Characterization of titanium dioxide nanoparticle removal in simulated drinking water treatment processes

This study characterized the fate of nano-TiO₂ in both powder (TiO₂(P)) and suspension (TiO₂(S)) forms in simulated drinking water treatments. Nano-TiO₂ solutions of 0.1, 1.0, and 10mg/L were prepared with deionized water and raw waters from the Changxing and Fengshan Water Treatment Plants in Taiwan to assess the effects of water matrices on nano-TiO₂ behavior during water treatment. After the laboratory simulated water treatment, including pre-chlorination, coagulation, sedimentation, filtration and post-chlorination, the residual Ti concentration ranged from 2.7 to 47.4% in different treatment units and overall removal efficiency was between 52.6% and 97.3% in all cases except for nano-TiO₂ at concentration of 0.1mg/L. Overall removal efficiency for the TiO₂ at 10mg/L concentration ranged from 9.3 to 53.5%. Sedimentation (after coagulation) and filtration were the most important processes for removing nano-TiO₂ due in part to particle agglomeration, which was confirmed by size distribution and zeta potential measurements. The size of nano-TiO₂ increased from 21-36nm to 4490nm in the supernatant after sedimentation, and subsequent filtration treatment further removed all agglomerates at size >1μm. Zeta potential revealed interactions between nano-TiO₂ particles and anionic functional groups or negatively-charged natural organic matters, leading to a decrease in surface charge. After sedimentation and filtration, the zeta potential of supernatants and filtrates were close to zero, meaning the absence of nanoparticles. The highest Ti removal after sedimentation occurred in Fengshan raw water due to higher ionic strength and coagulant dosage applied. On the other hand, the surfactant additives in TiO₂(S) promoted dispersion of nano-TiO₂ particles, which in turn led to lower particle removal. SEM images of nanoparticles after chlorination or coagulation revealed the coverage of nano-TiO₂ particles by viscous substances and formation of colloidal structures.