Effects of nano-Ag particles loading on TiO₂ photocatalytic reduction of selenate ions

The photocatalytic reduction of selenate Se(VI) ions was studied using unmodified TiO₂ and Ag-loaded TiO₂ (Ag-TiO₂) photocatalysts. In the presence of formic acid, both the TiO₂ and Ag-TiO₂ photocatalysts were effective in reducing Se(VI). The reaction proceeded through the reduction of Se(VI) ions to elemental selenium Se and then to hydrogen selenide gas (H₂Se). When unmodified TiO₂ photocatalyst was used, the Se formed from the reduction of Se(VI) was further reduced to Se^2- in the form of H₂Se upon the exhaustion of Se(VI) in solution. In the presence of the Ag-TiO₂ photocatalysts, hydrogen selenide gas was generated simultaneously with the reduction of Se(VI). It was found that the maximum Se(VI) reduction rate occurred at pH 3.5 and at 2.0at.% Ag loading while the maximum hydrogen selenide gas generation occurred at pH 3.5 and at 2.0at.% Ag loading. The simultaneous reduction of Se(VI) to hydrogen selenide gas can be attributed to efficient charge separation due to the mediation of photogenerated electrons by the Ag particles. A mechanism is proposed in terms of the TiO₂-Ag-Se electronic interaction during UV irradiation.