Phase transformation of crystalline iron oxides and their adsorption abilities for Pb and Cd

• Iron oxides were prepared by calcination based on a facile sol–gel synthesis.
• Batch sorption experiments were conducted towards Pb2+ and Cd2+ adsorption.
• Iron oxide transformed from Fe3O4 to γ-Fe2O3 and finally to α-Fe2O3.
• Fe3O4 had a better adsorption behavior than that of (α + γ)-Fe2O3 and α-Fe2O3.
• The performance mechanisms were analysed by XRD, XPS and zeta potential measurements.

The current study developed a new perspective whether crystalline structures influence the adsorption of heavy metals on magnetic iron oxides synthesized by applying surfactant sodium dodecyl sulfate (SDS)-templated strategy. The products were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive analysis of X-ray (EDAX), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transformed infrared spectroscopy (FT-IR), and ζ-potential analyzer. Most SDS were removed after calcination and the phase of iron oxide transformed from amorphous Fe3O4 to (α + γ)-Fe2O3 and finally to highly crystalline α-Fe2O3, followed by decreasing surface area and pore volume with increasing calcining temperatures. Amorphous Fe3O4 exhibited a better adsorption capacity towards Pb (II) and Cd (II) than that of (α + γ)-Fe2O3 and α-Fe2O3, suggesting that metals adsorption by amorphous iron oxide was superior to that owns high degree of crystallization. The removal performances containing ion exchange, hydrolysis of heavy metals, electrostatic attraction and chemical binding or specific interaction were consistent with XPS and other measurements. After five adsorption–desorption cycles, all synthesized iron oxides still maintained above 79% removal efficiency for both Pb (II) and Cd (II). This study provides a promising potential for the application of iron oxides as adsorbents for heavy metals treatment in wastewater.

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