Improved visible light-driven photocatalytic degradation of an industrial dye Acid Orange 7 using metal-free sulfur-doped graphitic carbon nitride

Abstract

Sulfur-doped graphitic carbon nitride (SGCN) is prepared by thermal polymerization and utilized for industrial Acid Orange 7 (AO-7) degradation. Under visible light irradiation, the degradation pathway is investigated using liquid chromatography–mass spectrometry (LC–MS) at different time intervals. The degradation efficiency and reaction kinetics are checked by UV-visible spectroscopy. The degradation rate for SGCN (98%) is much higher than that for pristine graphitic carbon nitride (45%) in 70 min under visible light irradiation. SGCN shows higher reaction rate constants (k = 0.0439 min⁻¹) and shorter t_1/2 (t_1/2 = ln 2/k) of 15 min than GCN (k = 0.0089 min⁻¹ and t_1/2 (t_1/2 = ln 2/k) of 77 min). The reaction kinetics follow pseudo-first-order kinetics with R² values of 0.984 and 0.987 for GCN and SGCN, respectively. A biocompatibility test is also performed on seeds that exhibited significant growth, similar to the control sample. Results suggest that the solution treated with SGCN has notable potential for cultivation. Effects of coexisting anions and cations are also checked using an AO-7 solution, indicating that coexisting ions lowered the SGCN photocatalytic efficiency. Trapping agents are added to determine the reactive species, and results reveal that superoxide radicals (˙O₂⁻) are the main reactive species, while ˙OH, e⁻, and h⁺ play a minor role in AO-7 degradation. The degradation efficiency of the catalysts in tap water reaches up to 81%. SGCN also shows 70% and 81% degradation efficiency for cationic methylene blue (MB) and anionic Congo red (CR), respectively, suggesting its versatility in degrading multiple organic pollutants.