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

Abstract

Water pollution by dyes is a major health and environmental issue. Photosensitive organic dyes produce hydrated electrons, eaq−, singlet oxygen, 1O2, and superoxide anion radicals, O2·−. O2·− catalyzes H2O2 transformation into hydroxyl radicals ·OH, which can be used to degrade wastewater under sunlight. Here, we measured eaq−, 1O2, O2·− and ·OH generated under visible light irradiation of photosensitive organic dyes in the presence of the auxiliary Fe3+, the 1,5-diphenylcarbazide molecular probe, the NH2OH molecular probe and the dimethyl sulphoxide probe. Results show that single photosensitive organic dyes are not degraded in the visible light/H2O2 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 O2·− in single photosensitive organic dye solutions were lower, of 0.015 (ΔA510), than that in mixed brown solutions, of 0.113. The color-mixing effect is explained mainly by higher ·OH generation from increased O2·− production. Findings imply that sunlight irradiation is a promising way to clean colorful and dye-containing wastewater.