Effects of adsorbed inorganic anions on the magnetic properties of calcination-prepared porous maghemite

Porous maghemite (PM) was prepared by two-step calcinations. The crystallographic and magnetic changes of PM upon SO42− and CO32− adsorption were investigated. The crystallite size (~ 27 nm) and PM texture remain unchanged after adsorption. Fourier transform infrared spectra showed that SO42− and CO32− are adsorbed on the PM mainly through outer- and inner-sphere surface complexations, respectively. X-ray photoelectron spectra showed the increase and decrease of binding energy for Fe 2p3/2 electrons after SO42− and CO32− adsorption, respectively. Sulfate adsorption increases the magnetic susceptibility (χ) of PM by ~ 4.5%, and decreases the saturation magnetization (Ms) of PM by 24.6%. By comparison, carbonate adsorption decreases χ by ~ 3.7% and increases Ms by 17.8%. The magnetic changes upon SO42− and CO32− adsorption are attributed both to the electronegativity of anions and the size distribution of PM.