α-Sulfur as a metal-free catalyst to activate peroxymonosulfate under visible light irradiation for decolorization

While transition metals have been frequently used to activate peroxymonosulfate (PMS) for chemical oxidation reactions, recently metal-free activation of PMS has also drawn great attention considering that no metal is required and the environmental impact can be minimized. In this study, orthorhombic alpha-sulfur (alpha-S), for the first time, is employed as a metal-free photocatalyst to activate PMS under visible (vis) light irradiation. To study the activation of PMS by the alpha-S/vis process, decolorization of rhodamine B (RhB) dye is selected as a model reaction. Parameters affecting the decolorization were investigated, including alpha-S loading, PMS dosage, temperature, pH, salt and inhibitors. The decolorization using PMS activated by alpha-S/vis was much faster than the self-activation of PMS. A higher alpha-S loading also facilitated the activation of PMS; however, over-loading of alpha-S led to the shielding effect, thereby decreasing the decolorization extent. Higher PMS dosages and temperatures were both preferable for the decolorization. While the decolorization was improved under acidic conditions, the activation of PMS was hindered under alkaline conditions. When high concentrations of NaCl were added to RhB solutions, the decolorization extent still remained almost the same. Electron paramagnetic resonance (EPR) spectroscopic analysis was performed to probe into the mechanism of PMS activated by the alpha-S/vis process. The alpha-S/vis process was found to be recyclable and stable over multiple cycles, even though alpha-S did not undergo any regeneration treatments. Considering these features, the alpha-S/vis process appears to be a promising and environmentally friendly process to activate PMS for chemical oxidation reactions.