Color-mixing effects of photosensitive organic dyes initiated by superoxide anion radicals under visible-light irradiation

Water pollution by dyes is a major health and environmental issue. Photosensitive organic dyes produce hydrated electrons, e aq − , singlet oxygen, 1 O 2 , and superoxide anion radicals, O 2 · − . O 2 · − catalyzes H 2 O 2 transformation into hydroxyl radicals ·OH, which can be used to degrade wastewater under sunlight. Here, we measured e aq − , 1 O 2 , O 2 · − and ·OH generated under visible light irradiation of photosensitive organic dyes in the presence of the auxiliary Fe 3+ , the 1,5-diphenylcarbazide molecular probe, the NH 2 OH molecular probe and the dimethyl sulphoxide probe. Results show that single photosensitive organic dyes are not degraded in the visible light/H 2 O 2 system. However, they were degraded when mixed in a certain proportion to produce a brown coloration, a phenomenon named the ‘color-mixing effect.’ Likewise, the average yields of O 2 · − in single photosensitive organic dye solutions were lower, of 0.015 (Δ A 510 ), than that in mixed brown solutions, of 0.113. The color-mixing effect is explained mainly by higher ·OH generation from increased O 2 · − production. Findings imply that sunlight irradiation is a promising way to clean colorful and dye-containing wastewater.