Visible light oxidation of inorganic sulfide by modified Nb-containing photocatalysts

Inorganic sulfide, a pollutant present in massive quantities in effluents from different industries is an extremely toxic and corrosive contaminant that causes several economic and environmental damages. For these reasons, this study brings a promising and environmentally friendly technique of heterogeneous photocatalysis using visible light as an energy source, to transform inorganic sulfides into less toxic forms of sulfur, such as sulfate (SO ₄ ²⁻ ), thiosulfates (S ₂ O ₃ ²⁻ ). Niobium(V) oxides were modified with cobalt species and showed a bandgap in a wide range of visible light. The results show that the sulfide oxidation reactions are all pseudo-first order and that the Co-containing material with the highest amount of Co ³⁺ species presents lower activation energy (E _a ). UV-Vis spectroscopy reveals that all modified materials photooxidized more than 88% of a 4000 ppm S ²⁻ solution, at 25ºC, reaching almost 100% oxidation in just 2h by the best material, mNb _2 − x Co _x O _5, maintain up to 80% of catalytic capacity for 6 reaction cycles. Raman analyses confirmed the presence of SO ₄ ²⁻ and S ₂ O ₃ ²⁻ species after the reactions. Mechanisms studies have shown that the most important reactive species for the photooxidation is superoxide radical (O ₂ ⁻ •), but the reaction occurs through a combination involving these radicals and singlet oxygen species (O ₂ *).