Iron-impregnated nanosized silica and alumina: Implications for contaminant transformation
Aqueous iron and prevailing mineral oxides such as silicon and alumina interact to create a vast amount of iron-enriched surfaces with considerable surface adsorption capacity and redox activity in the natural environment. Considering the abundance and benign nature of these oxides and also the significant role of iron in both adsorption and redox transformation, iron-immobilized silica and alumina are of great interest in the field of recalcitrant contaminant transformation. In this study, nanoscale silica aerogel and alumina aerogel which act as proxies of colloidal silica and alumina solid phase were impregnated with low concentrations of iron by immersing them into dilute aqueous solutions of Fe(II) or Fe(III) ions. The resultant solid phases were investigated for catalytic activity and adsorption potential with benzoate as the probe compound. Both impregnated solids can catalyze Fenton-like reactions in the presence of hydrogen peroxide. pH is influential in Fenton-like reactions that are catalyzed by the impregnated solids, and benzoate transformation can be hindered by the catalytic failure to generate hydroxyl radicals in neutral and alkaline conditions. Structure of interfacial iron species after impregnated with ferrous precursor includes isolated Fe, ferric oligomers and big precipitates of ferric oxides, whereas configuration of impregnation resulted ferric precursor contains primarily ferric oligomer and precipitate. Among three types of impregnation, isolated Fe and oligomers are good catalyst for Fenton reactions, yet big precipitates are good adsorbents. Silica as a substrate promotes isolated-iron-type impregnation, while alumina prefers formation of oligomers.