Effects of Al Content and Synthesis Temperature on Al-Substituted Fe Oxides

Abstract

Four series of Al-substituted Fe oxides with different Al contents were synthesized under different temperatures and characterized to study the effects of Al substitution on the crystal properties and bioaccessibility of Fe oxides. The UV-visible diffuse-reflectance spectra and their first derivatives showed that color changes from red to white were associated with the visible absorbance of Fe oxides and Al oxides. The X-ray diffraction results showed that the presence of Al in Fe oxides could hinder the formation of maghemite and hematite in addition to the crystal transformation from maghemite to hematite during sintering. The Brunauer-Emmett-Teller surface areas of Al-substituted Fe oxides increased with increasing Al content and with decreasing sintering temperatures. Fourier transform infrared spectra results showed that gibbsite formed with high Al contents. The crystal structure, crystallinity, and the surface area of Al-substituted Fe oxides heavily depended on the Al content and the sintering temperature. The effects of Al on the hydroxylamine-reducible Fe(III) of Al-substituted Fe oxides were also investigated. The results showed that the hydroxylamine-reducible Fe(III) of Al-substituted Fe oxides increased with increasing Al content for the Fe oxides with a high degree of crystallization when synthesized at higher temperatures (>300°C) but decreased with increasing Al content for Fe oxides with a low sintering temperature (180°C). The effect of the Al content on hydroxylamine-reducible Fe(III) was attributable to the crystal structure, crystallinity, and the properties of surface structures—all of which can provide detailed information needed to understand the effect of Al on the iron reactivity of natural soils or sediments.