Visible light-driven oxidation of arsenite, sulfite and thiazine dyes: A new strategy for using waste to treat waste

Recently, the advanced oxidation processes (AOPs) based on sulfate radical (SO4•⁻) using sulfite has received increasing concerns. Metal-free activation of sulfite is still in urgent demands from the perspective of cleaner production since the reported efficient systems containing sulfite has been relied on the transition metals. In this work, methylene blue (MB⁺), a well-known photosensitizer and thiazine dye contaminant, was utilized to activate sulfite for the efficient photooxidation of arsenite (As(III)) to arsenate (As(V)) under visible light (LED lamps with wavelength 640 nm) and sunlight, respectively. The initial oxidation rates of 5–40 μM As(III) are 28-fold–116-fold as that of the decolorization of 0.27 μM MB⁺ with 50 μM sulfite at pH 7.3. The maximum stoichiometric ratio MB⁺: As(III): sulfite was obtained to be 1 : 142: 247, suggesting an efficient utilization of both MB⁺ and sulfite as waste resources. The electrical energy per order (EE/O) index was calculated to be 1.83 × 10−3 kWh L−1 for the visible light-MB+-sulfite system, which is much less than those reported systems. Oxysulfur radicals (mainly peroxymonosulfate radical, SO5•⁻) produced in this photochemical system are responsible for As(III) oxidation. The other two common thiazine dyes (thionine and Azure B) are also capable of activating sulfite for As(III) oxidation due to the same chromophores. Thus the novel strategy of using waste sulfite and the thiazine dyes is promising for As(III) oxidation under sunlight, with which low-concentration wastes of thiazine dye and sulfite can be utilized to achieve the goal of cleaner production.