engleski

Surface reactions in aqueous solutions of insoluble oxides (TiO2) and poorly soluble oxides (SiO2)

Mineral/aqueous electrolyte interfaces play an important role in numerous environmental and technological processes. At the metal oxide surfaces the surface complexation and adsorption of ions and molecules take place. However, in some cases dissolution, crystal growth, surface transformation and degradation may occur. Two oxides were selected for this purpose, titania (TiO2), the naturally occurring insoluble oxide, and silica (SiO2), most commonly found in nature as quartz. Surface charge of insoluble metal oxide TiO2 depends on protonation and deprotonation reactions, while for SiO2 the interfacial water molecules additionally affect the electrical charge. The solubility of silicon dioxide in water strongly depends on its crystalline form, particle size and pH value. The surface properties of TiO2 and SiO2, namely surface charge, surface potential, and electroneutrality points were examined by means of acid-base potentiometric titration, potentiometric mass titration and electrophoresis. The size distribution was measured by dynamic and static light scattering. Surface properties were determined under different compositions of electrolyte solutions (ionic strength and pH) in which the examined surfaces are insoluble but also under conditions in which dissolution occurs. Mineral dissolution can be either diffusion controlled or kinetically controlled by surface reactions. During the dissolution of the mineral surface, the formation of electrical interfacial layer and formation of the charged surface groups take place which influence diffusion and distribution of ions between the interfacial layer and bulk solution. The dissolution process was examined by electrophoretic and calorimetric measurements as well as time monitoring of particle size distribution by static light scattering. The aim of this research is to obtain detailed mechanism and coupling of surface reactions and dissolution at titania and silica aqueous electrolyte interfaces.

Mineral/aqueous electrolyte interfaces ; TiO2 ; SiO2 ; surface complexation ; adsorption of ions and molecules ; dissolution ; crystal growth ; surface transformation ; surface degradation

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