Simultaneous conversion of dye and hexavalent chromium in visible light-illuminated aqueous solution of polyoxometalate as an electron transfer catalyst

This study reports the simultaneous conversion of dye and hexavalent chromium (Cr(VI)) in visible light-illuminated polyoxometalate (POM) solution. SiW₁₂O₄₀^4- (SiW₁₂^4-) was used as a homogeneous catalyst that mediates the electron transfer from excited dye to heavy metal ion in the dye/Cr(VI)/POM ternary system. Rhodamine B (RhB), methylene blue (MB), and acid orange 7 (AO7) were chosen as test dyes. The conversion rate of dye and Cr(VI) in visible light-illuminated dye/Cr(VI)/POM system was strongly affected by the charge property of dye. The simultaneous conversion of cationic (RhB and MB) dye and anionic Cr(VI) were synergically enhanced in the presence of SiW₁₂^4-. POM successfully plays the role of the electron transfer mediator from the excited cationic dye to Cr(VI). However, the conversion of anionic dye (AO7) and Cr(VI) was negatively affected by the addition of SiW₁₂^4-. The observation that the initial UV/vis absorption spectrum of the dye/Cr(VI)/POM solution (with RhB or MB) was changed with increasing the concentration of SiW₁₂^4- indicates the complex formation between POM and cationic dyes. However, no such complex formation was observed with AO7. As for RhB, the dye degradation mechanism in the ternary RhB/Cr(VI)/POM solution was markedly different from that in the binary RhB/POM solution. The effects of various experimental parameters and the proposed mechanisms are discussed in detail.