Investigating the behavior and catalytic activity of TiO2 nanomaterials in soil extracts

Titanium dioxide nanoparticles (TiO₂ NPs) are increasingly used as photocatalysts, or as additives in personal care products, paints or food packaging, owing to their specific and individual properties. TiO₂ NPs can enter the environment through corrosion and degradation of end-of-life products and surfaces bearing these entities. Additionally, the agricultural application of sewage sludge, presumed to be the primary sink for engineered TiO₂ through the treatment of wastewater, has become a widespread fertilizing practice. The primary environmental medium receiving unwanted TiO₂NPs as pollutants is the soil environment. Hence, studying the behavior and interactions of such NPs with the soil and the soil solution is a prerequisite for apprehending the ecological risk related to such emerging pollutants. The interactions of TiO₂ NPs with the soil solution and solid phases -adsorption, aggregation, dissolution and sedimentation- are governed by the physicochemical characteristics of the nanoparticles (size, shape, surface properties, crystal structures) as well as the properties of the soil (pH, salt and organic matter contents).

            Our study focuses on performing laboratory experiments in soil extracts, as model soil solutions, of a neutral phaeozem, an acidic regosol and a solonetz (pH>9) with engineered TiO₂ NPs: 89% anatase (20-25 nm) and 11% rutile (50 nm), pure rutile and pure anatase. The goal is twofold i) investigating the changes of the photocatalytic activity of TiO₂ NPsfollowing their interactions with the different soil solutions, and ii) studying the degradation of the dissolved natural organic material in the soil solution due to the addition of TiO₂ catalysts. Three different types of soils have been selected for the experiments displaying variable pH and salt contents (acidic and neutral soils with low salt contents, and alkaline soils with high salt contents) to examine the effect of the pH, ionic strength and salt concentrations in the soil extracts on the outcome of the experiments. The reacted NPs will be characterized by SEM for potential changes in the particle morphology, XRD for analyzing the changes induced in its crystal structure, and IR spectroscopy for examining the changes occurred on the surface of the NPs (adsorption of organic molecules from the soil solution, changes in hydrophylicity) and DRS (changes in their optical properties). The photocatalytic activity following NP interactions with the soil solutions will be evaluated by monitoring phenol degradation in a reactor. The impact of the TiO₂ NPs on the degradation of natural organic matter will be assessed by total organic carbon measurements. Our preliminary results show that depending on the crystal structure of the applied TiO₂ NP, the pH of the soil solution may significantly change and there are visible changes of the NPs' properties. These results will promote our understanding of potential environmental risks related to engineered nanoparticles released into soils and groundwater.