SIZE DEPENDENT SURFACE COMPLEXATION OF TITANIUM DIOXIDE NANOPARTICLES

Mineral nanoparticles are present in all compartments of the environment. Due to their small size, they have an increased roughness and curvature of the surface, a large specific surface area, and a high abundance of attractive binding sites, which are properties that result in high reactivity (e.g. surface complexation). Interactions that occur at the surface of nanoparticles are important for understanding and predicting nanoparticle fate and behavior, as well as characterizing the potential risks for the environment and human health. Surface complexation also plays a significant role in the transport of metals and organic contaminants in natural systems.

The surface charge of nanoparticles is sometimes different than the surface charge of corresponding larger microparticles. In the present study, stable TiO₂ (anatase) suspensions of well-defined particle sizes (<30 nm) have been synthesized and extensively characterized, and their surface complexation is studied as a function of particle size, pH and ionic strength. Selected phenolic compounds possessing several functional groups, e.g. carboxyl and hydroxyl groups, are used as model substances to mimic the interactions of nanoparticles with natural organic matter. The aim is to mechanistically understand these interactions that are known to govern nanoparticle fate and transport in the environment. Adsorption experiments are combined with potentiometric titrations, electrophoretic mobility measurements and light scattering techniques.

Classic surface complexation models are built on certain assumptions that may not be valid for nanoparticles. Therefore, the Charge Distribution - Multisite Complexation (CD‑MUSIC) model established for microparticles is compared with the Corrected Debye-Hückel theory of surface complexation (CDH-SC), which is generic but also applicable to modeling the surface charging of nanoparticles with a diameter smaller than 25 nm. The theoretical models obtained in this work will be used to describe and predict surface complexation on nanoparticles in the presence of NOM adsorption in different aquatic environments.