Effects of the adsorption of NOM model molecules on the aggregation of TiO2 nanoparticles in aqueous suspensions

Interaction of synthetic TiO2 (anatase) nanoparticles in aqueous suspension at pH 5 was investigated as a function of time in the presence of various organic molecules in terms of adsorption and aggregation behaviour. zeta-potential and average particle diameter were determined with electrophoretic and dynamic light scattering, respectively, while batch adsorption experiments were used to quantify the amount of organic ligand adsorbed to the TiO2 NP. An IR spectroscopic study was carried out at pH 2.8 and 5 to gain information about the interactions of the adsorbed molecules with the TiO2 surface on the molecular level. Furthermore, DLVO calculations provided information about the interaction energies between particles and their tendency to aggregate under some experimental conditions. Colloidal stability of TiO2 NPs in the presence of organic molecules was studied during a time period of up to 90 days. Results showed that ligands with different functional groups may interact differently with the surface depending on the type and position of available surface sites, the molecular structure of the ligand and suspension pH. Adsorption, hydrodynamic diameter and zeta-potential were affected by the ligand concentration in all tested systems. Increased concentration gave rise to increased adsorption, while.potential decreased and charge inversion was observed for all tested molecules at pH 5. IR spectroscopic study showed the formation of inner sphere and/or outer sphere complexes depending on pH and type of organic ligand. According to DLVO calculations, the critical coagulation concentration (CCC) indicated a trend of increasing colloidal stability with increased concentration of SRFA at pH 5, which was in agreement with the experimental data.