Abstract
The presence of natural organic matter (NOM) in water can negatively affect water quality if NOM is present in excessive amounts during certain stages of water treatment. It is known that NOM constituents serve as precursors for DBPs (disinfection by-products) during the disinfection step of the water treatment process. Removal of natural organic matter (NOM) before disinfection is therefore essential to control DBP formation; however, owing to the heterogeneity and complexity of NOM, most water treatment processes are unable to attain complete removal of NOM from water sources. There is therefore a great need to develop methods that can effectively degrade NOM into smaller and relatively harmless compounds that are easily removed during water treatment processes. In this study, multi-walled carbon nanotubes/nitrogen, palladium co-doped titanium dioxide (MWCNT/N, Pd co-doped TiO2) (0.5–5% MWCNTs) were fabricated for the enhancement of NOM degradation under visible-light illumination. Different MWCNT/N, Pd co-doped TiO2 (also referred to as CT) nanocomposites were synthesised via a modified sol-gel method and characterised using ultraviolet visible (UV-Vis), X-ray diffraction spectroscopy (XRD), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), Fourier transform infrared (FTIR) Spectroscopy and energy-dispersive X-ray spectroscopy (EDS) analysis. The UV-Vis results showed that introducing the MWCNTs on the nanocomposites was accompanied by a red shift, with CT (0.5% MWCNTs) occurring at a longer wavelength than the others. The efficiency and effectiveness of the synthesised nanocomposites was quantified through measuring the NOM degradation rate in raw water samples collected from two South African water treatment plants. These two drinking water treatment plants were selected as the raw water treated in these plants contains different NOM fractions: the raw water treated at Midvaal (MV) Water Company contains transphilic NOM fractions, while that treated at Plettenberg Bay (P) Water Treatment Plant contains high amounts of hydrophobic NOM. The changes in the amount of NOM in a sample were monitored by measuring the UV absorbance at 254 nm (UVA254) as there is a strong correlation between absorbance at this wavelength and the aromatic content. The highest photocatalytic activity was observed with CT (0.5% MWCNTs) which attained 69.4% UVA254 reduction for MV and 97.7% UVA254 reduction for P raw waters. Compared to conventional methods applied by the two water treatment plants of interest, NOM removal efficiency (in terms of UVA254 reduction) by CT (0.5% MWCNTs) increased by up to 9.4% and 11.5% for MV and P water treatment plants, respectively. The enhanced photocatalytic activity of this CT nanocomposite (0.5% MWCNTs) is attributable to the large surface area of the synthesised nanocomposites which allows huge amounts of NOM to be adsorbed onto the surface of the TiO2, thereby increasing their photodegradation. It is also due to the synergistic effect of N and Pd which enhanced the photoactivity of the TiO2 in the MWCNT/N, Pd co-doped TiO2 composite. Overall, it is clear that the use of the conventional processes is not effective in removing NOM from water. Findings obtained show that CT (0.5% MWCNTs) nanocomposite is a promising future NOM removal method that could complement the available water treatment processes in enhancing the removal of the aromatic component of NOM in water.
Published Version
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