Doped and immobilized titanium dioxide photocatalysts as a potential source of nitrosamine formation

Immobilized and visible-light-active titanium dioxide (TiO₂) is widely used for water treatment. However, the accelerated generation of degradation byproducts is a potential risk of TiO₂-based photocatalysis. This study aimed to investigate the structural effect of engineered TiO₂ samples on the formation of major nitrosamines during photocatalysis. The nitrogen-containing impurities and leached metal ions from doped-TiO₂ samples could exacerbate nitrosamine formation potential (FP) in distilled water, secondary effluent, and chloraminated water. Doped-TiO₂ with 2-ethylimidazole, trimethylamine, triethylamine, and N-carbon nanotubes could leach in the range of 47-64 ng L^-1 nitrosamines (including N-nitrosomethylethylamine, N-nitrosodiethylamine, N-nitrosodimethylamine, and N-nitrosopyrrolidine) even under dark conditions. Furthermore, we investigated the role of metal dopants on nitrosamine-FP during the chloramination of precursors such as dimethylamine and microcystin-LR. Metal ions such as Cu that leached from the metal-doped catalysts may catalyze the nitrosamine-FP. Therefore, pre-purification (washing) and immobilization of doped-TiO₂ samples on substrates are suggested to remove a considerable amount of nitrosamines. However, during the prolonged tryout, the selection of substrates was critical. Polymeric supports, such as polyimide and polyvinylpyrrolidone, can produce up to 85 ng L^-1 nitrosamine, whereas TiO₂ immobilized onto steel mesh can remove nitrosamine formation during photocatalytic oxidation followed by chloramination. This study systematically screened a diverse range of dopants, supports, and solvents in engineered TiO₂ photocatalysts, in 61 samples, and provided novel insights into their effect on nitrosamine formation.