Efficient hexavalent chromium reduction via non-metallic composite photocatalysts: insights into g-C3N4, carbon quantum dots, and graphene aerogel synergy

In this study, a non-metallic composite photocatalyst was synthesized using nanomaterial carbon quantum dots (C dots) combined with graphitic carbon nitride (g-C₃N₄) and graphene aerogel (GA). The composite catalysts effectively reduced the energy gap and enhanced the utilization of visible light by ultraviolet–visible spectrometer. From fluorescence spectrum analysis, the result revealed that C dots, when irradiated with 340 nm excitation light, emitted fluorescence at 460 nm. Using dynamic light scattering analyses, the results confirmed that the C dots had a size range of approximately 3–8 nm, indicating successful preparation and purification processes. Using the Taguchi method, the optimal synthesis conditions for the composite photocatalysts were determined to be a temperature of 160 °C, a preparation time of 8 h, 15 mL of C dots, and a graphene oxide to g-C₃N₄ weight ratio of 1:1. Under these above conditions, the composite photocatalyst completely reduced Cr(VI) in water under UV light within 60 min, demonstrating high photocatalytic efficiency. The stability and reusability of the catalyst were confirmed through cycling tests, which showed maintained effectiveness after five cycles. Biological toxicity tests using Daphnia magna, the results indicated that the highly toxic Cr(VI) was rendered non-toxic after treatment with the composite photocatalyst. These results confirm that the g-C₃N₄/C dots/GA photocatalyst was successfully synthesized and is capable of reducing aqueous hexavalent chromium, thus achieving the goal of reducing pollutant toxicity.