Coating of silica sand with goethite: preparation and analytical identification

Abstract

Preparation and characterisation of silica sand coated with well defined iron oxides is described. The procedure employs a heterogeneous suspension reaction in which the silica sand is mixed with an aqueous suspension of the iron oxide with adjusted pH and ionic strength. The coating reaction was observed to occur with different iron oxides (e.g. goethite, hematite) on cristobalite as well as quartz surfaces. The coating reaction of silica sand (cristobalite) with goethite was studied as a function of pH and ionic strength in detail. The extent of this coating reaction shows a characteristic increase with increasing pH and an abrupt decrease at pH values above the point of zero charge of goethite. This behavior can be explained with a simple electrostatic model, which is based on the assumption that the charge of the adsorbed goethite particles neutralizes the charge of the silica surface. The surface coatings consist of well characterized and crystalline iron oxides which were investigated using scanning electron microscopy and Fourier transform infrared (FTIR) spectroscopy. Detachment experiments with coated sand by goethite show, that the iron oxide coating adheres very strongly to the silica surface. Based on results from X-ray photoelectron spectroscopy (XPS) we propose that the formation of chemical FeOSi bonds is responsible for the stability of the coating.