Simultaneous and synergistic conversion of dyes and heavy metal ions in aqueous TiO₂ suspensions under visible-light illumination

This study reports synergistic effects in the simultaneous conversion of dyes and heavy metal ions in aqueous TiO₂/dye/metal ion systems (ternary components) under visible light (λ > 420 nm). TiO ₂/Cr(VI)/Acid Orange 7 (AO7), TiO₂/Cr(VI)/Rhodamine B (RhB), TiO₂/Ag⁺/AO7, and TiO₂/Ag ⁺/RhB were chosen as test systems. Although dyes can be degraded in TiO₂ suspensions under visible light, their removal rates were markedly enhanced in the presence of metal ions. Similarly, the reduction rates of metal ions in visible-light-illuminated TiO₂ suspensions were negligible, but they were highly accelerated with dyes present. In particular, the synergistic effect in the ternary system of TiO₂/Cr(VI)/AO7 was outstanding. The presence of dissolved oxygen increased the photoreduction rate of Cr(VI) despite the fact that Cr(VI) and O₂ are competing electron acceptors. This is ascribed to in-situ photogenerated H₂O₂ from O₂, which acts as a reductant of Cr(VI). RhB and Ag⁺ ions could be also converted simultaneously under visible light both in the presence and absence of TiO₂. The visible-light-induced reduction of Ag⁺ did not occur at all in TiO²/Ag⁺ system, but it was enabled in both TiO₂/Ag⁺/RhB and TiO ₂/Ag⁺/AO7 to generate Ag particles. On the other hand, the binary systems of Cr(VI)/AO7, Ag⁺/AO7, and Ag⁺/RhB show significant visible-light activities for the conversion of both dye and metal ion. In this case, metal ions and dyes seem to form complexes that induce intracomplex electron transfers upon visible-light absorption. The Cr(VI)/RhB system, however, exhibited insignificant visible-light reactivity.