Photodegradation of Azo Dyes in Sunlight Promoted by Nitrogen–Sulfur–Phosphorus Codoped Carbon Dots

Removal of organic contaminants such as azo dyes is highly desirable because of their toxicity, stability, and mutagenic properties. Herein an efficient method for the photodegradation of two well-known model azo dyes, namely metanil yellow (MY) and methyl orange (MO), under sunlight by multiple-heteroatom-doped carbon dots (CD) (nitrogen-, sulfur-, and phosphorus-doped-CD (NSP-CD)) is presented. The photodegradation results favored sunlight as a sustainable light source, as its value of t_1/2 is significantly less than that of artificial light (100 W tungsten bulb). Various parameters, such as the concentrations of individual dyes (20-100 ppm), a mixture of both dyes, changes in pH, and foreign/interfering ions, were investigated to understand the photocatalytic activity of NSP-CD. Under sunlight, photodegradation of ∼20 ppm of dye was observed in ∼60 min (for MY) and ∼90 min (for MO), respectively. A comparative NMR investigation was performed to confirm the photodegradation of the complex structural framework of azo dyes by NSP-CD. Moreover, to explore the real-life utility of the process, sunlight-promoted photodegradation experiments were conducted by externally spiking four different types of industrial samples. Spiking of MY and MO dyes with concentrations in the range of ∼15-23 ppm showed similar results of photodegradation.