Preparation of alumina-iron oxide compounds by gel evaporation method and its simultaneous uptake properties for Ni²⁺, NH₄⁺ and H₂PO₄^-

Fe₂O₃/Al₂O₃ powders with a range of Fe/Al compositions were prepared by a gel evaporation method to investigate the effect of alumina on the product phases, magnetic properties and simultaneous adsorption of Ni²⁺ (a model heavy metal cation), NH₄⁺ (a model eutrophication-related cation) and H₂PO₄^- (a model harmful anion). Precursor gels were prepared by dissolving Fe(NO₃)₃·9H₂O and Al(NO₃)₃·9H₂O in ethylene glycol, evaporating to dryness, grinding and heating at 300-1000 °C for 5 h. The crystalline products were γ-Fe₂O₃ (maghemite), formed at 300-600 °C, or α-Fe₂O₃ (hematite) and AlFeO₃, formed >600 °C. The temperatures of the phase change from γ-Fe₂O₃ to α-Fe₂O₃ increased with increasing alumina additions. The resulting lattice parameters suggest that Al³⁺ is incorporated into these phases up to about 15 mol.% at 300 °C, falling to 11 mol.% in the γ-Fe₂O₃ formed at 600 °C. The α-Fe₂O₃ formed at 700 °C contained 6 mol.% Al, increasing to 14 mol.% at 1000 °C. The magnetic properties of the samples were measured using a vibrating sample magnetometer. The saturation magnetization values of the γ-Fe₂O₃-containing samples increased with the addition of alumina to a maximum value of 61 emu/g in the sample containing 95 mol.% Fe₂O₃ heated at 400 °C. The simultaneous adsorption of Ni²⁺, NH₄⁺ and H₂PO₄^- from water was investigated by a batch method. The highest adsorption values were found for the sample containing 80 mol.% Fe₂O₃ heated at 600 °C, which contained both γ-Fe₂O₃ and α-Fe₂O₃. It was therefore concluded that the addition of alumina to iron oxide affects the crystalline phases and phase changes, and enhances the simultaneous cation and anion uptake ability of the materials.