate onto amine functionalized nano-sized magnetic polymer adsorbents: mechanism and magnetic effects

Abstract

A series of tetraethylenepentamine-functionalized core–shell structured nano magnetic Fe₃O₄ polymers (TEPA-Fe₃O₄-NMPs) with different amounts of the magnetic core were synthesized and characterized by XRD, EA, VSM, FTIR and XPS. Their applications as adsorbents for phosphate removal from aqueous solutions were studied. The adsorption mechanism and magnetic effects were intensively investigated. The adsorption processes of phosphate by TEPA-Fe₃O₄-NMPs were found to be highly pH dependent and related to the content of Fe₃O₄ magnetic core in the adsorbents. The optimized pH value was found to be 3.0 for TEPA-Fe₃O₄-NMPs, while that of TEPA-Fe₃O₄-NMPs-0, which was without the Fe₃O₄ magnetic core, was found to be 2.5. Kinetic studies showed that the adsorption of phosphate by TEPA-Fe₃O₄-NMPs followed a pseudo-second-order model, with the adsorption rate constant, k₂, between 0.00274–0.0241 g mg⁻¹ min⁻¹, suggesting a chemisorption process. Activation energies (E_a) for phosphate removal varied with the content of Fe₃O₄ magnetic core, and were found to be 38.9–16.5 kJ mol⁻¹, indicating that the diffusion process might be the rate-controlling step. Thermodynamic studies suggested that the adsorption processes fit the Langmuir isotherm well with the optimized maximum adsorption capacities of phosphate onto TEPA-Fe₃O₄-NMPs obtained when the content of Fe₃O₄ in TEPA-Fe₃O₄-NMPs was 14.55%. The Langmuir constants of apparent heat change, K_L, were found to be 0.0142–0.0461 L mg⁻¹, and varied with the content of Fe₃O₄ magnetic core as well. FTIR and XPS analytical results of the adsorbents before and after phosphate adsorption suggested that phosphate had been successfully adsorbed onto TEPA-Fe₃O₄-NMPs via electrostatic attraction. The existence of the magnetic core might be favorable for mass transfer to accelerate the adsorption process.