Effect of ion (Al, Fe and Zn) co-doped TiO₂ nanotubes on photocatalytic degradation of humic acids under UV/ozonation for drinking water purification

O₃/UV/TiO₂ was used to effectively decompose humic acids (HAs) in drinking water. A series of Al-doped, Fe-doped, Zn-doped and co-doped TiO₂ nanotubes were successfully synthesized by a hydrothermal method. According to the characterization of ion-doped TiO₂ nanotubes, Al³⁺ and Fe³⁺ ions occupied substitutional positions in the crystal lattice, and Zn²⁺ ions were partially dispersed in the bulk of the TiO₂ nanotubes, with ZnO formed on the surface of the TiO₂. The calcination temperature and doping concentration could affect the anatase phase weight fractions, average crystallite sizes, Brunauer–Emmett–Teller (BET) surface area, bandgap energy, and photocatalytic activity of the catalysts. For single ion-doped TiO₂ nanotubes, the best photocatalytic activities were achieved when the ion-doping amount was 1.0%, and the optimal calcination temperatures for Al-, Fe- or Zn-doped TiO₂ were 600°C, 550°C and 550°C, respectively. The highest HA removal efficiency of 80.1% was achieved in the presence of 550°C calcined, 1.0% co-doped TiO₂ nanotubes with an Fe:Zn ratio of 1:1 (atomic percent), with second-order rate constant of 0.0394 L/(min·mg). The addition of a third ion had little effect on the photocatalytic activity. The pollutants in filter influent from No. 9 waterworks in Beijing could be efficiently removed, with UV₂₅₄ and total organic carbon (TOC) removal efficiencies of 57.2% and 44.7%, respectively.