Photocatalytic activity of graphene oxide quantum dots in an effluent from a South African wastewater treatment plant

Graphene oxide quantum dots (GOQDs) were used as photocatalysts under visible light for the discoloration of synthetic and real wastewater spiked with organic azo dye. The GOQDs were synthesized using a one-pot method involving the pyrolysis of citric acid. The characterization of the synthesized GOQDs using XPS, FTIR, XRD, Raman, UV-Vis, FEEM, Zetasizer and TEM confirmed the successful synthesis of the nanosized materials. The results of the photocatalytic experiments showed efficient degradation of the dye, which amounted to 60% and 98% over a period of 180 min for the synthetic water and spiked real wastewater, respectively. The pseudo-first-order model was found to be a better fit for both the synthetic and spiked real wastewater samples. The coefficient of correlation (R2) of 0.991 and 0.996 was obtained for the synthetic and real wastewaters, respectively. The rate of discoloration decreased with increasing concentration of the dyes. The data demonstrated that an increase in the reaction rate constant resulted in faster dye discoloration and higher catalytic activity and yielded shorter half-lives. The degradation of dye was attributed to a combination of photocatalytic mechanism and photosensitization degradation emanating from the self-degradation of photoexcited dyes. The reactive oxygen radicals and holes played a role in the degradation of the dye molecules. Additionally, the photolytic contribution was minor, while the photocatalytic activity of GOQDs was the major to the degradation process. The results showed that GOQDs can be used in the treatment of wastewater effluent for the detoxification and discoloration of the harmful dye.