COATING OF SILICA SAND WITH GOETHITE - PREPARATION AND ANALYTICAL IDENTIFICATION

Preparation and characterisation of silica sand coated with well defin ed iron oxides is described. The procedure employs a heterogeneous sus pension reaction in which the silica sand is mixed with an aqueous sus pension of the iron oxide with adjusted pH and ionic strength. The coa ting reaction was observed to occur with different iron oxides (e.g. g oethite, hematite) on cristobalite as well as quartz surfaces. The coa ting reaction of silica sand (cristobalite) with goethile was studied as a function of pH and ionic strength in detail. The extent of this c oating reaction shows a characteristic increase with increasing pH and an abrupt decrease at pH values above the point of zero charge of goe thite. This behavior can be explained with a simple electrostatic mode l, which is based on the assumption that the charge of the adsorbed go ethite particles neutralizes the charge of the silica surface. The sur face coatings consist of well characterized and crystalline iron oxide s which were investigated using scanning electron microscopy and Fouri er transform infrared (FTIR) spectroscopy. Detachment experiments with coated sand by goethite show, that the iron oxide coating adheres ver y strongly to the silica surface. Based on results from X-ray photoele ctron spectroscopy (XPS) we propose that the formation of chemical Fe- O-Si bonds is responsible for the stability of the coating.