Simultaneous and Synergistic Conversion of Dyes and Heavy Metal Ions in Aqueous TiO_2 Suspensions under Visible-Light Illumination

摘要

This study reports synergistic effects in the simultaneous conversion of dyes and heavy metal ions in aqueous TiO_2/ dye/metal ion systems(ternary components)under visible light(A > 420 nm).TiO_2/Cr(VI)/Acid Orange 7(A07),TiO_2/ Cr(VI)/Rhodamine B(RhB),TiO_2/Ag~+/A07,and TiO_2/Ag+/RhB were chosen as test systems.Although dyes can be degraded in TiO_2 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 TiO_2 suspensions were negligible,but they were highly accelerated with dyes present.In particular,the synergistic effect in the ternary system of TiO_2/ Cr(VI)/A07 was outstanding.The presence of dissolved oxygen increased the photoreduction rate of Cr(VI)despite the fact that Cr(VI)and O_2 are competing electron acceptors.This is ascribed to in-situ photogenerated H_2O_2 from O_2,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 TiO_2.The visible-light-induced reduction of Ag~+ did not occur at all in TiO_2/Ag~+ system,but it was enabled in both TiO_2/Ag~+/ RhB and TiOz/Ag+/A07 to generate Ag particles.On the other hand,the binary systems of Cr(VI)/A07,Ag~+/A07,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.