Photocatalytic degradation of N-nitrosodimethylamine: Mechanism, product distribution, and TiO₂ surface modification

The photocatalytic degradation (PCD) reaction of N-nitrosodimethylamine (NDMA) in water was investigated using pure and surface-modified TiO ₂. The PCD products of NDMA were methylamine (MA), dimethylamine (DMA), nitrite, nitrate, and ammonium, and their distribution could be changed by modifying the surface of TiO₂. The PCD reaction of NDMA seems to be initiated mostly by OH radicals, not valence band holes, because the addition of excess oxalates (hole scavengers) only moderately retarded the PCD rate. The presence of oxalate, however, enabled a new reductive transformation path in which the CO₂^-• radicals generated from the oxalate converted NDMA into DMA. In acidic suspensions of pure TiO₂, the formation of MA was highly favored over DMA and NH₃, whereas all degradation products (MA, DMA, and NH₃) were generated at comparable concentrations at basic pH. It is suggested that there are three parallel paths depending on the position of the initial attack of OH radical on NDMA and the product distribution is closely related with which path is favored under a specific condition. DMA production is related to the OH radical attack on the nitrosyl nitrogen. Platinum deposition, silica loading, Nafion coating, and surface fluorination were tested to investigate the effects of TiO₂ surface modification on the product distribution. The surface platinization of TiO₂ had little effect on the PCD reaction of NDMA under air-equilibrated conditions but accelerated the PCD reaction under deaerated conditions. An enhanced PCD reaction of NDMA was achieved with the silica-loaded TiO₂ and Nafion-coated TiO₂, both of which favored the formation of DMA over MA. The PCD of NDMA on surface-fluorinated TiO₂ was also highly enhanced but favored the formation of MA over the formation of DMA.