Effect of 17β-estradiol on stability and mobility of TiO₂ rutile nanoparticles

Contaminants including titanium dioxide nanoparticles (n-TiO₂), as well as organic wastewater contaminants (OWCs), have been detected in wastewater treatment plant effluents, however, no information is yet available on how OWCs may modify the surface properties of TiO₂ nanoparticles, or influence their stability in water and subsequent mobility in porous media. In this study, 17β-estradiol (E2) was chosen as a representative OWC to investigate the interaction between OWCs and n-TiO₂. Batch and kinetic sorption experiments and Fourier Transform Infrared (FTIR) Spectrometer measurements confirmed that E2 was quickly sorbed onto the surface of n-TiO₂ aggregates in water. Aggregation experiments showed that the presence of E2 has a minor influence on the size of n-TiO₂ aggregates under lower ionic strength conditions at natural pH. In high ionic strength solution, the presence of E2 led to an increased average hydrodynamic diameter and a wider distribution of n-TiO₂ aggregate sizes. Interaction energy analyses indicated that steric repulsion likely contributed to the stability of the n-TiO₂ suspension in the presence of E2. Mobility analysis based on the clean bed filtration theory indicated that the impact of E2 on the mobility of n-TiO₂ in porous media is minimal in comparison to the influence of solution ionic strength.