Removal of dyes and bacteria from wastewater using green synthesised metal decorated carbon based materials

Abstract

Water pollution associated with dyes and bacteria has become a concern to researchers due to lack of quality sanitation. South Africa falls amongst the water scarce countries. Thus, it is important to develop materials that can effectively remove a variety of pollutants such as dyes and bacteria. Titanium dioxide (TiO2) and Zinc Oxide (ZnO) are materials that have been shown to have great potential in removing these pollutants since they have both the antibacterial and photocatalytic properties. However, due to their large bandgap, fast recombination rate, selective activity and the use of toxic solvents during the synthesis, a need was identified to improve these materials. In this study, the use of Monsonia burkeana (MB) plant extract for the synthesis of TiO2 and ZnO was conducted, to ensure that environmentally safe products were produced and to assist in the antibacterial activity. Also, carbon nanomaterials were loaded on these metal oxides to assist with the reduction of the recombination rate and increase the active sites to ensure that an enhanced photodegradation process took place. Green synthesised TiO2 and ZnO as antibacterial agents and photocatalyst were investigated and compared. Moreover, carbon based materials decorated with green synthesised metal oxides for photocatalytic degradation of dyes was also conducted. In the first section, TiO2 and ZnO nanoparticles were synthesised using MB extract as a reducing agent. The synthesised TiO2 and ZnO nanoparticles were characterised using UV-vis, FTIR, TEM, SAED, SEM, XRD and TGA. These materials were then tested for their antibacterial and photocatalytic potential. From the results, LC-MS and FTIR provided evidence of the compounds, functional groups and elements that contributed to the mechanism of metal oxide formation. The UV-vis of TiO2 and ZnO nanoparticles exhibited absorbance peaks at 327 and 325 nm, respectively, confirming their formation. In support, FTIR showed vibration bands at the fingerprint region belonging to the metal oxides. SEM showed a spherical shape of TiO2 nanoparticles whereas ZnO nanoparticles exhibited a hexagonal shape. Particle size analysis showed that ZnO nanoparticles had a broad size distribution from 5 to 35 nm and TiO2 had a distribution size from 2 to 18 nm. Thereafter, the metal oxides were tested for their antibacterial and photocatalytic activity. Upon testing their antibacterial potential, ZnO nanoparticles were active against all the four bacterial strains, Staphylococcus iii aureus, Pseudomonas aeruginosa, Enterococcus faecalis and Escherichia coli, while for TiO2 nanoparticles, the strains were resistant and only partially active against Escherichia coli. The water samples collected from tap, sewage, pond and river were tested for their presence of total pollution, Escherichia coli, total coliform and Enterobacteriaceae strains. TiO2 nanoparticles inhibited most of the strains compared to ZnO nanoparticles. For photocatalytic degradation using Methylene blue (MB) as a model pollutant, the TiO2 nanoparticles showed a higher photocatalytic activity than ZnO nanoparticles with a percentage degradation of 85.5% and 61.3%, respectively. The optimum degradation was obtained at pH (10), dosage (60 mg), concentration (20 ppm) and time (120 min). For reusability, the stability behaviour of TiO2 and ZnO nanoparticles gradually decreased with an increase in the number of testing cycles. To improve the activity of TiO2 nanoparticles, carbon based materials (Carbon spheres (CSs) and Carbon nanofibers (CNFs)) were loaded with 5, 10 and 20% of TiO2 and tested for their photocatalytic activity. The 20%TiO2/CSs composite, degraded 88.5% of MB in solution at 120 min. The addition of a low bandgap material, CuO on to the composites (TiO2/CNFs and TiO2/CSs) did not improve the photocatalytic degradation of MB. This study has shown that low cost and safe materials can be produced and can be used to inhibit and degrade various pollutants.