Graphene oxide quantum dots-based nanocomposites for wastewater treatment

Abstract

The presence of new emerging pollutants in freshwater systems poses potentially serious threats to human health, food production, ecosystems and hinder economic growth. Emerging personal care products and pharmaceuticals, pesticides, wide range of industrial chemicals, as well as climate change, all pose new threats to water quality. With largely long-term effects on people, the environment and ecosystems being unclear. The main aim of the thesis was to synthesize graphene oxide quantum dots (GQDs) using a facile one-pot method through the pyrolysis of citric acid and deploy them in the removal of emerging wastewater pollutants. GQDs were initially applied alone as photocatalysts in the photodegradation of pollutants in real wastewater spiked with an organic dye (Brilliant black). The photodegradation and removal efficiency of the GQDs were improved by conjugating differently shaped zinc oxide (ZnO) nanoparticles to the GQDs. The design strategy and approach were based on the GQDs to acting as reservoirs for the photogenerated electrons that could subsequently enhance the lifetime of the electrons, exploiting the ability of GQDs to absorb light as well as the π → π∗ transition in GQDs that allow more harvesting of sunlight. Upon conjugation of the GQDS with ZnO, there was a significant decrease in the band gaps, along with an improvement in photocatalytic efficiency. The conjugates of ZnO-GQDs showed enhanced bacterial inhibition against the bacterial strains tested (Bacillus cereus (B. cereus), Pseudomonas aeruginosa (P. aeruginosa), Staphylococcus aureus (S. aureus) and Escherichia coli (E. coli)). When PAA was employed as a synergistic oxidant on the GQDs; radicals that were not prone to quenching in complex water matrices were produced (acetylperoxyl and peroxyl radicals CH3C(=O)OO• and CH3C(=O)O•). PAA exhibited disinfectant capabilities that were not entirely dependent on pH and did not generate toxic disinfection by-products (DBPs) in the treated effluent. To mitigate the limitations of a slurry photocatalytic system, the GQDs were embedded in polyethersulfone (PES), and the fabricated membranes were tested in a hybrid-filtration-advanced oxidation processes where the performance of the fabricated membrane was compared to that of a commercial membrane and water quality parameters were evaluated to determine the quality of the water. GQDs were found to be promising materials for conjugation with other photocatalysts to form heterojunctions with increased photocatalytic activity under visible light irradiation. When embedded in polymeric membranes, the activity of the GQDs was not lost and the membrane could perform a dual AOPs/filtration function. This is important for the application of the membranes in real wastewater treatment applications etc.